Date: Sun, 17 Mar 2024 11:31:14 -0700
From: Noah Misch <noah@leadboat.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>,
	Michael Paquier <michael@paquier.xyz>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Autogenerate some wait events code and documentation
Message-ID: <20240317183114.16@rfd.leadboat.com>
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On Wed, Jul 05, 2023 at 10:57:19AM +0900, Michael Paquier wrote:
> I have applied it.

I like the new developer experience of adding a wait event.  After release of
v17, how should we approach back-patching an event, like was done in commits
8fa4a1a 1396b5c 78c0f85?  Each of those commits put the new event at the end
of its released-branch wait_event.h enum.  In v17,
generate-wait_event_types.pl sorts events to position them.  Adding an event
will renumber others, which can make an extension report the wrong event until
recompiled.  Extensions citus, pg_bulkload, and vector refer to static events.
If a back-patch added WAIT_EVENT_MESSAGE_QUEUE_SOMETHING_NEW, an old-build
pg_bulkload report of WAIT_EVENT_PARALLEL_CREATE_INDEX_SCAN would show up in
pg_stat_activity as WAIT_EVENT_PARALLEL_BITMAP_SCAN.  (WAIT_EVENT_EXTENSION is
not part of a generated enum, fortunately.)  Some options:

1. Don't back-patch wait events to v17+.  Use the closest existing event.
2. Let wait_event_names.txt back-patches control the enum order.  For example,
   a line could have an annotation that controls its position relative to the
   auto-sorted lines.  For another example, the generator could stop sorting.
3. Accept the renumbering, because the consequence isn't that horrible.

Option (3) is worse than (1), but I don't have a recommendation between (1)
and (2).  I tend to like (1), a concern being the ease of accidental
violations.  If we had the ABI compliance checking that
https://postgr.es/m/flat/CAH2-Wzk7tvgLXzOZ8a22aF-gmO5gHojWTYRvAk5ZgOvTrcEQeg@mail.gmail.com
explored, (1) would be plenty safe.  Should anything change here, or not?