Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

On Fri, Sep 26, 2025 at 12:46 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Thu, Sep 25, 2025 at 4:57 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Tue, Sep 23, 2025 at 3:28 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > >
> > > I've attached the updated patch. It incorporates all comments I got so
> > > far and implements to lazily disable logical decoding. It's used only
> > > when the process tries to disable logical decoding during process
> > > exit.
> > >
> >
> > I am resuming the review now. I agree with the discussion of lazily
> > disabling logical decoding on ERROR or process-exit for temp-slot.
> >
> > Few initial comments:
>
> Thank you for the comments!
>
> >
> > 1)
> > I see that on standby too, during proc-exit, we set 'pending_disable'.
> > But it never resets it, as DisableLogicalDecodingIfNecessary is no-op
> > on standby. And thus the checkpoint keeps on attempting to reset it
> > everytime. Do we even need to set it on standby?
> >
> > Logfile has repeated: 'start completing pending logical decoding
> > disable request'
>
> Ugh, I missed that part. I think that standbys should not delegate the
> deactivation to the checkpointer uless the deactivation is actually
> required.
>
> > 2)
> > + ereport(LOG,
> > + (errmsg("skip disabling logical decoding as during process exit")));
> >
> > 'as' not needed.
>
> I've fixed the above two points and attached the new version patch.
>

Thanks.

1)
Currently, in the existing implementation, if a promotion is in
progress (delay_status_change = true) and, during that time, a process
exits (causing a temporary slot to be released), then on the standby,
we may end up setting pending_disable. As a result, the checkpointer
will have to wait for the transition to complete before it can proceed
with disabling logical decoding (if needed).

a)
This means the checkpoint may be delayed further, depending on how
long it takes for all processes to respond to ProcSignalBarrier().

b)
Additionally, consider the case where the promotion fails midway
(after UpdateLogicalDecodingStatusEndOfRecovery). If the checkpointer
still sees RecoveryInProgress and delay_status_change as true, could
it end up waiting indefinitely for the transition to complete? In my
testing, when promotion fails and the startup process exits, it
usually causes the rest of the processes, including the checkpointer,
to terminate as well. So, it seems that a dangling pending_disable
state may not actually occur on standby in practice.

I believe scenario (b) can't really happen, but I still wanted to
check with you.

I am not sure if (a) is a real concern — what’s your take on it?

2)
As per discussion in [1], there was a proposal to implement lazily
disabling decoding both in ERROR and proc-exit scenarios. But I see it
only implemented in proc-exit scenario. Are we planning to do it for
ERROR as well?

[1]: https://www.postgresql.org/message-id/CAA4eK1JVNbb-OT1PO%3DiOFG1KA__Q83n8cLZoDjF2yA1rZyvCnA%40mail.gmail.com

thanks
Shveta