Re: Synchronizing slots from primary to standby

On Tue, Apr 2, 2024 at 1:54 PM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> On Tue, Apr 02, 2024 at 07:20:46AM +0000, Zhijie Hou (Fujitsu) wrote:
> > I added one test in 040_standby_failover_slots_sync.pl in 0002 patch, which can
> > reproduce the data loss issue consistently on my machine.
>
> Thanks!
>
> > It may not reproduce
> > in some rare cases if concurrent xl_running_xacts are written by bgwriter, but
> > I think it's still valuable if it can verify the fix in most cases.
>
> What about adding a "wait" injection point in LogStandbySnapshot() to prevent
> checkpointer/bgwriter to log a standby snapshot? Something among those lines:
>
>        if (AmCheckpointerProcess() || AmBackgroundWriterProcess())
>                INJECTION_POINT("bgw-log-standby-snapshot");
>
> And make use of it in the test, something like:
>
>        $node_primary->safe_psql('postgres',
>                "SELECT injection_points_attach('bgw-log-standby-snapshot', 'wait');");
>

Sometimes we want the checkpoint to log the standby snapshot as we
need it at a predictable time, maybe one can use
pg_log_standby_snapshot() instead of that. Can we add an injection
point as a separate patch/commit after a bit more discussion? I want
to discuss this in a separate thread so that later we should not get
an objection to adding an injection_point at this location. One other
idea to make such tests predictable is to add a developer-specific GUC
say debug_bg_log_standby_snapshot or something like that but injection
point sounds like a better idea.

--
With Regards,
Amit Kapila.