Re: Exit walsender before confirming remote flush in logical replication

From: Michael Paquier <michael(at)paquier(dot)xyz>
To: Andrey Silitskiy <a(dot)silitskiy(at)postgrespro(dot)ru>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Vitaly Davydov <v(dot)davydov(at)postgrespro(dot)ru>, "Hayato Kuroda (Fujitsu)" <kuroda(dot)hayato(at)fujitsu(dot)com>, "Takamichi Osumi (Fujitsu)" <osumi(dot)takamichi(at)fujitsu(dot)com>, "sawada(dot)mshk(at)gmail(dot)com" <sawada(dot)mshk(at)gmail(dot)com>, "peter(dot)eisentraut(at)enterprisedb(dot)com" <peter(dot)eisentraut(at)enterprisedb(dot)com>, "dilipbalaut(at)gmail(dot)com" <dilipbalaut(at)gmail(dot)com>, "andres(at)anarazel(dot)de" <andres(at)anarazel(dot)de>, "amit(dot)kapila16(at)gmail(dot)com" <amit(dot)kapila16(at)gmail(dot)com>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, Peter Smith <smithpb2250(at)gmail(dot)com>, Greg Sabino Mullane <htamfids(at)gmail(dot)com>, Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Subject: Re: Exit walsender before confirming remote flush in logical replication
Date: 2026-02-02 07:55:59
Message-ID: aYBYj_RaGxFoHdXz@paquier.xyz
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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

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