From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Robert Haas <robertmhaas@gmail.com>,
        Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Issue with cancel_before_shmem_exit while searching to remove a
 particular registered exit callbacks
In-reply-to: <20200811001102.bwcwmrwgirxyec6i@alap3.anarazel.de>
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 <20200807212022.wocdoo2mmzhjsjm3@alap3.anarazel.de>
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 <3195298.1597070661@sss.pgh.pa.us>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Mon, 10 Aug 2020 17:11:02 -0700"
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Date: Mon, 10 Aug 2020 20:46:02 -0400
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Andres Freund <andres@anarazel.de> writes:
> I think there's two different aspects here: Having before_shmem_exit(),
> and having cancel_before_shmem_exit(). We could just not have the
> latter, and instead use a separate list for PG_ENSURE_ERROR_CLEANUP
> internally. With the callback for PG_ENSURE_ERROR_CLEANUP calling those
> from its private list.  There's no other uses of
> cancel_before_shmem_exit afaict.

It's certainly arguable that PG_ENSURE_ERROR_CLEANUP is a special
snowflake and needs to use a separate mechanism.  What is not real clear
to me is why there are any other callers that must use before_shmem_exit
rather than on_shmem_exit --- IOW, except for P_E_E_C's use, I have never
been persuaded that the former callback list should exist at all.  The
expectation for on_shmem_exit is that callbacks correspond to system
service modules that are initialized in a particular order, and can safely
be torn down in the reverse order.  Why can't the existing callers just
make even-later entries into that same callback list?

			regards, tom lane