From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jacob Champion <jacob.champion@enterprisedb.com>
cc: Jelte Fennema-Nio <postgres@jeltef.nl>,
 pgsql-hackers@lists.postgresql.org,
        Andrew Dunstan <andrew@dunslane.net>
Subject: Re: pgsql: libpq: Grease the protocol by default
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Comments: In-reply-to Jacob Champion <jacob.champion@enterprisedb.com>
	message dated "Mon, 23 Feb 2026 15:41:46 -0800"
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Date: Mon, 23 Feb 2026 19:45:05 -0500
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Jacob Champion <jacob.champion@enterprisedb.com> writes:
> Something like the attached (tested only against 9.2 so far)? I would
> plan to backpatch after feature freeze is lifted.

This does not fix it for me on a local buildfarm instance.  I still get
the same failure during 9.2-to-HEAD cross-version upgrade:

=3D=3D~_~=3D=3D=3D-=3D-=3D=3D=3D~_~=3D=3D /home/buildfarm/bf-data/fs-upgra=
de.tester/HEAD/REL9_2_STABLE-dump1.log =3D=3D~_~=3D=3D=3D-=3D-=3D=3D=3D~_~=
=3D=3D
pg_dumpall: error: connection to server on socket "/home/buildfarm/bf-data=
/tmp/buildfarm-0fIcMg/.s.PGSQL.5700" failed: FATAL:  unsupported frontend =
protocol 3.9999: server supports 1.0 to 3.0
        This indicates a bug in either the server being contacted
        or a proxy handling the connection. Please consider
        reporting this to the maintainers of that software.
        For more information, including instructions on how to
        work around this issue for now, visit
                https://www.postgresql.org/docs/devel/libpq-connect.html#L=
IBPQ-CONNECT-MAX-PROTOCOL-VERSION

It's not obvious to me where the problem lies.  I can replicate
the failure by trying to use HEAD's psql to connect to a 9.2
server, but adding -d "max_protocol_version=3D3.0" makes psql happy,
so why not pg_dumpall?

Also: I was initially baffled why you thought this needs
back-patching, but I guess you have one eye on packagers like
Debian who think they can make older versions use newer libpq.so.
It'd be good to spell out that reasoning in the commit message.

			regards, tom lane