Horton

Re: BackendKeyData is mandatory?

Tatsuo Ishii <ishii@postgresql.org>

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From: Tatsuo Ishii <ishii@postgresql.org>
To: tgl@sss.pgh.pa.us
Cc: pgsql-hackers@postgresql.org
Date: 2025-06-17T12:41:31Z
Lists: pgsql-hackers 
>> My question is, BackendKeyData is mandatory or not. Currently
>> Pgpool-II raises a fatal error if BackendKeyData is not sent before
>> ReadyForQuery arrives. This is because without the message, frontend
>> cannot send a CancelRequest message later on, as there's no secret
>> key.
> 
> As you say, without BackendKeyData it's impossible to send a query
> cancel, so we expect the server will always send that.

That's my understanding too.

>> I heard that some "PostgreSQL compatible" servers do not send
>> BackendKeyData message to frontend. I wonder if this is a protocol
>> violation.
> 
> I'd say so.  Maybe whoever that is doesn't care to support query
> cancel.  They're within their rights to do that I guess, but
> Pgpool-II does not have to support the case.  (A less incompatible
> way of not supporting query cancel is to send dummy BackendKeyData
> values and then just ignore cancel requests.  So I don't see that
> you need to do anything towards this goal, if it is a goal and
> not merely a broken implementation.)

Agreed.
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp

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()