Re: [HACKERS] make async slave to wait for lsn to be replayed - Mailing list pgsql-hackers

From Alexander Korotkov
Subject Re: [HACKERS] make async slave to wait for lsn to be replayed
Date
Msg-id CAPpHfduYkve0sw-qy4aCCmJv_MXfuuAQ7wyRQsX8NjaLVKDE1Q@mail.gmail.com
Whole thread Raw
In response to Re: [HACKERS] make async slave to wait for lsn to be replayed  (Alexander Korotkov <aekorotkov@gmail.com>)
List pgsql-hackers
On Tue, Aug 6, 2024 at 11:18 AM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> On Tue, Aug 6, 2024 at 8:36 AM Michael Paquier <michael@paquier.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

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