Re: Fix assert failure when decoding XLOG_PARAMETER_CHANGE on primary

On Tue, Feb 4, 2025 at 10:20 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Tue, Feb 4, 2025 at 12:59 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Sun, Feb 2, 2025 at 8:11 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Jan 24, 2025 at 4:05 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > >
> > > > When a standby replays a XLOG_PARAMETER_CHANGE record that lowers
> > > > wal_level from logical, we invalidate all logical slots only when the
> > > > standby is in hot standby mode:
> > > >
> > > > if (InRecovery && InHotStandby &&
> > > > xlrec.wal_level < WAL_LEVEL_LOGICAL &&
> > > > wal_level >= WAL_LEVEL_LOGICAL)
> > > > InvalidateObsoleteReplicationSlots(RS_INVAL_WAL_LEVEL,
> > > > 0, InvalidOid,
> > > > InvalidTransactionId);
> > > >
> > > > However, it's possible that this record is replayed when not in hot
> > > > standby mode and the slot is used after the promotion. In this case,
> > > > the following Assert in xlog_decode() fails:
> > > >
> > > > /*
> > > > * This can occur only on a standby, as a primary would
> > > > * not allow to restart after changing wal_level < logical
> > > > * if there is pre-existing logical slot.
> > > > */
> > >
> > > Shouldn't we do similar to what this comment indicates on standby? We
> > > can disallow to start the server as standby, if the hot_standby is off
> > > and there is a pre-existing logical slot.
> >
> > It seems like a better idea. I thought we could pass StandbyMode to
> > StartupReplicationSlots() and check if there is a pre-existing logical
> > slot, but it would break the ABI compatibility. It might not be a
> > problem in practice as StartupReplicationSlots() is normally used only
> > by the startup process. But if we want to avoid that we can introduce
> > a new function for that.
>
> Since StandbyMode is exposed, we don't need to change the function
> signature. I'll update and submit the patch soon.
>

I've attached the updated patch. The fix needs to be back-patched to
v16 where logical decoding on standby was introduced.

Regards,

--
Masahiko Sawada
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