From 47995afe3af628c5f33eba2dbdbabd50f9f7a4d7 Mon Sep 17 00:00:00 2001
From: Fujii Masao <fujii@postgresql.org>
Date: Fri, 17 Oct 2025 12:52:18 +0900
Subject: [PATCH v1] Fix lag columns in pg_stat_replication not advancing when
 replay LSN stalls.

---
 src/backend/replication/walsender.c | 45 ++++++++++++++++++-----------
 1 file changed, 28 insertions(+), 17 deletions(-)

diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 59822f22b8d..d3ef136129f 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -233,6 +233,7 @@ typedef struct
 	int			write_head;
 	int			read_heads[NUM_SYNC_REP_WAIT_MODE];
 	WalTimeSample last_read[NUM_SYNC_REP_WAIT_MODE];
+	WalTimeSample overflowed[NUM_SYNC_REP_WAIT_MODE];
 } LagTracker;
 
 static LagTracker *lag_tracker;
@@ -4207,7 +4208,6 @@ WalSndKeepaliveIfNecessary(void)
 static void
 LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time)
 {
-	bool		buffer_full;
 	int			new_write_head;
 	int			i;
 
@@ -4229,25 +4229,19 @@ LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time)
 	 * of space.
 	 */
 	new_write_head = (lag_tracker->write_head + 1) % LAG_TRACKER_BUFFER_SIZE;
-	buffer_full = false;
 	for (i = 0; i < NUM_SYNC_REP_WAIT_MODE; ++i)
 	{
+		/*
+		 * If the buffer is full, move the slowest reader to a separate
+		 * overflow entry and free its space in the buffer so the write head
+		 * can advance.
+		 */
 		if (new_write_head == lag_tracker->read_heads[i])
-			buffer_full = true;
-	}
-
-	/*
-	 * If the buffer is full, for now we just rewind by one slot and overwrite
-	 * the last sample, as a simple (if somewhat uneven) way to lower the
-	 * sampling rate.  There may be better adaptive compaction algorithms.
-	 */
-	if (buffer_full)
-	{
-		new_write_head = lag_tracker->write_head;
-		if (lag_tracker->write_head > 0)
-			lag_tracker->write_head--;
-		else
-			lag_tracker->write_head = LAG_TRACKER_BUFFER_SIZE - 1;
+		{
+			lag_tracker->overflowed[i] =
+				lag_tracker->buffer[lag_tracker->read_heads[i]];
+			lag_tracker->read_heads[i] = -1;
+		}
 	}
 
 	/* Store a sample at the current write head position. */
@@ -4274,6 +4268,23 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
 {
 	TimestampTz time = 0;
 
+	/*
+	 * If 'lsn' has not passed the WAL position stored in the overflow entry,
+	 * return the elapsed time (in microseconds) since the saved local flush
+	 * time. Otherwise, resume using the buffer to control the read head and
+	 * compute the elapsed time.
+	 */
+	if (lag_tracker->read_heads[head] == -1)
+	{
+		if (lag_tracker->overflowed[head].lsn > lsn)
+			return now - lag_tracker->overflowed[head].time;
+
+		time = lag_tracker->overflowed[head].time;
+		lag_tracker->last_read[head] = lag_tracker->overflowed[head];
+		lag_tracker->read_heads[head] =
+			(lag_tracker->write_head + 1) % LAG_TRACKER_BUFFER_SIZE;
+	}
+
 	/* Read all unread samples up to this LSN or end of buffer. */
 	while (lag_tracker->read_heads[head] != lag_tracker->write_head &&
 		   lag_tracker->buffer[lag_tracker->read_heads[head]].lsn <= lsn)
-- 
2.50.1