From 7b5b8e53026ecdcdccc07029a38d35e8d13985fd Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Wed, 27 Dec 2023 16:41:47 -0500
Subject: [PATCH v1 3/5] Add LSNTimeline to PgStat_WalStats

Add a globally maintained instance of the new LSNTimeline to
PgStat_WalStats and add utility functions for maintaining and accessing
it. This commit does not insert new values to the timeline or use the
helpers to access it.
---
 src/backend/utils/activity/pgstat_wal.c | 48 +++++++++++++++++++++----
 src/include/pgstat.h                    |  6 ++++
 2 files changed, 48 insertions(+), 6 deletions(-)

diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index ba40aad258..594185acb9 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -36,10 +36,10 @@ static WalUsage prevWalUsage;
 
 
 static void lsntime_absorb(LSNTime *a, const LSNTime *b);
-void lsntime_insert(LSNTimeline *timeline, TimestampTz time, XLogRecPtr lsn);
+static void lsntime_insert(LSNTimeline *timeline, TimestampTz time, XLogRecPtr lsn);
 
-XLogRecPtr estimate_lsn_at_time(const LSNTimeline *timeline, TimestampTz time);
-TimestampTz estimate_time_at_lsn(const LSNTimeline *timeline, XLogRecPtr lsn);
+static XLogRecPtr estimate_lsn_at_time(const LSNTimeline *timeline, TimestampTz time);
+static TimestampTz estimate_time_at_lsn(const LSNTimeline *timeline, XLogRecPtr lsn);
 
 /*
  * Calculate how much WAL usage counters have increased and update
@@ -222,7 +222,7 @@ lsntime_absorb(LSNTime *a, const LSNTime *b)
  * Insert a new LSNTime into the LSNTimeline in the first element with spare
  * capacity.
  */
-void
+static void
 lsntime_insert(LSNTimeline *timeline, TimestampTz time,
 			   XLogRecPtr lsn)
 {
@@ -284,7 +284,7 @@ lsntime_insert(LSNTimeline *timeline, TimestampTz time,
  * Translate time to a LSN using the provided timeline. The timeline will not
  * be modified.
  */
-XLogRecPtr
+static XLogRecPtr
 estimate_lsn_at_time(const LSNTimeline *timeline, TimestampTz time)
 {
 	TimestampTz time_elapsed;
@@ -336,7 +336,7 @@ estimate_lsn_at_time(const LSNTimeline *timeline, TimestampTz time)
  * Translate lsn to a time using the provided timeline. The timeline will not
  * be modified.
  */
-TimestampTz
+static TimestampTz
 estimate_time_at_lsn(const LSNTimeline *timeline, XLogRecPtr lsn)
 {
 	TimestampTz time_elapsed;
@@ -383,3 +383,39 @@ estimate_time_at_lsn(const LSNTimeline *timeline, XLogRecPtr lsn)
 		return 0;
 	return result;
 }
+
+XLogRecPtr
+pgstat_wal_estimate_lsn_at_time(TimestampTz time)
+{
+	XLogRecPtr	result;
+	PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	result = estimate_lsn_at_time(&stats_shmem->stats.timeline, time);
+	LWLockRelease(&stats_shmem->lock);
+
+	return result;
+}
+
+TimestampTz
+pgstat_wal_estimate_time_at_lsn(XLogRecPtr lsn)
+{
+	TimestampTz result;
+	PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	result = estimate_time_at_lsn(&stats_shmem->stats.timeline, lsn);
+	LWLockRelease(&stats_shmem->lock);
+
+	return result;
+}
+
+void
+pgstat_wal_update_lsntimeline(TimestampTz time, XLogRecPtr lsn)
+{
+	PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	lsntime_insert(&stats_shmem->stats.timeline, time, lsn);
+	LWLockRelease(&stats_shmem->lock);
+}
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index ddbe320bf3..dd914e606e 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -472,6 +472,7 @@ typedef struct PgStat_WalStats
 	PgStat_Counter wal_sync;
 	PgStat_Counter wal_write_time;
 	PgStat_Counter wal_sync_time;
+	LSNTimeline timeline;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
@@ -754,6 +755,11 @@ extern void pgstat_execute_transactional_drops(int ndrops, struct xl_xact_stats_
 extern void pgstat_report_wal(bool force);
 extern PgStat_WalStats *pgstat_fetch_stat_wal(void);
 
+/* Helpers for maintaining the LSNTimeline */
+extern XLogRecPtr pgstat_wal_estimate_lsn_at_time(TimestampTz time);
+extern TimestampTz pgstat_wal_estimate_time_at_lsn(XLogRecPtr lsn);
+extern void pgstat_wal_update_lsntimeline(TimestampTz time, XLogRecPtr lsn);
+
 
 /*
  * Variables in pgstat.c
-- 
2.37.2