Re: Bug: WAIT FOR LSN crashes with assertion failure inside PL/pgSQL DO blocks and procedures

On Thu, Apr 9, 2026 at 10:27 AM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
> On Wed, Apr 8, 2026 at 11:00 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
>> On Thu, Apr 9, 2026 at 5:03 AM SATYANARAYANA NARLAPURAM
>> <satyanarlapuram@gmail.com> wrote:
>> > An assertion failure (server crash in assert-enabled builds) occurs when WAIT FOR LSN ... INTO is used inside PL/pgSQL DO blocks or within void procedures.
>> >
>> > Repro:
>> >
>> > -- Run this on a standby
>> >
>> > CREATE PROCEDURE test_wait()
>> >   LANGUAGE plpgsql AS $$
>> >   DECLARE
>> >     result text;
>> >   BEGIN
>> >     WAIT FOR LSN '0/1234' INTO result;
>> >     RAISE NOTICE '%', result;
>> >   END;
>> >   $$;
>> >   CALL test_wait();
>> >
>> >
>> > The WAIT FOR itself succeeds, but the very next PL/pgSQL statement that requires a snapshot crashes the backend with:
>> >
>> >   TRAP: failed Assert("portal->portalSnapshot == NULL"),
>> >   File: "pquery.c", Line: 1776
>> >
>> > Attached patches for both the test case and a potential fix. Please review.
>>
>> Thank you for reporting.  But I doubt the fix is correct.  Even that
>> this particular might work OK, I don't think it's safe to release
>> snapshots belonging to functions/procedures: it might affect them.  I
>> tend to think we must forbid wrapping WAIT FOR LSN with
>> functions/procedures.  I'll explore more on this today.
>
>
> Agreed, attached a v2 patch with your suggestion on preventing it running
> from procedures.

Thank you.  I've slightly revised your patch.  I'm going to push it if
no objections.

------
Regards,
Alexander Korotkov
Supabase