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

On Thu, Nov 27, 2025 at 11:00 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Thu, Nov 27, 2025 at 4:33 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > A few more comments:
> >
> >
> > 10)
> > +# Promote standby3, increasing effective_wal_level to 'logical' as
> > its wal_level
> > +# is set to 'logical'.
> > +$standby3->promote;
> > +
> > +# Check if effective_wal_level is increased to 'logical' on the
> > cascaded standby.
> > +$standby3->wait_for_replay_catchup($cascade);
> > +test_wal_level($cascade, "replica|logical",
> > + "effective_wal_level got increased to 'logical' on standby as the
> > new primary has wal_level='logical'"
> > +);
> >
> > The message is slightly confusing due to usage of both 'standby' and
> > 'new primary'. Can we make it:
> > effective_wal_level got increased to 'logical' as the new primary has
> > wal_level='logical'
> >
>
> Upon reconsideration, we can keep it as is. I understand the intent now.

Okay.

>
> ===
>
> I tried to test possible race-scenarios (known to me), they seem to
> work well, except one minor thing:
>
> Let's say there is slot1 present, backend1 is trying to drop slot1 and
> backend2 is trying to create slot2.
>
> DisableLogicalDecodingIfNecessary() first kicks in and reaches the
> stage where it has disabled and released the lock. Before it could
> EmitSignal and log, EnsureLogicalDecodingEnabled() kicks in and
> completes its execution.
> In such a case we end up with reverse LOG messages in log file:
>
> 09:47:16.489 IST LOG: logical decoding is enabled upon creating a new
> logical replication slot
> 09:47:17.484 IST LOG: logical decoding is disabled because there are
> no valid logical replication slots
>
> while logical decoding is actually enabled in the system.
>
> Shall we check 'if (!LogicalDecodingCtl->xlog_logical_info)' before
> logging in DisableLogicalDecodingIfNecessary()?
>

In DisableLogicalDecodingIfNecessary(), we have (without comments):

if (!LogicalDecodingCtl->xlog_logical_info || CheckLogicalSlotExists())
{
LogicalDecodingCtl->pending_disable = false;
LWLockRelease(LogicalDecodingControlLock);
return;
}

START_CRIT_SECTION();

LogicalDecodingCtl->logical_decoding_enabled = false;
write_logical_decoding_status_update_record(false);
LogicalDecodingCtl->xlog_logical_info = false;
LogicalDecodingCtl->pending_disable = false;

LWLockRelease(LogicalDecodingControlLock);

END_CRIT_SECTION();

EmitProcSignalBarrier(PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO);

ereport(LOG,
errmsg("logical decoding is disabled because there are no
valid logical replication slots"));

Does it make sense to reorder them to the following?

START_CRIT_SECTION();

LogicalDecodingCtl->logical_decoding_enabled = false;
write_logical_decoding_status_update_record(false);
LogicalDecodingCtl->xlog_logical_info = false;
LogicalDecodingCtl->pending_disable = false;

END_CRIT_SECTION();

ereport(LOG,
errmsg("logical decoding is disabled because there are no
valid logical replication slots"));

LWLockRelease(LogicalDecodingControlLock);

Regards,

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