Horton

Re: BackendKeyData is mandatory?

Jelte Fennema-Nio <postgres@jeltef.nl>

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From: Jelte Fennema-Nio <postgres@jeltef.nl>
To: Jacob Champion <jacob.champion@enterprisedb.com>
Cc: Tatsuo Ishii <ishii@postgresql.org>, hlinnaka@iki.fi, peter@eisentraut.org, pgsql-hackers@postgresql.org
Date: 2025-06-23T16:23:53Z
Lists: pgsql-hackers 
On Mon, 23 Jun 2025 at 18:02, Jacob Champion
<jacob.champion@enterprisedb.com> wrote:
> If anyone today is relying on "backend-key-less" connection, this is
> potentially a breaking change. For example, psycopg2 now complains:
>
>     psycopg2.OperationalError: can't get cancellation key


It's not super clear what you're referring to with "this" in your
first sentence.

To be clear, I'm not saying we should start throwing errors for things
in libpq that weren't errors before. But more as: I don't expect many
client authors to have considered the scenario of no BackendKeyData.
Because either an author believes the BackendKeyData is optional, in
which case they shouldn't send a CancelRequest for those connections.
Or they think it is required, in which case throwing an error seems
like the sensible thing to do. And the implementations in PgBouncer
and PG17 is neither, i.e. it won't throw an error, but instead of
doing nothing it will do something clearly useless.

Commits

  1. Revert unnecessary check for NULL

  2. libpq: Be strict about cancel key lengths

  3. libpq: Handle OOM by disconnecting instead of hanging or skipping msgs

  4. libpq: Complain about missing BackendKeyData later with PGgetCancel()