Pluggable cumulative statistics

From 4c065f73cb744d1735c01e6f276d658853810f2e Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 13 Jun 2024 13:25:05 +0900
Subject: [PATCH v2 1/5] Switch PgStat_Kind from enum to uint32

A follow-up patch is planned to make this counter extensible, and
keeping a trace of the kind behind a type is useful in the internal
routines used by pgstats.  While on it, switch pgstat_is_kind_valid() to
use PgStat_Kind, to be more consistent with its callers.
---
 src/include/pgstat.h                | 35 ++++++++++++++---------------
 src/backend/utils/activity/pgstat.c |  6 ++---
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2136239710..2d30fadaf1 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -32,26 +32,25 @@
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"
 
 /* The types of statistics entries */
-typedef enum PgStat_Kind
-{
-	/* use 0 for INVALID, to catch zero-initialized data */
-	PGSTAT_KIND_INVALID = 0,
+#define PgStat_Kind uint32
 
-	/* stats for variable-numbered objects */
-	PGSTAT_KIND_DATABASE,		/* database-wide statistics */
-	PGSTAT_KIND_RELATION,		/* per-table statistics */
-	PGSTAT_KIND_FUNCTION,		/* per-function statistics */
-	PGSTAT_KIND_REPLSLOT,		/* per-slot statistics */
-	PGSTAT_KIND_SUBSCRIPTION,	/* per-subscription statistics */
+/* use 0 for INVALID, to catch zero-initialized data */
+#define PGSTAT_KIND_INVALID 0
 
-	/* stats for fixed-numbered objects */
-	PGSTAT_KIND_ARCHIVER,
-	PGSTAT_KIND_BGWRITER,
-	PGSTAT_KIND_CHECKPOINTER,
-	PGSTAT_KIND_IO,
-	PGSTAT_KIND_SLRU,
-	PGSTAT_KIND_WAL,
-} PgStat_Kind;
+/* stats for variable-numbered objects */
+#define PGSTAT_KIND_DATABASE	1	/* database-wide statistics */
+#define PGSTAT_KIND_RELATION	2	/* per-table statistics */
+#define PGSTAT_KIND_FUNCTION	3	/* per-function statistics */
+#define PGSTAT_KIND_REPLSLOT	4	/* per-slot statistics */
+#define PGSTAT_KIND_SUBSCRIPTION	5	/* per-subscription statistics */
+
+/* stats for fixed-numbered objects */
+#define PGSTAT_KIND_ARCHIVER	6
+#define PGSTAT_KIND_BGWRITER	7
+#define PGSTAT_KIND_CHECKPOINTER	8
+#define PGSTAT_KIND_IO	9
+#define PGSTAT_KIND_SLRU	10
+#define PGSTAT_KIND_WAL	11
 
 #define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
 #define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index dcc2ad8d95..d558cc1414 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -173,7 +173,7 @@ static void pgstat_prep_snapshot(void);
 static void pgstat_build_snapshot(void);
 static void pgstat_build_snapshot_fixed(PgStat_Kind kind);
 
-static inline bool pgstat_is_kind_valid(int ikind);
+static inline bool pgstat_is_kind_valid(PgStat_Kind kind);
 
 
 /* ----------
@@ -1254,9 +1254,9 @@ pgstat_get_kind_from_str(char *kind_str)
 }
 
 static inline bool
-pgstat_is_kind_valid(int ikind)
+pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
+	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
 }
 
 const PgStat_KindInfo *
-- 
2.45.1

From 87d3c9a0f8197bcc962b8b74f85d5710a3a0dba2 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 13 Jun 2024 13:25:48 +0900
Subject: [PATCH v2 2/5] Replace hardcoded identifiers in pgstats file by
 variables

This changes three variable types:
- N for named entries.
- S for entries identified by a hash.
- E for end-of-file
---
 src/backend/utils/activity/pgstat.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index d558cc1414..f03fee7cd5 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -127,6 +127,13 @@
 
 #define PGSTAT_SNAPSHOT_HASH_SIZE	512
 
+/* ---------
+ * Identifiers in stats file.
+ * ---------
+ */
+#define PGSTAT_FILE_END		'E' /* end of file */
+#define PGSTAT_FILE_NAME	'N' /* stats entry identified by name */
+#define PGSTAT_FILE_SYSTEM	'S' /* stats entry identified by PgStat_HashKey */
 
 /* hash table for statistics snapshots entry */
 typedef struct PgStat_SnapshotEntry
@@ -1408,7 +1415,7 @@ pgstat_write_statsfile(void)
 		if (!kind_info->to_serialized_name)
 		{
 			/* normal stats entry, identified by PgStat_HashKey */
-			fputc('S', fpout);
+			fputc(PGSTAT_FILE_SYSTEM, fpout);
 			write_chunk_s(fpout, &ps->key);
 		}
 		else
@@ -1418,7 +1425,7 @@ pgstat_write_statsfile(void)
 
 			kind_info->to_serialized_name(&ps->key, shstats, &name);
 
-			fputc('N', fpout);
+			fputc(PGSTAT_FILE_NAME, fpout);
 			write_chunk_s(fpout, &ps->key.kind);
 			write_chunk_s(fpout, &name);
 		}
@@ -1435,7 +1442,7 @@ pgstat_write_statsfile(void)
 	 * pgstat.stat with it.  The ferror() check replaces testing for error
 	 * after each individual fputc or fwrite (in write_chunk()) above.
 	 */
-	fputc('E', fpout);
+	fputc(PGSTAT_FILE_END, fpout);
 
 	if (ferror(fpout))
 	{
@@ -1572,8 +1579,8 @@ pgstat_read_statsfile(void)
 
 		switch (t)
 		{
-			case 'S':
-			case 'N':
+			case PGSTAT_FILE_SYSTEM:
+			case PGSTAT_FILE_NAME:
 				{
 					PgStat_HashKey key;
 					PgStatShared_HashEntry *p;
@@ -1581,7 +1588,7 @@ pgstat_read_statsfile(void)
 
 					CHECK_FOR_INTERRUPTS();
 
-					if (t == 'S')
+					if (t == PGSTAT_FILE_SYSTEM)
 					{
 						/* normal stats entry, identified by PgStat_HashKey */
 						if (!read_chunk_s(fpin, &key))
@@ -1647,8 +1654,8 @@ pgstat_read_statsfile(void)
 
 					break;
 				}
-			case 'E':
-				/* check that 'E' actually signals end of file */
+			case PGSTAT_FILE_END:
+				/* check that PGSTAT_FILE_END actually signals end of file */
 				if (fgetc(fpin) != EOF)
 					goto error;
 
-- 
2.45.1

From 10ecf14b8bc51d870e2aaccb57386c3fda91615e Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 20 Jun 2024 09:19:54 +0900
Subject: [PATCH v2 3/5] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats
---
 src/include/pgstat.h                |  32 +++++-
 src/include/utils/pgstat_internal.h |   4 +-
 src/backend/utils/activity/pgstat.c | 171 +++++++++++++++++++++++++---
 src/backend/utils/adt/pgstatfuncs.c |   2 +-
 4 files changed, 189 insertions(+), 20 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2d30fadaf1..59abd422c3 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -52,9 +52,35 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+#define PGSTAT_KIND_MAX	256	/* Maximum ID allowed */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index dbbca31602..0fcee1ff3d 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -446,7 +446,7 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX_BUILTIN + 1];
 
 	PgStat_ArchiverStats archiver;
 
@@ -503,6 +503,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index f03fee7cd5..5b18d78782 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for out-of-core modules to define custom statistics kinds,
+ * that can use the same properties as any in-core stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all statistics kinds defined in
+ * core, and pgstat_kind_custom_infos for custom kinds registered at startup
+ * by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -249,7 +257,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -261,7 +269,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -399,6 +407,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -1051,7 +1068,7 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
@@ -1248,22 +1265,33 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID;	/* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
@@ -1271,7 +1299,94 @@ pgstat_get_kind_info(PgStat_Kind kind)
 {
 	Assert(pgstat_is_kind_valid(kind));
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
+
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL &&
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	Assert(false);
+	return NULL;				/* keep compiler quiet */
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * These are not supported for now, as these point out to fixed areas of
+	 * shared memory.
+	 */
+	if (kind_info->fixed_amount)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics property is invalid"),
+				 errhint("Custom cumulative statistics cannot use a fixed amount of data.")));
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+							   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom resource manager \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom resource manager \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1349,7 +1464,7 @@ pgstat_write_statsfile(void)
 
 	/*
 	 * XXX: The following could now be generalized to just iterate over
-	 * pgstat_kind_infos instead of knowing about the different kinds of
+	 * pgstat_kind_builtin_infos instead of knowing about the different kinds of
 	 * stats.
 	 */
 
@@ -1405,6 +1520,17 @@ pgstat_write_statsfile(void)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1529,8 +1655,8 @@ pgstat_read_statsfile(void)
 
 	/*
 	 * XXX: The following could now be generalized to just iterate over
-	 * pgstat_kind_infos instead of knowing about the different kinds of
-	 * stats.
+	 * pgstat_kind_builtin_infos instead of knowing about the different kinds
+	 * of stats.
 	 */
 
 	/*
@@ -1639,7 +1765,7 @@ pgstat_read_statsfile(void)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u",
 							 key.kind, key.dboid, key.objoid);
 						goto error;
 					}
@@ -1692,8 +1818,8 @@ pgstat_reset_after_failure(void)
 {
 	TimestampTz ts = GetCurrentTimestamp();
 
-	/* reset fixed-numbered stats */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	/* reset fixed-numbered stats for built-in */
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
@@ -1703,6 +1829,21 @@ pgstat_reset_after_failure(void)
 		kind_info->reset_all_cb(ts);
 	}
 
+	/* do the same for custom stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+		{
+			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+			if (!kind_info)
+				continue;
+			if (!kind_info->fixed_amount)
+				continue;
+			kind_info->reset_all_cb(ts);
+		}
+	}
+
 	/* and drop variable-numbered ones */
 	pgstat_drop_all_entries();
 }
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.1

From e74cbbdedbb1cf4945b401057e5e111116bdba22 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 13 Jun 2024 13:37:33 +0900
Subject: [PATCH v2 4/5] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 63 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 1d0b65193e..a1c2319857 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3682,6 +3682,69 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in
+     <filename>src/test/modules/injection_points/injection_stats.c</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.1

From c798aba13372aa2afdf6b9452fa28b8915e6043e Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 20 Jun 2024 09:18:35 +0900
Subject: [PATCH v2 5/5] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 194 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 +++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  50 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 324 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 31bd787994..9d84d9aaf3 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -9,9 +12,13 @@ PGFILEDESC = "injection_points - facility for injection points"
 REGRESS = injection_points
 REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index c16a33b08d..deaf47d8ae 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -54,3 +54,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index 5c44625d1d..46bec5059c 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -400,5 +414,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..c37b0b33d3
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,194 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+#define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 8e1b5b4539..612216c144 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -37,4 +38,12 @@ tests += {
     # The injection points are cluster-wide, so disable installcheck
     'runningcheck': false,
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..f8d90a3869
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,50 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres',
+	"SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres',
+	"SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 61ad417cde..481baee16d 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2116,6 +2116,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2147,6 +2148,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.1

From 2286f57cddd6e5b1cf7b6f771c9f08f9acb2a937 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 13:49:53 +0900
Subject: [PATCH v3 4/6] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 63 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index f3a3e4e2f8..7e34c5020b 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3686,6 +3686,69 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in
+     <filename>src/test/modules/injection_points/injection_stats.c</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.2

From 143d38f0292e08cde279a3073a5ea45844601cc7 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 13:58:58 +0900
Subject: [PATCH v3 5/6] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 194 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 +++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  48 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 322 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 2ffd2f77ed..7b9cd12a2a 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -11,9 +14,13 @@ REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
 ISOLATION = inplace
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index c16a33b08d..deaf47d8ae 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -54,3 +54,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index 1b695a1820..f65b34ce82 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -400,5 +414,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..c37b0b33d3
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,194 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+#define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 3c23c14d81..a52fe5121e 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -42,4 +43,12 @@ tests += {
       'inplace',
     ],
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..7d5a96e522
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,48 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index e6c1caf649..b81c137479 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2116,6 +2116,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2147,6 +2148,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.2

From 3cda5fe5588a20c3f037334a4879289ca57ec222 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 18:26:25 +0900
Subject: [PATCH v3 6/6] Add support for fixed-numbered statistics in pluggable
 pgstats facility

These kinds of stats (like WAL, bgwriter, checkpointer, etc.) are stored
in shared memory with a fixed amount of entries.  With the previous
commits in place, this commit only lifts a past restriction in place.

The meat of the change is in the test module injection_points, that
provides an example of how to implement fixed-numbered stats kinds.
---
 src/backend/utils/activity/pgstat.c           |   9 --
 .../injection_points--1.0.sql                 |  11 ++
 .../injection_points/injection_points.c       |   3 +
 .../injection_points/injection_stats.c        | 153 +++++++++++++++++-
 .../injection_points/injection_stats.h        |   3 +
 .../modules/injection_points/t/001_stats.pl   |  11 +-
 6 files changed, 178 insertions(+), 12 deletions(-)

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 994aa2ba51..d87999d091 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -1395,15 +1395,6 @@ pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
 				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
 				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
 
-	/*
-	 * These are not supported for now, as these point out to fixed areas of
-	 * shared memory.
-	 */
-	if (kind_info->fixed_amount)
-		ereport(ERROR,
-				(errmsg("custom cumulative statistics property is invalid"),
-				 errhint("Custom cumulative statistics cannot use a fixed amount of data.")));
-
 	/*
 	 * If pgstat_kind_custom_infos is not available yet, allocate it.
 	 */
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index deaf47d8ae..a52b7a6b7c 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -64,3 +64,14 @@ CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
 RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
+
+--
+-- injection_points_stats_fixed()
+--
+-- Reports fixed-numbered statistics for injection points.
+CREATE FUNCTION injection_points_stats_fixed(OUT numattach int8,
+   OUT numdetach int8,
+   OUT numrun int8)
+RETURNS record
+AS 'MODULE_PATHNAME', 'injection_points_stats_fixed'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index f65b34ce82..c71ad0d367 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -297,6 +297,7 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		condition.pid = MyProcPid;
 	}
 
+	pgstat_report_inj_fixed(1, 0, 0);
 	InjectionPointAttach(name, "injection_points", function, &condition,
 						 sizeof(InjectionPointCondition));
 
@@ -325,6 +326,7 @@ injection_points_run(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 0, 1);
 	INJECTION_POINT(name);
 
 	PG_RETURN_VOID();
@@ -401,6 +403,7 @@ injection_points_detach(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 1, 0);
 	if (!InjectionPointDetach(name))
 		elog(ERROR, "could not detach injection point \"%s\"", name);
 
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
index c37b0b33d3..53440f5184 100644
--- a/src/test/modules/injection_points/injection_stats.c
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -17,12 +17,13 @@
 #include "fmgr.h"
 
 #include "common/hashfn.h"
+#include "funcapi.h"
 #include "injection_stats.h"
 #include "pgstat.h"
 #include "utils/builtins.h"
 #include "utils/pgstat_internal.h"
 
-/* Structures for statistics of injection points */
+/* Structures for statistics of injection points, variable-size */
 typedef struct PgStat_StatInjEntry
 {
 	PgStat_Counter numcalls;	/* number of times point has been run */
@@ -35,6 +36,9 @@ typedef struct PgStatShared_InjectionPoint
 } PgStatShared_InjectionPoint;
 
 static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+static void injection_stats_fixed_init_shmem_cb(void *stats);
+static void injection_stats_fixed_reset_all_cb(TimestampTz ts);
+static void injection_stats_fixed_snapshot_cb(void);
 
 static const PgStat_KindInfo injection_stats = {
 	.name = "injection_points",
@@ -50,18 +54,49 @@ static const PgStat_KindInfo injection_stats = {
 	.flush_pending_cb = injection_stats_flush_cb,
 };
 
+/* Structures for statistics of injection points, fixed-size */
+typedef struct PgStat_StatInjFixedEntry
+{
+	PgStat_Counter numattach;	/* number of points attached */
+	PgStat_Counter numdetach;	/* number of points detached */
+	PgStat_Counter numrun;		/* number of points run */
+	TimestampTz stat_reset_timestamp;
+} PgStat_StatInjFixedEntry;
+
+typedef struct PgStatShared_InjectionPointFixed
+{
+	LWLock	lock;	/* protects all the counters */
+	uint32	changecount;
+	PgStat_StatInjFixedEntry stats;
+	PgStat_StatInjFixedEntry reset_offset;
+} PgStatShared_InjectionPointFixed;
+
+static const PgStat_KindInfo injection_stats_fixed = {
+	.name = "injection_points_fixed",
+	.fixed_amount = true,
+
+	.shared_size = sizeof(PgStat_StatInjFixedEntry),
+	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPointFixed *) 0)->stats),
+
+	.init_shmem_cb = injection_stats_fixed_init_shmem_cb,
+	.reset_all_cb = injection_stats_fixed_reset_all_cb,
+	.snapshot_cb = injection_stats_fixed_snapshot_cb,
+};
+
 /*
  * Compute stats entry idx from point name with a 4-byte hash.
  */
 #define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
 
 #define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+#define PGSTAT_KIND_INJECTION_FIXED	(PGSTAT_KIND_EXPERIMENTAL + 1)
 
 /* Track if stats are loaded */
 static bool inj_stats_loaded = false;
 
 /*
- * Callback for stats handling
+ * Callbacks for stats handling
  */
 static bool
 injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
@@ -79,6 +114,59 @@ injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	return true;
 }
 
+static void
+injection_stats_fixed_init_shmem_cb(void *stats)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+static void
+injection_stats_fixed_reset_all_cb(TimestampTz ts)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_INJECTION_FIXED];
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
+	LWLockRelease(&stats_shmem->lock);
+}
+
+static void
+injection_stats_fixed_snapshot_cb(void)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_INJECTION_FIXED];
+	PgStat_StatInjFixedEntry *stat_snap = (PgStat_StatInjFixedEntry *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_INJECTION_FIXED];
+	PgStat_StatInjFixedEntry *reset_offset = &stats_shmem->reset_offset;
+	PgStat_StatInjFixedEntry reset;
+
+	pgstat_copy_changecounted_stats(stat_snap,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
+	LWLockRelease(&stats_shmem->lock);
+
+	/* compensate by reset offsets */
+#define FIXED_COMP(fld) stat_snap->fld -= reset.fld;
+	FIXED_COMP(numattach);
+	FIXED_COMP(numdetach);
+	FIXED_COMP(numrun);
+#undef FIXED_COMP
+}
+
 /*
  * Support function for the SQL-callable pgstat* functions.  Returns
  * a pointer to the injection point statistics struct.
@@ -105,6 +193,7 @@ void
 pgstat_register_inj(void)
 {
 	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+	pgstat_register_kind(PGSTAT_KIND_INJECTION_FIXED, &injection_stats_fixed);
 
 	/* mark stats as loaded */
 	inj_stats_loaded = true;
@@ -176,6 +265,30 @@ pgstat_report_inj(const char *name)
 	pgstat_unlock_entry(entry_ref);
 }
 
+/*
+ * Report fixed number of statistics for an injection point.
+ */
+void
+pgstat_report_inj_fixed(uint32 numattach,
+						uint32 numdetach,
+						uint32 numrun)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	stats_shmem = (PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_INJECTION_FIXED];
+
+	pgstat_begin_changecount_write(&stats_shmem->changecount);
+	stats_shmem->stats.numattach += numattach;
+	stats_shmem->stats.numdetach += numdetach;
+	stats_shmem->stats.numrun += numrun;
+	pgstat_end_changecount_write(&stats_shmem->changecount);
+}
+
 /*
  * SQL function returning the number of times an injection point
  * has been called.
@@ -192,3 +305,39 @@ injection_points_stats_numcalls(PG_FUNCTION_ARGS)
 
 	PG_RETURN_INT64(entry->numcalls);
 }
+
+/*
+ * SQL function returning fixed-numbered statistics for injection points.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_fixed);
+Datum
+injection_points_stats_fixed(PG_FUNCTION_ARGS)
+{
+	TupleDesc	tupdesc;
+	Datum		values[3] = {0};
+	bool		nulls[3] = {0};
+	PgStat_StatInjFixedEntry *stats;
+
+	pgstat_snapshot_fixed(PGSTAT_KIND_INJECTION_FIXED);
+	stats = pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_INJECTION_FIXED];
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(3);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "numattach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "numdetach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "numrun",
+					   INT8OID, -1, 0);
+	BlessTupleDesc(tupdesc);
+
+	values[0] = Int64GetDatum(stats->numattach);
+	values[1] = Int64GetDatum(stats->numdetach);
+	values[2] = Int64GetDatum(stats->numrun);
+	nulls[0] = false;
+	nulls[1] = false;
+	nulls[2] = false;
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
index 3e99705483..cf68b25f7b 100644
--- a/src/test/modules/injection_points/injection_stats.h
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -19,5 +19,8 @@ extern void pgstat_register_inj(void);
 extern void pgstat_create_inj(const char *name);
 extern void pgstat_drop_inj(const char *name);
 extern void pgstat_report_inj(const char *name);
+extern void pgstat_report_inj_fixed(uint32 numattach,
+									uint32 numdetach,
+									uint32 numrun);
 
 #endif
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 7d5a96e522..e3c69b94ca 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -31,18 +31,27 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats');
 
 # Restart the node cleanly, stats should still be around.
 $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats after clean restart');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats after clean restart');
 
 # On crash the stats are gone.
 $node->stop('immediate');
 $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
-is($numcalls, '', 'number of stats after clean restart');
+is($numcalls, '', 'number of stats after crash');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '0|0|0', 'number of fixed stats after crash');
 
 done_testing();
-- 
2.45.2

From 596424c30af63e96f3ab4274541ab6cb6cfaab0e Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 13:26:22 +0900
Subject: [PATCH v3 1/6] Rework handling of fixed-numbered statistics in
 pgstats

This commit removes fixed-numbered statistics structures from
PgStat_ShmemControl, the central shared memory structure of pgstats and
PgStat_Snapshot, in charge of holding stats snapshots for backends,
based on copies of what is in shared memory.

This has the advantage to simplify the read and write of the pgstats
file by not tracking the types of fixed-numbered stats in pgstat.c
itself, replacing it with what PgStat_KindInfo already knows for each
one of them.

This refactoring makes possible the introduction of more pluggable APIs
into pgstats for fixed-numbered stats.  The following changes are
applied to the internal structures:
- PgStat_ShmemControl holds an array of void* indexed by
PGSTAT_NUM_KINDS, pointing to shared memory areas allocated for each
fixed-numbered stats.  Each entry is allocated a size corresponding to
PgStat_KindInfo->shared_size.
- PgStat_Snapshot holds an array of void* also indexed by
PGSTAT_NUM_KINDS, pointing to the fixed stats stored in the snapshots.
These have a size of PgStat_KindInfo->shared_data_len.
- Fixed numbered stats now set shared_size, so as
- A callback is added to initialize the shared memory assigned to each
fixed-numbered stats, consisting of LWLock initializations for the
current types of stats.  So this initialization is moved out of pgstat.c
into each stats kind file.
---
 src/include/utils/pgstat_internal.h           |  52 +++----
 src/backend/utils/activity/pgstat.c           | 130 ++++++++++--------
 src/backend/utils/activity/pgstat_archiver.c  |  23 +++-
 src/backend/utils/activity/pgstat_bgwriter.c  |  26 +++-
 .../utils/activity/pgstat_checkpointer.c      |  26 +++-
 src/backend/utils/activity/pgstat_io.c        |  44 ++++--
 src/backend/utils/activity/pgstat_shmem.c     |  33 +++--
 src/backend/utils/activity/pgstat_slru.c      |  25 +++-
 src/backend/utils/activity/pgstat_wal.c       |  26 +++-
 9 files changed, 250 insertions(+), 135 deletions(-)

diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index e21ef4e2c9..b8b2152d71 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -195,16 +195,11 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
-	 * PgStatShared_HashEntry->body).
+	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
+	 * the size of an entry in PgStat_ShmemControl->fixed_data.
 	 */
 	uint32		shared_size;
 
-	/*
-	 * The offset of the statistics struct in the containing shared memory
-	 * control structure PgStat_ShmemControl, for fixed-numbered statistics.
-	 */
-	uint32		shared_ctl_off;
-
 	/*
 	 * The offset/size of statistics inside the shared stats entry. Used when
 	 * [de-]serializing statistics to / from disk respectively. Separate from
@@ -245,6 +240,13 @@ typedef struct PgStat_KindInfo
 									   const PgStatShared_Common *header, NameData *name);
 	bool		(*from_serialized_name) (const NameData *name, PgStat_HashKey *key);
 
+	/*
+	 * For fixed-numbered statistics: Initialize shared memory state.
+	 *
+	 * "stats" is the pointer to the allocated shared memory area.
+	 */
+	void		(*init_shmem_cb) (void *stats);
+
 	/*
 	 * For fixed-numbered statistics: Reset All.
 	 */
@@ -425,14 +427,12 @@ typedef struct PgStat_ShmemControl
 	pg_atomic_uint64 gc_request_count;
 
 	/*
-	 * Stats data for fixed-numbered objects.
+	 * Stats data for fixed-numbered objects, indexed by PgStat_Kind.
+	 *
+	 * Each entry has a size of PgStat_KindInfo->shared_size.
 	 */
-	PgStatShared_Archiver archiver;
-	PgStatShared_BgWriter bgwriter;
-	PgStatShared_Checkpointer checkpointer;
-	PgStatShared_IO io;
-	PgStatShared_SLRU slru;
-	PgStatShared_Wal wal;
+	void	   *fixed_data[PGSTAT_NUM_KINDS];
+
 } PgStat_ShmemControl;
 
 
@@ -446,19 +446,13 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
+	/*
+	 * Data in snapshot for fixed-numbered statistics, indexed by PgStat_Kind.
+	 * Each entry is allocated in TopMemoryContext, for a size of
+	 * shared_data_len.
+	 */
 	bool		fixed_valid[PGSTAT_NUM_KINDS];
-
-	PgStat_ArchiverStats archiver;
-
-	PgStat_BgWriterStats bgwriter;
-
-	PgStat_CheckpointerStats checkpointer;
-
-	PgStat_IO	io;
-
-	PgStat_SLRUStats slru[SLRU_NUM_ELEMENTS];
-
-	PgStat_WalStats wal;
+	void	   *fixed_data[PGSTAT_NUM_KINDS];
 
 	/* to free snapshot in bulk */
 	MemoryContext context;
@@ -522,6 +516,7 @@ extern void pgstat_snapshot_fixed(PgStat_Kind kind);
  * Functions in pgstat_archiver.c
  */
 
+extern void pgstat_archiver_init_shmem_cb(void *stats);
 extern void pgstat_archiver_reset_all_cb(TimestampTz ts);
 extern void pgstat_archiver_snapshot_cb(void);
 
@@ -530,6 +525,7 @@ extern void pgstat_archiver_snapshot_cb(void);
  * Functions in pgstat_bgwriter.c
  */
 
+extern void pgstat_bgwriter_init_shmem_cb(void *stats);
 extern void pgstat_bgwriter_reset_all_cb(TimestampTz ts);
 extern void pgstat_bgwriter_snapshot_cb(void);
 
@@ -538,6 +534,7 @@ extern void pgstat_bgwriter_snapshot_cb(void);
  * Functions in pgstat_checkpointer.c
  */
 
+extern void pgstat_checkpointer_init_shmem_cb(void *stats);
 extern void pgstat_checkpointer_reset_all_cb(TimestampTz ts);
 extern void pgstat_checkpointer_snapshot_cb(void);
 
@@ -568,6 +565,7 @@ extern bool pgstat_function_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
  */
 
 extern bool pgstat_flush_io(bool nowait);
+extern void pgstat_io_init_shmem_cb(void *stats);
 extern void pgstat_io_reset_all_cb(TimestampTz ts);
 extern void pgstat_io_snapshot_cb(void);
 
@@ -626,6 +624,7 @@ extern PgStatShared_Common *pgstat_init_entry(PgStat_Kind kind,
  */
 
 extern bool pgstat_slru_flush(bool nowait);
+extern void pgstat_slru_init_shmem_cb(void *stats);
 extern void pgstat_slru_reset_all_cb(TimestampTz ts);
 extern void pgstat_slru_snapshot_cb(void);
 
@@ -638,6 +637,7 @@ extern bool pgstat_flush_wal(bool nowait);
 extern void pgstat_init_wal(void);
 extern bool pgstat_have_pending_wal(void);
 
+extern void pgstat_wal_init_shmem_cb(void *stats);
 extern void pgstat_wal_reset_all_cb(TimestampTz ts);
 extern void pgstat_wal_snapshot_cb(void);
 
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index c37c11b2ec..13ddbcdcfb 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -132,6 +132,7 @@
  * ---------
  */
 #define PGSTAT_FILE_ENTRY_END	'E' /* end of file */
+#define PGSTAT_FILE_ENTRY_FIXED	'F' /* fixed-numbered stats entry */
 #define PGSTAT_FILE_ENTRY_NAME	'N' /* stats entry identified by name */
 #define PGSTAT_FILE_ENTRY_HASH	'S' /* stats entry identified by
 									 * PgStat_HashKey */
@@ -173,6 +174,8 @@ typedef struct PgStat_SnapshotEntry
 static void pgstat_write_statsfile(void);
 static void pgstat_read_statsfile(void);
 
+static void pgstat_init_snapshot(void);
+
 static void pgstat_reset_after_failure(void);
 
 static bool pgstat_flush_pending_entries(bool nowait);
@@ -349,10 +352,11 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
-		.shared_ctl_off = offsetof(PgStat_ShmemControl, archiver),
+		.shared_size = sizeof(PgStatShared_Archiver),
 		.shared_data_off = offsetof(PgStatShared_Archiver, stats),
 		.shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
 
+		.init_shmem_cb = pgstat_archiver_init_shmem_cb,
 		.reset_all_cb = pgstat_archiver_reset_all_cb,
 		.snapshot_cb = pgstat_archiver_snapshot_cb,
 	},
@@ -362,10 +366,11 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
-		.shared_ctl_off = offsetof(PgStat_ShmemControl, bgwriter),
+		.shared_size = sizeof(PgStatShared_BgWriter),
 		.shared_data_off = offsetof(PgStatShared_BgWriter, stats),
 		.shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
 
+		.init_shmem_cb = pgstat_bgwriter_init_shmem_cb,
 		.reset_all_cb = pgstat_bgwriter_reset_all_cb,
 		.snapshot_cb = pgstat_bgwriter_snapshot_cb,
 	},
@@ -375,10 +380,11 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
-		.shared_ctl_off = offsetof(PgStat_ShmemControl, checkpointer),
+		.shared_size = sizeof(PgStatShared_Checkpointer),
 		.shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
 		.shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
 
+		.init_shmem_cb = pgstat_checkpointer_init_shmem_cb,
 		.reset_all_cb = pgstat_checkpointer_reset_all_cb,
 		.snapshot_cb = pgstat_checkpointer_snapshot_cb,
 	},
@@ -388,10 +394,11 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
-		.shared_ctl_off = offsetof(PgStat_ShmemControl, io),
+		.shared_size = sizeof(PgStatShared_IO),
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
 
+		.init_shmem_cb = pgstat_io_init_shmem_cb,
 		.reset_all_cb = pgstat_io_reset_all_cb,
 		.snapshot_cb = pgstat_io_snapshot_cb,
 	},
@@ -401,10 +408,11 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
-		.shared_ctl_off = offsetof(PgStat_ShmemControl, slru),
+		.shared_size = sizeof(PgStatShared_SLRU),
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
 
+		.init_shmem_cb = pgstat_slru_init_shmem_cb,
 		.reset_all_cb = pgstat_slru_reset_all_cb,
 		.snapshot_cb = pgstat_slru_snapshot_cb,
 	},
@@ -414,10 +422,11 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
-		.shared_ctl_off = offsetof(PgStat_ShmemControl, wal),
+		.shared_size = sizeof(PgStatShared_Wal),
 		.shared_data_off = offsetof(PgStatShared_Wal, stats),
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
 
+		.init_shmem_cb = pgstat_wal_init_shmem_cb,
 		.reset_all_cb = pgstat_wal_reset_all_cb,
 		.snapshot_cb = pgstat_wal_snapshot_cb,
 	},
@@ -571,6 +580,8 @@ pgstat_initialize(void)
 
 	pgstat_attach_shmem();
 
+	pgstat_init_snapshot();
+
 	pgstat_init_wal();
 
 	/* Set up a process-exit hook to clean up */
@@ -982,6 +993,22 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 	Assert(pgStatLocal.snapshot.fixed_valid[kind]);
 }
 
+static void
+pgstat_init_snapshot(void)
+{
+	/* Initialize fixed-numbered statistics data in snapshots */
+	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info->fixed_amount)
+			continue;
+
+		pgStatLocal.snapshot.fixed_data[kind] =
+			MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
+	}
+}
+
 static void
 pgstat_prep_snapshot(void)
 {
@@ -1371,47 +1398,25 @@ pgstat_write_statsfile(void)
 	format_id = PGSTAT_FILE_FORMAT_ID;
 	write_chunk_s(fpout, &format_id);
 
-	/*
-	 * XXX: The following could now be generalized to just iterate over
-	 * pgstat_kind_infos instead of knowing about the different kinds of
-	 * stats.
-	 */
+	/* Write various stats structs with fixed number of objects */
+	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	{
+		char	   *ptr;
+		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-	/*
-	 * Write archiver stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_ARCHIVER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.archiver);
+		if (!info->fixed_amount)
+			continue;
 
-	/*
-	 * Write bgwriter stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_BGWRITER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.bgwriter);
+		Assert(info->shared_size != 0 && info->shared_data_len != 0);
 
-	/*
-	 * Write checkpointer stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_CHECKPOINTER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.checkpointer);
+		/* prepare snapshot data and write it */
+		pgstat_build_snapshot_fixed(kind);
+		ptr = pgStatLocal.snapshot.fixed_data[kind];
 
-	/*
-	 * Write IO stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_IO);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.io);
-
-	/*
-	 * Write SLRU stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_SLRU);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.slru);
-
-	/*
-	 * Write WAL stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_WAL);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.wal);
+		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
+		write_chunk_s(fpout, &kind);
+		write_chunk(fpout, ptr, info->shared_data_len);
+	}
 
 	/*
 	 * Walk through the stats entries
@@ -1551,22 +1556,6 @@ pgstat_read_statsfile(void)
 		format_id != PGSTAT_FILE_FORMAT_ID)
 		goto error;
 
-	/* Read various stats structs with fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
-	{
-		char	   *ptr;
-		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
-
-		if (!info->fixed_amount)
-			continue;
-
-		Assert(info->shared_ctl_off != 0);
-
-		ptr = ((char *) shmem) + info->shared_ctl_off + info->shared_data_off;
-		if (!read_chunk(fpin, ptr, info->shared_data_len))
-			goto error;
-	}
-
 	/*
 	 * We found an existing statistics file. Read it and put all the hash
 	 * table entries into place.
@@ -1577,6 +1566,29 @@ pgstat_read_statsfile(void)
 
 		switch (t)
 		{
+			case PGSTAT_FILE_ENTRY_FIXED:
+				{
+					PgStat_Kind kind;
+					const PgStat_KindInfo *info;
+					char	   *ptr;
+
+					if (!read_chunk_s(fpin, &kind))
+						goto error;
+
+					if (!pgstat_is_kind_valid(kind))
+						goto error;
+
+					info = pgstat_get_kind_info(kind);
+					Assert(info->fixed_amount);
+
+					/* Load back stats into shared memory */
+					ptr = ((char *) shmem->fixed_data[kind]) + info->shared_data_off;
+					if (!read_chunk(fpin, ptr,
+									info->shared_data_len))
+						goto error;
+
+					break;
+				}
 			case PGSTAT_FILE_ENTRY_HASH:
 			case PGSTAT_FILE_ENTRY_NAME:
 				{
diff --git a/src/backend/utils/activity/pgstat_archiver.c b/src/backend/utils/activity/pgstat_archiver.c
index 66398b20e5..99cd461f81 100644
--- a/src/backend/utils/activity/pgstat_archiver.c
+++ b/src/backend/utils/activity/pgstat_archiver.c
@@ -27,7 +27,8 @@
 void
 pgstat_report_archiver(const char *xlog, bool failed)
 {
-	PgStatShared_Archiver *stats_shmem = &pgStatLocal.shmem->archiver;
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_ARCHIVER];
 	TimestampTz now = GetCurrentTimestamp();
 
 	pgstat_begin_changecount_write(&stats_shmem->changecount);
@@ -59,13 +60,23 @@ pgstat_fetch_stat_archiver(void)
 {
 	pgstat_snapshot_fixed(PGSTAT_KIND_ARCHIVER);
 
-	return &pgStatLocal.snapshot.archiver;
+	return (PgStat_ArchiverStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_ARCHIVER];
+}
+
+void
+pgstat_archiver_init_shmem_cb(void *stats)
+{
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
 }
 
 void
 pgstat_archiver_reset_all_cb(TimestampTz ts)
 {
-	PgStatShared_Archiver *stats_shmem = &pgStatLocal.shmem->archiver;
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_ARCHIVER];
 
 	/* see explanation above PgStatShared_Archiver for the reset protocol */
 	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
@@ -80,8 +91,10 @@ pgstat_archiver_reset_all_cb(TimestampTz ts)
 void
 pgstat_archiver_snapshot_cb(void)
 {
-	PgStatShared_Archiver *stats_shmem = &pgStatLocal.shmem->archiver;
-	PgStat_ArchiverStats *stat_snap = &pgStatLocal.snapshot.archiver;
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_ARCHIVER];
+	PgStat_ArchiverStats *stat_snap = (PgStat_ArchiverStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_ARCHIVER];
 	PgStat_ArchiverStats *reset_offset = &stats_shmem->reset_offset;
 	PgStat_ArchiverStats reset;
 
diff --git a/src/backend/utils/activity/pgstat_bgwriter.c b/src/backend/utils/activity/pgstat_bgwriter.c
index 7d2432e4fa..778d3f0b0e 100644
--- a/src/backend/utils/activity/pgstat_bgwriter.c
+++ b/src/backend/utils/activity/pgstat_bgwriter.c
@@ -29,7 +29,8 @@ PgStat_BgWriterStats PendingBgWriterStats = {0};
 void
 pgstat_report_bgwriter(void)
 {
-	PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
+	PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_BGWRITER];
 	static const PgStat_BgWriterStats all_zeroes;
 
 	Assert(!pgStatLocal.shmem->is_shutdown);
@@ -72,13 +73,23 @@ pgstat_fetch_stat_bgwriter(void)
 {
 	pgstat_snapshot_fixed(PGSTAT_KIND_BGWRITER);
 
-	return &pgStatLocal.snapshot.bgwriter;
+	return (PgStat_BgWriterStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_BGWRITER];
+}
+
+void
+pgstat_bgwriter_init_shmem_cb(void *stats)
+{
+	PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
 }
 
 void
 pgstat_bgwriter_reset_all_cb(TimestampTz ts)
 {
-	PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
+	PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_BGWRITER];
 
 	/* see explanation above PgStatShared_BgWriter for the reset protocol */
 	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
@@ -93,11 +104,14 @@ pgstat_bgwriter_reset_all_cb(TimestampTz ts)
 void
 pgstat_bgwriter_snapshot_cb(void)
 {
-	PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
+	PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_BGWRITER];
+	PgStat_BgWriterStats *stat_snap = (PgStat_BgWriterStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_BGWRITER];
 	PgStat_BgWriterStats *reset_offset = &stats_shmem->reset_offset;
 	PgStat_BgWriterStats reset;
 
-	pgstat_copy_changecounted_stats(&pgStatLocal.snapshot.bgwriter,
+	pgstat_copy_changecounted_stats(stat_snap,
 									&stats_shmem->stats,
 									sizeof(stats_shmem->stats),
 									&stats_shmem->changecount);
@@ -107,7 +121,7 @@ pgstat_bgwriter_snapshot_cb(void)
 	LWLockRelease(&stats_shmem->lock);
 
 	/* compensate by reset offsets */
-#define BGWRITER_COMP(fld) pgStatLocal.snapshot.bgwriter.fld -= reset.fld;
+#define BGWRITER_COMP(fld) stat_snap->fld -= reset.fld;
 	BGWRITER_COMP(buf_written_clean);
 	BGWRITER_COMP(maxwritten_clean);
 	BGWRITER_COMP(buf_alloc);
diff --git a/src/backend/utils/activity/pgstat_checkpointer.c b/src/backend/utils/activity/pgstat_checkpointer.c
index 30a8110e38..6e86255b8d 100644
--- a/src/backend/utils/activity/pgstat_checkpointer.c
+++ b/src/backend/utils/activity/pgstat_checkpointer.c
@@ -31,7 +31,8 @@ pgstat_report_checkpointer(void)
 {
 	/* We assume this initializes to zeroes */
 	static const PgStat_CheckpointerStats all_zeroes;
-	PgStatShared_Checkpointer *stats_shmem = &pgStatLocal.shmem->checkpointer;
+	PgStatShared_Checkpointer *stats_shmem = (PgStatShared_Checkpointer *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_CHECKPOINTER];
 
 	Assert(!pgStatLocal.shmem->is_shutdown);
 	pgstat_assert_is_up();
@@ -81,13 +82,23 @@ pgstat_fetch_stat_checkpointer(void)
 {
 	pgstat_snapshot_fixed(PGSTAT_KIND_CHECKPOINTER);
 
-	return &pgStatLocal.snapshot.checkpointer;
+	return (PgStat_CheckpointerStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_CHECKPOINTER];
+}
+
+void
+pgstat_checkpointer_init_shmem_cb(void *stats)
+{
+	PgStatShared_Checkpointer *stats_shmem = (PgStatShared_Checkpointer *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
 }
 
 void
 pgstat_checkpointer_reset_all_cb(TimestampTz ts)
 {
-	PgStatShared_Checkpointer *stats_shmem = &pgStatLocal.shmem->checkpointer;
+	PgStatShared_Checkpointer *stats_shmem = (PgStatShared_Checkpointer *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_CHECKPOINTER];
 
 	/* see explanation above PgStatShared_Checkpointer for the reset protocol */
 	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
@@ -102,11 +113,14 @@ pgstat_checkpointer_reset_all_cb(TimestampTz ts)
 void
 pgstat_checkpointer_snapshot_cb(void)
 {
-	PgStatShared_Checkpointer *stats_shmem = &pgStatLocal.shmem->checkpointer;
+	PgStatShared_Checkpointer *stats_shmem = (PgStatShared_Checkpointer *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_CHECKPOINTER];
+	PgStat_CheckpointerStats *stat_snap = (PgStat_CheckpointerStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_CHECKPOINTER];
 	PgStat_CheckpointerStats *reset_offset = &stats_shmem->reset_offset;
 	PgStat_CheckpointerStats reset;
 
-	pgstat_copy_changecounted_stats(&pgStatLocal.snapshot.checkpointer,
+	pgstat_copy_changecounted_stats(stat_snap,
 									&stats_shmem->stats,
 									sizeof(stats_shmem->stats),
 									&stats_shmem->changecount);
@@ -116,7 +130,7 @@ pgstat_checkpointer_snapshot_cb(void)
 	LWLockRelease(&stats_shmem->lock);
 
 	/* compensate by reset offsets */
-#define CHECKPOINTER_COMP(fld) pgStatLocal.snapshot.checkpointer.fld -= reset.fld;
+#define CHECKPOINTER_COMP(fld) stat_snap->fld -= reset.fld;
 	CHECKPOINTER_COMP(num_timed);
 	CHECKPOINTER_COMP(num_requested);
 	CHECKPOINTER_COMP(restartpoints_timed);
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index 9d6e067382..6cbbc2094a 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -158,7 +158,8 @@ pgstat_fetch_stat_io(void)
 {
 	pgstat_snapshot_fixed(PGSTAT_KIND_IO);
 
-	return &pgStatLocal.snapshot.io;
+	return (PgStat_IO *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_IO];
 }
 
 /*
@@ -174,13 +175,16 @@ pgstat_flush_io(bool nowait)
 {
 	LWLock	   *bktype_lock;
 	PgStat_BktypeIO *bktype_shstats;
+	PgStatShared_IO *stat_shmem;
 
 	if (!have_iostats)
 		return false;
 
-	bktype_lock = &pgStatLocal.shmem->io.locks[MyBackendType];
-	bktype_shstats =
-		&pgStatLocal.shmem->io.stats.stats[MyBackendType];
+	stat_shmem = (PgStatShared_IO *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_IO];
+
+	bktype_lock = &stat_shmem->locks[MyBackendType];
+	bktype_shstats = &stat_shmem->stats.stats[MyBackendType];
 
 	if (!nowait)
 		LWLockAcquire(bktype_lock, LW_EXCLUSIVE);
@@ -251,13 +255,25 @@ pgstat_get_io_object_name(IOObject io_object)
 	pg_unreachable();
 }
 
+void
+pgstat_io_init_shmem_cb(void *stats)
+{
+	PgStatShared_IO *stat_shmem = (PgStatShared_IO *) stats;
+
+	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
+		LWLockInitialize(&stat_shmem->locks[i], LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_io_reset_all_cb(TimestampTz ts)
 {
+	PgStatShared_IO *stat_shmem = (PgStatShared_IO *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_IO];
+
 	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
 	{
-		LWLock	   *bktype_lock = &pgStatLocal.shmem->io.locks[i];
-		PgStat_BktypeIO *bktype_shstats = &pgStatLocal.shmem->io.stats.stats[i];
+		LWLock	   *bktype_lock = &stat_shmem->locks[i];
+		PgStat_BktypeIO *bktype_shstats = &stat_shmem->stats.stats[i];
 
 		LWLockAcquire(bktype_lock, LW_EXCLUSIVE);
 
@@ -266,7 +282,7 @@ pgstat_io_reset_all_cb(TimestampTz ts)
 		 * the reset timestamp as well.
 		 */
 		if (i == 0)
-			pgStatLocal.shmem->io.stats.stat_reset_timestamp = ts;
+			stat_shmem->stats.stat_reset_timestamp = ts;
 
 		memset(bktype_shstats, 0, sizeof(*bktype_shstats));
 		LWLockRelease(bktype_lock);
@@ -276,11 +292,16 @@ pgstat_io_reset_all_cb(TimestampTz ts)
 void
 pgstat_io_snapshot_cb(void)
 {
+	PgStatShared_IO *stat_shmem = (PgStatShared_IO *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_IO];
+	PgStat_IO  *stat_snap = (PgStat_IO *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_IO];
+
 	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
 	{
-		LWLock	   *bktype_lock = &pgStatLocal.shmem->io.locks[i];
-		PgStat_BktypeIO *bktype_shstats = &pgStatLocal.shmem->io.stats.stats[i];
-		PgStat_BktypeIO *bktype_snap = &pgStatLocal.snapshot.io.stats[i];
+		LWLock	   *bktype_lock = &stat_shmem->locks[i];
+		PgStat_BktypeIO *bktype_shstats = &stat_shmem->stats.stats[i];
+		PgStat_BktypeIO *bktype_snap = &stat_snap->stats[i];
 
 		LWLockAcquire(bktype_lock, LW_SHARED);
 
@@ -289,8 +310,7 @@ pgstat_io_snapshot_cb(void)
 		 * the reset timestamp as well.
 		 */
 		if (i == 0)
-			pgStatLocal.snapshot.io.stat_reset_timestamp =
-				pgStatLocal.shmem->io.stats.stat_reset_timestamp;
+			stat_snap->stat_reset_timestamp = stat_shmem->stats.stat_reset_timestamp;
 
 		/* using struct assignment due to better type safety */
 		*bktype_snap = *bktype_shstats;
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 634b967820..bd62c4b72a 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -131,6 +131,19 @@ StatsShmemSize(void)
 	sz = MAXALIGN(sizeof(PgStat_ShmemControl));
 	sz = add_size(sz, pgstat_dsa_init_size());
 
+	/* Add shared memory for all the fixed-numbered statistics */
+	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info->fixed_amount)
+			continue;
+
+		Assert(kind_info->shared_size != 0);
+
+		sz += MAXALIGN(kind_info->shared_size);
+	}
+
 	return sz;
 }
 
@@ -196,17 +209,19 @@ StatsShmemInit(void)
 
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
+		/* initialize fixed-numbered statistics */
+		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		{
+			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		/* initialize fixed-numbered stats */
-		LWLockInitialize(&ctl->archiver.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->bgwriter.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->checkpointer.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->slru.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->wal.lock, LWTRANCHE_PGSTATS_DATA);
+			if (!kind_info->fixed_amount)
+				continue;
 
-		for (int i = 0; i < BACKEND_NUM_TYPES; i++)
-			LWLockInitialize(&ctl->io.locks[i],
-							 LWTRANCHE_PGSTATS_DATA);
+			Assert(kind_info->shared_size != 0);
+
+			ctl->fixed_data[kind] = ShmemAlloc(kind_info->shared_size);
+			kind_info->init_shmem_cb(ctl->fixed_data[kind]);
+		}
 	}
 	else
 	{
diff --git a/src/backend/utils/activity/pgstat_slru.c b/src/backend/utils/activity/pgstat_slru.c
index 56ea1c3378..d50589c1de 100644
--- a/src/backend/utils/activity/pgstat_slru.c
+++ b/src/backend/utils/activity/pgstat_slru.c
@@ -106,7 +106,8 @@ pgstat_fetch_slru(void)
 {
 	pgstat_snapshot_fixed(PGSTAT_KIND_SLRU);
 
-	return pgStatLocal.snapshot.slru;
+	return (PgStat_SLRUStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_SLRU];
 }
 
 /*
@@ -155,7 +156,8 @@ pgstat_get_slru_index(const char *name)
 bool
 pgstat_slru_flush(bool nowait)
 {
-	PgStatShared_SLRU *stats_shmem = &pgStatLocal.shmem->slru;
+	PgStatShared_SLRU *stats_shmem = (PgStatShared_SLRU *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_SLRU];
 	int			i;
 
 	if (!have_slrustats)
@@ -192,6 +194,14 @@ pgstat_slru_flush(bool nowait)
 	return false;
 }
 
+void
+pgstat_slru_init_shmem_cb(void *stats)
+{
+	PgStatShared_SLRU *stats_shmem = (PgStatShared_SLRU *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_slru_reset_all_cb(TimestampTz ts)
 {
@@ -202,12 +212,14 @@ pgstat_slru_reset_all_cb(TimestampTz ts)
 void
 pgstat_slru_snapshot_cb(void)
 {
-	PgStatShared_SLRU *stats_shmem = &pgStatLocal.shmem->slru;
+	PgStatShared_SLRU *stats_shmem = (PgStatShared_SLRU *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_SLRU];
+	PgStat_SLRUStats *stat_snap = (PgStat_SLRUStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_SLRU];
 
 	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
 
-	memcpy(pgStatLocal.snapshot.slru, &stats_shmem->stats,
-		   sizeof(stats_shmem->stats));
+	memcpy(stat_snap, &stats_shmem->stats, sizeof(stats_shmem->stats));
 
 	LWLockRelease(&stats_shmem->lock);
 }
@@ -237,7 +249,8 @@ get_slru_entry(int slru_idx)
 static void
 pgstat_reset_slru_counter_internal(int index, TimestampTz ts)
 {
-	PgStatShared_SLRU *stats_shmem = &pgStatLocal.shmem->slru;
+	PgStatShared_SLRU *stats_shmem = (PgStatShared_SLRU *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_SLRU];
 
 	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
 
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 0e374f133a..030b339e47 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -68,7 +68,8 @@ pgstat_fetch_stat_wal(void)
 {
 	pgstat_snapshot_fixed(PGSTAT_KIND_WAL);
 
-	return &pgStatLocal.snapshot.wal;
+	return (PgStat_WalStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_WAL];
 }
 
 /*
@@ -81,7 +82,8 @@ pgstat_fetch_stat_wal(void)
 bool
 pgstat_flush_wal(bool nowait)
 {
-	PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
+	PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_WAL];
 	WalUsage	wal_usage_diff = {0};
 
 	Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
@@ -163,10 +165,20 @@ pgstat_have_pending_wal(void)
 		PendingWalStats.wal_sync != 0;
 }
 
+void
+pgstat_wal_init_shmem_cb(void *stats)
+{
+	PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+
 void
 pgstat_wal_reset_all_cb(TimestampTz ts)
 {
-	PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
+	PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_WAL];
 
 	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
 	memset(&stats_shmem->stats, 0, sizeof(stats_shmem->stats));
@@ -177,10 +189,12 @@ pgstat_wal_reset_all_cb(TimestampTz ts)
 void
 pgstat_wal_snapshot_cb(void)
 {
-	PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
+	PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *)
+		pgStatLocal.shmem->fixed_data[PGSTAT_KIND_WAL];
+	PgStat_WalStats *stat_snap = (PgStat_WalStats *)
+		pgStatLocal.snapshot.fixed_data[PGSTAT_KIND_WAL];
 
 	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
-	memcpy(&pgStatLocal.snapshot.wal, &stats_shmem->stats,
-		   sizeof(pgStatLocal.snapshot.wal));
+	memcpy(stat_snap, &stats_shmem->stats, sizeof(PgStat_WalStats));
 	LWLockRelease(&stats_shmem->lock);
 }
-- 
2.45.2

From 790d900cca796a82aa3761da25597bf1496c2615 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 13:39:01 +0900
Subject: [PATCH v3 2/6] Switch PgStat_Kind from enum to uint32

A follow-up patch is planned to make this counter extensible, and
keeping a trace of the kind behind a type is useful in the internal
routines used by pgstats.  While on it, switch pgstat_is_kind_valid() to
use PgStat_Kind, to be more consistent with its callers.
---
 src/include/pgstat.h                | 35 ++++++++++++++---------------
 src/backend/utils/activity/pgstat.c |  6 ++---
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2136239710..2d30fadaf1 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -32,26 +32,25 @@
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"
 
 /* The types of statistics entries */
-typedef enum PgStat_Kind
-{
-	/* use 0 for INVALID, to catch zero-initialized data */
-	PGSTAT_KIND_INVALID = 0,
+#define PgStat_Kind uint32
 
-	/* stats for variable-numbered objects */
-	PGSTAT_KIND_DATABASE,		/* database-wide statistics */
-	PGSTAT_KIND_RELATION,		/* per-table statistics */
-	PGSTAT_KIND_FUNCTION,		/* per-function statistics */
-	PGSTAT_KIND_REPLSLOT,		/* per-slot statistics */
-	PGSTAT_KIND_SUBSCRIPTION,	/* per-subscription statistics */
+/* use 0 for INVALID, to catch zero-initialized data */
+#define PGSTAT_KIND_INVALID 0
 
-	/* stats for fixed-numbered objects */
-	PGSTAT_KIND_ARCHIVER,
-	PGSTAT_KIND_BGWRITER,
-	PGSTAT_KIND_CHECKPOINTER,
-	PGSTAT_KIND_IO,
-	PGSTAT_KIND_SLRU,
-	PGSTAT_KIND_WAL,
-} PgStat_Kind;
+/* stats for variable-numbered objects */
+#define PGSTAT_KIND_DATABASE	1	/* database-wide statistics */
+#define PGSTAT_KIND_RELATION	2	/* per-table statistics */
+#define PGSTAT_KIND_FUNCTION	3	/* per-function statistics */
+#define PGSTAT_KIND_REPLSLOT	4	/* per-slot statistics */
+#define PGSTAT_KIND_SUBSCRIPTION	5	/* per-subscription statistics */
+
+/* stats for fixed-numbered objects */
+#define PGSTAT_KIND_ARCHIVER	6
+#define PGSTAT_KIND_BGWRITER	7
+#define PGSTAT_KIND_CHECKPOINTER	8
+#define PGSTAT_KIND_IO	9
+#define PGSTAT_KIND_SLRU	10
+#define PGSTAT_KIND_WAL	11
 
 #define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
 #define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 13ddbcdcfb..fb27272e86 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -184,7 +184,7 @@ static void pgstat_prep_snapshot(void);
 static void pgstat_build_snapshot(void);
 static void pgstat_build_snapshot_fixed(PgStat_Kind kind);
 
-static inline bool pgstat_is_kind_valid(int ikind);
+static inline bool pgstat_is_kind_valid(PgStat_Kind kind);
 
 
 /* ----------
@@ -1312,9 +1312,9 @@ pgstat_get_kind_from_str(char *kind_str)
 }
 
 static inline bool
-pgstat_is_kind_valid(int ikind)
+pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
+	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
 }
 
 const PgStat_KindInfo *
-- 
2.45.2

From 173976d81c0adeeef8768d708099b0b5f4584144 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 15:04:45 +0900
Subject: [PATCH v3 3/6] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats

As of this patch, fixed-numbered stats kind (like WAL, archiver,
bgwriter) are not supported, still some infrastructure is added to make
its support easier, as the fixed_data areas for the shmem and snapshot
structures are extended to cover custom and builtin stats kinds.
---
 src/include/pgstat.h                      |  35 ++++-
 src/include/utils/pgstat_internal.h       |   8 +-
 src/backend/utils/activity/pgstat.c       | 161 +++++++++++++++++++---
 src/backend/utils/activity/pgstat_shmem.c |   8 +-
 src/backend/utils/adt/pgstatfuncs.c       |   2 +-
 5 files changed, 186 insertions(+), 28 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2d30fadaf1..8d523607a4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -34,6 +34,10 @@
 /* The types of statistics entries */
 #define PgStat_Kind uint32
 
+/* Range of IDs allowed, for built-in and custom kinds */
+#define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
+#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
 
@@ -52,9 +56,34 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index b8b2152d71..01b43a80e0 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -431,7 +431,7 @@ typedef struct PgStat_ShmemControl
 	 *
 	 * Each entry has a size of PgStat_KindInfo->shared_size.
 	 */
-	void	   *fixed_data[PGSTAT_NUM_KINDS];
+	void	   *fixed_data[PGSTAT_KIND_MAX + 1];
 
 } PgStat_ShmemControl;
 
@@ -451,8 +451,8 @@ typedef struct PgStat_Snapshot
 	 * Each entry is allocated in TopMemoryContext, for a size of
 	 * shared_data_len.
 	 */
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
-	void	   *fixed_data[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX + 1];
+	void	   *fixed_data[PGSTAT_KIND_MAX + 1];
 
 	/* to free snapshot in bulk */
 	MemoryContext context;
@@ -497,6 +497,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index fb27272e86..994aa2ba51 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for external modules to define custom statistics kinds,
+ * that can use the same properties as any built-in stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
+ * defined, and pgstat_kind_custom_infos for custom kinds registered at
+ * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -253,7 +261,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -265,7 +273,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -432,6 +440,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -997,11 +1014,11 @@ static void
 pgstat_init_snapshot(void)
 {
 	/* Initialize fixed-numbered statistics data in snapshots */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		pgStatLocal.snapshot.fixed_data[kind] =
@@ -1102,10 +1119,12 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 		{
 			Assert(kind_info->snapshot_cb == NULL);
@@ -1299,30 +1318,125 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
 pgstat_get_kind_info(PgStat_Kind kind)
 {
-	Assert(pgstat_is_kind_valid(kind));
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL ||
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	return NULL;
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * These are not supported for now, as these point out to fixed areas of
+	 * shared memory.
+	 */
+	if (kind_info->fixed_amount)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics property is invalid"),
+				 errhint("Custom cumulative statistics cannot use a fixed amount of data.")));
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+								   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom cumulative statistics \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1399,12 +1513,12 @@ pgstat_write_statsfile(void)
 	write_chunk_s(fpout, &format_id);
 
 	/* Write various stats structs with fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		char	   *ptr;
 		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-		if (!info->fixed_amount)
+		if (!info || !info->fixed_amount)
 			continue;
 
 		Assert(info->shared_size != 0 && info->shared_data_len != 0);
@@ -1434,6 +1548,17 @@ pgstat_write_statsfile(void)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1649,7 +1774,7 @@ pgstat_read_statsfile(void)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u",
 							 key.kind, key.dboid, key.objoid);
 						goto error;
 					}
@@ -1706,12 +1831,12 @@ pgstat_reset_after_failure(void)
 {
 	TimestampTz ts = GetCurrentTimestamp();
 
-	/* reset fixed-numbered stats */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	/* reset fixed-numbered stats for built-in */
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		kind_info->reset_all_cb(ts);
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index bd62c4b72a..0b50d58cce 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -132,10 +132,12 @@ StatsShmemSize(void)
 	sz = add_size(sz, pgstat_dsa_init_size());
 
 	/* Add shared memory for all the fixed-numbered statistics */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 			continue;
 
@@ -210,11 +212,11 @@ StatsShmemInit(void)
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
 		/* initialize fixed-numbered statistics */
-		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 		{
 			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-			if (!kind_info->fixed_amount)
+			if (!kind_info || !kind_info->fixed_amount)
 				continue;
 
 			Assert(kind_info->shared_size != 0);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.2

From b21fd6cfd839595d0987674402a8a266350b0192 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 14:03:17 +0900
Subject: [PATCH v4 5/8] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats

This is able to support fixed-numbered (like WAL, archiver, bgwriter)
and variable-numbered stats kinds.
---
 src/include/pgstat.h                      |  35 +++-
 src/include/utils/pgstat_internal.h       |  22 ++-
 src/backend/utils/activity/pgstat.c       | 231 +++++++++++++++++++---
 src/backend/utils/activity/pgstat_shmem.c |  31 ++-
 src/backend/utils/adt/pgstatfuncs.c       |   2 +-
 5 files changed, 287 insertions(+), 34 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2d30fadaf1..8d523607a4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -34,6 +34,10 @@
 /* The types of statistics entries */
 #define PgStat_Kind uint32
 
+/* Range of IDs allowed, for built-in and custom kinds */
+#define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
+#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
 
@@ -52,9 +56,34 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 778f625ca1..39f63362a3 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -195,7 +195,8 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
-	 * PgStatShared_HashEntry->body).
+	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
+	 * the size of an entry in PgStat_ShmemControl->custom_data.
 	 */
 	uint32		shared_size;
 
@@ -446,6 +447,13 @@ typedef struct PgStat_ShmemControl
 	PgStatShared_IO io;
 	PgStatShared_SLRU slru;
 	PgStatShared_Wal wal;
+
+	/*
+	 * Custom stats data with fixed-numbered objects, indexed by (PgStat_Kind
+	 * - PGSTAT_KIND_CUSTOM_MIN).
+	 */
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 } PgStat_ShmemControl;
 
 
@@ -459,7 +467,7 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX_BUILTIN + 1];
 
 	PgStat_ArchiverStats archiver;
 
@@ -473,6 +481,14 @@ typedef struct PgStat_Snapshot
 
 	PgStat_WalStats wal;
 
+	/*
+	 * Data in snapshot for custom fixed-numbered statistics, indexed by
+	 * (PgStat_Kind - PGSTAT_KIND_CUSTOM_MIN).  Each entry is allocated in
+	 * TopMemoryContext, for a size of shared_data_len.
+	 */
+	bool		custom_valid[PGSTAT_KIND_CUSTOM_SIZE];
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 	/* to free snapshot in bulk */
 	MemoryContext context;
 	struct pgstat_snapshot_hash *stats;
@@ -516,6 +532,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index f572a87a61..3b85fe0fc8 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for external modules to define custom statistics kinds,
+ * that can use the same properties as any built-in stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
+ * defined, and pgstat_kind_custom_infos for custom kinds registered at
+ * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -174,6 +182,8 @@ typedef struct PgStat_SnapshotEntry
 static void pgstat_write_statsfile(void);
 static void pgstat_read_statsfile(void);
 
+static void pgstat_init_snapshot_fixed(void);
+
 static void pgstat_reset_after_failure(void);
 
 static bool pgstat_flush_pending_entries(bool nowait);
@@ -251,7 +261,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -263,7 +273,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -436,6 +446,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -586,6 +605,8 @@ pgstat_initialize(void)
 
 	pgstat_init_wal();
 
+	pgstat_init_snapshot_fixed();
+
 	/* Set up a process-exit hook to clean up */
 	before_shmem_exit(pgstat_shutdown_hook, 0);
 
@@ -829,6 +850,8 @@ pgstat_clear_snapshot(void)
 
 	memset(&pgStatLocal.snapshot.fixed_valid, 0,
 		   sizeof(pgStatLocal.snapshot.fixed_valid));
+	memset(&pgStatLocal.snapshot.custom_valid, 0,
+		   sizeof(pgStatLocal.snapshot.custom_valid));
 	pgStatLocal.snapshot.stats = NULL;
 	pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_NONE;
 
@@ -992,7 +1015,29 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 	else
 		pgstat_build_snapshot_fixed(kind);
 
-	Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	if (pgstat_is_kind_builtin(kind))
+		Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	else if (pgstat_is_kind_custom(kind))
+		Assert(pgStatLocal.snapshot.custom_valid[kind - PGSTAT_KIND_CUSTOM_MIN]);
+}
+
+static void
+pgstat_init_snapshot_fixed(void)
+{
+	/*
+	 * Initialize fixed-numbered statistics data in snapshots, only for custom
+	 * stats kinds.
+	 */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info || !kind_info->fixed_amount)
+			continue;
+
+		pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN] =
+			MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
+	}
 }
 
 static void
@@ -1088,10 +1133,12 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 		{
 			Assert(kind_info->snapshot_cb == NULL);
@@ -1108,6 +1155,20 @@ static void
 pgstat_build_snapshot_fixed(PgStat_Kind kind)
 {
 	const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+	int			idx;
+	bool	   *valid;
+
+	/* Position in fixed_valid or custom_valid */
+	if (pgstat_is_kind_builtin(kind))
+	{
+		idx = kind;
+		valid = pgStatLocal.snapshot.fixed_valid;
+	}
+	else
+	{
+		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+		valid = pgStatLocal.snapshot.custom_valid;
+	}
 
 	Assert(kind_info->fixed_amount);
 	Assert(kind_info->snapshot_cb != NULL);
@@ -1115,21 +1176,21 @@ pgstat_build_snapshot_fixed(PgStat_Kind kind)
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
 	{
 		/* rebuild every time */
-		pgStatLocal.snapshot.fixed_valid[kind] = false;
+		valid[idx] = false;
 	}
-	else if (pgStatLocal.snapshot.fixed_valid[kind])
+	else if (valid[idx])
 	{
 		/* in snapshot mode we shouldn't get called again */
 		Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
 		return;
 	}
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
+	Assert(!valid[idx]);
 
 	kind_info->snapshot_cb();
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
-	pgStatLocal.snapshot.fixed_valid[kind] = true;
+	Assert(!valid[idx]);
+	valid[idx] = true;
 }
 
 
@@ -1285,30 +1346,127 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
 pgstat_get_kind_info(PgStat_Kind kind)
 {
-	Assert(pgstat_is_kind_valid(kind));
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL ||
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	return NULL;
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * Check some data for fixed-numbered stats.
+	 */
+	if (kind_info->fixed_amount)
+	{
+		if (kind_info->shared_size == 0)
+			ereport(ERROR,
+					(errmsg("custom cumulative statistics property is invalid"),
+					 errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+	}
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+								   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom cumulative statistics \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1385,18 +1543,22 @@ pgstat_write_statsfile(void)
 	write_chunk_s(fpout, &format_id);
 
 	/* Write various stats structs for fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		char	   *ptr;
 		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-		if (!info->fixed_amount)
+		if (!info || !info->fixed_amount)
 			continue;
 
-		Assert(info->snapshot_ctl_off != 0);
+		if (pgstat_is_kind_builtin(kind))
+			Assert(info->snapshot_ctl_off != 0);
 
 		pgstat_build_snapshot_fixed(kind);
-		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		if (pgstat_is_kind_builtin(kind))
+			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		else
+			ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];
 
 		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
 		write_chunk_s(fpout, &kind);
@@ -1419,6 +1581,17 @@ pgstat_write_statsfile(void)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1566,8 +1739,16 @@ pgstat_read_statsfile(void)
 					Assert(info->fixed_amount);
 
 					/* Load back stats into shared memory */
-					ptr = ((char *) shmem) + info->shared_ctl_off +
-						info->shared_data_off;
+					if (pgstat_is_kind_builtin(kind))
+						ptr = ((char *) shmem) + info->shared_ctl_off +
+							info->shared_data_off;
+					else
+					{
+						int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+						ptr = ((char *) shmem->custom_data[idx]) +
+							info->shared_data_off;
+					}
 
 					if (!read_chunk(fpin, ptr,
 									info->shared_data_len))
@@ -1635,7 +1816,7 @@ pgstat_read_statsfile(void)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u",
 							 key.kind, key.dboid, key.objoid);
 						goto error;
 					}
@@ -1693,11 +1874,11 @@ pgstat_reset_after_failure(void)
 	TimestampTz ts = GetCurrentTimestamp();
 
 	/* reset fixed-numbered stats */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		kind_info->reset_all_cb(ts);
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 256f177f5a..c8c5a5e98e 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -131,6 +131,21 @@ StatsShmemSize(void)
 	sz = MAXALIGN(sizeof(PgStat_ShmemControl));
 	sz = add_size(sz, pgstat_dsa_init_size());
 
+	/* Add shared memory for all the custom fixed-numbered statistics */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info)
+			continue;
+		if (!kind_info->fixed_amount)
+			continue;
+
+		Assert(kind_info->shared_size != 0);
+
+		sz += MAXALIGN(kind_info->shared_size);
+	}
+
 	return sz;
 }
 
@@ -197,15 +212,25 @@ StatsShmemInit(void)
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
 		/* initialize fixed-numbered stats */
-		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 		{
 			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 			char	   *ptr;
 
-			if (!kind_info->fixed_amount)
+			if (!kind_info || !kind_info->fixed_amount)
 				continue;
 
-			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			if (pgstat_is_kind_builtin(kind))
+				ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			else
+			{
+				int			idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+				Assert(kind_info->shared_size != 0);
+				ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
+				ptr = ctl->custom_data[idx];
+			}
+
 			kind_info->init_shmem_cb((void *) ptr);
 		}
 	}
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.2

From f79cd779b66489b9ffc1c45e4e775c376f722ab2 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 13:49:53 +0900
Subject: [PATCH v4 6/8] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 63 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 756a9d07fb..878722af51 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3700,6 +3700,69 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in
+     <filename>src/test/modules/injection_points/injection_stats.c</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.2

From 9f6ec2e76ffbe5c45d94b957f3d30ee79a77368a Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 13:36:06 +0900
Subject: [PATCH v4 7/8] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 194 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 +++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  48 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 322 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 2ffd2f77ed..7b9cd12a2a 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -11,9 +14,13 @@ REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
 ISOLATION = inplace
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index e275c2cf5b..747a64e812 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -64,3 +64,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index b6c8e89324..eb411b9d44 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -417,5 +431,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..c37b0b33d3
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,194 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+#define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 3c23c14d81..a52fe5121e 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -42,4 +43,12 @@ tests += {
       'inplace',
     ],
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..7d5a96e522
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,48 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 9320e4d808..c2211d9804 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2117,6 +2117,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2148,6 +2149,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.2

From db9dc4179f56e6448b28a0898a228761013bf765 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 13:44:14 +0900
Subject: [PATCH v4 8/8] injection_points: Add example for fixed-numbered
 statistics

This acts as a template to show what can be achieved with fixed-numbered
stats (like WAL, bgwriter, etc.) for pluggable cumulative statistics.
---
 .../injection_points--1.0.sql                 |  11 ++
 .../injection_points/injection_points.c       |   3 +
 .../injection_points/injection_stats.c        | 155 +++++++++++++++++-
 .../injection_points/injection_stats.h        |   3 +
 .../modules/injection_points/t/001_stats.pl   |  11 +-
 5 files changed, 180 insertions(+), 3 deletions(-)

diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index 747a64e812..fa0b1d06ae 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -74,3 +74,14 @@ CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
 RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
+
+--
+-- injection_points_stats_fixed()
+--
+-- Reports fixed-numbered statistics for injection points.
+CREATE FUNCTION injection_points_stats_fixed(OUT numattach int8,
+   OUT numdetach int8,
+   OUT numrun int8)
+RETURNS record
+AS 'MODULE_PATHNAME', 'injection_points_stats_fixed'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index eb411b9d44..f65bd83cc3 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -297,6 +297,7 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		condition.pid = MyProcPid;
 	}
 
+	pgstat_report_inj_fixed(1, 0, 0);
 	InjectionPointAttach(name, "injection_points", function, &condition,
 						 sizeof(InjectionPointCondition));
 
@@ -342,6 +343,7 @@ injection_points_run(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 0, 1);
 	INJECTION_POINT(name);
 
 	PG_RETURN_VOID();
@@ -418,6 +420,7 @@ injection_points_detach(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 1, 0);
 	if (!InjectionPointDetach(name))
 		elog(ERROR, "could not detach injection point \"%s\"", name);
 
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
index c37b0b33d3..69560b7fdc 100644
--- a/src/test/modules/injection_points/injection_stats.c
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -17,12 +17,13 @@
 #include "fmgr.h"
 
 #include "common/hashfn.h"
+#include "funcapi.h"
 #include "injection_stats.h"
 #include "pgstat.h"
 #include "utils/builtins.h"
 #include "utils/pgstat_internal.h"
 
-/* Structures for statistics of injection points */
+/* Structures for statistics of injection points, variable-size */
 typedef struct PgStat_StatInjEntry
 {
 	PgStat_Counter numcalls;	/* number of times point has been run */
@@ -35,6 +36,9 @@ typedef struct PgStatShared_InjectionPoint
 } PgStatShared_InjectionPoint;
 
 static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+static void injection_stats_fixed_init_shmem_cb(void *stats);
+static void injection_stats_fixed_reset_all_cb(TimestampTz ts);
+static void injection_stats_fixed_snapshot_cb(void);
 
 static const PgStat_KindInfo injection_stats = {
 	.name = "injection_points",
@@ -50,18 +54,51 @@ static const PgStat_KindInfo injection_stats = {
 	.flush_pending_cb = injection_stats_flush_cb,
 };
 
+/* Structures for statistics of injection points, fixed-size */
+typedef struct PgStat_StatInjFixedEntry
+{
+	PgStat_Counter numattach;	/* number of points attached */
+	PgStat_Counter numdetach;	/* number of points detached */
+	PgStat_Counter numrun;		/* number of points run */
+	TimestampTz stat_reset_timestamp;
+} PgStat_StatInjFixedEntry;
+
+typedef struct PgStatShared_InjectionPointFixed
+{
+	LWLock	lock;	/* protects all the counters */
+	uint32	changecount;
+	PgStat_StatInjFixedEntry stats;
+	PgStat_StatInjFixedEntry reset_offset;
+} PgStatShared_InjectionPointFixed;
+
+static const PgStat_KindInfo injection_stats_fixed = {
+	.name = "injection_points_fixed",
+	.fixed_amount = true,
+
+	.shared_size = sizeof(PgStat_StatInjFixedEntry),
+	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPointFixed *) 0)->stats),
+
+	.init_shmem_cb = injection_stats_fixed_init_shmem_cb,
+	.reset_all_cb = injection_stats_fixed_reset_all_cb,
+	.snapshot_cb = injection_stats_fixed_snapshot_cb,
+};
+
 /*
  * Compute stats entry idx from point name with a 4-byte hash.
  */
 #define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
 
 #define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+#define PGSTAT_KIND_INJECTION_FIXED	(PGSTAT_KIND_EXPERIMENTAL + 1)
+/* Position of fixed-numbered data in internal structures */
+#define PGSTAT_KIND_INJECTION_IDX	(PGSTAT_KIND_INJECTION_FIXED - PGSTAT_KIND_CUSTOM_MIN)
 
 /* Track if stats are loaded */
 static bool inj_stats_loaded = false;
 
 /*
- * Callback for stats handling
+ * Callbacks for stats handling
  */
 static bool
 injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
@@ -79,6 +116,59 @@ injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	return true;
 }
 
+static void
+injection_stats_fixed_init_shmem_cb(void *stats)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+static void
+injection_stats_fixed_reset_all_cb(TimestampTz ts)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
+	LWLockRelease(&stats_shmem->lock);
+}
+
+static void
+injection_stats_fixed_snapshot_cb(void)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+	PgStat_StatInjFixedEntry *stat_snap = (PgStat_StatInjFixedEntry *)
+		pgStatLocal.snapshot.custom_data[PGSTAT_KIND_INJECTION_IDX];
+	PgStat_StatInjFixedEntry *reset_offset = &stats_shmem->reset_offset;
+	PgStat_StatInjFixedEntry reset;
+
+	pgstat_copy_changecounted_stats(stat_snap,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
+	LWLockRelease(&stats_shmem->lock);
+
+	/* compensate by reset offsets */
+#define FIXED_COMP(fld) stat_snap->fld -= reset.fld;
+	FIXED_COMP(numattach);
+	FIXED_COMP(numdetach);
+	FIXED_COMP(numrun);
+#undef FIXED_COMP
+}
+
 /*
  * Support function for the SQL-callable pgstat* functions.  Returns
  * a pointer to the injection point statistics struct.
@@ -105,6 +195,7 @@ void
 pgstat_register_inj(void)
 {
 	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+	pgstat_register_kind(PGSTAT_KIND_INJECTION_FIXED, &injection_stats_fixed);
 
 	/* mark stats as loaded */
 	inj_stats_loaded = true;
@@ -176,6 +267,30 @@ pgstat_report_inj(const char *name)
 	pgstat_unlock_entry(entry_ref);
 }
 
+/*
+ * Report fixed number of statistics for an injection point.
+ */
+void
+pgstat_report_inj_fixed(uint32 numattach,
+						uint32 numdetach,
+						uint32 numrun)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	stats_shmem = (PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	pgstat_begin_changecount_write(&stats_shmem->changecount);
+	stats_shmem->stats.numattach += numattach;
+	stats_shmem->stats.numdetach += numdetach;
+	stats_shmem->stats.numrun += numrun;
+	pgstat_end_changecount_write(&stats_shmem->changecount);
+}
+
 /*
  * SQL function returning the number of times an injection point
  * has been called.
@@ -192,3 +307,39 @@ injection_points_stats_numcalls(PG_FUNCTION_ARGS)
 
 	PG_RETURN_INT64(entry->numcalls);
 }
+
+/*
+ * SQL function returning fixed-numbered statistics for injection points.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_fixed);
+Datum
+injection_points_stats_fixed(PG_FUNCTION_ARGS)
+{
+	TupleDesc	tupdesc;
+	Datum		values[3] = {0};
+	bool		nulls[3] = {0};
+	PgStat_StatInjFixedEntry *stats;
+
+	pgstat_snapshot_fixed(PGSTAT_KIND_INJECTION_FIXED);
+	stats = pgStatLocal.snapshot.custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(3);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "numattach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "numdetach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "numrun",
+					   INT8OID, -1, 0);
+	BlessTupleDesc(tupdesc);
+
+	values[0] = Int64GetDatum(stats->numattach);
+	values[1] = Int64GetDatum(stats->numdetach);
+	values[2] = Int64GetDatum(stats->numrun);
+	nulls[0] = false;
+	nulls[1] = false;
+	nulls[2] = false;
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
index 3e99705483..cf68b25f7b 100644
--- a/src/test/modules/injection_points/injection_stats.h
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -19,5 +19,8 @@ extern void pgstat_register_inj(void);
 extern void pgstat_create_inj(const char *name);
 extern void pgstat_drop_inj(const char *name);
 extern void pgstat_report_inj(const char *name);
+extern void pgstat_report_inj_fixed(uint32 numattach,
+									uint32 numdetach,
+									uint32 numrun);
 
 #endif
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 7d5a96e522..e3c69b94ca 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -31,18 +31,27 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats');
 
 # Restart the node cleanly, stats should still be around.
 $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats after clean restart');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats after clean restart');
 
 # On crash the stats are gone.
 $node->stop('immediate');
 $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
-is($numcalls, '', 'number of stats after clean restart');
+is($numcalls, '', 'number of stats after crash');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '0|0|0', 'number of fixed stats after crash');
 
 done_testing();
-- 
2.45.2

From d9fe88f22478012cac305612d06304c724c0265d Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 11:21:42 +0900
Subject: [PATCH v4 1/8] Use pgstat_kind_infos to write fixed shared statistics

This is similar to 9004abf6206e, but this time for the write part.  This
requires the addition to an offset in PgStat_KindInfo to track the
location of the stats to write in PgStat_Snapshot.
---
 src/include/utils/pgstat_internal.h |  6 ++++
 src/backend/utils/activity/pgstat.c | 56 ++++++++++-------------------
 2 files changed, 24 insertions(+), 38 deletions(-)

diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index e21ef4e2c9..43d6deb03c 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -199,6 +199,12 @@ typedef struct PgStat_KindInfo
 	 */
 	uint32		shared_size;
 
+	/*
+	 * The offset of the statistics struct in the cached statistics snapshot
+	 * PgStat_Snapshot, for fixed-numbered statistics.
+	 */
+	uint32		snapshot_ctl_off;
+
 	/*
 	 * The offset of the statistics struct in the containing shared memory
 	 * control structure PgStat_ShmemControl, for fixed-numbered statistics.
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index c37c11b2ec..65b937a85f 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -349,6 +349,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, archiver),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, archiver),
 		.shared_data_off = offsetof(PgStatShared_Archiver, stats),
 		.shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
@@ -362,6 +363,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, bgwriter),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, bgwriter),
 		.shared_data_off = offsetof(PgStatShared_BgWriter, stats),
 		.shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
@@ -375,6 +377,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, checkpointer),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, checkpointer),
 		.shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
 		.shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
@@ -388,6 +391,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, io),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, io),
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
@@ -401,6 +405,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, slru),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, slru),
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
@@ -414,6 +419,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, wal),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, wal),
 		.shared_data_off = offsetof(PgStatShared_Wal, stats),
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
@@ -1371,47 +1377,21 @@ pgstat_write_statsfile(void)
 	format_id = PGSTAT_FILE_FORMAT_ID;
 	write_chunk_s(fpout, &format_id);
 
-	/*
-	 * XXX: The following could now be generalized to just iterate over
-	 * pgstat_kind_infos instead of knowing about the different kinds of
-	 * stats.
-	 */
+	/* Write various stats structs for fixed number of objects */
+	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	{
+		char	   *ptr;
+		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-	/*
-	 * Write archiver stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_ARCHIVER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.archiver);
+		if (!info->fixed_amount)
+			continue;
 
-	/*
-	 * Write bgwriter stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_BGWRITER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.bgwriter);
+		Assert(info->snapshot_ctl_off != 0);
 
-	/*
-	 * Write checkpointer stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_CHECKPOINTER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.checkpointer);
-
-	/*
-	 * Write IO stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_IO);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.io);
-
-	/*
-	 * Write SLRU stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_SLRU);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.slru);
-
-	/*
-	 * Write WAL stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_WAL);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.wal);
+		pgstat_build_snapshot_fixed(kind);
+		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		write_chunk(fpout, ptr, info->shared_data_len);
+	}
 
 	/*
 	 * Walk through the stats entries
-- 
2.45.2

From 9491b4eabe583d9fde97cb702ec88990d8b5b54d Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 11:41:25 +0900
Subject: [PATCH v4 2/8] Add PgStat_KindInfo.init_shmem_cb

This new callback gives fixed-numbered stats the possibility to take
actions based on the area of shared memory they have been allocated.

This removes from pgstat_shmem.c any knowledge specific to the types
of fixed-numbered stats, and the initializations happen in their own
files.

This is handy to make this area of the code more pluggable (as in custom
stats kinds with a fixed number of objects).
---
 src/include/utils/pgstat_internal.h           | 13 +++++++++++++
 src/backend/utils/activity/pgstat.c           |  6 ++++++
 src/backend/utils/activity/pgstat_archiver.c  |  8 ++++++++
 src/backend/utils/activity/pgstat_bgwriter.c  |  8 ++++++++
 .../utils/activity/pgstat_checkpointer.c      |  8 ++++++++
 src/backend/utils/activity/pgstat_io.c        |  9 +++++++++
 src/backend/utils/activity/pgstat_shmem.c     | 19 ++++++++++---------
 src/backend/utils/activity/pgstat_slru.c      |  8 ++++++++
 src/backend/utils/activity/pgstat_wal.c       |  8 ++++++++
 9 files changed, 78 insertions(+), 9 deletions(-)

diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 43d6deb03c..778f625ca1 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -251,6 +251,13 @@ typedef struct PgStat_KindInfo
 									   const PgStatShared_Common *header, NameData *name);
 	bool		(*from_serialized_name) (const NameData *name, PgStat_HashKey *key);
 
+	/*
+	 * For fixed-numbered statistics: Initialize shared memory state.
+	 *
+	 * "stats" is the pointer to the allocated shared memory area.
+	 */
+	void		(*init_shmem_cb) (void *stats);
+
 	/*
 	 * For fixed-numbered statistics: Reset All.
 	 */
@@ -528,6 +535,7 @@ extern void pgstat_snapshot_fixed(PgStat_Kind kind);
  * Functions in pgstat_archiver.c
  */
 
+extern void pgstat_archiver_init_shmem_cb(void *stats);
 extern void pgstat_archiver_reset_all_cb(TimestampTz ts);
 extern void pgstat_archiver_snapshot_cb(void);
 
@@ -536,6 +544,7 @@ extern void pgstat_archiver_snapshot_cb(void);
  * Functions in pgstat_bgwriter.c
  */
 
+extern void pgstat_bgwriter_init_shmem_cb(void *stats);
 extern void pgstat_bgwriter_reset_all_cb(TimestampTz ts);
 extern void pgstat_bgwriter_snapshot_cb(void);
 
@@ -544,6 +553,7 @@ extern void pgstat_bgwriter_snapshot_cb(void);
  * Functions in pgstat_checkpointer.c
  */
 
+extern void pgstat_checkpointer_init_shmem_cb(void *stats);
 extern void pgstat_checkpointer_reset_all_cb(TimestampTz ts);
 extern void pgstat_checkpointer_snapshot_cb(void);
 
@@ -574,6 +584,7 @@ extern bool pgstat_function_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
  */
 
 extern bool pgstat_flush_io(bool nowait);
+extern void pgstat_io_init_shmem_cb(void *stats);
 extern void pgstat_io_reset_all_cb(TimestampTz ts);
 extern void pgstat_io_snapshot_cb(void);
 
@@ -632,6 +643,7 @@ extern PgStatShared_Common *pgstat_init_entry(PgStat_Kind kind,
  */
 
 extern bool pgstat_slru_flush(bool nowait);
+extern void pgstat_slru_init_shmem_cb(void *stats);
 extern void pgstat_slru_reset_all_cb(TimestampTz ts);
 extern void pgstat_slru_snapshot_cb(void);
 
@@ -644,6 +656,7 @@ extern bool pgstat_flush_wal(bool nowait);
 extern void pgstat_init_wal(void);
 extern bool pgstat_have_pending_wal(void);
 
+extern void pgstat_wal_init_shmem_cb(void *stats);
 extern void pgstat_wal_reset_all_cb(TimestampTz ts);
 extern void pgstat_wal_snapshot_cb(void);
 
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 65b937a85f..ef435167f7 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -354,6 +354,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Archiver, stats),
 		.shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
 
+		.init_shmem_cb = pgstat_archiver_init_shmem_cb,
 		.reset_all_cb = pgstat_archiver_reset_all_cb,
 		.snapshot_cb = pgstat_archiver_snapshot_cb,
 	},
@@ -368,6 +369,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_BgWriter, stats),
 		.shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
 
+		.init_shmem_cb = pgstat_bgwriter_init_shmem_cb,
 		.reset_all_cb = pgstat_bgwriter_reset_all_cb,
 		.snapshot_cb = pgstat_bgwriter_snapshot_cb,
 	},
@@ -382,6 +384,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
 		.shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
 
+		.init_shmem_cb = pgstat_checkpointer_init_shmem_cb,
 		.reset_all_cb = pgstat_checkpointer_reset_all_cb,
 		.snapshot_cb = pgstat_checkpointer_snapshot_cb,
 	},
@@ -396,6 +399,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
 
+		.init_shmem_cb = pgstat_io_init_shmem_cb,
 		.reset_all_cb = pgstat_io_reset_all_cb,
 		.snapshot_cb = pgstat_io_snapshot_cb,
 	},
@@ -410,6 +414,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
 
+		.init_shmem_cb = pgstat_slru_init_shmem_cb,
 		.reset_all_cb = pgstat_slru_reset_all_cb,
 		.snapshot_cb = pgstat_slru_snapshot_cb,
 	},
@@ -424,6 +429,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Wal, stats),
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
 
+		.init_shmem_cb = pgstat_wal_init_shmem_cb,
 		.reset_all_cb = pgstat_wal_reset_all_cb,
 		.snapshot_cb = pgstat_wal_snapshot_cb,
 	},
diff --git a/src/backend/utils/activity/pgstat_archiver.c b/src/backend/utils/activity/pgstat_archiver.c
index 66398b20e5..60e04711c4 100644
--- a/src/backend/utils/activity/pgstat_archiver.c
+++ b/src/backend/utils/activity/pgstat_archiver.c
@@ -62,6 +62,14 @@ pgstat_fetch_stat_archiver(void)
 	return &pgStatLocal.snapshot.archiver;
 }
 
+void
+pgstat_archiver_init_shmem_cb(void *stats)
+{
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_archiver_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_bgwriter.c b/src/backend/utils/activity/pgstat_bgwriter.c
index 7d2432e4fa..364a7a2024 100644
--- a/src/backend/utils/activity/pgstat_bgwriter.c
+++ b/src/backend/utils/activity/pgstat_bgwriter.c
@@ -75,6 +75,14 @@ pgstat_fetch_stat_bgwriter(void)
 	return &pgStatLocal.snapshot.bgwriter;
 }
 
+void
+pgstat_bgwriter_init_shmem_cb(void *stats)
+{
+	PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_bgwriter_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_checkpointer.c b/src/backend/utils/activity/pgstat_checkpointer.c
index 30a8110e38..bbfc9c7e18 100644
--- a/src/backend/utils/activity/pgstat_checkpointer.c
+++ b/src/backend/utils/activity/pgstat_checkpointer.c
@@ -84,6 +84,14 @@ pgstat_fetch_stat_checkpointer(void)
 	return &pgStatLocal.snapshot.checkpointer;
 }
 
+void
+pgstat_checkpointer_init_shmem_cb(void *stats)
+{
+	PgStatShared_Checkpointer *stats_shmem = (PgStatShared_Checkpointer *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_checkpointer_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index 9d6e067382..8af55989ee 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -251,6 +251,15 @@ pgstat_get_io_object_name(IOObject io_object)
 	pg_unreachable();
 }
 
+void
+pgstat_io_init_shmem_cb(void *stats)
+{
+	PgStatShared_IO *stat_shmem = (PgStatShared_IO *) stats;
+
+	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
+		LWLockInitialize(&stat_shmem->locks[i], LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_io_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 634b967820..256f177f5a 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -196,17 +196,18 @@ StatsShmemInit(void)
 
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
-
 		/* initialize fixed-numbered stats */
-		LWLockInitialize(&ctl->archiver.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->bgwriter.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->checkpointer.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->slru.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->wal.lock, LWTRANCHE_PGSTATS_DATA);
+		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		{
+			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+			char	   *ptr;
 
-		for (int i = 0; i < BACKEND_NUM_TYPES; i++)
-			LWLockInitialize(&ctl->io.locks[i],
-							 LWTRANCHE_PGSTATS_DATA);
+			if (!kind_info->fixed_amount)
+				continue;
+
+			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			kind_info->init_shmem_cb((void *) ptr);
+		}
 	}
 	else
 	{
diff --git a/src/backend/utils/activity/pgstat_slru.c b/src/backend/utils/activity/pgstat_slru.c
index 56ea1c3378..6f922a85bf 100644
--- a/src/backend/utils/activity/pgstat_slru.c
+++ b/src/backend/utils/activity/pgstat_slru.c
@@ -192,6 +192,14 @@ pgstat_slru_flush(bool nowait)
 	return false;
 }
 
+void
+pgstat_slru_init_shmem_cb(void *stats)
+{
+	PgStatShared_SLRU *stats_shmem = (PgStatShared_SLRU *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_slru_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 0e374f133a..e2a3f6b865 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -163,6 +163,14 @@ pgstat_have_pending_wal(void)
 		PendingWalStats.wal_sync != 0;
 }
 
+void
+pgstat_wal_init_shmem_cb(void *stats)
+{
+	PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_wal_reset_all_cb(TimestampTz ts)
 {
-- 
2.45.2

From aec71d0c382689a6527de8769fa7670f615e18e9 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 12:24:44 +0900
Subject: [PATCH v4 3/8] Add a new 'F' type of entry in pgstats file

This new entry type is used for fixed-numbered statistics, making an
upcoming change to add support for custom pluggable stats kinds more
palatable.
---
 src/backend/utils/activity/pgstat.c | 44 ++++++++++++++++++-----------
 1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index ef435167f7..ed32ee4b6f 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -132,6 +132,7 @@
  * ---------
  */
 #define PGSTAT_FILE_ENTRY_END	'E' /* end of file */
+#define PGSTAT_FILE_ENTRY_FIXED	'F' /* fixed-numbered stats entry */
 #define PGSTAT_FILE_ENTRY_NAME	'N' /* stats entry identified by name */
 #define PGSTAT_FILE_ENTRY_HASH	'S' /* stats entry identified by
 									 * PgStat_HashKey */
@@ -1396,6 +1397,9 @@ pgstat_write_statsfile(void)
 
 		pgstat_build_snapshot_fixed(kind);
 		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+
+		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
+		write_chunk_s(fpout, &kind);
 		write_chunk(fpout, ptr, info->shared_data_len);
 	}
 
@@ -1537,22 +1541,6 @@ pgstat_read_statsfile(void)
 		format_id != PGSTAT_FILE_FORMAT_ID)
 		goto error;
 
-	/* Read various stats structs with fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
-	{
-		char	   *ptr;
-		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
-
-		if (!info->fixed_amount)
-			continue;
-
-		Assert(info->shared_ctl_off != 0);
-
-		ptr = ((char *) shmem) + info->shared_ctl_off + info->shared_data_off;
-		if (!read_chunk(fpin, ptr, info->shared_data_len))
-			goto error;
-	}
-
 	/*
 	 * We found an existing statistics file. Read it and put all the hash
 	 * table entries into place.
@@ -1563,6 +1551,30 @@ pgstat_read_statsfile(void)
 
 		switch (t)
 		{
+			case PGSTAT_FILE_ENTRY_FIXED:
+				{
+					PgStat_Kind kind;
+					const PgStat_KindInfo *info;
+					char	   *ptr;
+
+					if (!read_chunk_s(fpin, &kind))
+						goto error;
+
+					if (!pgstat_is_kind_valid(kind))
+						goto error;
+					info = pgstat_get_kind_info(kind);
+					Assert(info->fixed_amount);
+
+					/* Load back stats into shared memory */
+					ptr = ((char *) shmem) + info->shared_ctl_off +
+						info->shared_data_off;
+
+					if (!read_chunk(fpin, ptr,
+									info->shared_data_len))
+						goto error;
+
+					break;
+				}
 			case PGSTAT_FILE_ENTRY_HASH:
 			case PGSTAT_FILE_ENTRY_NAME:
 				{
-- 
2.45.2

From cc18cfc3180a70695820f7824534a2dd27821dd1 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 12:28:26 +0900
Subject: [PATCH v4 4/8] Switch PgStat_Kind from enum to uint32

A follow-up patch is planned to make this counter extensible, and
keeping a trace of the kind behind a type is useful in the internal
routines used by pgstats.  While on it, switch pgstat_is_kind_valid() to
use PgStat_Kind, to be more consistent with its callers.
---
 src/include/pgstat.h                | 35 ++++++++++++++---------------
 src/backend/utils/activity/pgstat.c |  6 ++---
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2136239710..2d30fadaf1 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -32,26 +32,25 @@
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"
 
 /* The types of statistics entries */
-typedef enum PgStat_Kind
-{
-	/* use 0 for INVALID, to catch zero-initialized data */
-	PGSTAT_KIND_INVALID = 0,
+#define PgStat_Kind uint32
 
-	/* stats for variable-numbered objects */
-	PGSTAT_KIND_DATABASE,		/* database-wide statistics */
-	PGSTAT_KIND_RELATION,		/* per-table statistics */
-	PGSTAT_KIND_FUNCTION,		/* per-function statistics */
-	PGSTAT_KIND_REPLSLOT,		/* per-slot statistics */
-	PGSTAT_KIND_SUBSCRIPTION,	/* per-subscription statistics */
+/* use 0 for INVALID, to catch zero-initialized data */
+#define PGSTAT_KIND_INVALID 0
 
-	/* stats for fixed-numbered objects */
-	PGSTAT_KIND_ARCHIVER,
-	PGSTAT_KIND_BGWRITER,
-	PGSTAT_KIND_CHECKPOINTER,
-	PGSTAT_KIND_IO,
-	PGSTAT_KIND_SLRU,
-	PGSTAT_KIND_WAL,
-} PgStat_Kind;
+/* stats for variable-numbered objects */
+#define PGSTAT_KIND_DATABASE	1	/* database-wide statistics */
+#define PGSTAT_KIND_RELATION	2	/* per-table statistics */
+#define PGSTAT_KIND_FUNCTION	3	/* per-function statistics */
+#define PGSTAT_KIND_REPLSLOT	4	/* per-slot statistics */
+#define PGSTAT_KIND_SUBSCRIPTION	5	/* per-subscription statistics */
+
+/* stats for fixed-numbered objects */
+#define PGSTAT_KIND_ARCHIVER	6
+#define PGSTAT_KIND_BGWRITER	7
+#define PGSTAT_KIND_CHECKPOINTER	8
+#define PGSTAT_KIND_IO	9
+#define PGSTAT_KIND_SLRU	10
+#define PGSTAT_KIND_WAL	11
 
 #define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
 #define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index ed32ee4b6f..f572a87a61 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -182,7 +182,7 @@ static void pgstat_prep_snapshot(void);
 static void pgstat_build_snapshot(void);
 static void pgstat_build_snapshot_fixed(PgStat_Kind kind);
 
-static inline bool pgstat_is_kind_valid(int ikind);
+static inline bool pgstat_is_kind_valid(PgStat_Kind kind);
 
 
 /* ----------
@@ -1298,9 +1298,9 @@ pgstat_get_kind_from_str(char *kind_str)
 }
 
 static inline bool
-pgstat_is_kind_valid(int ikind)
+pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
+	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
 }
 
 const PgStat_KindInfo *
-- 
2.45.2

From 390fdbc5b3fb1f25df809d0541507b886f1b15ec Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Tue, 9 Jul 2024 04:52:04 +0000
Subject: [PATCH v1] Renaming two fields in PgStat_KindInfo

Renaming shared_data_off to shared_stats_off and shared_data_len to
shared_stats_len. "data" seems too generic and started to be confusing since
b68b29bc8f makes use of the "_len" one in the snapshot context. Using "stats"
instead as that's what those fields are linked to.
---
 src/backend/utils/activity/pgstat.c | 56 ++++++++++++++---------------
 src/include/utils/pgstat_internal.h | 12 +++----
 2 files changed, 34 insertions(+), 34 deletions(-)
  84.2% src/backend/utils/activity/
  15.7% src/include/utils/

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 65b937a85f..bb777423fb 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -274,8 +274,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.accessed_across_databases = true,
 
 		.shared_size = sizeof(PgStatShared_Database),
-		.shared_data_off = offsetof(PgStatShared_Database, stats),
-		.shared_data_len = sizeof(((PgStatShared_Database *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_Database, stats),
+		.shared_stats_len = sizeof(((PgStatShared_Database *) 0)->stats),
 		.pending_size = sizeof(PgStat_StatDBEntry),
 
 		.flush_pending_cb = pgstat_database_flush_cb,
@@ -288,8 +288,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.fixed_amount = false,
 
 		.shared_size = sizeof(PgStatShared_Relation),
-		.shared_data_off = offsetof(PgStatShared_Relation, stats),
-		.shared_data_len = sizeof(((PgStatShared_Relation *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_Relation, stats),
+		.shared_stats_len = sizeof(((PgStatShared_Relation *) 0)->stats),
 		.pending_size = sizeof(PgStat_TableStatus),
 
 		.flush_pending_cb = pgstat_relation_flush_cb,
@@ -302,8 +302,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.fixed_amount = false,
 
 		.shared_size = sizeof(PgStatShared_Function),
-		.shared_data_off = offsetof(PgStatShared_Function, stats),
-		.shared_data_len = sizeof(((PgStatShared_Function *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_Function, stats),
+		.shared_stats_len = sizeof(((PgStatShared_Function *) 0)->stats),
 		.pending_size = sizeof(PgStat_FunctionCounts),
 
 		.flush_pending_cb = pgstat_function_flush_cb,
@@ -317,8 +317,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.accessed_across_databases = true,
 
 		.shared_size = sizeof(PgStatShared_ReplSlot),
-		.shared_data_off = offsetof(PgStatShared_ReplSlot, stats),
-		.shared_data_len = sizeof(((PgStatShared_ReplSlot *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_ReplSlot, stats),
+		.shared_stats_len = sizeof(((PgStatShared_ReplSlot *) 0)->stats),
 
 		.reset_timestamp_cb = pgstat_replslot_reset_timestamp_cb,
 		.to_serialized_name = pgstat_replslot_to_serialized_name_cb,
@@ -333,8 +333,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.accessed_across_databases = true,
 
 		.shared_size = sizeof(PgStatShared_Subscription),
-		.shared_data_off = offsetof(PgStatShared_Subscription, stats),
-		.shared_data_len = sizeof(((PgStatShared_Subscription *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_Subscription, stats),
+		.shared_stats_len = sizeof(((PgStatShared_Subscription *) 0)->stats),
 		.pending_size = sizeof(PgStat_BackendSubEntry),
 
 		.flush_pending_cb = pgstat_subscription_flush_cb,
@@ -351,8 +351,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, archiver),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, archiver),
-		.shared_data_off = offsetof(PgStatShared_Archiver, stats),
-		.shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_Archiver, stats),
+		.shared_stats_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
 
 		.reset_all_cb = pgstat_archiver_reset_all_cb,
 		.snapshot_cb = pgstat_archiver_snapshot_cb,
@@ -365,8 +365,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, bgwriter),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, bgwriter),
-		.shared_data_off = offsetof(PgStatShared_BgWriter, stats),
-		.shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_BgWriter, stats),
+		.shared_stats_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
 
 		.reset_all_cb = pgstat_bgwriter_reset_all_cb,
 		.snapshot_cb = pgstat_bgwriter_snapshot_cb,
@@ -379,8 +379,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, checkpointer),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, checkpointer),
-		.shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
-		.shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_Checkpointer, stats),
+		.shared_stats_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
 
 		.reset_all_cb = pgstat_checkpointer_reset_all_cb,
 		.snapshot_cb = pgstat_checkpointer_snapshot_cb,
@@ -393,8 +393,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, io),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, io),
-		.shared_data_off = offsetof(PgStatShared_IO, stats),
-		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_IO, stats),
+		.shared_stats_len = sizeof(((PgStatShared_IO *) 0)->stats),
 
 		.reset_all_cb = pgstat_io_reset_all_cb,
 		.snapshot_cb = pgstat_io_snapshot_cb,
@@ -407,8 +407,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, slru),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, slru),
-		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
-		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_SLRU, stats),
+		.shared_stats_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
 
 		.reset_all_cb = pgstat_slru_reset_all_cb,
 		.snapshot_cb = pgstat_slru_snapshot_cb,
@@ -421,8 +421,8 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, wal),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, wal),
-		.shared_data_off = offsetof(PgStatShared_Wal, stats),
-		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
+		.shared_stats_off = offsetof(PgStatShared_Wal, stats),
+		.shared_stats_len = sizeof(((PgStatShared_Wal *) 0)->stats),
 
 		.reset_all_cb = pgstat_wal_reset_all_cb,
 		.snapshot_cb = pgstat_wal_snapshot_cb,
@@ -910,15 +910,15 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, Oid objoid)
 	 * repeated accesses.
 	 */
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
-		stats_data = palloc(kind_info->shared_data_len);
+		stats_data = palloc(kind_info->shared_stats_len);
 	else
 		stats_data = MemoryContextAlloc(pgStatLocal.snapshot.context,
-										kind_info->shared_data_len);
+										kind_info->shared_stats_len);
 
 	pgstat_lock_entry_shared(entry_ref, false);
 	memcpy(stats_data,
 		   pgstat_get_entry_data(kind, entry_ref->shared_stats),
-		   kind_info->shared_data_len);
+		   kind_info->shared_stats_len);
 	pgstat_unlock_entry(entry_ref);
 
 	if (pgstat_fetch_consistency > PGSTAT_FETCH_CONSISTENCY_NONE)
@@ -1390,7 +1390,7 @@ pgstat_write_statsfile(void)
 
 		pgstat_build_snapshot_fixed(kind);
 		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
-		write_chunk(fpout, ptr, info->shared_data_len);
+		write_chunk(fpout, ptr, info->shared_stats_len);
 	}
 
 	/*
@@ -1542,8 +1542,8 @@ pgstat_read_statsfile(void)
 
 		Assert(info->shared_ctl_off != 0);
 
-		ptr = ((char *) shmem) + info->shared_ctl_off + info->shared_data_off;
-		if (!read_chunk(fpin, ptr, info->shared_data_len))
+		ptr = ((char *) shmem) + info->shared_ctl_off + info->shared_stats_off;
+		if (!read_chunk(fpin, ptr, info->shared_stats_len))
 			goto error;
 	}
 
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 43d6deb03c..9246998ac6 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -217,8 +217,8 @@ typedef struct PgStat_KindInfo
 	 * shared_size because [de-]serialization may not include in-memory state
 	 * like lwlocks.
 	 */
-	uint32		shared_data_off;
-	uint32		shared_data_len;
+	uint32		shared_stats_off;
+	uint32		shared_stats_len;
 
 	/*
 	 * The size of the pending data for this kind. E.g. how large
@@ -789,22 +789,22 @@ pgstat_hash_hash_key(const void *d, size_t size, void *arg)
 }
 
 /*
- * The length of the data portion of a shared memory stats entry (i.e. without
+ * The length of the stats portion of a shared memory stats entry (i.e. without
  * transient data such as refcounts, lwlocks, ...).
  */
 static inline size_t
 pgstat_get_entry_len(PgStat_Kind kind)
 {
-	return pgstat_get_kind_info(kind)->shared_data_len;
+	return pgstat_get_kind_info(kind)->shared_stats_len;
 }
 
 /*
- * Returns a pointer to the data portion of a shared memory stats entry.
+ * Returns a pointer to the stats portion of a shared memory stats entry.
  */
 static inline void *
 pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry)
 {
-	size_t		off = pgstat_get_kind_info(kind)->shared_data_off;
+	size_t		off = pgstat_get_kind_info(kind)->shared_stats_off;
 
 	Assert(off != 0 && off < PG_UINT32_MAX);
 
-- 
2.34.1

From 7b878d78a8ea42a509ee929802c5598d23981634 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Tue, 9 Jul 2024 15:24:40 +0900
Subject: [PATCH v5 1/7] Add PgStat_KindInfo.init_shmem_cb

This new callback gives fixed-numbered stats the possibility to take
actions based on the area of shared memory they have been allocated.

This removes from pgstat_shmem.c any knowledge specific to the types
of fixed-numbered stats, and the initializations happen in their own
files.

This is handy to make this area of the code more pluggable (as in custom
stats kinds with a fixed number of objects).
---
 src/include/utils/pgstat_internal.h           | 13 +++++++++++++
 src/backend/utils/activity/pgstat.c           |  6 ++++++
 src/backend/utils/activity/pgstat_archiver.c  |  8 ++++++++
 src/backend/utils/activity/pgstat_bgwriter.c  |  8 ++++++++
 .../utils/activity/pgstat_checkpointer.c      |  8 ++++++++
 src/backend/utils/activity/pgstat_io.c        |  9 +++++++++
 src/backend/utils/activity/pgstat_shmem.c     | 19 ++++++++++---------
 src/backend/utils/activity/pgstat_slru.c      |  8 ++++++++
 src/backend/utils/activity/pgstat_wal.c       |  8 ++++++++
 9 files changed, 78 insertions(+), 9 deletions(-)

diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 43d6deb03c..778f625ca1 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -251,6 +251,13 @@ typedef struct PgStat_KindInfo
 									   const PgStatShared_Common *header, NameData *name);
 	bool		(*from_serialized_name) (const NameData *name, PgStat_HashKey *key);
 
+	/*
+	 * For fixed-numbered statistics: Initialize shared memory state.
+	 *
+	 * "stats" is the pointer to the allocated shared memory area.
+	 */
+	void		(*init_shmem_cb) (void *stats);
+
 	/*
 	 * For fixed-numbered statistics: Reset All.
 	 */
@@ -528,6 +535,7 @@ extern void pgstat_snapshot_fixed(PgStat_Kind kind);
  * Functions in pgstat_archiver.c
  */
 
+extern void pgstat_archiver_init_shmem_cb(void *stats);
 extern void pgstat_archiver_reset_all_cb(TimestampTz ts);
 extern void pgstat_archiver_snapshot_cb(void);
 
@@ -536,6 +544,7 @@ extern void pgstat_archiver_snapshot_cb(void);
  * Functions in pgstat_bgwriter.c
  */
 
+extern void pgstat_bgwriter_init_shmem_cb(void *stats);
 extern void pgstat_bgwriter_reset_all_cb(TimestampTz ts);
 extern void pgstat_bgwriter_snapshot_cb(void);
 
@@ -544,6 +553,7 @@ extern void pgstat_bgwriter_snapshot_cb(void);
  * Functions in pgstat_checkpointer.c
  */
 
+extern void pgstat_checkpointer_init_shmem_cb(void *stats);
 extern void pgstat_checkpointer_reset_all_cb(TimestampTz ts);
 extern void pgstat_checkpointer_snapshot_cb(void);
 
@@ -574,6 +584,7 @@ extern bool pgstat_function_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
  */
 
 extern bool pgstat_flush_io(bool nowait);
+extern void pgstat_io_init_shmem_cb(void *stats);
 extern void pgstat_io_reset_all_cb(TimestampTz ts);
 extern void pgstat_io_snapshot_cb(void);
 
@@ -632,6 +643,7 @@ extern PgStatShared_Common *pgstat_init_entry(PgStat_Kind kind,
  */
 
 extern bool pgstat_slru_flush(bool nowait);
+extern void pgstat_slru_init_shmem_cb(void *stats);
 extern void pgstat_slru_reset_all_cb(TimestampTz ts);
 extern void pgstat_slru_snapshot_cb(void);
 
@@ -644,6 +656,7 @@ extern bool pgstat_flush_wal(bool nowait);
 extern void pgstat_init_wal(void);
 extern bool pgstat_have_pending_wal(void);
 
+extern void pgstat_wal_init_shmem_cb(void *stats);
 extern void pgstat_wal_reset_all_cb(TimestampTz ts);
 extern void pgstat_wal_snapshot_cb(void);
 
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 65b937a85f..ef435167f7 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -354,6 +354,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Archiver, stats),
 		.shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
 
+		.init_shmem_cb = pgstat_archiver_init_shmem_cb,
 		.reset_all_cb = pgstat_archiver_reset_all_cb,
 		.snapshot_cb = pgstat_archiver_snapshot_cb,
 	},
@@ -368,6 +369,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_BgWriter, stats),
 		.shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
 
+		.init_shmem_cb = pgstat_bgwriter_init_shmem_cb,
 		.reset_all_cb = pgstat_bgwriter_reset_all_cb,
 		.snapshot_cb = pgstat_bgwriter_snapshot_cb,
 	},
@@ -382,6 +384,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
 		.shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
 
+		.init_shmem_cb = pgstat_checkpointer_init_shmem_cb,
 		.reset_all_cb = pgstat_checkpointer_reset_all_cb,
 		.snapshot_cb = pgstat_checkpointer_snapshot_cb,
 	},
@@ -396,6 +399,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
 
+		.init_shmem_cb = pgstat_io_init_shmem_cb,
 		.reset_all_cb = pgstat_io_reset_all_cb,
 		.snapshot_cb = pgstat_io_snapshot_cb,
 	},
@@ -410,6 +414,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
 
+		.init_shmem_cb = pgstat_slru_init_shmem_cb,
 		.reset_all_cb = pgstat_slru_reset_all_cb,
 		.snapshot_cb = pgstat_slru_snapshot_cb,
 	},
@@ -424,6 +429,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Wal, stats),
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
 
+		.init_shmem_cb = pgstat_wal_init_shmem_cb,
 		.reset_all_cb = pgstat_wal_reset_all_cb,
 		.snapshot_cb = pgstat_wal_snapshot_cb,
 	},
diff --git a/src/backend/utils/activity/pgstat_archiver.c b/src/backend/utils/activity/pgstat_archiver.c
index 66398b20e5..60e04711c4 100644
--- a/src/backend/utils/activity/pgstat_archiver.c
+++ b/src/backend/utils/activity/pgstat_archiver.c
@@ -62,6 +62,14 @@ pgstat_fetch_stat_archiver(void)
 	return &pgStatLocal.snapshot.archiver;
 }
 
+void
+pgstat_archiver_init_shmem_cb(void *stats)
+{
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_archiver_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_bgwriter.c b/src/backend/utils/activity/pgstat_bgwriter.c
index 7d2432e4fa..364a7a2024 100644
--- a/src/backend/utils/activity/pgstat_bgwriter.c
+++ b/src/backend/utils/activity/pgstat_bgwriter.c
@@ -75,6 +75,14 @@ pgstat_fetch_stat_bgwriter(void)
 	return &pgStatLocal.snapshot.bgwriter;
 }
 
+void
+pgstat_bgwriter_init_shmem_cb(void *stats)
+{
+	PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_bgwriter_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_checkpointer.c b/src/backend/utils/activity/pgstat_checkpointer.c
index 30a8110e38..bbfc9c7e18 100644
--- a/src/backend/utils/activity/pgstat_checkpointer.c
+++ b/src/backend/utils/activity/pgstat_checkpointer.c
@@ -84,6 +84,14 @@ pgstat_fetch_stat_checkpointer(void)
 	return &pgStatLocal.snapshot.checkpointer;
 }
 
+void
+pgstat_checkpointer_init_shmem_cb(void *stats)
+{
+	PgStatShared_Checkpointer *stats_shmem = (PgStatShared_Checkpointer *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_checkpointer_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index 9d6e067382..8af55989ee 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -251,6 +251,15 @@ pgstat_get_io_object_name(IOObject io_object)
 	pg_unreachable();
 }
 
+void
+pgstat_io_init_shmem_cb(void *stats)
+{
+	PgStatShared_IO *stat_shmem = (PgStatShared_IO *) stats;
+
+	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
+		LWLockInitialize(&stat_shmem->locks[i], LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_io_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 634b967820..1c2b69d563 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -196,17 +196,18 @@ StatsShmemInit(void)
 
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
-
 		/* initialize fixed-numbered stats */
-		LWLockInitialize(&ctl->archiver.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->bgwriter.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->checkpointer.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->slru.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->wal.lock, LWTRANCHE_PGSTATS_DATA);
+		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		{
+			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+			char	   *ptr;
 
-		for (int i = 0; i < BACKEND_NUM_TYPES; i++)
-			LWLockInitialize(&ctl->io.locks[i],
-							 LWTRANCHE_PGSTATS_DATA);
+			if (!kind_info->fixed_amount)
+				continue;
+
+			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			kind_info->init_shmem_cb(ptr);
+		}
 	}
 	else
 	{
diff --git a/src/backend/utils/activity/pgstat_slru.c b/src/backend/utils/activity/pgstat_slru.c
index 56ea1c3378..6f922a85bf 100644
--- a/src/backend/utils/activity/pgstat_slru.c
+++ b/src/backend/utils/activity/pgstat_slru.c
@@ -192,6 +192,14 @@ pgstat_slru_flush(bool nowait)
 	return false;
 }
 
+void
+pgstat_slru_init_shmem_cb(void *stats)
+{
+	PgStatShared_SLRU *stats_shmem = (PgStatShared_SLRU *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_slru_reset_all_cb(TimestampTz ts)
 {
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 0e374f133a..e2a3f6b865 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -163,6 +163,14 @@ pgstat_have_pending_wal(void)
 		PendingWalStats.wal_sync != 0;
 }
 
+void
+pgstat_wal_init_shmem_cb(void *stats)
+{
+	PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_wal_reset_all_cb(TimestampTz ts)
 {
-- 
2.45.2

From f3b4d76a6c732b4b84c02f9700188b2a6a4c32a7 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 12:24:44 +0900
Subject: [PATCH v5 2/7] Add a new 'F' type of entry in pgstats file

This new entry type is used for fixed-numbered statistics, making an
upcoming change to add support for custom pluggable stats kinds more
palatable.
---
 src/backend/utils/activity/pgstat.c | 44 ++++++++++++++++++-----------
 1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index ef435167f7..ed32ee4b6f 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -132,6 +132,7 @@
  * ---------
  */
 #define PGSTAT_FILE_ENTRY_END	'E' /* end of file */
+#define PGSTAT_FILE_ENTRY_FIXED	'F' /* fixed-numbered stats entry */
 #define PGSTAT_FILE_ENTRY_NAME	'N' /* stats entry identified by name */
 #define PGSTAT_FILE_ENTRY_HASH	'S' /* stats entry identified by
 									 * PgStat_HashKey */
@@ -1396,6 +1397,9 @@ pgstat_write_statsfile(void)
 
 		pgstat_build_snapshot_fixed(kind);
 		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+
+		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
+		write_chunk_s(fpout, &kind);
 		write_chunk(fpout, ptr, info->shared_data_len);
 	}
 
@@ -1537,22 +1541,6 @@ pgstat_read_statsfile(void)
 		format_id != PGSTAT_FILE_FORMAT_ID)
 		goto error;
 
-	/* Read various stats structs with fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
-	{
-		char	   *ptr;
-		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
-
-		if (!info->fixed_amount)
-			continue;
-
-		Assert(info->shared_ctl_off != 0);
-
-		ptr = ((char *) shmem) + info->shared_ctl_off + info->shared_data_off;
-		if (!read_chunk(fpin, ptr, info->shared_data_len))
-			goto error;
-	}
-
 	/*
 	 * We found an existing statistics file. Read it and put all the hash
 	 * table entries into place.
@@ -1563,6 +1551,30 @@ pgstat_read_statsfile(void)
 
 		switch (t)
 		{
+			case PGSTAT_FILE_ENTRY_FIXED:
+				{
+					PgStat_Kind kind;
+					const PgStat_KindInfo *info;
+					char	   *ptr;
+
+					if (!read_chunk_s(fpin, &kind))
+						goto error;
+
+					if (!pgstat_is_kind_valid(kind))
+						goto error;
+					info = pgstat_get_kind_info(kind);
+					Assert(info->fixed_amount);
+
+					/* Load back stats into shared memory */
+					ptr = ((char *) shmem) + info->shared_ctl_off +
+						info->shared_data_off;
+
+					if (!read_chunk(fpin, ptr,
+									info->shared_data_len))
+						goto error;
+
+					break;
+				}
 			case PGSTAT_FILE_ENTRY_HASH:
 			case PGSTAT_FILE_ENTRY_NAME:
 				{
-- 
2.45.2

From 3e390bf999f7e107d2cc03ba6714b6d07c437883 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 12:28:26 +0900
Subject: [PATCH v5 3/7] Switch PgStat_Kind from enum to uint32

A follow-up patch is planned to make this counter extensible, and
keeping a trace of the kind behind a type is useful in the internal
routines used by pgstats.  While on it, switch pgstat_is_kind_valid() to
use PgStat_Kind, to be more consistent with its callers.
---
 src/include/pgstat.h                | 35 ++++++++++++++---------------
 src/backend/utils/activity/pgstat.c |  6 ++---
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2136239710..2d30fadaf1 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -32,26 +32,25 @@
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"
 
 /* The types of statistics entries */
-typedef enum PgStat_Kind
-{
-	/* use 0 for INVALID, to catch zero-initialized data */
-	PGSTAT_KIND_INVALID = 0,
+#define PgStat_Kind uint32
 
-	/* stats for variable-numbered objects */
-	PGSTAT_KIND_DATABASE,		/* database-wide statistics */
-	PGSTAT_KIND_RELATION,		/* per-table statistics */
-	PGSTAT_KIND_FUNCTION,		/* per-function statistics */
-	PGSTAT_KIND_REPLSLOT,		/* per-slot statistics */
-	PGSTAT_KIND_SUBSCRIPTION,	/* per-subscription statistics */
+/* use 0 for INVALID, to catch zero-initialized data */
+#define PGSTAT_KIND_INVALID 0
 
-	/* stats for fixed-numbered objects */
-	PGSTAT_KIND_ARCHIVER,
-	PGSTAT_KIND_BGWRITER,
-	PGSTAT_KIND_CHECKPOINTER,
-	PGSTAT_KIND_IO,
-	PGSTAT_KIND_SLRU,
-	PGSTAT_KIND_WAL,
-} PgStat_Kind;
+/* stats for variable-numbered objects */
+#define PGSTAT_KIND_DATABASE	1	/* database-wide statistics */
+#define PGSTAT_KIND_RELATION	2	/* per-table statistics */
+#define PGSTAT_KIND_FUNCTION	3	/* per-function statistics */
+#define PGSTAT_KIND_REPLSLOT	4	/* per-slot statistics */
+#define PGSTAT_KIND_SUBSCRIPTION	5	/* per-subscription statistics */
+
+/* stats for fixed-numbered objects */
+#define PGSTAT_KIND_ARCHIVER	6
+#define PGSTAT_KIND_BGWRITER	7
+#define PGSTAT_KIND_CHECKPOINTER	8
+#define PGSTAT_KIND_IO	9
+#define PGSTAT_KIND_SLRU	10
+#define PGSTAT_KIND_WAL	11
 
 #define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
 #define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index ed32ee4b6f..f572a87a61 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -182,7 +182,7 @@ static void pgstat_prep_snapshot(void);
 static void pgstat_build_snapshot(void);
 static void pgstat_build_snapshot_fixed(PgStat_Kind kind);
 
-static inline bool pgstat_is_kind_valid(int ikind);
+static inline bool pgstat_is_kind_valid(PgStat_Kind kind);
 
 
 /* ----------
@@ -1298,9 +1298,9 @@ pgstat_get_kind_from_str(char *kind_str)
 }
 
 static inline bool
-pgstat_is_kind_valid(int ikind)
+pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
+	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
 }
 
 const PgStat_KindInfo *
-- 
2.45.2

From 57429e5aa03efbe8ba2afc3946c69fd99d963786 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 14:03:17 +0900
Subject: [PATCH v5 4/7] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats

This is able to support fixed-numbered (like WAL, archiver, bgwriter)
and variable-numbered stats kinds.
---
 src/include/pgstat.h                      |  35 +++-
 src/include/utils/pgstat_internal.h       |  22 ++-
 src/backend/utils/activity/pgstat.c       | 231 +++++++++++++++++++---
 src/backend/utils/activity/pgstat_shmem.c |  31 ++-
 src/backend/utils/adt/pgstatfuncs.c       |   2 +-
 5 files changed, 287 insertions(+), 34 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 2d30fadaf1..8d523607a4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -34,6 +34,10 @@
 /* The types of statistics entries */
 #define PgStat_Kind uint32
 
+/* Range of IDs allowed, for built-in and custom kinds */
+#define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
+#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
 
@@ -52,9 +56,34 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 778f625ca1..39f63362a3 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -195,7 +195,8 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
-	 * PgStatShared_HashEntry->body).
+	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
+	 * the size of an entry in PgStat_ShmemControl->custom_data.
 	 */
 	uint32		shared_size;
 
@@ -446,6 +447,13 @@ typedef struct PgStat_ShmemControl
 	PgStatShared_IO io;
 	PgStatShared_SLRU slru;
 	PgStatShared_Wal wal;
+
+	/*
+	 * Custom stats data with fixed-numbered objects, indexed by (PgStat_Kind
+	 * - PGSTAT_KIND_CUSTOM_MIN).
+	 */
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 } PgStat_ShmemControl;
 
 
@@ -459,7 +467,7 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX_BUILTIN + 1];
 
 	PgStat_ArchiverStats archiver;
 
@@ -473,6 +481,14 @@ typedef struct PgStat_Snapshot
 
 	PgStat_WalStats wal;
 
+	/*
+	 * Data in snapshot for custom fixed-numbered statistics, indexed by
+	 * (PgStat_Kind - PGSTAT_KIND_CUSTOM_MIN).  Each entry is allocated in
+	 * TopMemoryContext, for a size of shared_data_len.
+	 */
+	bool		custom_valid[PGSTAT_KIND_CUSTOM_SIZE];
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 	/* to free snapshot in bulk */
 	MemoryContext context;
 	struct pgstat_snapshot_hash *stats;
@@ -516,6 +532,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index f572a87a61..3b85fe0fc8 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for external modules to define custom statistics kinds,
+ * that can use the same properties as any built-in stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
+ * defined, and pgstat_kind_custom_infos for custom kinds registered at
+ * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -174,6 +182,8 @@ typedef struct PgStat_SnapshotEntry
 static void pgstat_write_statsfile(void);
 static void pgstat_read_statsfile(void);
 
+static void pgstat_init_snapshot_fixed(void);
+
 static void pgstat_reset_after_failure(void);
 
 static bool pgstat_flush_pending_entries(bool nowait);
@@ -251,7 +261,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -263,7 +273,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -436,6 +446,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -586,6 +605,8 @@ pgstat_initialize(void)
 
 	pgstat_init_wal();
 
+	pgstat_init_snapshot_fixed();
+
 	/* Set up a process-exit hook to clean up */
 	before_shmem_exit(pgstat_shutdown_hook, 0);
 
@@ -829,6 +850,8 @@ pgstat_clear_snapshot(void)
 
 	memset(&pgStatLocal.snapshot.fixed_valid, 0,
 		   sizeof(pgStatLocal.snapshot.fixed_valid));
+	memset(&pgStatLocal.snapshot.custom_valid, 0,
+		   sizeof(pgStatLocal.snapshot.custom_valid));
 	pgStatLocal.snapshot.stats = NULL;
 	pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_NONE;
 
@@ -992,7 +1015,29 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 	else
 		pgstat_build_snapshot_fixed(kind);
 
-	Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	if (pgstat_is_kind_builtin(kind))
+		Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	else if (pgstat_is_kind_custom(kind))
+		Assert(pgStatLocal.snapshot.custom_valid[kind - PGSTAT_KIND_CUSTOM_MIN]);
+}
+
+static void
+pgstat_init_snapshot_fixed(void)
+{
+	/*
+	 * Initialize fixed-numbered statistics data in snapshots, only for custom
+	 * stats kinds.
+	 */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info || !kind_info->fixed_amount)
+			continue;
+
+		pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN] =
+			MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
+	}
 }
 
 static void
@@ -1088,10 +1133,12 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 		{
 			Assert(kind_info->snapshot_cb == NULL);
@@ -1108,6 +1155,20 @@ static void
 pgstat_build_snapshot_fixed(PgStat_Kind kind)
 {
 	const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+	int			idx;
+	bool	   *valid;
+
+	/* Position in fixed_valid or custom_valid */
+	if (pgstat_is_kind_builtin(kind))
+	{
+		idx = kind;
+		valid = pgStatLocal.snapshot.fixed_valid;
+	}
+	else
+	{
+		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+		valid = pgStatLocal.snapshot.custom_valid;
+	}
 
 	Assert(kind_info->fixed_amount);
 	Assert(kind_info->snapshot_cb != NULL);
@@ -1115,21 +1176,21 @@ pgstat_build_snapshot_fixed(PgStat_Kind kind)
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
 	{
 		/* rebuild every time */
-		pgStatLocal.snapshot.fixed_valid[kind] = false;
+		valid[idx] = false;
 	}
-	else if (pgStatLocal.snapshot.fixed_valid[kind])
+	else if (valid[idx])
 	{
 		/* in snapshot mode we shouldn't get called again */
 		Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
 		return;
 	}
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
+	Assert(!valid[idx]);
 
 	kind_info->snapshot_cb();
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
-	pgStatLocal.snapshot.fixed_valid[kind] = true;
+	Assert(!valid[idx]);
+	valid[idx] = true;
 }
 
 
@@ -1285,30 +1346,127 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
 pgstat_get_kind_info(PgStat_Kind kind)
 {
-	Assert(pgstat_is_kind_valid(kind));
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL ||
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	return NULL;
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * Check some data for fixed-numbered stats.
+	 */
+	if (kind_info->fixed_amount)
+	{
+		if (kind_info->shared_size == 0)
+			ereport(ERROR,
+					(errmsg("custom cumulative statistics property is invalid"),
+					 errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+	}
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+								   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom cumulative statistics \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1385,18 +1543,22 @@ pgstat_write_statsfile(void)
 	write_chunk_s(fpout, &format_id);
 
 	/* Write various stats structs for fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		char	   *ptr;
 		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-		if (!info->fixed_amount)
+		if (!info || !info->fixed_amount)
 			continue;
 
-		Assert(info->snapshot_ctl_off != 0);
+		if (pgstat_is_kind_builtin(kind))
+			Assert(info->snapshot_ctl_off != 0);
 
 		pgstat_build_snapshot_fixed(kind);
-		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		if (pgstat_is_kind_builtin(kind))
+			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		else
+			ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];
 
 		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
 		write_chunk_s(fpout, &kind);
@@ -1419,6 +1581,17 @@ pgstat_write_statsfile(void)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1566,8 +1739,16 @@ pgstat_read_statsfile(void)
 					Assert(info->fixed_amount);
 
 					/* Load back stats into shared memory */
-					ptr = ((char *) shmem) + info->shared_ctl_off +
-						info->shared_data_off;
+					if (pgstat_is_kind_builtin(kind))
+						ptr = ((char *) shmem) + info->shared_ctl_off +
+							info->shared_data_off;
+					else
+					{
+						int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+						ptr = ((char *) shmem->custom_data[idx]) +
+							info->shared_data_off;
+					}
 
 					if (!read_chunk(fpin, ptr,
 									info->shared_data_len))
@@ -1635,7 +1816,7 @@ pgstat_read_statsfile(void)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u",
 							 key.kind, key.dboid, key.objoid);
 						goto error;
 					}
@@ -1693,11 +1874,11 @@ pgstat_reset_after_failure(void)
 	TimestampTz ts = GetCurrentTimestamp();
 
 	/* reset fixed-numbered stats */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		kind_info->reset_all_cb(ts);
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 1c2b69d563..ca3d3eecfb 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -131,6 +131,21 @@ StatsShmemSize(void)
 	sz = MAXALIGN(sizeof(PgStat_ShmemControl));
 	sz = add_size(sz, pgstat_dsa_init_size());
 
+	/* Add shared memory for all the custom fixed-numbered statistics */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info)
+			continue;
+		if (!kind_info->fixed_amount)
+			continue;
+
+		Assert(kind_info->shared_size != 0);
+
+		sz += MAXALIGN(kind_info->shared_size);
+	}
+
 	return sz;
 }
 
@@ -197,15 +212,25 @@ StatsShmemInit(void)
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
 		/* initialize fixed-numbered stats */
-		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 		{
 			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 			char	   *ptr;
 
-			if (!kind_info->fixed_amount)
+			if (!kind_info || !kind_info->fixed_amount)
 				continue;
 
-			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			if (pgstat_is_kind_builtin(kind))
+				ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			else
+			{
+				int			idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+				Assert(kind_info->shared_size != 0);
+				ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
+				ptr = ctl->custom_data[idx];
+			}
+
 			kind_info->init_shmem_cb(ptr);
 		}
 	}
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.2

From aca83deecd4a7ed78b2fc35dde373534cf26abc0 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 13:49:53 +0900
Subject: [PATCH v5 5/7] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 63 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 756a9d07fb..878722af51 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3700,6 +3700,69 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in
+     <filename>src/test/modules/injection_points/injection_stats.c</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.2

From a26f486e0bea4fe73d3b134284415ce4054cbd04 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 13:36:06 +0900
Subject: [PATCH v5 6/7] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 194 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 +++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  48 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 322 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 2ffd2f77ed..7b9cd12a2a 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -11,9 +14,13 @@ REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
 ISOLATION = inplace
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index e275c2cf5b..747a64e812 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -64,3 +64,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index b6c8e89324..eb411b9d44 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -417,5 +431,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..c37b0b33d3
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,194 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+#define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 3c23c14d81..a52fe5121e 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -42,4 +43,12 @@ tests += {
       'inplace',
     ],
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..7d5a96e522
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,48 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 635e6d6e21..5206029e96 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2118,6 +2118,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2149,6 +2150,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.2

From 52c75a4504605094bc47c1e5b47553de8db338a1 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 13:44:14 +0900
Subject: [PATCH v5 7/7] injection_points: Add example for fixed-numbered
 statistics

This acts as a template to show what can be achieved with fixed-numbered
stats (like WAL, bgwriter, etc.) for pluggable cumulative statistics.
---
 .../injection_points--1.0.sql                 |  11 ++
 .../injection_points/injection_points.c       |   3 +
 .../injection_points/injection_stats.c        | 155 +++++++++++++++++-
 .../injection_points/injection_stats.h        |   3 +
 .../modules/injection_points/t/001_stats.pl   |  11 +-
 5 files changed, 180 insertions(+), 3 deletions(-)

diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index 747a64e812..fa0b1d06ae 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -74,3 +74,14 @@ CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
 RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
+
+--
+-- injection_points_stats_fixed()
+--
+-- Reports fixed-numbered statistics for injection points.
+CREATE FUNCTION injection_points_stats_fixed(OUT numattach int8,
+   OUT numdetach int8,
+   OUT numrun int8)
+RETURNS record
+AS 'MODULE_PATHNAME', 'injection_points_stats_fixed'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index eb411b9d44..f65bd83cc3 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -297,6 +297,7 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		condition.pid = MyProcPid;
 	}
 
+	pgstat_report_inj_fixed(1, 0, 0);
 	InjectionPointAttach(name, "injection_points", function, &condition,
 						 sizeof(InjectionPointCondition));
 
@@ -342,6 +343,7 @@ injection_points_run(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 0, 1);
 	INJECTION_POINT(name);
 
 	PG_RETURN_VOID();
@@ -418,6 +420,7 @@ injection_points_detach(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 1, 0);
 	if (!InjectionPointDetach(name))
 		elog(ERROR, "could not detach injection point \"%s\"", name);
 
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
index c37b0b33d3..69560b7fdc 100644
--- a/src/test/modules/injection_points/injection_stats.c
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -17,12 +17,13 @@
 #include "fmgr.h"
 
 #include "common/hashfn.h"
+#include "funcapi.h"
 #include "injection_stats.h"
 #include "pgstat.h"
 #include "utils/builtins.h"
 #include "utils/pgstat_internal.h"
 
-/* Structures for statistics of injection points */
+/* Structures for statistics of injection points, variable-size */
 typedef struct PgStat_StatInjEntry
 {
 	PgStat_Counter numcalls;	/* number of times point has been run */
@@ -35,6 +36,9 @@ typedef struct PgStatShared_InjectionPoint
 } PgStatShared_InjectionPoint;
 
 static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+static void injection_stats_fixed_init_shmem_cb(void *stats);
+static void injection_stats_fixed_reset_all_cb(TimestampTz ts);
+static void injection_stats_fixed_snapshot_cb(void);
 
 static const PgStat_KindInfo injection_stats = {
 	.name = "injection_points",
@@ -50,18 +54,51 @@ static const PgStat_KindInfo injection_stats = {
 	.flush_pending_cb = injection_stats_flush_cb,
 };
 
+/* Structures for statistics of injection points, fixed-size */
+typedef struct PgStat_StatInjFixedEntry
+{
+	PgStat_Counter numattach;	/* number of points attached */
+	PgStat_Counter numdetach;	/* number of points detached */
+	PgStat_Counter numrun;		/* number of points run */
+	TimestampTz stat_reset_timestamp;
+} PgStat_StatInjFixedEntry;
+
+typedef struct PgStatShared_InjectionPointFixed
+{
+	LWLock	lock;	/* protects all the counters */
+	uint32	changecount;
+	PgStat_StatInjFixedEntry stats;
+	PgStat_StatInjFixedEntry reset_offset;
+} PgStatShared_InjectionPointFixed;
+
+static const PgStat_KindInfo injection_stats_fixed = {
+	.name = "injection_points_fixed",
+	.fixed_amount = true,
+
+	.shared_size = sizeof(PgStat_StatInjFixedEntry),
+	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPointFixed *) 0)->stats),
+
+	.init_shmem_cb = injection_stats_fixed_init_shmem_cb,
+	.reset_all_cb = injection_stats_fixed_reset_all_cb,
+	.snapshot_cb = injection_stats_fixed_snapshot_cb,
+};
+
 /*
  * Compute stats entry idx from point name with a 4-byte hash.
  */
 #define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
 
 #define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+#define PGSTAT_KIND_INJECTION_FIXED	(PGSTAT_KIND_EXPERIMENTAL + 1)
+/* Position of fixed-numbered data in internal structures */
+#define PGSTAT_KIND_INJECTION_IDX	(PGSTAT_KIND_INJECTION_FIXED - PGSTAT_KIND_CUSTOM_MIN)
 
 /* Track if stats are loaded */
 static bool inj_stats_loaded = false;
 
 /*
- * Callback for stats handling
+ * Callbacks for stats handling
  */
 static bool
 injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
@@ -79,6 +116,59 @@ injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	return true;
 }
 
+static void
+injection_stats_fixed_init_shmem_cb(void *stats)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+static void
+injection_stats_fixed_reset_all_cb(TimestampTz ts)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
+	LWLockRelease(&stats_shmem->lock);
+}
+
+static void
+injection_stats_fixed_snapshot_cb(void)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+	PgStat_StatInjFixedEntry *stat_snap = (PgStat_StatInjFixedEntry *)
+		pgStatLocal.snapshot.custom_data[PGSTAT_KIND_INJECTION_IDX];
+	PgStat_StatInjFixedEntry *reset_offset = &stats_shmem->reset_offset;
+	PgStat_StatInjFixedEntry reset;
+
+	pgstat_copy_changecounted_stats(stat_snap,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
+	LWLockRelease(&stats_shmem->lock);
+
+	/* compensate by reset offsets */
+#define FIXED_COMP(fld) stat_snap->fld -= reset.fld;
+	FIXED_COMP(numattach);
+	FIXED_COMP(numdetach);
+	FIXED_COMP(numrun);
+#undef FIXED_COMP
+}
+
 /*
  * Support function for the SQL-callable pgstat* functions.  Returns
  * a pointer to the injection point statistics struct.
@@ -105,6 +195,7 @@ void
 pgstat_register_inj(void)
 {
 	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+	pgstat_register_kind(PGSTAT_KIND_INJECTION_FIXED, &injection_stats_fixed);
 
 	/* mark stats as loaded */
 	inj_stats_loaded = true;
@@ -176,6 +267,30 @@ pgstat_report_inj(const char *name)
 	pgstat_unlock_entry(entry_ref);
 }
 
+/*
+ * Report fixed number of statistics for an injection point.
+ */
+void
+pgstat_report_inj_fixed(uint32 numattach,
+						uint32 numdetach,
+						uint32 numrun)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	stats_shmem = (PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	pgstat_begin_changecount_write(&stats_shmem->changecount);
+	stats_shmem->stats.numattach += numattach;
+	stats_shmem->stats.numdetach += numdetach;
+	stats_shmem->stats.numrun += numrun;
+	pgstat_end_changecount_write(&stats_shmem->changecount);
+}
+
 /*
  * SQL function returning the number of times an injection point
  * has been called.
@@ -192,3 +307,39 @@ injection_points_stats_numcalls(PG_FUNCTION_ARGS)
 
 	PG_RETURN_INT64(entry->numcalls);
 }
+
+/*
+ * SQL function returning fixed-numbered statistics for injection points.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_fixed);
+Datum
+injection_points_stats_fixed(PG_FUNCTION_ARGS)
+{
+	TupleDesc	tupdesc;
+	Datum		values[3] = {0};
+	bool		nulls[3] = {0};
+	PgStat_StatInjFixedEntry *stats;
+
+	pgstat_snapshot_fixed(PGSTAT_KIND_INJECTION_FIXED);
+	stats = pgStatLocal.snapshot.custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(3);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "numattach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "numdetach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "numrun",
+					   INT8OID, -1, 0);
+	BlessTupleDesc(tupdesc);
+
+	values[0] = Int64GetDatum(stats->numattach);
+	values[1] = Int64GetDatum(stats->numdetach);
+	values[2] = Int64GetDatum(stats->numrun);
+	nulls[0] = false;
+	nulls[1] = false;
+	nulls[2] = false;
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
index 3e99705483..cf68b25f7b 100644
--- a/src/test/modules/injection_points/injection_stats.h
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -19,5 +19,8 @@ extern void pgstat_register_inj(void);
 extern void pgstat_create_inj(const char *name);
 extern void pgstat_drop_inj(const char *name);
 extern void pgstat_report_inj(const char *name);
+extern void pgstat_report_inj_fixed(uint32 numattach,
+									uint32 numdetach,
+									uint32 numrun);
 
 #endif
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 7d5a96e522..e3c69b94ca 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -31,18 +31,27 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats');
 
 # Restart the node cleanly, stats should still be around.
 $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats after clean restart');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats after clean restart');
 
 # On crash the stats are gone.
 $node->stop('immediate');
 $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
-is($numcalls, '', 'number of stats after clean restart');
+is($numcalls, '', 'number of stats after crash');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '0|0|0', 'number of fixed stats after crash');
 
 done_testing();
-- 
2.45.2

From 2cb5aa81d0d93a910934105756d893d2c0586ea2 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 12:28:26 +0900
Subject: [PATCH v6 1/5] Switch PgStat_Kind from enum to uint32

A follow-up patch is planned to make this counter extensible, and
keeping a trace of the kind behind a type is useful in the internal
routines used by pgstats.  While on it, switch pgstat_is_kind_valid() to
use PgStat_Kind, to be more consistent with its callers.
---
 src/include/pgstat.h                | 35 ++++++++++++++---------------
 src/backend/utils/activity/pgstat.c |  6 ++---
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 6b99bb8aad..cc97274708 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -32,26 +32,25 @@
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"
 
 /* The types of statistics entries */
-typedef enum PgStat_Kind
-{
-	/* use 0 for INVALID, to catch zero-initialized data */
-	PGSTAT_KIND_INVALID = 0,
+#define PgStat_Kind uint32
 
-	/* stats for variable-numbered objects */
-	PGSTAT_KIND_DATABASE,		/* database-wide statistics */
-	PGSTAT_KIND_RELATION,		/* per-table statistics */
-	PGSTAT_KIND_FUNCTION,		/* per-function statistics */
-	PGSTAT_KIND_REPLSLOT,		/* per-slot statistics */
-	PGSTAT_KIND_SUBSCRIPTION,	/* per-subscription statistics */
+/* use 0 for INVALID, to catch zero-initialized data */
+#define PGSTAT_KIND_INVALID 0
 
-	/* stats for fixed-numbered objects */
-	PGSTAT_KIND_ARCHIVER,
-	PGSTAT_KIND_BGWRITER,
-	PGSTAT_KIND_CHECKPOINTER,
-	PGSTAT_KIND_IO,
-	PGSTAT_KIND_SLRU,
-	PGSTAT_KIND_WAL,
-} PgStat_Kind;
+/* stats for variable-numbered objects */
+#define PGSTAT_KIND_DATABASE	1	/* database-wide statistics */
+#define PGSTAT_KIND_RELATION	2	/* per-table statistics */
+#define PGSTAT_KIND_FUNCTION	3	/* per-function statistics */
+#define PGSTAT_KIND_REPLSLOT	4	/* per-slot statistics */
+#define PGSTAT_KIND_SUBSCRIPTION	5	/* per-subscription statistics */
+
+/* stats for fixed-numbered objects */
+#define PGSTAT_KIND_ARCHIVER	6
+#define PGSTAT_KIND_BGWRITER	7
+#define PGSTAT_KIND_CHECKPOINTER	8
+#define PGSTAT_KIND_IO	9
+#define PGSTAT_KIND_SLRU	10
+#define PGSTAT_KIND_WAL	11
 
 #define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
 #define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index c0ec9e8195..805712d414 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -182,7 +182,7 @@ static void pgstat_prep_snapshot(void);
 static void pgstat_build_snapshot(void);
 static void pgstat_build_snapshot_fixed(PgStat_Kind kind);
 
-static inline bool pgstat_is_kind_valid(int ikind);
+static inline bool pgstat_is_kind_valid(PgStat_Kind kind);
 
 
 /* ----------
@@ -1298,9 +1298,9 @@ pgstat_get_kind_from_str(char *kind_str)
 }
 
 static inline bool
-pgstat_is_kind_valid(int ikind)
+pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
+	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
 }
 
 const PgStat_KindInfo *
-- 
2.45.2

From 0869c45608b7acb73afa4afc7bf2e481c239d705 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 14:03:17 +0900
Subject: [PATCH v6 2/5] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats

This is able to support fixed-numbered (like WAL, archiver, bgwriter)
and variable-numbered stats kinds.
---
 src/include/pgstat.h                      |  35 +++-
 src/include/utils/pgstat_internal.h       |  22 ++-
 src/backend/utils/activity/pgstat.c       | 231 +++++++++++++++++++---
 src/backend/utils/activity/pgstat_shmem.c |  31 ++-
 src/backend/utils/adt/pgstatfuncs.c       |   2 +-
 5 files changed, 287 insertions(+), 34 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index cc97274708..21c3705ce4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -34,6 +34,10 @@
 /* The types of statistics entries */
 #define PgStat_Kind uint32
 
+/* Range of IDs allowed, for built-in and custom kinds */
+#define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
+#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
 
@@ -52,9 +56,34 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 778f625ca1..39f63362a3 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -195,7 +195,8 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
-	 * PgStatShared_HashEntry->body).
+	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
+	 * the size of an entry in PgStat_ShmemControl->custom_data.
 	 */
 	uint32		shared_size;
 
@@ -446,6 +447,13 @@ typedef struct PgStat_ShmemControl
 	PgStatShared_IO io;
 	PgStatShared_SLRU slru;
 	PgStatShared_Wal wal;
+
+	/*
+	 * Custom stats data with fixed-numbered objects, indexed by (PgStat_Kind
+	 * - PGSTAT_KIND_CUSTOM_MIN).
+	 */
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 } PgStat_ShmemControl;
 
 
@@ -459,7 +467,7 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX_BUILTIN + 1];
 
 	PgStat_ArchiverStats archiver;
 
@@ -473,6 +481,14 @@ typedef struct PgStat_Snapshot
 
 	PgStat_WalStats wal;
 
+	/*
+	 * Data in snapshot for custom fixed-numbered statistics, indexed by
+	 * (PgStat_Kind - PGSTAT_KIND_CUSTOM_MIN).  Each entry is allocated in
+	 * TopMemoryContext, for a size of shared_data_len.
+	 */
+	bool		custom_valid[PGSTAT_KIND_CUSTOM_SIZE];
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 	/* to free snapshot in bulk */
 	MemoryContext context;
 	struct pgstat_snapshot_hash *stats;
@@ -516,6 +532,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 805712d414..aceb4bed00 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for external modules to define custom statistics kinds,
+ * that can use the same properties as any built-in stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
+ * defined, and pgstat_kind_custom_infos for custom kinds registered at
+ * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -174,6 +182,8 @@ typedef struct PgStat_SnapshotEntry
 static void pgstat_write_statsfile(void);
 static void pgstat_read_statsfile(void);
 
+static void pgstat_init_snapshot_fixed(void);
+
 static void pgstat_reset_after_failure(void);
 
 static bool pgstat_flush_pending_entries(bool nowait);
@@ -251,7 +261,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -263,7 +273,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -436,6 +446,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -586,6 +605,8 @@ pgstat_initialize(void)
 
 	pgstat_init_wal();
 
+	pgstat_init_snapshot_fixed();
+
 	/* Set up a process-exit hook to clean up */
 	before_shmem_exit(pgstat_shutdown_hook, 0);
 
@@ -829,6 +850,8 @@ pgstat_clear_snapshot(void)
 
 	memset(&pgStatLocal.snapshot.fixed_valid, 0,
 		   sizeof(pgStatLocal.snapshot.fixed_valid));
+	memset(&pgStatLocal.snapshot.custom_valid, 0,
+		   sizeof(pgStatLocal.snapshot.custom_valid));
 	pgStatLocal.snapshot.stats = NULL;
 	pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_NONE;
 
@@ -992,7 +1015,29 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 	else
 		pgstat_build_snapshot_fixed(kind);
 
-	Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	if (pgstat_is_kind_builtin(kind))
+		Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	else if (pgstat_is_kind_custom(kind))
+		Assert(pgStatLocal.snapshot.custom_valid[kind - PGSTAT_KIND_CUSTOM_MIN]);
+}
+
+static void
+pgstat_init_snapshot_fixed(void)
+{
+	/*
+	 * Initialize fixed-numbered statistics data in snapshots, only for custom
+	 * stats kinds.
+	 */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info || !kind_info->fixed_amount)
+			continue;
+
+		pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN] =
+			MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
+	}
 }
 
 static void
@@ -1088,10 +1133,12 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 		{
 			Assert(kind_info->snapshot_cb == NULL);
@@ -1108,6 +1155,20 @@ static void
 pgstat_build_snapshot_fixed(PgStat_Kind kind)
 {
 	const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+	int			idx;
+	bool	   *valid;
+
+	/* Position in fixed_valid or custom_valid */
+	if (pgstat_is_kind_builtin(kind))
+	{
+		idx = kind;
+		valid = pgStatLocal.snapshot.fixed_valid;
+	}
+	else
+	{
+		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+		valid = pgStatLocal.snapshot.custom_valid;
+	}
 
 	Assert(kind_info->fixed_amount);
 	Assert(kind_info->snapshot_cb != NULL);
@@ -1115,21 +1176,21 @@ pgstat_build_snapshot_fixed(PgStat_Kind kind)
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
 	{
 		/* rebuild every time */
-		pgStatLocal.snapshot.fixed_valid[kind] = false;
+		valid[idx] = false;
 	}
-	else if (pgStatLocal.snapshot.fixed_valid[kind])
+	else if (valid[idx])
 	{
 		/* in snapshot mode we shouldn't get called again */
 		Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
 		return;
 	}
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
+	Assert(!valid[idx]);
 
 	kind_info->snapshot_cb();
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
-	pgStatLocal.snapshot.fixed_valid[kind] = true;
+	Assert(!valid[idx]);
+	valid[idx] = true;
 }
 
 
@@ -1285,30 +1346,127 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
 pgstat_get_kind_info(PgStat_Kind kind)
 {
-	Assert(pgstat_is_kind_valid(kind));
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL ||
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	return NULL;
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * Check some data for fixed-numbered stats.
+	 */
+	if (kind_info->fixed_amount)
+	{
+		if (kind_info->shared_size == 0)
+			ereport(ERROR,
+					(errmsg("custom cumulative statistics property is invalid"),
+					 errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+	}
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+								   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom cumulative statistics \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1385,18 +1543,22 @@ pgstat_write_statsfile(void)
 	write_chunk_s(fpout, &format_id);
 
 	/* Write various stats structs for fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		char	   *ptr;
 		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-		if (!info->fixed_amount)
+		if (!info || !info->fixed_amount)
 			continue;
 
-		Assert(info->snapshot_ctl_off != 0);
+		if (pgstat_is_kind_builtin(kind))
+			Assert(info->snapshot_ctl_off != 0);
 
 		pgstat_build_snapshot_fixed(kind);
-		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		if (pgstat_is_kind_builtin(kind))
+			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		else
+			ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];
 
 		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
 		write_chunk_s(fpout, &kind);
@@ -1419,6 +1581,17 @@ pgstat_write_statsfile(void)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1570,8 +1743,16 @@ pgstat_read_statsfile(void)
 						goto error;
 
 					/* Load back stats into shared memory */
-					ptr = ((char *) shmem) + info->shared_ctl_off +
-						info->shared_data_off;
+					if (pgstat_is_kind_builtin(kind))
+						ptr = ((char *) shmem) + info->shared_ctl_off +
+							info->shared_data_off;
+					else
+					{
+						int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+						ptr = ((char *) shmem->custom_data[idx]) +
+							info->shared_data_off;
+					}
 
 					if (!read_chunk(fpin, ptr, info->shared_data_len))
 						goto error;
@@ -1638,7 +1819,7 @@ pgstat_read_statsfile(void)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u",
 							 key.kind, key.dboid, key.objoid);
 						goto error;
 					}
@@ -1696,11 +1877,11 @@ pgstat_reset_after_failure(void)
 	TimestampTz ts = GetCurrentTimestamp();
 
 	/* reset fixed-numbered stats */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		kind_info->reset_all_cb(ts);
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 1c2b69d563..ca3d3eecfb 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -131,6 +131,21 @@ StatsShmemSize(void)
 	sz = MAXALIGN(sizeof(PgStat_ShmemControl));
 	sz = add_size(sz, pgstat_dsa_init_size());
 
+	/* Add shared memory for all the custom fixed-numbered statistics */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info)
+			continue;
+		if (!kind_info->fixed_amount)
+			continue;
+
+		Assert(kind_info->shared_size != 0);
+
+		sz += MAXALIGN(kind_info->shared_size);
+	}
+
 	return sz;
 }
 
@@ -197,15 +212,25 @@ StatsShmemInit(void)
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
 		/* initialize fixed-numbered stats */
-		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 		{
 			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 			char	   *ptr;
 
-			if (!kind_info->fixed_amount)
+			if (!kind_info || !kind_info->fixed_amount)
 				continue;
 
-			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			if (pgstat_is_kind_builtin(kind))
+				ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			else
+			{
+				int			idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+				Assert(kind_info->shared_size != 0);
+				ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
+				ptr = ctl->custom_data[idx];
+			}
+
 			kind_info->init_shmem_cb(ptr);
 		}
 	}
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.2

From 6081492b5d0e603496dcf6edfe667bd431d8b86f Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 3 Jul 2024 13:49:53 +0900
Subject: [PATCH v6 3/5] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 63 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 756a9d07fb..878722af51 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3700,6 +3700,69 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in
+     <filename>src/test/modules/injection_points/injection_stats.c</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.2

From ae6acc8de61ebd9e1c040b46c859b4d075f70e2a Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 13:36:06 +0900
Subject: [PATCH v6 4/5] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 194 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 +++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  48 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 322 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 2ffd2f77ed..7b9cd12a2a 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -11,9 +14,13 @@ REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
 ISOLATION = inplace
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index e275c2cf5b..747a64e812 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -64,3 +64,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index b6c8e89324..eb411b9d44 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -417,5 +431,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..c37b0b33d3
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,194 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+#define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 3c23c14d81..a52fe5121e 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -42,4 +43,12 @@ tests += {
       'inplace',
     ],
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..7d5a96e522
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,48 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 635e6d6e21..5206029e96 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2118,6 +2118,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2149,6 +2150,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.2

From f3a4855aa5356c7d21f656f37d8799b91ba85b52 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 13:44:14 +0900
Subject: [PATCH v6 5/5] injection_points: Add example for fixed-numbered
 statistics

This acts as a template to show what can be achieved with fixed-numbered
stats (like WAL, bgwriter, etc.) for pluggable cumulative statistics.
---
 .../injection_points--1.0.sql                 |  11 ++
 .../injection_points/injection_points.c       |   3 +
 .../injection_points/injection_stats.c        | 155 +++++++++++++++++-
 .../injection_points/injection_stats.h        |   3 +
 .../modules/injection_points/t/001_stats.pl   |  11 +-
 5 files changed, 180 insertions(+), 3 deletions(-)

diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index 747a64e812..fa0b1d06ae 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -74,3 +74,14 @@ CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
 RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
+
+--
+-- injection_points_stats_fixed()
+--
+-- Reports fixed-numbered statistics for injection points.
+CREATE FUNCTION injection_points_stats_fixed(OUT numattach int8,
+   OUT numdetach int8,
+   OUT numrun int8)
+RETURNS record
+AS 'MODULE_PATHNAME', 'injection_points_stats_fixed'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index eb411b9d44..f65bd83cc3 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -297,6 +297,7 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		condition.pid = MyProcPid;
 	}
 
+	pgstat_report_inj_fixed(1, 0, 0);
 	InjectionPointAttach(name, "injection_points", function, &condition,
 						 sizeof(InjectionPointCondition));
 
@@ -342,6 +343,7 @@ injection_points_run(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 0, 1);
 	INJECTION_POINT(name);
 
 	PG_RETURN_VOID();
@@ -418,6 +420,7 @@ injection_points_detach(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 1, 0);
 	if (!InjectionPointDetach(name))
 		elog(ERROR, "could not detach injection point \"%s\"", name);
 
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
index c37b0b33d3..69560b7fdc 100644
--- a/src/test/modules/injection_points/injection_stats.c
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -17,12 +17,13 @@
 #include "fmgr.h"
 
 #include "common/hashfn.h"
+#include "funcapi.h"
 #include "injection_stats.h"
 #include "pgstat.h"
 #include "utils/builtins.h"
 #include "utils/pgstat_internal.h"
 
-/* Structures for statistics of injection points */
+/* Structures for statistics of injection points, variable-size */
 typedef struct PgStat_StatInjEntry
 {
 	PgStat_Counter numcalls;	/* number of times point has been run */
@@ -35,6 +36,9 @@ typedef struct PgStatShared_InjectionPoint
 } PgStatShared_InjectionPoint;
 
 static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+static void injection_stats_fixed_init_shmem_cb(void *stats);
+static void injection_stats_fixed_reset_all_cb(TimestampTz ts);
+static void injection_stats_fixed_snapshot_cb(void);
 
 static const PgStat_KindInfo injection_stats = {
 	.name = "injection_points",
@@ -50,18 +54,51 @@ static const PgStat_KindInfo injection_stats = {
 	.flush_pending_cb = injection_stats_flush_cb,
 };
 
+/* Structures for statistics of injection points, fixed-size */
+typedef struct PgStat_StatInjFixedEntry
+{
+	PgStat_Counter numattach;	/* number of points attached */
+	PgStat_Counter numdetach;	/* number of points detached */
+	PgStat_Counter numrun;		/* number of points run */
+	TimestampTz stat_reset_timestamp;
+} PgStat_StatInjFixedEntry;
+
+typedef struct PgStatShared_InjectionPointFixed
+{
+	LWLock	lock;	/* protects all the counters */
+	uint32	changecount;
+	PgStat_StatInjFixedEntry stats;
+	PgStat_StatInjFixedEntry reset_offset;
+} PgStatShared_InjectionPointFixed;
+
+static const PgStat_KindInfo injection_stats_fixed = {
+	.name = "injection_points_fixed",
+	.fixed_amount = true,
+
+	.shared_size = sizeof(PgStat_StatInjFixedEntry),
+	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPointFixed *) 0)->stats),
+
+	.init_shmem_cb = injection_stats_fixed_init_shmem_cb,
+	.reset_all_cb = injection_stats_fixed_reset_all_cb,
+	.snapshot_cb = injection_stats_fixed_snapshot_cb,
+};
+
 /*
  * Compute stats entry idx from point name with a 4-byte hash.
  */
 #define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
 
 #define PGSTAT_KIND_INJECTION	PGSTAT_KIND_EXPERIMENTAL
+#define PGSTAT_KIND_INJECTION_FIXED	(PGSTAT_KIND_EXPERIMENTAL + 1)
+/* Position of fixed-numbered data in internal structures */
+#define PGSTAT_KIND_INJECTION_IDX	(PGSTAT_KIND_INJECTION_FIXED - PGSTAT_KIND_CUSTOM_MIN)
 
 /* Track if stats are loaded */
 static bool inj_stats_loaded = false;
 
 /*
- * Callback for stats handling
+ * Callbacks for stats handling
  */
 static bool
 injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
@@ -79,6 +116,59 @@ injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	return true;
 }
 
+static void
+injection_stats_fixed_init_shmem_cb(void *stats)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+static void
+injection_stats_fixed_reset_all_cb(TimestampTz ts)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
+	LWLockRelease(&stats_shmem->lock);
+}
+
+static void
+injection_stats_fixed_snapshot_cb(void)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+	PgStat_StatInjFixedEntry *stat_snap = (PgStat_StatInjFixedEntry *)
+		pgStatLocal.snapshot.custom_data[PGSTAT_KIND_INJECTION_IDX];
+	PgStat_StatInjFixedEntry *reset_offset = &stats_shmem->reset_offset;
+	PgStat_StatInjFixedEntry reset;
+
+	pgstat_copy_changecounted_stats(stat_snap,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
+	LWLockRelease(&stats_shmem->lock);
+
+	/* compensate by reset offsets */
+#define FIXED_COMP(fld) stat_snap->fld -= reset.fld;
+	FIXED_COMP(numattach);
+	FIXED_COMP(numdetach);
+	FIXED_COMP(numrun);
+#undef FIXED_COMP
+}
+
 /*
  * Support function for the SQL-callable pgstat* functions.  Returns
  * a pointer to the injection point statistics struct.
@@ -105,6 +195,7 @@ void
 pgstat_register_inj(void)
 {
 	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+	pgstat_register_kind(PGSTAT_KIND_INJECTION_FIXED, &injection_stats_fixed);
 
 	/* mark stats as loaded */
 	inj_stats_loaded = true;
@@ -176,6 +267,30 @@ pgstat_report_inj(const char *name)
 	pgstat_unlock_entry(entry_ref);
 }
 
+/*
+ * Report fixed number of statistics for an injection point.
+ */
+void
+pgstat_report_inj_fixed(uint32 numattach,
+						uint32 numdetach,
+						uint32 numrun)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	stats_shmem = (PgStatShared_InjectionPointFixed *)
+		pgStatLocal.shmem->custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	pgstat_begin_changecount_write(&stats_shmem->changecount);
+	stats_shmem->stats.numattach += numattach;
+	stats_shmem->stats.numdetach += numdetach;
+	stats_shmem->stats.numrun += numrun;
+	pgstat_end_changecount_write(&stats_shmem->changecount);
+}
+
 /*
  * SQL function returning the number of times an injection point
  * has been called.
@@ -192,3 +307,39 @@ injection_points_stats_numcalls(PG_FUNCTION_ARGS)
 
 	PG_RETURN_INT64(entry->numcalls);
 }
+
+/*
+ * SQL function returning fixed-numbered statistics for injection points.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_fixed);
+Datum
+injection_points_stats_fixed(PG_FUNCTION_ARGS)
+{
+	TupleDesc	tupdesc;
+	Datum		values[3] = {0};
+	bool		nulls[3] = {0};
+	PgStat_StatInjFixedEntry *stats;
+
+	pgstat_snapshot_fixed(PGSTAT_KIND_INJECTION_FIXED);
+	stats = pgStatLocal.snapshot.custom_data[PGSTAT_KIND_INJECTION_IDX];
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(3);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "numattach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "numdetach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "numrun",
+					   INT8OID, -1, 0);
+	BlessTupleDesc(tupdesc);
+
+	values[0] = Int64GetDatum(stats->numattach);
+	values[1] = Int64GetDatum(stats->numdetach);
+	values[2] = Int64GetDatum(stats->numrun);
+	nulls[0] = false;
+	nulls[1] = false;
+	nulls[2] = false;
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
index 3e99705483..cf68b25f7b 100644
--- a/src/test/modules/injection_points/injection_stats.h
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -19,5 +19,8 @@ extern void pgstat_register_inj(void);
 extern void pgstat_create_inj(const char *name);
 extern void pgstat_drop_inj(const char *name);
 extern void pgstat_report_inj(const char *name);
+extern void pgstat_report_inj_fixed(uint32 numattach,
+									uint32 numdetach,
+									uint32 numrun);
 
 #endif
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 7d5a96e522..e3c69b94ca 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -31,18 +31,27 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats');
 
 # Restart the node cleanly, stats should still be around.
 $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats after clean restart');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats after clean restart');
 
 # On crash the stats are gone.
 $node->stop('immediate');
 $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
-is($numcalls, '', 'number of stats after clean restart');
+is($numcalls, '', 'number of stats after crash');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '0|0|0', 'number of fixed stats after crash');
 
 done_testing();
-- 
2.45.2

From 67fdfa3eeef3e089a7b99b97f47f6c1e64b501cf Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 11 Jul 2024 12:26:05 +0000
Subject: [PATCH v1] Fix comment on top of pgstat_read_statsfile()

The stats are read from the file and then loaded into the shared stats hash (
for non fixed amount stats) or into the fixed amount stats. Previous comment
was mentioning only the shared stats hash case.
---
 src/backend/utils/activity/pgstat.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
 100.0% src/backend/utils/activity/

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index c0ec9e8195..a7b8ecc1b6 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -1495,10 +1495,11 @@ read_chunk(FILE *fpin, void *ptr, size_t len)
 #define read_chunk_s(fpin, ptr) read_chunk(fpin, ptr, sizeof(*ptr))
 
 /*
- * Reads in existing statistics file into the shared stats hash.
+ * Reads in existing statistics file into the shared stats hash (for non fixed
+ * amount stats) or into the fixed amount stats.
  *
- * This function is called in the only process that is accessing the shared
- * stats so locking is not required.
+ * This function is called in the only process that is accessing the stats
+ * so locking is not required.
  */
 static void
 pgstat_read_statsfile(void)
-- 
2.34.1

From 63ea23477e13322587bf100ddc85ffecfbe1a627 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:22:54 +0900
Subject: [PATCH v7 5/6] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 197 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 ++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  48 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 325 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 2ffd2f77ed..7b9cd12a2a 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -11,9 +14,13 @@ REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
 ISOLATION = inplace
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index 0f280419a5..b98d571889 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -74,3 +74,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index 15f9d0233c..acec4e95b0 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -431,5 +445,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..78042074ff
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,197 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+/*
+ * Kind ID reserved for statistics of injection points.
+ */
+#define PGSTAT_KIND_INJECTION	129
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 3c23c14d81..a52fe5121e 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -42,4 +43,12 @@ tests += {
       'inplace',
     ],
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..7d5a96e522
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,48 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index b4d7f9217c..c0625b8830 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2119,6 +2119,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2150,6 +2151,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.2

From cd82e799d2bd2d05d217b2eaec646e0298677302 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:44:33 +0900
Subject: [PATCH v7 6/6] injection_points: Add example for fixed-numbered
 statistics

This acts as a template to show what can be achieved with fixed-numbered
stats (like WAL, bgwriter, etc.) for pluggable cumulative statistics.
---
 src/test/modules/injection_points/Makefile    |   3 +-
 .../injection_points--1.0.sql                 |  11 +
 .../injection_points/injection_points.c       |   4 +
 .../injection_points/injection_stats.h        |   7 +
 .../injection_points/injection_stats_fixed.c  | 192 ++++++++++++++++++
 src/test/modules/injection_points/meson.build |   1 +
 .../modules/injection_points/t/001_stats.pl   |  11 +-
 7 files changed, 227 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats_fixed.c

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 7b9cd12a2a..ed28cd13a8 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -4,7 +4,8 @@ MODULE_big = injection_points
 OBJS = \
 	$(WIN32RES) \
 	injection_points.o \
-	injection_stats.o
+	injection_stats.o \
+	injection_stats_fixed.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index b98d571889..1b2a4938a9 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -84,3 +84,14 @@ CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
 RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
+
+--
+-- injection_points_stats_fixed()
+--
+-- Reports fixed-numbered statistics for injection points.
+CREATE FUNCTION injection_points_stats_fixed(OUT numattach int8,
+   OUT numdetach int8,
+   OUT numrun int8)
+RETURNS record
+AS 'MODULE_PATHNAME', 'injection_points_stats_fixed'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index acec4e95b0..dc02be1bbf 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -297,6 +297,7 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		condition.pid = MyProcPid;
 	}
 
+	pgstat_report_inj_fixed(1, 0, 0);
 	InjectionPointAttach(name, "injection_points", function, &condition,
 						 sizeof(InjectionPointCondition));
 
@@ -342,6 +343,7 @@ injection_points_run(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 0, 1);
 	INJECTION_POINT(name);
 
 	PG_RETURN_VOID();
@@ -432,6 +434,7 @@ injection_points_detach(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 1, 0);
 	if (!InjectionPointDetach(name))
 		elog(ERROR, "could not detach injection point \"%s\"", name);
 
@@ -459,4 +462,5 @@ _PG_init(void)
 		return;
 
 	pgstat_register_inj();
+	pgstat_register_inj_fixed();
 }
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
index 3e99705483..d519f29f83 100644
--- a/src/test/modules/injection_points/injection_stats.h
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -15,9 +15,16 @@
 #ifndef INJECTION_STATS
 #define INJECTION_STATS
 
+/* injection_stats.c */
 extern void pgstat_register_inj(void);
 extern void pgstat_create_inj(const char *name);
 extern void pgstat_drop_inj(const char *name);
 extern void pgstat_report_inj(const char *name);
 
+/* injection_stats_fixed.c */
+extern void pgstat_register_inj_fixed(void);
+extern void pgstat_report_inj_fixed(uint32 numattach,
+									uint32 numdetach,
+									uint32 numrun);
+
 #endif
diff --git a/src/test/modules/injection_points/injection_stats_fixed.c b/src/test/modules/injection_points/injection_stats_fixed.c
new file mode 100644
index 0000000000..75639328a8
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats_fixed.c
@@ -0,0 +1,192 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats_fixed.c
+ *		Code for fixed-numbered statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats_fixed.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "funcapi.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points, fixed-size */
+typedef struct PgStat_StatInjFixedEntry
+{
+	PgStat_Counter numattach;	/* number of points attached */
+	PgStat_Counter numdetach;	/* number of points detached */
+	PgStat_Counter numrun;		/* number of points run */
+	TimestampTz stat_reset_timestamp;
+} PgStat_StatInjFixedEntry;
+
+typedef struct PgStatShared_InjectionPointFixed
+{
+	LWLock	lock;	/* protects all the counters */
+	uint32	changecount;
+	PgStat_StatInjFixedEntry stats;
+	PgStat_StatInjFixedEntry reset_offset;
+} PgStatShared_InjectionPointFixed;
+
+/* Callbacks for fixed-numbered stats */
+static void injection_stats_fixed_init_shmem_cb(void *stats);
+static void injection_stats_fixed_reset_all_cb(TimestampTz ts);
+static void injection_stats_fixed_snapshot_cb(void);
+
+static const PgStat_KindInfo injection_stats_fixed = {
+	.name = "injection_points_fixed",
+	.fixed_amount = true,
+
+	.shared_size = sizeof(PgStat_StatInjFixedEntry),
+	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPointFixed *) 0)->stats),
+
+	.init_shmem_cb = injection_stats_fixed_init_shmem_cb,
+	.reset_all_cb = injection_stats_fixed_reset_all_cb,
+	.snapshot_cb = injection_stats_fixed_snapshot_cb,
+};
+
+/*
+ * Kind ID reserved for statistics of injection points.
+ */
+#define PGSTAT_KIND_INJECTION_FIXED	130
+
+/* Track if fixed-numbered stats are loaded */
+static bool inj_fixed_loaded = false;
+
+static void
+injection_stats_fixed_init_shmem_cb(void *stats)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+static void
+injection_stats_fixed_reset_all_cb(TimestampTz ts)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
+	LWLockRelease(&stats_shmem->lock);
+}
+
+static void
+injection_stats_fixed_snapshot_cb(void)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+	PgStat_StatInjFixedEntry *stat_snap =
+		pgstat_get_custom_snapshot_data(PGSTAT_KIND_INJECTION_FIXED);
+	PgStat_StatInjFixedEntry *reset_offset = &stats_shmem->reset_offset;
+	PgStat_StatInjFixedEntry reset;
+
+	pgstat_copy_changecounted_stats(stat_snap,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
+	LWLockRelease(&stats_shmem->lock);
+
+	/* compensate by reset offsets */
+#define FIXED_COMP(fld) stat_snap->fld -= reset.fld;
+	FIXED_COMP(numattach);
+	FIXED_COMP(numdetach);
+	FIXED_COMP(numrun);
+#undef FIXED_COMP
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj_fixed(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION_FIXED, &injection_stats_fixed);
+
+	/* mark stats as loaded */
+	inj_fixed_loaded = true;
+}
+
+/*
+ * Report fixed number of statistics for an injection point.
+ */
+void
+pgstat_report_inj_fixed(uint32 numattach,
+						uint32 numdetach,
+						uint32 numrun)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem;
+
+	/* leave if disabled */
+	if (!inj_fixed_loaded)
+		return;
+
+	stats_shmem = pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	pgstat_begin_changecount_write(&stats_shmem->changecount);
+	stats_shmem->stats.numattach += numattach;
+	stats_shmem->stats.numdetach += numdetach;
+	stats_shmem->stats.numrun += numrun;
+	pgstat_end_changecount_write(&stats_shmem->changecount);
+}
+
+/*
+ * SQL function returning fixed-numbered statistics for injection points.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_fixed);
+Datum
+injection_points_stats_fixed(PG_FUNCTION_ARGS)
+{
+	TupleDesc	tupdesc;
+	Datum		values[3] = {0};
+	bool		nulls[3] = {0};
+	PgStat_StatInjFixedEntry *stats;
+
+	if (!inj_fixed_loaded)
+		PG_RETURN_NULL();
+
+	pgstat_snapshot_fixed(PGSTAT_KIND_INJECTION_FIXED);
+	stats = pgstat_get_custom_snapshot_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(3);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "numattach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "numdetach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "numrun",
+					   INT8OID, -1, 0);
+	BlessTupleDesc(tupdesc);
+
+	values[0] = Int64GetDatum(stats->numattach);
+	values[1] = Int64GetDatum(stats->numdetach);
+	values[2] = Int64GetDatum(stats->numrun);
+	nulls[0] = false;
+	nulls[1] = false;
+	nulls[2] = false;
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index a52fe5121e..c9e357f644 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -7,6 +7,7 @@ endif
 injection_points_sources = files(
   'injection_points.c',
   'injection_stats.c',
+  'injection_stats_fixed.c',
 )
 
 if host_system == 'windows'
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 7d5a96e522..e3c69b94ca 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -31,18 +31,27 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats');
 
 # Restart the node cleanly, stats should still be around.
 $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats after clean restart');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats after clean restart');
 
 # On crash the stats are gone.
 $node->stop('immediate');
 $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
-is($numcalls, '', 'number of stats after clean restart');
+is($numcalls, '', 'number of stats after crash');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '0|0|0', 'number of fixed stats after crash');
 
 done_testing();
-- 
2.45.2

From 54aa07f04489aca5dc2fed02121639d5fb453abd Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 12:28:26 +0900
Subject: [PATCH v7 1/6] Switch PgStat_Kind from enum to uint32

A follow-up patch is planned to make this counter extensible, and
keeping a trace of the kind behind a type is useful in the internal
routines used by pgstats.  While on it, switch pgstat_is_kind_valid() to
use PgStat_Kind, to be more consistent with its callers.
---
 src/include/pgstat.h                | 35 ++++++++++++++---------------
 src/backend/utils/activity/pgstat.c |  6 ++---
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 6b99bb8aad..cc97274708 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -32,26 +32,25 @@
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"
 
 /* The types of statistics entries */
-typedef enum PgStat_Kind
-{
-	/* use 0 for INVALID, to catch zero-initialized data */
-	PGSTAT_KIND_INVALID = 0,
+#define PgStat_Kind uint32
 
-	/* stats for variable-numbered objects */
-	PGSTAT_KIND_DATABASE,		/* database-wide statistics */
-	PGSTAT_KIND_RELATION,		/* per-table statistics */
-	PGSTAT_KIND_FUNCTION,		/* per-function statistics */
-	PGSTAT_KIND_REPLSLOT,		/* per-slot statistics */
-	PGSTAT_KIND_SUBSCRIPTION,	/* per-subscription statistics */
+/* use 0 for INVALID, to catch zero-initialized data */
+#define PGSTAT_KIND_INVALID 0
 
-	/* stats for fixed-numbered objects */
-	PGSTAT_KIND_ARCHIVER,
-	PGSTAT_KIND_BGWRITER,
-	PGSTAT_KIND_CHECKPOINTER,
-	PGSTAT_KIND_IO,
-	PGSTAT_KIND_SLRU,
-	PGSTAT_KIND_WAL,
-} PgStat_Kind;
+/* stats for variable-numbered objects */
+#define PGSTAT_KIND_DATABASE	1	/* database-wide statistics */
+#define PGSTAT_KIND_RELATION	2	/* per-table statistics */
+#define PGSTAT_KIND_FUNCTION	3	/* per-function statistics */
+#define PGSTAT_KIND_REPLSLOT	4	/* per-slot statistics */
+#define PGSTAT_KIND_SUBSCRIPTION	5	/* per-subscription statistics */
+
+/* stats for fixed-numbered objects */
+#define PGSTAT_KIND_ARCHIVER	6
+#define PGSTAT_KIND_BGWRITER	7
+#define PGSTAT_KIND_CHECKPOINTER	8
+#define PGSTAT_KIND_IO	9
+#define PGSTAT_KIND_SLRU	10
+#define PGSTAT_KIND_WAL	11
 
 #define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
 #define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 2e22bf2707..e3058b32c0 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -182,7 +182,7 @@ static void pgstat_prep_snapshot(void);
 static void pgstat_build_snapshot(void);
 static void pgstat_build_snapshot_fixed(PgStat_Kind kind);
 
-static inline bool pgstat_is_kind_valid(int ikind);
+static inline bool pgstat_is_kind_valid(PgStat_Kind kind);
 
 
 /* ----------
@@ -1298,9 +1298,9 @@ pgstat_get_kind_from_str(char *kind_str)
 }
 
 static inline bool
-pgstat_is_kind_valid(int ikind)
+pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
+	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
 }
 
 const PgStat_KindInfo *
-- 
2.45.2

From 07c4be73bb31c06b1c44b2457ab5425f2426cc8c Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 8 Jul 2024 14:03:17 +0900
Subject: [PATCH v7 2/6] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats

This is able to support fixed-numbered (like WAL, archiver, bgwriter)
and variable-numbered stats kinds.
---
 src/include/pgstat.h                      |  35 +++-
 src/include/utils/pgstat_internal.h       |  22 ++-
 src/backend/utils/activity/pgstat.c       | 231 +++++++++++++++++++---
 src/backend/utils/activity/pgstat_shmem.c |  31 ++-
 src/backend/utils/adt/pgstatfuncs.c       |   2 +-
 5 files changed, 287 insertions(+), 34 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index cc97274708..21c3705ce4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -34,6 +34,10 @@
 /* The types of statistics entries */
 #define PgStat_Kind uint32
 
+/* Range of IDs allowed, for built-in and custom kinds */
+#define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
+#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
 
@@ -52,9 +56,34 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 778f625ca1..39f63362a3 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -195,7 +195,8 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
-	 * PgStatShared_HashEntry->body).
+	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
+	 * the size of an entry in PgStat_ShmemControl->custom_data.
 	 */
 	uint32		shared_size;
 
@@ -446,6 +447,13 @@ typedef struct PgStat_ShmemControl
 	PgStatShared_IO io;
 	PgStatShared_SLRU slru;
 	PgStatShared_Wal wal;
+
+	/*
+	 * Custom stats data with fixed-numbered objects, indexed by (PgStat_Kind
+	 * - PGSTAT_KIND_CUSTOM_MIN).
+	 */
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 } PgStat_ShmemControl;
 
 
@@ -459,7 +467,7 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX_BUILTIN + 1];
 
 	PgStat_ArchiverStats archiver;
 
@@ -473,6 +481,14 @@ typedef struct PgStat_Snapshot
 
 	PgStat_WalStats wal;
 
+	/*
+	 * Data in snapshot for custom fixed-numbered statistics, indexed by
+	 * (PgStat_Kind - PGSTAT_KIND_CUSTOM_MIN).  Each entry is allocated in
+	 * TopMemoryContext, for a size of shared_data_len.
+	 */
+	bool		custom_valid[PGSTAT_KIND_CUSTOM_SIZE];
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 	/* to free snapshot in bulk */
 	MemoryContext context;
 	struct pgstat_snapshot_hash *stats;
@@ -516,6 +532,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index e3058b32c0..bb70b81e49 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for external modules to define custom statistics kinds,
+ * that can use the same properties as any built-in stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
+ * defined, and pgstat_kind_custom_infos for custom kinds registered at
+ * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -174,6 +182,8 @@ typedef struct PgStat_SnapshotEntry
 static void pgstat_write_statsfile(void);
 static void pgstat_read_statsfile(void);
 
+static void pgstat_init_snapshot_fixed(void);
+
 static void pgstat_reset_after_failure(void);
 
 static bool pgstat_flush_pending_entries(bool nowait);
@@ -251,7 +261,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -263,7 +273,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -436,6 +446,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -586,6 +605,8 @@ pgstat_initialize(void)
 
 	pgstat_init_wal();
 
+	pgstat_init_snapshot_fixed();
+
 	/* Set up a process-exit hook to clean up */
 	before_shmem_exit(pgstat_shutdown_hook, 0);
 
@@ -829,6 +850,8 @@ pgstat_clear_snapshot(void)
 
 	memset(&pgStatLocal.snapshot.fixed_valid, 0,
 		   sizeof(pgStatLocal.snapshot.fixed_valid));
+	memset(&pgStatLocal.snapshot.custom_valid, 0,
+		   sizeof(pgStatLocal.snapshot.custom_valid));
 	pgStatLocal.snapshot.stats = NULL;
 	pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_NONE;
 
@@ -992,7 +1015,29 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 	else
 		pgstat_build_snapshot_fixed(kind);
 
-	Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	if (pgstat_is_kind_builtin(kind))
+		Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	else if (pgstat_is_kind_custom(kind))
+		Assert(pgStatLocal.snapshot.custom_valid[kind - PGSTAT_KIND_CUSTOM_MIN]);
+}
+
+static void
+pgstat_init_snapshot_fixed(void)
+{
+	/*
+	 * Initialize fixed-numbered statistics data in snapshots, only for custom
+	 * stats kinds.
+	 */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info || !kind_info->fixed_amount)
+			continue;
+
+		pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN] =
+			MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
+	}
 }
 
 static void
@@ -1088,10 +1133,12 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 		{
 			Assert(kind_info->snapshot_cb == NULL);
@@ -1108,6 +1155,20 @@ static void
 pgstat_build_snapshot_fixed(PgStat_Kind kind)
 {
 	const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+	int			idx;
+	bool	   *valid;
+
+	/* Position in fixed_valid or custom_valid */
+	if (pgstat_is_kind_builtin(kind))
+	{
+		idx = kind;
+		valid = pgStatLocal.snapshot.fixed_valid;
+	}
+	else
+	{
+		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+		valid = pgStatLocal.snapshot.custom_valid;
+	}
 
 	Assert(kind_info->fixed_amount);
 	Assert(kind_info->snapshot_cb != NULL);
@@ -1115,21 +1176,21 @@ pgstat_build_snapshot_fixed(PgStat_Kind kind)
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
 	{
 		/* rebuild every time */
-		pgStatLocal.snapshot.fixed_valid[kind] = false;
+		valid[idx] = false;
 	}
-	else if (pgStatLocal.snapshot.fixed_valid[kind])
+	else if (valid[idx])
 	{
 		/* in snapshot mode we shouldn't get called again */
 		Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
 		return;
 	}
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
+	Assert(!valid[idx]);
 
 	kind_info->snapshot_cb();
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
-	pgStatLocal.snapshot.fixed_valid[kind] = true;
+	Assert(!valid[idx]);
+	valid[idx] = true;
 }
 
 
@@ -1285,30 +1346,127 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
 pgstat_get_kind_info(PgStat_Kind kind)
 {
-	Assert(pgstat_is_kind_valid(kind));
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL ||
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	return NULL;
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * Check some data for fixed-numbered stats.
+	 */
+	if (kind_info->fixed_amount)
+	{
+		if (kind_info->shared_size == 0)
+			ereport(ERROR,
+					(errmsg("custom cumulative statistics property is invalid"),
+					 errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+	}
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+								   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom cumulative statistics \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1388,18 +1546,22 @@ pgstat_write_statsfile(void)
 	write_chunk_s(fpout, &format_id);
 
 	/* Write various stats structs for fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		char	   *ptr;
 		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-		if (!info->fixed_amount)
+		if (!info || !info->fixed_amount)
 			continue;
 
-		Assert(info->snapshot_ctl_off != 0);
+		if (pgstat_is_kind_builtin(kind))
+			Assert(info->snapshot_ctl_off != 0);
 
 		pgstat_build_snapshot_fixed(kind);
-		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		if (pgstat_is_kind_builtin(kind))
+			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		else
+			ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];
 
 		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
 		write_chunk_s(fpout, &kind);
@@ -1422,6 +1584,17 @@ pgstat_write_statsfile(void)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1570,8 +1743,16 @@ pgstat_read_statsfile(void)
 						goto error;
 
 					/* Load back stats into shared memory */
-					ptr = ((char *) shmem) + info->shared_ctl_off +
-						info->shared_data_off;
+					if (pgstat_is_kind_builtin(kind))
+						ptr = ((char *) shmem) + info->shared_ctl_off +
+							info->shared_data_off;
+					else
+					{
+						int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+						ptr = ((char *) shmem->custom_data[idx]) +
+							info->shared_data_off;
+					}
 
 					if (!read_chunk(fpin, ptr, info->shared_data_len))
 						goto error;
@@ -1638,7 +1819,7 @@ pgstat_read_statsfile(void)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u",
 							 key.kind, key.dboid, key.objoid);
 						goto error;
 					}
@@ -1696,11 +1877,11 @@ pgstat_reset_after_failure(void)
 	TimestampTz ts = GetCurrentTimestamp();
 
 	/* reset fixed-numbered stats */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		kind_info->reset_all_cb(ts);
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 1c2b69d563..ca3d3eecfb 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -131,6 +131,21 @@ StatsShmemSize(void)
 	sz = MAXALIGN(sizeof(PgStat_ShmemControl));
 	sz = add_size(sz, pgstat_dsa_init_size());
 
+	/* Add shared memory for all the custom fixed-numbered statistics */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info)
+			continue;
+		if (!kind_info->fixed_amount)
+			continue;
+
+		Assert(kind_info->shared_size != 0);
+
+		sz += MAXALIGN(kind_info->shared_size);
+	}
+
 	return sz;
 }
 
@@ -197,15 +212,25 @@ StatsShmemInit(void)
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
 		/* initialize fixed-numbered stats */
-		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 		{
 			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 			char	   *ptr;
 
-			if (!kind_info->fixed_amount)
+			if (!kind_info || !kind_info->fixed_amount)
 				continue;
 
-			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			if (pgstat_is_kind_builtin(kind))
+				ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			else
+			{
+				int			idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+				Assert(kind_info->shared_size != 0);
+				ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
+				ptr = ctl->custom_data[idx];
+			}
+
 			kind_info->init_shmem_cb(ptr);
 		}
 	}
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.2

From e475faaa8943356379923433f80125efbe68b9d0 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:10:35 +0900
Subject: [PATCH v7 3/6] Add helper routines to retrieve custom data for
 fixed-numbered stats

This is useful for extensions to get snapshot and shmem data for custom
statistics, so as these don't need to know about the current snapshots.
---
 src/include/utils/pgstat_internal.h | 32 +++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 39f63362a3..6bbd49388c 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -525,6 +525,8 @@ static inline int pgstat_cmp_hash_key(const void *a, const void *b, size_t size,
 static inline uint32 pgstat_hash_hash_key(const void *d, size_t size, void *arg);
 static inline size_t pgstat_get_entry_len(PgStat_Kind kind);
 static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry);
+static inline void *pgstat_get_custom_shmem_data(PgStat_Kind kind);
+static inline void *pgstat_get_custom_snapshot_data(PgStat_Kind kind);
 
 
 /*
@@ -842,4 +844,34 @@ pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry)
 	return ((char *) (entry)) + off;
 }
 
+/*
+ * Returns a pointer to the shared memory area of custom stats for
+ * fixed-numbered statistics.
+ */
+static inline void *
+pgstat_get_custom_shmem_data(PgStat_Kind kind)
+{
+	int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	Assert(pgstat_is_kind_custom(kind));
+	Assert(pgstat_get_kind_info(kind)->fixed_amount);
+
+	return pgStatLocal.shmem->custom_data[idx];
+}
+
+/*
+ * Returns a pointer to the portion of custom data for fixed-numbered
+ * statistics in the current snapshot.
+ */
+static inline void *
+pgstat_get_custom_snapshot_data(PgStat_Kind kind)
+{
+	int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	Assert(pgstat_is_kind_custom(kind));
+	Assert(pgstat_get_kind_info(kind)->fixed_amount);
+
+	return pgStatLocal.snapshot.custom_data[idx];
+}
+
 #endif							/* PGSTAT_INTERNAL_H */
-- 
2.45.2

From 75e499b54c7c6ab79dadffe6eacb943df0a55038 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:14:58 +0900
Subject: [PATCH v7 4/6] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 62 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 7e92e89846..839c78c102 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3703,6 +3703,68 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in <filename>src/test/modules/injection_points</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.2

From af5a2456f90bf02b993c7c718e45a4cd6d863c79 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Fri, 2 Aug 2024 05:42:52 +0900
Subject: [PATCH v3 1/5] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats

This is able to support fixed-numbered (like WAL, archiver, bgwriter)
and variable-numbered stats kinds.
---
 src/include/pgstat.h                      |  35 +++-
 src/include/utils/pgstat_internal.h       |  22 ++-
 src/backend/utils/activity/pgstat.c       | 231 +++++++++++++++++++---
 src/backend/utils/activity/pgstat_shmem.c |  31 ++-
 src/backend/utils/adt/pgstatfuncs.c       |   2 +-
 5 files changed, 287 insertions(+), 34 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index f84e9fdeca..cd93a0109b 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -35,6 +35,10 @@
 /* The types of statistics entries */
 #define PgStat_Kind uint32
 
+/* Range of IDs allowed, for built-in and custom kinds */
+#define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
+#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
 
@@ -53,9 +57,34 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 778f625ca1..39f63362a3 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -195,7 +195,8 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
-	 * PgStatShared_HashEntry->body).
+	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
+	 * the size of an entry in PgStat_ShmemControl->custom_data.
 	 */
 	uint32		shared_size;
 
@@ -446,6 +447,13 @@ typedef struct PgStat_ShmemControl
 	PgStatShared_IO io;
 	PgStatShared_SLRU slru;
 	PgStatShared_Wal wal;
+
+	/*
+	 * Custom stats data with fixed-numbered objects, indexed by (PgStat_Kind
+	 * - PGSTAT_KIND_CUSTOM_MIN).
+	 */
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 } PgStat_ShmemControl;
 
 
@@ -459,7 +467,7 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX_BUILTIN + 1];
 
 	PgStat_ArchiverStats archiver;
 
@@ -473,6 +481,14 @@ typedef struct PgStat_Snapshot
 
 	PgStat_WalStats wal;
 
+	/*
+	 * Data in snapshot for custom fixed-numbered statistics, indexed by
+	 * (PgStat_Kind - PGSTAT_KIND_CUSTOM_MIN).  Each entry is allocated in
+	 * TopMemoryContext, for a size of shared_data_len.
+	 */
+	bool		custom_valid[PGSTAT_KIND_CUSTOM_SIZE];
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 	/* to free snapshot in bulk */
 	MemoryContext context;
 	struct pgstat_snapshot_hash *stats;
@@ -516,6 +532,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 143478aca0..9bc8baca9c 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for external modules to define custom statistics kinds,
+ * that can use the same properties as any built-in stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
+ * defined, and pgstat_kind_custom_infos for custom kinds registered at
+ * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -175,6 +183,8 @@ typedef struct PgStat_SnapshotEntry
 static void pgstat_write_statsfile(XLogRecPtr redo);
 static void pgstat_read_statsfile(XLogRecPtr redo);
 
+static void pgstat_init_snapshot_fixed(void);
+
 static void pgstat_reset_after_failure(void);
 
 static bool pgstat_flush_pending_entries(bool nowait);
@@ -252,7 +262,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -264,7 +274,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -437,6 +447,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -587,6 +606,8 @@ pgstat_initialize(void)
 
 	pgstat_init_wal();
 
+	pgstat_init_snapshot_fixed();
+
 	/* Set up a process-exit hook to clean up */
 	before_shmem_exit(pgstat_shutdown_hook, 0);
 
@@ -830,6 +851,8 @@ pgstat_clear_snapshot(void)
 
 	memset(&pgStatLocal.snapshot.fixed_valid, 0,
 		   sizeof(pgStatLocal.snapshot.fixed_valid));
+	memset(&pgStatLocal.snapshot.custom_valid, 0,
+		   sizeof(pgStatLocal.snapshot.custom_valid));
 	pgStatLocal.snapshot.stats = NULL;
 	pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_NONE;
 
@@ -993,7 +1016,29 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 	else
 		pgstat_build_snapshot_fixed(kind);
 
-	Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	if (pgstat_is_kind_builtin(kind))
+		Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	else if (pgstat_is_kind_custom(kind))
+		Assert(pgStatLocal.snapshot.custom_valid[kind - PGSTAT_KIND_CUSTOM_MIN]);
+}
+
+static void
+pgstat_init_snapshot_fixed(void)
+{
+	/*
+	 * Initialize fixed-numbered statistics data in snapshots, only for custom
+	 * stats kinds.
+	 */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info || !kind_info->fixed_amount)
+			continue;
+
+		pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN] =
+			MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
+	}
 }
 
 static void
@@ -1089,10 +1134,12 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (PgStat_Kind kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 		{
 			Assert(kind_info->snapshot_cb == NULL);
@@ -1109,6 +1156,20 @@ static void
 pgstat_build_snapshot_fixed(PgStat_Kind kind)
 {
 	const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+	int			idx;
+	bool	   *valid;
+
+	/* Position in fixed_valid or custom_valid */
+	if (pgstat_is_kind_builtin(kind))
+	{
+		idx = kind;
+		valid = pgStatLocal.snapshot.fixed_valid;
+	}
+	else
+	{
+		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+		valid = pgStatLocal.snapshot.custom_valid;
+	}
 
 	Assert(kind_info->fixed_amount);
 	Assert(kind_info->snapshot_cb != NULL);
@@ -1116,21 +1177,21 @@ pgstat_build_snapshot_fixed(PgStat_Kind kind)
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
 	{
 		/* rebuild every time */
-		pgStatLocal.snapshot.fixed_valid[kind] = false;
+		valid[idx] = false;
 	}
-	else if (pgStatLocal.snapshot.fixed_valid[kind])
+	else if (valid[idx])
 	{
 		/* in snapshot mode we shouldn't get called again */
 		Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
 		return;
 	}
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
+	Assert(!valid[idx]);
 
 	kind_info->snapshot_cb();
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
-	pgStatLocal.snapshot.fixed_valid[kind] = true;
+	Assert(!valid[idx]);
+	valid[idx] = true;
 }
 
 
@@ -1286,30 +1347,127 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (PgStat_Kind kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (PgStat_Kind kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
 pgstat_get_kind_info(PgStat_Kind kind)
 {
-	Assert(pgstat_is_kind_valid(kind));
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL ||
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	return NULL;
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * Check some data for fixed-numbered stats.
+	 */
+	if (kind_info->fixed_amount)
+	{
+		if (kind_info->shared_size == 0)
+			ereport(ERROR,
+					(errmsg("custom cumulative statistics property is invalid"),
+					 errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+	}
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+								   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom cumulative statistics \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1393,18 +1551,22 @@ pgstat_write_statsfile(XLogRecPtr redo)
 	write_chunk_s(fpout, &redo);
 
 	/* Write various stats structs for fixed number of objects */
-	for (PgStat_Kind kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		char	   *ptr;
 		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-		if (!info->fixed_amount)
+		if (!info || !info->fixed_amount)
 			continue;
 
-		Assert(info->snapshot_ctl_off != 0);
+		if (pgstat_is_kind_builtin(kind))
+			Assert(info->snapshot_ctl_off != 0);
 
 		pgstat_build_snapshot_fixed(kind);
-		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		if (pgstat_is_kind_builtin(kind))
+			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		else
+			ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];
 
 		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
 		write_chunk_s(fpout, &kind);
@@ -1427,6 +1589,17 @@ pgstat_write_statsfile(XLogRecPtr redo)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1613,8 +1786,16 @@ pgstat_read_statsfile(XLogRecPtr redo)
 					}
 
 					/* Load back stats into shared memory */
-					ptr = ((char *) shmem) + info->shared_ctl_off +
-						info->shared_data_off;
+					if (pgstat_is_kind_builtin(kind))
+						ptr = ((char *) shmem) + info->shared_ctl_off +
+							info->shared_data_off;
+					else
+					{
+						int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+						ptr = ((char *) shmem->custom_data[idx]) +
+							info->shared_data_off;
+					}
 
 					if (!read_chunk(fpin, ptr, info->shared_data_len))
 					{
@@ -1711,7 +1892,7 @@ pgstat_read_statsfile(XLogRecPtr redo)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u of type %c",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u of type %c",
 							 key.kind, key.dboid, key.objoid, t);
 						goto error;
 					}
@@ -1777,11 +1958,11 @@ pgstat_reset_after_failure(void)
 	TimestampTz ts = GetCurrentTimestamp();
 
 	/* reset fixed-numbered stats */
-	for (PgStat_Kind kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		kind_info->reset_all_cb(ts);
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 2d5f7d46de..c0ac9e4bbc 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -131,6 +131,21 @@ StatsShmemSize(void)
 	sz = MAXALIGN(sizeof(PgStat_ShmemControl));
 	sz = add_size(sz, pgstat_dsa_init_size());
 
+	/* Add shared memory for all the custom fixed-numbered statistics */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info)
+			continue;
+		if (!kind_info->fixed_amount)
+			continue;
+
+		Assert(kind_info->shared_size != 0);
+
+		sz += MAXALIGN(kind_info->shared_size);
+	}
+
 	return sz;
 }
 
@@ -197,15 +212,25 @@ StatsShmemInit(void)
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
 		/* initialize fixed-numbered stats */
-		for (PgStat_Kind kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 		{
 			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 			char	   *ptr;
 
-			if (!kind_info->fixed_amount)
+			if (!kind_info || !kind_info->fixed_amount)
 				continue;
 
-			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			if (pgstat_is_kind_builtin(kind))
+				ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			else
+			{
+				int			idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+				Assert(kind_info->shared_size != 0);
+				ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
+				ptr = ctl->custom_data[idx];
+			}
+
 			kind_info->init_shmem_cb(ptr);
 		}
 	}
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.2

From 5bcbe74bdb930ce1bd46791419e5fdd195b5f0d4 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:10:35 +0900
Subject: [PATCH v3 2/5] Add helper routines to retrieve custom data for
 fixed-numbered stats

This is useful for extensions to get snapshot and shmem data for custom
statistics, so as these don't need to know about the current snapshots.
---
 src/include/utils/pgstat_internal.h | 32 +++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 39f63362a3..6bbd49388c 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -525,6 +525,8 @@ static inline int pgstat_cmp_hash_key(const void *a, const void *b, size_t size,
 static inline uint32 pgstat_hash_hash_key(const void *d, size_t size, void *arg);
 static inline size_t pgstat_get_entry_len(PgStat_Kind kind);
 static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry);
+static inline void *pgstat_get_custom_shmem_data(PgStat_Kind kind);
+static inline void *pgstat_get_custom_snapshot_data(PgStat_Kind kind);
 
 
 /*
@@ -842,4 +844,34 @@ pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry)
 	return ((char *) (entry)) + off;
 }
 
+/*
+ * Returns a pointer to the shared memory area of custom stats for
+ * fixed-numbered statistics.
+ */
+static inline void *
+pgstat_get_custom_shmem_data(PgStat_Kind kind)
+{
+	int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	Assert(pgstat_is_kind_custom(kind));
+	Assert(pgstat_get_kind_info(kind)->fixed_amount);
+
+	return pgStatLocal.shmem->custom_data[idx];
+}
+
+/*
+ * Returns a pointer to the portion of custom data for fixed-numbered
+ * statistics in the current snapshot.
+ */
+static inline void *
+pgstat_get_custom_snapshot_data(PgStat_Kind kind)
+{
+	int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	Assert(pgstat_is_kind_custom(kind));
+	Assert(pgstat_get_kind_info(kind)->fixed_amount);
+
+	return pgStatLocal.snapshot.custom_data[idx];
+}
+
 #endif							/* PGSTAT_INTERNAL_H */
-- 
2.45.2

From 99d3799031bd7d8f6f19880c356337fe14208e9a Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:14:58 +0900
Subject: [PATCH v3 3/5] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 62 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index bf76490cbc..fbdbdbd883 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3864,6 +3864,68 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in <filename>src/test/modules/injection_points</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.2

From 47dacf574ece0f3b348c02e2a78ef602c3f14a23 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:22:54 +0900
Subject: [PATCH v3 4/5] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 197 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 ++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  48 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 325 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 2ffd2f77ed..7b9cd12a2a 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -11,9 +14,13 @@ REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
 ISOLATION = inplace
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index 0f280419a5..b98d571889 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -74,3 +74,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index 15f9d0233c..acec4e95b0 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -431,5 +445,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..78042074ff
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,197 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+/*
+ * Kind ID reserved for statistics of injection points.
+ */
+#define PGSTAT_KIND_INJECTION	129
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 3c23c14d81..a52fe5121e 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -42,4 +43,12 @@ tests += {
       'inplace',
     ],
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..7d5a96e522
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,48 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 8de9978ad8..48febe4698 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2119,6 +2119,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2150,6 +2151,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.2

From 29774effafad952be4a6e9d6aafc8b0638dbe90d Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 18 Jul 2024 14:44:33 +0900
Subject: [PATCH v3 5/5] injection_points: Add example for fixed-numbered
 statistics

This acts as a template to show what can be achieved with fixed-numbered
stats (like WAL, bgwriter, etc.) for pluggable cumulative statistics.
---
 src/test/modules/injection_points/Makefile    |   3 +-
 .../injection_points--1.0.sql                 |  11 +
 .../injection_points/injection_points.c       |   4 +
 .../injection_points/injection_stats.h        |   7 +
 .../injection_points/injection_stats_fixed.c  | 192 ++++++++++++++++++
 src/test/modules/injection_points/meson.build |   1 +
 .../modules/injection_points/t/001_stats.pl   |  11 +-
 7 files changed, 227 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats_fixed.c

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 7b9cd12a2a..ed28cd13a8 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -4,7 +4,8 @@ MODULE_big = injection_points
 OBJS = \
 	$(WIN32RES) \
 	injection_points.o \
-	injection_stats.o
+	injection_stats.o \
+	injection_stats_fixed.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index b98d571889..1b2a4938a9 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -84,3 +84,14 @@ CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
 RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
+
+--
+-- injection_points_stats_fixed()
+--
+-- Reports fixed-numbered statistics for injection points.
+CREATE FUNCTION injection_points_stats_fixed(OUT numattach int8,
+   OUT numdetach int8,
+   OUT numrun int8)
+RETURNS record
+AS 'MODULE_PATHNAME', 'injection_points_stats_fixed'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index acec4e95b0..dc02be1bbf 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -297,6 +297,7 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		condition.pid = MyProcPid;
 	}
 
+	pgstat_report_inj_fixed(1, 0, 0);
 	InjectionPointAttach(name, "injection_points", function, &condition,
 						 sizeof(InjectionPointCondition));
 
@@ -342,6 +343,7 @@ injection_points_run(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 0, 1);
 	INJECTION_POINT(name);
 
 	PG_RETURN_VOID();
@@ -432,6 +434,7 @@ injection_points_detach(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 1, 0);
 	if (!InjectionPointDetach(name))
 		elog(ERROR, "could not detach injection point \"%s\"", name);
 
@@ -459,4 +462,5 @@ _PG_init(void)
 		return;
 
 	pgstat_register_inj();
+	pgstat_register_inj_fixed();
 }
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
index 3e99705483..d519f29f83 100644
--- a/src/test/modules/injection_points/injection_stats.h
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -15,9 +15,16 @@
 #ifndef INJECTION_STATS
 #define INJECTION_STATS
 
+/* injection_stats.c */
 extern void pgstat_register_inj(void);
 extern void pgstat_create_inj(const char *name);
 extern void pgstat_drop_inj(const char *name);
 extern void pgstat_report_inj(const char *name);
 
+/* injection_stats_fixed.c */
+extern void pgstat_register_inj_fixed(void);
+extern void pgstat_report_inj_fixed(uint32 numattach,
+									uint32 numdetach,
+									uint32 numrun);
+
 #endif
diff --git a/src/test/modules/injection_points/injection_stats_fixed.c b/src/test/modules/injection_points/injection_stats_fixed.c
new file mode 100644
index 0000000000..75639328a8
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats_fixed.c
@@ -0,0 +1,192 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats_fixed.c
+ *		Code for fixed-numbered statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats_fixed.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "funcapi.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points, fixed-size */
+typedef struct PgStat_StatInjFixedEntry
+{
+	PgStat_Counter numattach;	/* number of points attached */
+	PgStat_Counter numdetach;	/* number of points detached */
+	PgStat_Counter numrun;		/* number of points run */
+	TimestampTz stat_reset_timestamp;
+} PgStat_StatInjFixedEntry;
+
+typedef struct PgStatShared_InjectionPointFixed
+{
+	LWLock	lock;	/* protects all the counters */
+	uint32	changecount;
+	PgStat_StatInjFixedEntry stats;
+	PgStat_StatInjFixedEntry reset_offset;
+} PgStatShared_InjectionPointFixed;
+
+/* Callbacks for fixed-numbered stats */
+static void injection_stats_fixed_init_shmem_cb(void *stats);
+static void injection_stats_fixed_reset_all_cb(TimestampTz ts);
+static void injection_stats_fixed_snapshot_cb(void);
+
+static const PgStat_KindInfo injection_stats_fixed = {
+	.name = "injection_points_fixed",
+	.fixed_amount = true,
+
+	.shared_size = sizeof(PgStat_StatInjFixedEntry),
+	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPointFixed *) 0)->stats),
+
+	.init_shmem_cb = injection_stats_fixed_init_shmem_cb,
+	.reset_all_cb = injection_stats_fixed_reset_all_cb,
+	.snapshot_cb = injection_stats_fixed_snapshot_cb,
+};
+
+/*
+ * Kind ID reserved for statistics of injection points.
+ */
+#define PGSTAT_KIND_INJECTION_FIXED	130
+
+/* Track if fixed-numbered stats are loaded */
+static bool inj_fixed_loaded = false;
+
+static void
+injection_stats_fixed_init_shmem_cb(void *stats)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+static void
+injection_stats_fixed_reset_all_cb(TimestampTz ts)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
+	LWLockRelease(&stats_shmem->lock);
+}
+
+static void
+injection_stats_fixed_snapshot_cb(void)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+	PgStat_StatInjFixedEntry *stat_snap =
+		pgstat_get_custom_snapshot_data(PGSTAT_KIND_INJECTION_FIXED);
+	PgStat_StatInjFixedEntry *reset_offset = &stats_shmem->reset_offset;
+	PgStat_StatInjFixedEntry reset;
+
+	pgstat_copy_changecounted_stats(stat_snap,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
+	LWLockRelease(&stats_shmem->lock);
+
+	/* compensate by reset offsets */
+#define FIXED_COMP(fld) stat_snap->fld -= reset.fld;
+	FIXED_COMP(numattach);
+	FIXED_COMP(numdetach);
+	FIXED_COMP(numrun);
+#undef FIXED_COMP
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj_fixed(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION_FIXED, &injection_stats_fixed);
+
+	/* mark stats as loaded */
+	inj_fixed_loaded = true;
+}
+
+/*
+ * Report fixed number of statistics for an injection point.
+ */
+void
+pgstat_report_inj_fixed(uint32 numattach,
+						uint32 numdetach,
+						uint32 numrun)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem;
+
+	/* leave if disabled */
+	if (!inj_fixed_loaded)
+		return;
+
+	stats_shmem = pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	pgstat_begin_changecount_write(&stats_shmem->changecount);
+	stats_shmem->stats.numattach += numattach;
+	stats_shmem->stats.numdetach += numdetach;
+	stats_shmem->stats.numrun += numrun;
+	pgstat_end_changecount_write(&stats_shmem->changecount);
+}
+
+/*
+ * SQL function returning fixed-numbered statistics for injection points.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_fixed);
+Datum
+injection_points_stats_fixed(PG_FUNCTION_ARGS)
+{
+	TupleDesc	tupdesc;
+	Datum		values[3] = {0};
+	bool		nulls[3] = {0};
+	PgStat_StatInjFixedEntry *stats;
+
+	if (!inj_fixed_loaded)
+		PG_RETURN_NULL();
+
+	pgstat_snapshot_fixed(PGSTAT_KIND_INJECTION_FIXED);
+	stats = pgstat_get_custom_snapshot_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(3);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "numattach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "numdetach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "numrun",
+					   INT8OID, -1, 0);
+	BlessTupleDesc(tupdesc);
+
+	values[0] = Int64GetDatum(stats->numattach);
+	values[1] = Int64GetDatum(stats->numdetach);
+	values[2] = Int64GetDatum(stats->numrun);
+	nulls[0] = false;
+	nulls[1] = false;
+	nulls[2] = false;
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index a52fe5121e..c9e357f644 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -7,6 +7,7 @@ endif
 injection_points_sources = files(
   'injection_points.c',
   'injection_stats.c',
+  'injection_stats_fixed.c',
 )
 
 if host_system == 'windows'
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 7d5a96e522..e3c69b94ca 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -31,18 +31,27 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats');
 
 # Restart the node cleanly, stats should still be around.
 $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats after clean restart');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats after clean restart');
 
 # On crash the stats are gone.
 $node->stop('immediate');
 $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
-is($numcalls, '', 'number of stats after clean restart');
+is($numcalls, '', 'number of stats after crash');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '0|0|0', 'number of fixed stats after crash');
 
 done_testing();
-- 
2.45.2