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Date: Mon, 25 May 2026 23:39:54 -0300
From: "Euler Taveira" <euler@eulerto.com>
To: "Michael Paquier" <michael@paquier.xyz>,
 "Postgres hackers" <pgsql-hackers@lists.postgresql.org>
Cc: =?UTF-8?Q?=C3=81lvaro_Herrera?= <alvherre@kurilemu.de>
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References: <ahP-JT4ZRPyobnLb@paquier.xyz> <ahQGnTqb7i0UUz70@paquier.xyz>
Subject: Re: NULL pointer dereference in syslogger with load_libraries() and
 -DEXEC_BACKEND at startup
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On Mon, May 25, 2026, at 5:21 AM, Michael Paquier wrote:
> On Mon, May 25, 2026 at 04:45:41PM +0900, Michael Paquier wrote:
>> I did not take the cycles necessary for a bisect, but it looks like
>> this has been around for a few months at least.  I have pinged
>> f3c9e341cdf1 as a safe startup point for now, so that's a 2026 issue.
>
> Well, well:
> 0c8e082fba8d36434552d3d7800abda54acafd57 is the first bad commit=20
> committer: =C3=81lvaro Herrera <alvherre@kurilemu.de>
> date: Wed, 4 Feb 2026 16:56:57 +0100
> Assign "backend" type earlier during process start-up=20
>
> I have also checked manually a revert of this commit, and saw that the
> problem is gone, so it does not look like I have messed up my bisect.
>

It seems I was too optimistic about this patch. Since the commit 0c8e082=
fba8
sets MyBackendType to B_LOGGER earlier, it breaks the following assumpti=
on in
syslogger.c.

        /*
         * If we're told to write to a structured log file, but it's not=
 open,
         * dump the data to syslogFile (which is always open) instead.  =
This can
         * happen if structured output is enabled after postmaster start=
 and we've
         * been unable to open logFile.  There are also race conditions =
during a
         * parameter change whereby backends might send us structured ou=
tput
         * before we open the logFile or after we close it.  Writing for=
matted
         * output to the regular log file isn't great, but it beats drop=
ping log
         * output on the floor.

It shouldn't assume syslogFile is always open. The send_message_to_serve=
r_log()
shouldn't be executing the following code path in this case.

    /* If in the syslogger process, try to write messages direct to file=
 */
    if (MyBackendType =3D=3D B_LOGGER)
        write_syslogger_file(buf.data, buf.len, LOG_DESTINATION_STDERR);

It could set MyBackendType only if child_type !=3D B_LOGGER during
launch_backend.c and set it at the same code path as in the past. Howeve=
r, I
consider this solution ugly and hackish.


--=20
Euler Taveira
EDB   https://www.enterprisedb.com/