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

Hi Alexander,

On Thu, Apr 9, 2026 at 5:28 AM Alexander Korotkov <aekorotkov(at)gmail(dot)com>
wrote:

> On Thu, Apr 9, 2026 at 10:27 AM SATYANARAYANA NARLAPURAM
> <satyanarlapuram(at)gmail(dot)com> wrote:
> > On Wed, Apr 8, 2026 at 11:00 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com>
> wrote:
> >> On Thu, Apr 9, 2026 at 5:03 AM SATYANARAYANA NARLAPURAM
> >> <satyanarlapuram(at)gmail(dot)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.
>

LGTM, thank you!