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Re: Exit walsender before confirming remote flush in logical replication

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Andrey Silitskiy <a.silitskiy@postgrespro.ru>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Vitaly Davydov <v.davydov@postgrespro.ru>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, "Takamichi Osumi (Fujitsu)" <osumi.takamichi@fujitsu.com>, "sawada.mshk@gmail.com" <sawada.mshk@gmail.com>, "peter.eisentraut@enterprisedb.com" <peter.eisentraut@enterprisedb.com>, "dilipbalaut@gmail.com" <dilipbalaut@gmail.com>, "andres@anarazel.de" <andres@anarazel.de>, "amit.kapila16@gmail.com" <amit.kapila16@gmail.com>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Greg Sabino Mullane <htamfids@gmail.com>, Alexander Korotkov <aekorotkov@gmail.com>, Fujii Masao <masao.fujii@gmail.com>
Date: 2026-02-02T07:55:59Z
Lists: pgsql-hackers 
On Mon, Feb 02, 2026 at 02:52:36AM +0700, Andrey Silitskiy wrote:
> +/*
> + * Shutdown walsender in immediate mode.
> + */
> +static void
> +WalSndDoneImmediate()
> +{
> +	QueryCompletion qc;
> +
> +	/* Try to inform receiver that XLOG streaming is done */
> +	SetQueryCompletion(&qc, CMDTAG_COPY, 0);
> +	EndCommand(&qc, DestRemote, false);

Couldn't that be potentially dangerous, particularly if
wal_sender_shutdown_mode is set to immediate, meaning that it applies
to all the WAL senders?  The WAL receiver side could be on a backend
with an older backend version than the WAL sender where this new GUC
exists.  Hence, a completion could be understood incorrectly by a WAL
receiver depending on how the receiving side is coded?  Assuming this
is merged into v19 in this shape, a receiver connecting to a newer
server would get a new bevahior compared to older versions, without
the receiver being aware of that.

I suspect that this option, as designed, is potentially a footgun that 
could surprise many users, especially as it is super aggressive in
shutting down everything on sight, unconditionally.
--
Michael

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Avoid blocking indefinitely while finishing walsender shutdown

  2. Add wal_sender_shutdown_timeout GUC to limit shutdown wait for replication

  3. pg_upgrade: Add --copy option

  4. Prevent possibility of panics during shutdown checkpoint.

  5. Support clean switchover.