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

Hi Satya, Alexander,

On Thu, Apr 9, 2026 at 2:38 PM Xuneng Zhou <xunengzhou(at)gmail(dot)com> wrote:
>
> On Thu, Apr 9, 2026 at 2:00 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
> >
> > Hi, Satya!
> >
> > 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.
> >

Opus, sorry for clicking send incidentally before typing anything...

I had looked at these patches before and didn’t see anything
particularly wrong except:
1. patch 1 unconditionally nulled ActivePortal->portalSnapshot
whenever it was non-NULL after the pop;
2. patch 2 used RAISE NOTICE after WAIT FOR, which seems not excercise
the bug straightforwardly.

I didn't realized the safety implications of releasing a procedure/DO
snapshot during PL execution. I’ve noticed several warnings in the
tree advising against this.

/*
* Ensure there's an active snapshot whilst we execute whatever's
* involved here. Note that this is *not* sufficient to make the
* world safe for TOAST pointers to be included in the returned data:
* the referenced data could have gone away while we didn't hold a
* snapshot. Hence, it's incumbent on PLs that can do COMMIT/ROLLBACK
* to not return TOAST pointers, unless those pointers were fetched
* after the last COMMIT/ROLLBACK in the procedure.
*
* XXX that is a really nasty, hard-to-test requirement. Is there a
* way to remove it?
*/
EnsurePortalSnapshotExists();

--
Best,
Xuneng