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From: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Date: Thu, 9 Apr 2026 14:37:44 -0700
Message-ID: 
 <CAHg+QDd4NK2zjW6hPmS+Xr9UUdy+DAtywHeQhvcG+BXBEX0nEA@mail.gmail.com>
Subject: Re: Bug: WAIT FOR LSN crashes with assertion failure inside PL/pgSQL
 DO blocks and procedures
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
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Hi Alexander,

On Thu, Apr 9, 2026 at 5:28=E2=80=AFAM Alexander Korotkov <aekorotkov@gmail=
.com>
wrote:

> On Thu, Apr 9, 2026 at 10:27=E2=80=AFAM SATYANARAYANA NARLAPURAM
> <satyanarlapuram@gmail.com> wrote:
> > On Wed, Apr 8, 2026 at 11:00=E2=80=AFPM Alexander Korotkov <aekorotkov@=
gmail.com>
> wrote:
> >> On Thu, Apr 9, 2026 at 5:03=E2=80=AFAM 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 vo=
id
> 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 =3D=3D 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 runni=
ng
> > from procedures.
>
> Thank you.  I've slightly revised your patch.  I'm going to push it if
> no objections.
>

LGTM, thank you!

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<div dir=3D"ltr"><div dir=3D"ltr">Hi Alexander,</div><br><div class=3D"gmai=
l_quote gmail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">On Thu=
, Apr 9, 2026 at 5:28=E2=80=AFAM Alexander Korotkov &lt;<a href=3D"mailto:a=
ekorotkov@gmail.com">aekorotkov@gmail.com</a>&gt; wrote:<br></div><blockquo=
te class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px =
solid rgb(204,204,204);padding-left:1ex">On Thu, Apr 9, 2026 at 10:27=E2=80=
=AFAM SATYANARAYANA NARLAPURAM<br>
&lt;<a href=3D"mailto:satyanarlapuram@gmail.com" target=3D"_blank">satyanar=
lapuram@gmail.com</a>&gt; wrote:<br>
&gt; On Wed, Apr 8, 2026 at 11:00=E2=80=AFPM Alexander Korotkov &lt;<a href=
=3D"mailto:aekorotkov@gmail.com" target=3D"_blank">aekorotkov@gmail.com</a>=
&gt; wrote:<br>
&gt;&gt; On Thu, Apr 9, 2026 at 5:03=E2=80=AFAM SATYANARAYANA NARLAPURAM<br=
>
&gt;&gt; &lt;<a href=3D"mailto:satyanarlapuram@gmail.com" target=3D"_blank"=
>satyanarlapuram@gmail.com</a>&gt; wrote:<br>
&gt;&gt; &gt; An assertion failure (server crash in assert-enabled builds) =
occurs when WAIT FOR LSN ... INTO is used inside PL/pgSQL DO blocks or with=
in void procedures.<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt; Repro:<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt; -- Run this on a standby<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt; CREATE PROCEDURE test_wait()<br>
&gt;&gt; &gt;=C2=A0 =C2=A0LANGUAGE plpgsql AS $$<br>
&gt;&gt; &gt;=C2=A0 =C2=A0DECLARE<br>
&gt;&gt; &gt;=C2=A0 =C2=A0 =C2=A0result text;<br>
&gt;&gt; &gt;=C2=A0 =C2=A0BEGIN<br>
&gt;&gt; &gt;=C2=A0 =C2=A0 =C2=A0WAIT FOR LSN &#39;0/1234&#39; INTO result;=
<br>
&gt;&gt; &gt;=C2=A0 =C2=A0 =C2=A0RAISE NOTICE &#39;%&#39;, result;<br>
&gt;&gt; &gt;=C2=A0 =C2=A0END;<br>
&gt;&gt; &gt;=C2=A0 =C2=A0$$;<br>
&gt;&gt; &gt;=C2=A0 =C2=A0CALL test_wait();<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt; The WAIT FOR itself succeeds, but the very next PL/pgSQL stat=
ement that requires a snapshot crashes the backend with:<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt;=C2=A0 =C2=A0TRAP: failed Assert(&quot;portal-&gt;portalSnapsh=
ot =3D=3D NULL&quot;),<br>
&gt;&gt; &gt;=C2=A0 =C2=A0File: &quot;pquery.c&quot;, Line: 1776<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt; Attached patches for both the test case and a potential fix. =
Please review.<br>
&gt;&gt;<br>
&gt;&gt; Thank you for reporting.=C2=A0 But I doubt the fix is correct.=C2=
=A0 Even that<br>
&gt;&gt; this particular might work OK, I don&#39;t think it&#39;s safe to =
release<br>
&gt;&gt; snapshots belonging to functions/procedures: it might affect them.=
=C2=A0 I<br>
&gt;&gt; tend to think we must forbid wrapping WAIT FOR LSN with<br>
&gt;&gt; functions/procedures.=C2=A0 I&#39;ll explore more on this today.<b=
r>
&gt;<br>
&gt;<br>
&gt; Agreed, attached a v2 patch with your suggestion on preventing it runn=
ing<br>
&gt; from procedures.<br>
<br>
Thank you.=C2=A0 I&#39;ve slightly revised your patch.=C2=A0 I&#39;m going =
to push it if<br>
no objections.<br></blockquote><div><br></div><div>LGTM, thank you!=C2=A0</=
div></div></div>

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