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

On Wed, Nov 12, 2025 at 3:42 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Wed, Nov 12, 2025 at 3:36 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Tue, Nov 11, 2025 at 6:05 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Nov 10, 2025 at 8:05 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > >
> > > > On Thu, Nov 6, 2025 at 4:32 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > >
> > > > >
> > > > > I've updated and rebased the patch.
> > > > >
> > > >
> > > > Thanks for the patch. Please find a few comments:
> > > >
> > > >
> > > > 1)
> > > > ReplicationSlotsDropDBSlots:
> > > >
> > > > + SpinLockAcquire(&s->mutex);
> > > > + invalidated = s->data.invalidated == RS_INVAL_NONE;
> > > > + SpinLockRelease(&s->mutex);
> > > > +
> > > > + /*
> > > > + * Count slots on other databases too so we can disable logical
> > > > + * decoding only if no slots in the cluster.
> > > > + */
> > > > + if (invalidated)
> > > > + n_valid_logicalslots++;
> > > >
> > > >
> > > > This seems confusing to me. Can we instead do:
> > > >
> > > > SpinLockAcquire(&s->mutex);
> > > > if (s->data.invalidated == RS_INVAL_NONE)
> > > > n_valid_logicalslots++;
> > > > SpinLockRelease(&s->mutex);
> > > >
> > > > 2)
> > > > InvalidateObsoleteReplicationSlots:
> > > >
> > > > + bool islogical = SlotIsLogical(s);
> > > >
> > > > /* Prevent invalidation of logical slots during binary upgrade */
> > > > if (SlotIsLogical(s) && IsBinaryUpgrade)
> > > > + {
> > > > + SpinLockAcquire(&s->mutex);
> > > > + if (s->data.invalidated == RS_INVAL_NONE)
> > > > + n_valid_logicalslots++;
> > > > + SpinLockRelease(&s->mutex);
> > > > +
> > > > continue;
> > > > + }
> > > >
> > > > We should use 'islogical' instead of SlotIsLogical here.
> > > >
> > > > 3)
> > > > InvalidateObsoleteReplicationSlots() is more robust now as we are
> > > > using both 'invalidated' and 'released_lock' flags but still nowhere
> > > > we guarantee that invalidated=true implies released_lock=true. Since
> > > > we jump to 'restart' label only if released_lock is true, it becomes
> > > > important to have an ASSERT which says invalidated=true implicitly
> > > > means released_lock=true or vice versa. Because at the end we go by
> > > > 'invalidated_logical' rather than 'released_lock' to decide about
> > > > logical-decoding disabling.
> > > >
> > > > In this logic:
> > > >
> > > > + if (InvalidatePossiblyObsoleteSlot(possible_causes, s, oldestLSN,
> > > > + dboid, snapshotConflictHorizon,
> > > > + &released_lock))
> > > > {
> > > > - /* if the lock was released, start from scratch */
> > > > - goto restart;
> > > > + /* Remember we have invalidated a physical or logical slot */
> > > > + invalidated = true;
> > > > +
> > > > + /*
> > > > + * Additionally, remember we have invalidated a logical slot too
> > > > + * as we can request disabling logical decoding later.
> > > > + */
> > > > + if (islogical)
> > > > + invalidated_logical = true;
> > > > }
> > > >
> > > > Shall we have an Assert(released_lock) if
> > > > InvalidatePossiblyObsoleteSlot returns true. Or any better way?
> > > >
> > > > 4)
> > > > + SpinLockAcquire(&s->mutex);
> > > > + if (s->data.invalidated == RS_INVAL_NONE)
> > > > + n_valid_logicalslots++;
> > > >
> > > > In the same function, isn't the above code problematic: Don't we need
> > > > 'islogical' check before incrementing 'n_valid_logicalslots',
> > > > otherwise it may wrongly count valid physical slots as well.
> > >
> > > Agreed with all the above points. Will fix and update the updated version.
> > >
> >
> > I've attached the updated version patch. I addressed all comments I
> > got so far, and made some cosmetic changes.
> >
>
> Thanks. A few comments:
>
> 1)
> Shall we update comments atop InvalidateObsoleteReplicationSlots() as
> well, similar to other functions. Something like:
>
> If it invalidates the last logical slot in the cluster, it requests to
> disable logical decoding.

Okay, added.

>
> 2)
> With the new sanity check (Assert(released_lock)) in
> InvalidateObsoleteReplicationSlots, we have made sure that whenever a
> slot is invalidated, we do release-lock. But we have not made sure
> that released_lock=true always implies a slot is invalidated. Looking
> at InvalidatePossiblyObsoleteSlot(), that seems to be the case always,
> but shall we have a sanity check in for this as well. Thoughts?
>

I think it's possible that InvalidatePossiblyObsoleteSlot() releases
the slot but doesn't invalidate it. For example, after it terminates
the process owning the slot, the slot gets dropped or its restart_lsn
(or xmin) gets advanced enough not to be invalidated. That's why we
have a separate bool variable releaesd_lock from the bool returned
from the function.

>
> 3)
> Regarding this new comment:
> + * deactivation. We cannot rely on the end-of-recovery to allow toggling
> + * of the logical decoding status because it's possible that concurrent
> + * processes write non-logical WAL records after the startup process
> + * enables logical decoding.
>
> Do we really want to say 'to allow toggling' here or 'to restrict
> toggling'. Let me know if I have misunderstood.

Will adjust the wordings. Thanks!

Regards,

--
Masahiko Sawada
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