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

Hi, Satya!

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.

------
Regards,
Alexander Korotkov
Supabase