Subject: [PATCH v1] Minimal fix for WAIT FOR ... MODE 'standby_flush'

From: Andres Freund <[email-masked]>
Date: Tue, 7 Apr 2026 09:11:07 -0400

The investigation into the negative test performance impact of 7e8aeb9e483
lead to discovering that there are a few issues with WAIT FOR.

This commit is just a minimal fix to prevent hangs in standby_flush mode, due
to WAIT FOR ... 'standby_flush' seeing a 0 LSN if a newly started walreceiver
does not receive any writes, because the stanby is already caught up.

There are several other issues and this is isn't necessarily the best fix. But
this way we get the hangs out of the way.

Reported-by: Tom Lane <[email-masked]>
Discussion: https://postgr.es/m/zqbppucpmkeqecfy4s5kscnru4tbk6khp3ozqz6ad2zijz354k@w4bdf4z3wqoz
---
 src/backend/replication/walreceiver.c      | 2 --
 src/backend/replication/walreceiverfuncs.c | 1 +
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index a437273cf9a..09fde92bfd7 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -242,8 +242,6 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
 
 	SpinLockRelease(&walrcv->mutex);
 
-	pg_atomic_write_u64(&WalRcv->writtenUpto, 0);
-
 	/* Arrange to clean up at walreceiver exit */
 	on_shmem_exit(WalRcvDie, PointerGetDatum(&startpointTLI));
 
diff --git a/src/backend/replication/walreceiverfuncs.c b/src/backend/replication/walreceiverfuncs.c
index 4e03e721872..bd5d47be964 100644
--- a/src/backend/replication/walreceiverfuncs.c
+++ b/src/backend/replication/walreceiverfuncs.c
@@ -321,6 +321,7 @@ RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo,
 		walrcv->flushedUpto = recptr;
 		walrcv->receivedTLI = tli;
 		walrcv->latestChunkStart = recptr;
+		pg_atomic_write_u64(&walrcv->writtenUpto, recptr);
 	}
 	walrcv->receiveStart = recptr;
 	walrcv->receiveStartTLI = tli;
-- 
2.53.0.1.gb2826b52eb

--ags3ltqh73mz7p2n--