Re: Implement waiting for wal lsn replay: reloaded

Hi,

On Tue, May 19, 2026 at 1:00 PM Alexander Lakhin <exclusion(at)gmail(dot)com> wrote:
>
> Hello Alexander and Xuneng,
>
> 06.04.2026 22:49, Alexander Korotkov wrote:
>
> Thank you, I've pushed your version of patchset. I made two minor
> corrections for patch #2: mention default mode value in the header
> comment, and fallback to polling on has_wal_read_bug sparc64+ext4 bug.
>
>
> I discovered a new test failure, that is apparently caused by new
> wait_for_catchup() implementation [1]:
> [06:20:23.110](1.069s) not ok 8 - check that the slot state changes to "extended"
> [06:20:23.110](0.001s) # Failed test 'check that the slot state changes to "extended"'
> # at /Users/ec2-user/bf/goldfish/HEAD/pgsql/src/test/recovery/t/019_replslot_limit.pl line 140.
> [06:20:23.111](0.000s) # got: 'unreserved'
> # expected: 'extended'
> [06:20:23.231](0.120s) not ok 9 - check that the slot state changes to "unreserved"
> [06:20:23.231](0.000s) # Failed test 'check that the slot state changes to "unreserved"'
> # at /Users/ec2-user/bf/goldfish/HEAD/pgsql/src/test/recovery/t/019_replslot_limit.pl line 152.
> [06:20:23.231](0.000s) # got: 'lost|'
> # expected: 'unreserved|t'
>
> I've managed to reproduce such failures with:
> diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
> index 07eac07b9ce..493ce92674e 100644
> --- a/src/backend/replication/walreceiver.c
> +++ b/src/backend/replication/walreceiver.c
> @@ -1143,2 +1143,3 @@ XLogWalRcvSendReply(bool force, bool requestReply, bool checkApply)
>
> +pg_usleep(10000);
> /* Get current timestamp. */
> diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
> index 04aa770d981..19cda3a6b51 100644
> --- a/src/backend/replication/walsender.c
> +++ b/src/backend/replication/walsender.c
> @@ -2521,2 +2521,3 @@ ProcessStandbyReplyMessage(void)
>
> +pg_usleep(100000);
> /* the caller already consumed the msgtype byte */
>
> Concretely, a loop:
> for i in {1..100}; do echo "ITERATION $i"; PROVE_TESTS="t/019*" make -s check -C src/test/recovery/ || break; done
> failed for me on iterations 2, 1, 7:
> ITERATION 7
> # +++ tap check in src/test/recovery +++
> t/019_replslot_limit.pl .. 8/?
> # Failed test 'check that the slot state changes to "extended"'
> # at t/019_replslot_limit.pl line 140.
> # got: 'unreserved'
> # expected: 'extended'
> t/019_replslot_limit.pl .. 26/? # Looks like you failed 1 test of 26.
> t/019_replslot_limit.pl .. Dubious, test returned 1 (wstat 256, 0x100)
> Failed 1/26 subtests
>
> With "WAIT FOR LSN" in wait_for_catchup() disabled, 100 iterations
> passed.
>
> Having extra logging added, I could see the key difference.
> Failed run:
> 2026-05-19 22:01:37.968 EEST client backend[3632148] 019_replslot_limit.pl LOG: !!!GetWALAvailability| targetLSN: 0/016C0000, targetSeg: 22, oldestSlotSeg: 23, oldestSegMaxWalSize: 24, oldestSeg: 22
> 2026-05-19 22:01:37.968 EEST client backend[3632148] 019_replslot_limit.pl STATEMENT: SELECT wal_status FROM pg_replication_slots WHERE slot_name = 'rep1'
> vs
> Successful run:
> 2026-05-19 22:04:18.102 EEST client backend[3633761] 019_replslot_limit.pl LOG: !!!GetWALAvailability| targetLSN: 0/01700000, targetSeg: 23, oldestSlotSeg: 23, oldestSegMaxWalSize: 24, oldestSeg: 23
> 2026-05-19 22:04:18.102 EEST client backend[3633761] 019_replslot_limit.pl STATEMENT: SELECT wal_status FROM pg_replication_slots WHERE slot_name = 'rep1'
>
> That is, with WAIT FOR LSN, primary in this test may advance
> slot->data.restart_lsn to the expected position after wait_for_catchup()
> returns.
>
> [1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=goldfish&dt=2026-05-13%2006%3A15%3A03

Thanks for reporting this issue.

I think this is related to the semantic change made earlier:
wait_for_catchup() now returns once the standby itself has reached the
target LSN, rather than waiting until the primary observes that
position via pg_stat_replication.

As a result, the primary may not yet have processed the standby
feedback needed to advance the slot's restart_lsn when
wait_for_catchup() returns.

Actually, I was aware of this semantic change and previously thought
it might be harmless. But this failure appears to disprove that. I'll
prepare a patch to fix this shortly.

--
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.