Re: [HACKERS] make async slave to wait for lsn to be replayed

On Tue, Aug 6, 2024 at 11:18 AM Alexander Korotkov <aekorotkov(at)gmail(dot)com>
wrote:
> On Tue, Aug 6, 2024 at 8:36 AM Michael Paquier <michael(at)paquier(dot)xyz>
wrote:
> > On Tue, Aug 06, 2024 at 05:17:10AM +0300, Alexander Korotkov wrote:
> > > The 0001 patch is intended to improve this situation. Actually, it's
> > > not right to just put RecoveryInProgress() after
> > > GetXLogReplayRecPtr(), because more wal could be replayed between
> > > these calls. Instead we need to recheck GetXLogReplayRecPtr() after
> > > getting negative result of RecoveryInProgress() because WAL replay
> > > position couldn't get updated after.
> > > 0002 patch comprises fix for the header comment of
WaitLSNSetLatches() function
> > > 0003 patch comprises tests for pg_wal_replay_wait() errors.
> >
> > Before adding more tests, could it be possible to stabilize what's in
> > the tree? drongo has reported one failure with the recovery test
> > 043_wal_replay_wait.pl introduced recently by 3c5db1d6b016:
> >
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2024-08-05%2004%3A24%3A54
>
> Thank you for pointing!
> Surely, I'll fix this before.

Something breaks in these lines during second iteration of the loop.
"SELECT pg_current_wal_insert_lsn()" has been queried from primary, but
standby didn't receive "CALL pg_wal_replay_wait('...');"

for (my $i = 0; $i < 5; $i++)
{
print($i);
$node_primary->safe_psql('postgres',
"INSERT INTO wait_test VALUES (${i});");
my $lsn =
$node_primary->safe_psql('postgres',
"SELECT pg_current_wal_insert_lsn()");
$psql_sessions[$i] = $node_standby1->background_psql('postgres');
$psql_sessions[$i]->query_until(
qr/start/, qq[
\\echo start
CALL pg_wal_replay_wait('${lsn}');
SELECT log_count(${i});
]);
}

I wonder what could it be. Probably something hangs inside launching
background psql... I'll investigate this more.

------
Regards,
Alexander Korotkov
Supabase