Re: Implement waiting for wal lsn replay: reloaded

On Thu, Jan 8, 2026 at 6:29 PM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> On Thu, Jan 8, 2026 at 10:19 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> > I see, you were right.  This is not related to the MyProc->xmin.
> > ResolveRecoveryConflictWithTablespace() calls
> > GetConflictingVirtualXIDs(InvalidTransactionId, InvalidOid).  That
> > would kill WAIT FOR LSN query independently on its xmin.
>
> I think the concern is valid --- conflicts like
> PROCSIG_RECOVERY_CONFLICT_SNAPSHOT could occur and terminate the
> backend if the timing is unlucky. It's more difficult to reproduce
> though. A check for the log containing "conflict with recovery" would
> likely catch these conflicts as well.

Yes, I found multiple reasons why xmin gets temporarily set during
processing of WAIT FOR LSN query.  I'll soon post a draft patch to fix
that.

> > I guess your
> > patch is the only way to go.  It's clumsy to wrap WAIT FOR LSN call
> > with retry loop, but it would still consume less resources than
> > polling.
> >
>
> Assuming recovery conflicts are relatively rare in tap tests, except
> for the explicitly designed tests like 031_recovery_conflict and the
> narrow timing window that the standby has not caught up while the wait
> for gets invoked, a simple fallback seems appropriate to me.

Yes, I see.  Seems acceptable given this seems the only feasible way to go.

------
Regards,
Alexander Korotkov
Supabase