Re: Assertion failure in SnapBuildInitialSnapshot()

On Sat, Jan 28, 2023 at 11:54 PM Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> Dear Amit, Sawada-san,
>
> I have also reproduced the failure on PG15 with some debug log, and I agreed that
> somebody changed procArray->replication_slot_xmin to InvalidTransactionId.
>
> > > The same assertion failure has been reported on another thread[1].
> > > Since I could reproduce this issue several times in my environment
> > > I've investigated the root cause.
> > >
> > > I think there is a race condition of updating
> > > procArray->replication_slot_xmin by CreateInitDecodingContext() and
> > > LogicalConfirmReceivedLocation().
> > >
> > > What I observed in the test was that a walsender process called:
> > > SnapBuildProcessRunningXacts()
> > > LogicalIncreaseXminForSlot()
> > > LogicalConfirmReceivedLocation()
> > > ReplicationSlotsComputeRequiredXmin(false).
> > >
> > > In ReplicationSlotsComputeRequiredXmin() it acquired the
> > > ReplicationSlotControlLock and got 0 as the minimum xmin since there
> > > was no wal sender having effective_xmin.
> > >
> >
> > What about the current walsender process which is processing
> > running_xacts via SnapBuildProcessRunningXacts()? Isn't that walsender
> > slot's effective_xmin have a non-zero value? If not, then why?
>
> Normal walsenders which are not for tablesync create a replication slot with
> NOEXPORT_SNAPSHOT option. I think in this case, CreateInitDecodingContext() is
> called with need_full_snapshot = false, and slot->effective_xmin is not updated.

Right. This is how we create a slot used by an apply worker.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com