Re: Implement waiting for wal lsn replay: reloaded

On Tue, May 19, 2026 at 8:30 PM Xuneng Zhou <xunengzhou(at)gmail(dot)com> wrote:
>
> 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.

After some consideration, 019_replslot_limit.pl appears to the better
place to place the fix rather than by restoring the old primary-side
polling barrier in wait_for_catchup().

The new wait_for_catchup() behavior is closer to its natural
semantics: for replay/write/flush modes, it waits until the standby
itself has reached the requested LSN. The old implementation used
pg_stat_replication on the primary, which also implied that the
primary had received and processed standby feedback. That was a useful
side effect for this test, but it is not required by most callers.

019_replslot_limit.pl is different because it checks primary-side slot
state. For a physical slot, restart_lsn advances only after the
primary's walsender processes standby feedback. So the test needs an
extra condition beyond ordinary standby catchup.

The patch makes that dependency explicit: wait for the standby to
replay the target LSN, then wait for the slot's restart_lsn on the
primary to pass the same LSN. This keeps wait_for_catchup() focused on
standby catchup while making the slot-specific synchronization visible
in the test that needs it.

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