Use replay LSN as target for cascading logical WAL senders

michaelpq · michaelpq · commit 87be749c716a · 2025-06-02T12:04:04.000+09:00
A cascading WAL sender doing logical decoding (as known as doing its work on a standby) has been using as flush LSN the value returned by GetStandbyFlushRecPtr() (last position safely flushed to disk). This is incorrect as such processes are only able to decode changes up to the LSN that has been replayed by the startup process. This commit changes cascading logical WAL senders to use the replay LSN, as returned by GetXLogReplayRecPtr(). This distinction is important particularly during shutdown, when WAL senders need to send any remaining available data to their clients, switching WAL senders to a caught-up state. Using the latest flush LSN rather than the replay LSN could cause the WAL senders to be stuck in an infinite loop preventing them to shut down, as the startup process does not run when WAL senders attempt to catch up, so they could keep waiting for work that would never happen. Backpatch down to v16, where logical decoding on standbys has been introduced. Author: Alexey Makhmutov <a.makhmutov@postgrespro.ru> Reviewed-by: Ajin Cherian <itsajin@gmail.com> Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com> Reviewed-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/52138028-7246-421c-9161-4fa108b88070@postgrespro.ru Backpatch-through: 16
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
@@ -3450,8 +3450,16 @@ XLogSendLogical(void)
 	if (flushPtr == InvalidXLogRecPtr ||
 		logical_decoding_ctx->reader->EndRecPtr >= flushPtr)
 	{
+		/*
+		 * For cascading logical WAL senders, we use the replay LSN instead of
+		 * the flush LSN, since logical decoding on a standby only processes
+		 * WAL that has been replayed.  This distinction becomes particularly
+		 * important during shutdown, as new WAL is no longer replayed and the
+		 * last replayed LSN marks the furthest point up to which decoding can
+		 * proceed.
+		 */
 		if (am_cascading_walsender)
-			flushPtr = GetStandbyFlushRecPtr(NULL);
+			flushPtr = GetXLogReplayRecPtr(NULL);
 		else
 			flushPtr = GetFlushRecPtr(NULL);
 	}
