Re: [PATCH] Release replication slot on error in SQL-callable slot functions

Hi Shveta,

On Tue, May 26, 2026 at 8:54 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:

> On Tue, May 26, 2026 at 1:41 PM SATYANARAYANA NARLAPURAM
> <satyanarlapuram(at)gmail(dot)com> wrote:
> >
> > Hi,
> >
> > On Mon, May 25, 2026 at 10:50 PM shveta malik <shveta(dot)malik(at)gmail(dot)com>
> wrote:
> >>
> >> On Tue, May 26, 2026 at 10:06 AM shveta malik <shveta(dot)malik(at)gmail(dot)com>
> wrote:
> >> >
> >> > On Tue, May 26, 2026 at 12:31 AM SATYANARAYANA NARLAPURAM
> >> > <satyanarlapuram(at)gmail(dot)com> wrote:
> >> > >
> >> > > Hi,
> >> > >
> >> > > On Mon, May 25, 2026 at 2:58 AM shveta malik <
> shveta(dot)malik(at)gmail(dot)com> wrote:
> >> > >>
> >> > >> On Mon, May 25, 2026 at 12:42 PM SATYANARAYANA NARLAPURAM
> >> > >> <satyanarlapuram(at)gmail(dot)com> wrote:
> >> > >> >
> >> > >> > Hi
> >> > >> >
> >> > >> > On Fri, May 22, 2026 at 2:16 AM shveta malik <
> shveta(dot)malik(at)gmail(dot)com> wrote:
> >> > >> >>
> >> > >> >> Thanks for reporting the issue. I could reproduce the same
> issue with
> >> > >> >> all these as well:
> >> > >> >>
> >> > >> >> pg_logical_slot_peek_changes
> >> > >> >> pg_logical_slot_get_binary_changes
> >> > >> >> pg_logical_slot_peek_binary_changes
> >> > >> >
> >> > >> >
> >> > >> > Please find the attached v2 patch that addressed these three
> cases as well.
> >> > >> >
> >> > >>
> >> > >> Thank You for addressuing these cases. A few comments:
> >> > >>
> >> > >> 1)
> >> > >>
> >> > >> +-- Test 2: session remains usable after the error
> (MyReplicationSlot cleared)
> >> > >>
> >> > >> It shoudl be part of 'Test 1' itself and thus should not be named
> as 'Test 2'
> >> > >>
> >> > >> 2)
> >> > >> --------
> >> > >> +-- Test 4: copy_replication_slot with max_replication_slots
> exceeded.
> >> > >> +-- We reduce max_replication_slots artificially by filling all
> remaining slots.
> >> > >> +-- Instead, trigger an error by copying to an already-existing
> name.
> >> > >> +DO $$
> >> > >> +BEGIN
> >> > >> + PERFORM pg_copy_logical_replication_slot('regression_slot_t3',
> >> > >> 'regression_slot_t3');
> >> > >> +EXCEPTION WHEN OTHERS THEN
> >> > >> + RAISE NOTICE 'caught: %', SQLERRM;
> >> > >> +END;
> >> > >> +$$;
> >> > >> +-- The original slot must still exist and be usable
> >> > >> +SELECT count(*) = 1 AS orig_slot_ok FROM pg_replication_slots
> >> > >> + WHERE slot_name = 'regression_slot_t3';
> >> > >> -----------
> >> > >>
> >> > >> I don't think we can hit the Assert with above test (at-least I
> could
> >> > >> not). Since creation of slot itself will fail as the slot with
> >> > >> same-name already exists, MyReplicationSlot will never be set and
> thus
> >> > >> Assert will not be hit. A better testcase will be below which
> fails
> >> > >> during LoadOutputPlugin() after slot-creation and
> MyReplicationSlot is
> >> > >> set already.
> >> > >>
> >> > >> SELECT pg_create_logical_replication_slot('src_slot',
> 'test_decoding');
> >> > >>
> >> > >> DO $$
> >> > >> BEGIN
> >> > >> PERFORM pg_copy_logical_replication_slot('src_slot', 'dst_slot',
> >> > >> false, 'nonexistent_plugin');
> >> > >> EXCEPTION WHEN others THEN
> >> > >> RAISE NOTICE 'caught: %', SQLERRM;
> >> > >> END $$;
> >> > >>
> >> > >> SELECT count(*) FROM pg_logical_slot_get_changes('src_slot', NULL,
> NULL);
> >> > >>
> >> > >> 3)
> >> > >> So overall these are the problematic APIs:
> >> > >>
> >> > >> pg_create_logical_replication_slot
> >> > >> pg_replication_slot_advance
> >> > >> pg_copy_logical_replication_slot
> >> > >> pg_logical_slot_peek_binary_changes
> >> > >> pg_logical_slot_peek_changes
> >> > >> pg_logical_slot_get_changes
> >> > >> pg_logical_slot_get_binary_changes
> >> > >>
> >> > >> First 3 are are mutually exclusive fixes fow which we have added
> >> > >> testcases. Last 4 are addressed by fixing common function
> >> > >> pg_logical_slot_get_changes_guts(). I think we should add a test
> case
> >> > >> for at-least any one of these APIs to cover
> >> > >> pg_logical_slot_get_changes_guts().
> >> > >
> >> > >
> >> > > Thanks for reviewing. Please review the attached v3 patch.
> >> > >
> >> >
> >> > A few trivial things:
> >> >
> >> > 1)
> >> > pg_replication_slot_advance:
> >> > + PG_TRY();
> >> > + {
> >> > + /* Acquire the slot so we "own" it */
> >> > + ReplicationSlotAcquire(NameStr(*slotname), true, true);
> >> > + /* A slot whose restart_lsn has never been reserved cannot be
> advanced */
> >> > + if (!XLogRecPtrIsValid(MyReplicationSlot->data.restart_lsn))
> >> >
> >> >
> >> > We can have a blank line after ReplicationSlotAcquire for better
> readability.
> >> >
> >> > 2)
> >> >
> >> > +SELECT 'init' FROM
> >> > pg_create_logical_replication_slot('regression_slot_t3',
> >> > 'test_decoding', true);
> >> > +SELECT count(*) = 1 AS slot_exists FROM pg_replication_slots
> >> > + WHERE slot_name = 'regression_slot_t3';
> >> >
> >> > The intent is not clear why are we checking existence of
> >> > regression_slot_t3? I think we can skip it (or else add a comment if
> >> > really needed). The success of previous
> >> > pg_create_logical_replication_slot is enough to confirm that session
> >> > is healthy to run other slot related queries.
> >> >
> >> > 3)
> >> > +SELECT pg_drop_replication_slot('regression_slot_phy');
> >> > +
> >> > +-- cleanup
> >> > +SELECT pg_drop_replication_slot('regression_slot_t3');
> >> >
> >> > We can move drop of 'regression_slot_phy' too under '-- cleanup'
> >> >
> >> > ~~
> >> >
> >> > I have no further comments other than the trivial things mentioned
> above.
> >> >
> >>
> >> Missed to inform this earlier, I am not able to apply any version of
> >> the patches shared so far with 'git am'. It gives error, 'patch -p1'
> >> works.
> >>
> >> git am
> v3-0001-Release-replication-slot-on-error-in-slot-SQL-functions.patch
> >> Patch format detection failed.
> >
> > Thanks , Shveta! Please find the attached v4 patch that addressed your
> comments.
> >
>
> Thank You for the patch.
> I noticed that we are creating regression_slot_t3 as a a temporary
> slot, is that intentional? I think creating a permanent slot here will
> be a better testcase.

No specific reason to use temp slot, ok to create a permanent slot too.

>
> I have made a few cosmetic changes for better readability along with
> creating the 'permanent' regression_slot_t3 slot. Please incorporate
> what you think is okay. I have no more comments.

Thank you for the changes and review.

Thanks,
Satya