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From: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Date: Thu, 9 Apr 2026 00:26:54 -0700
Message-ID: 
 <CAHg+QDevycEwWfTMJ6ADZQJR9YSMcsAf=Dg=vH2R8Efcx1NH1w@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 Alexnader,

On Wed, Apr 8, 2026 at 11:00=E2=80=AFPM Alexander Korotkov <aekorotkov@gmai=
l.com>
wrote:

> Hi, Satya!
>
> 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 whe=
n
> 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 =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 running
from procedures.

Thanks,
Satya

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<div dir=3D"ltr"><div dir=3D"ltr">Hi Alexnader,</div><br><div class=3D"gmai=
l_quote gmail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">On Wed=
, Apr 8, 2026 at 11:00=E2=80=AFPM Alexander Korotkov &lt;<a href=3D"mailto:=
aekorotkov@gmail.com">aekorotkov@gmail.com</a>&gt; wrote:<br></div><blockqu=
ote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px=
 solid rgb(204,204,204);padding-left:1ex">Hi, Satya!<br>
<br>
On Thu, Apr 9, 2026 at 5:03=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; An assertion failure (server crash in assert-enabled builds) occurs wh=
en WAIT FOR LSN ... INTO is used inside PL/pgSQL DO blocks or within void p=
rocedures.<br>
&gt;<br>
&gt; Repro:<br>
&gt;<br>
&gt; -- Run this on a standby<br>
&gt;<br>
&gt; CREATE PROCEDURE test_wait()<br>
&gt;=C2=A0 =C2=A0LANGUAGE plpgsql AS $$<br>
&gt;=C2=A0 =C2=A0DECLARE<br>
&gt;=C2=A0 =C2=A0 =C2=A0result text;<br>
&gt;=C2=A0 =C2=A0BEGIN<br>
&gt;=C2=A0 =C2=A0 =C2=A0WAIT FOR LSN &#39;0/1234&#39; INTO result;<br>
&gt;=C2=A0 =C2=A0 =C2=A0RAISE NOTICE &#39;%&#39;, result;<br>
&gt;=C2=A0 =C2=A0END;<br>
&gt;=C2=A0 =C2=A0$$;<br>
&gt;=C2=A0 =C2=A0CALL test_wait();<br>
&gt;<br>
&gt;<br>
&gt; The WAIT FOR itself succeeds, but the very next PL/pgSQL statement tha=
t requires a snapshot crashes the backend with:<br>
&gt;<br>
&gt;=C2=A0 =C2=A0TRAP: failed Assert(&quot;portal-&gt;portalSnapshot =3D=3D=
 NULL&quot;),<br>
&gt;=C2=A0 =C2=A0File: &quot;pquery.c&quot;, Line: 1776<br>
&gt;<br>
&gt; Attached patches for both the test case and a potential fix. Please re=
view.<br>
<br>
Thank you for reporting.=C2=A0 But I doubt the fix is correct.=C2=A0 Even t=
hat<br>
this particular might work OK, I don&#39;t think it&#39;s safe to release<b=
r>
snapshots belonging to functions/procedures: it might affect them.=C2=A0 I<=
br>
tend to think we must forbid wrapping WAIT FOR LSN with<br>
functions/procedures.=C2=A0 I&#39;ll explore more on this today.</blockquot=
e><div><br></div><div>Agreed, attached a v2 patch with your suggestion on p=
reventing it running</div><div>from procedures.</div><div><br></div><div>Th=
anks,</div><div>Satya=C2=A0</div></div></div>

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