From c0c2a3211bea705b49185a63975c86589b46871a Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Tue, 23 Jul 2024 12:40:08 +0900
Subject: [PATCH v5 1/2] Add redo LSN to pgstats file

This is used in the startup process to check that the file we are
loading is the one referring to the latest checkpoint.

Bump PGSTAT_FILE_FORMAT_ID.
---
 src/include/pgstat.h                |  5 +++--
 src/backend/access/transam/xlog.c   |  2 +-
 src/backend/utils/activity/pgstat.c | 35 +++++++++++++++++++++++------
 3 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 6b99bb8aad..043d39a565 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -11,6 +11,7 @@
 #ifndef PGSTAT_H
 #define PGSTAT_H
 
+#include "access/xlogdefs.h"
 #include "datatype/timestamp.h"
 #include "portability/instr_time.h"
 #include "postmaster/pgarch.h"	/* for MAX_XFN_CHARS */
@@ -235,7 +236,7 @@ typedef struct PgStat_TableXactStatus
  * ------------------------------------------------------------
  */
 
-#define PGSTAT_FILE_FORMAT_ID	0x01A5BCAD
+#define PGSTAT_FILE_FORMAT_ID	0x01A5BCAE
 
 typedef struct PgStat_ArchiverStats
 {
@@ -466,7 +467,7 @@ extern Size StatsShmemSize(void);
 extern void StatsShmemInit(void);
 
 /* Functions called during server startup / shutdown */
-extern void pgstat_restore_stats(void);
+extern void pgstat_restore_stats(XLogRecPtr redo);
 extern void pgstat_discard_stats(void);
 extern void pgstat_before_server_shutdown(int code, Datum arg);
 
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index f86f4b5c4b..b7bb4f9a31 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5652,7 +5652,7 @@ StartupXLOG(void)
 	if (didCrash)
 		pgstat_discard_stats();
 	else
-		pgstat_restore_stats();
+		pgstat_restore_stats(checkPoint.redo);
 
 	lastFullPageWrites = checkPoint.fullPageWrites;
 
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 228cdf73f7..65ae488e2a 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -94,6 +94,7 @@
 #include <unistd.h>
 
 #include "access/xact.h"
+#include "access/xlog.h"
 #include "lib/dshash.h"
 #include "pgstat.h"
 #include "port/atomics.h"
@@ -171,8 +172,8 @@ typedef struct PgStat_SnapshotEntry
  * ----------
  */
 
-static void pgstat_write_statsfile(void);
-static void pgstat_read_statsfile(void);
+static void pgstat_write_statsfile(XLogRecPtr redo);
+static void pgstat_read_statsfile(XLogRecPtr redo);
 
 static void pgstat_reset_after_failure(void);
 
@@ -448,9 +449,9 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
  * Should only be called by the startup process or in single user mode.
  */
 void
-pgstat_restore_stats(void)
+pgstat_restore_stats(XLogRecPtr redo)
 {
-	pgstat_read_statsfile();
+	pgstat_read_statsfile(redo);
 }
 
 /*
@@ -526,7 +527,7 @@ pgstat_before_server_shutdown(int code, Datum arg)
 	if (code == 0)
 	{
 		pgStatLocal.shmem->is_shutdown = true;
-		pgstat_write_statsfile();
+		pgstat_write_statsfile(GetRedoRecPtr());
 	}
 }
 
@@ -1349,7 +1350,7 @@ write_chunk(FILE *fpout, void *ptr, size_t len)
  * stats so locking is not required.
  */
 static void
-pgstat_write_statsfile(void)
+pgstat_write_statsfile(XLogRecPtr redo)
 {
 	FILE	   *fpout;
 	int32		format_id;
@@ -1387,6 +1388,9 @@ pgstat_write_statsfile(void)
 	format_id = PGSTAT_FILE_FORMAT_ID;
 	write_chunk_s(fpout, &format_id);
 
+	/* Write the redo LSN, used to cross check the file read */
+	write_chunk_s(fpout, &redo);
+
 	/* Write various stats structs for fixed number of objects */
 	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
 	{
@@ -1501,13 +1505,14 @@ read_chunk(FILE *fpin, void *ptr, size_t len)
  * stats so locking is not required.
  */
 static void
-pgstat_read_statsfile(void)
+pgstat_read_statsfile(XLogRecPtr redo)
 {
 	FILE	   *fpin;
 	int32		format_id;
 	bool		found;
 	const char *statfile = PGSTAT_STAT_PERMANENT_FILENAME;
 	PgStat_ShmemControl *shmem = pgStatLocal.shmem;
+	XLogRecPtr	file_redo;
 
 	/* shouldn't be called from postmaster */
 	Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
@@ -1550,6 +1555,22 @@ pgstat_read_statsfile(void)
 		goto error;
 	}
 
+	/*
+	 * Read the redo LSN stored in the file.
+	 */
+	if (!read_chunk_s(fpin, &file_redo))
+	{
+		elog(WARNING, "could not read redo LSN");
+		goto error;
+	}
+
+	if (file_redo != redo)
+	{
+		elog(WARNING, "found incorrect redo LSN %X/%X (expected %X/%X)",
+			 LSN_FORMAT_ARGS(file_redo), LSN_FORMAT_ARGS(redo));
+		goto error;
+	}
+
 	/*
 	 * We found an existing statistics file. Read it and put all the stats
 	 * data into place.
-- 
2.45.2