From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Michael Paquier <michael@paquier.xyz>, Andres Freund <andres@anarazel.de>,
        Alvaro Herrera <alvherre@2ndquadrant.com>,
        PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: TopoSort() fix
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Tue, 30 Jul 2019 12:46:41 -0400"
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Date: Tue, 30 Jul 2019 13:36:36 -0400
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Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Jul 30, 2019 at 10:27 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> (BTW, why aren't these functions just "parallel restricted"?)

> ...
> But it is really pretty arguable whether we should feel responsible
> for that problem.  We could just decide that if you're doing that, and
> you don't want the scenario described above to happen, you oughta mark
> the function that contains this logic at least PARALLEL RESTRICTED,
> and if you don't, then it's your fault for doing a dumb thing.  I
> believe when we were early on in the development of this we wanted to
> be very conservative lest, ah, someone accuse us of not locking things
> down well enough, but maybe at this point parallel query is a
> sufficiently well-established concept that we should lighten up on
> some cases where we took an overly-stringent line.  If we take that
> view, then I'm not sure why these functions couldn't be just marked
> PARALLEL SAFE.

No, there's a sufficient reason why we should force advisory locks
to be taken in the leader process, namely that the behavior is totally
different if we don't: they will disappear at the end of the parallel
worker run, not at end of transaction or session as documented.

However, that argument doesn't seem to be a reason why the advisory-lock
functions couldn't be parallel-restricted rather than parallel-unsafe.

In any case, my question at the moment is whether we need the belt-and-
suspenders-too approach of having both non-parallel-safe marking and an
explicit check inside these functions.  We've largely moved away from
hard-wired checks for e.g. superuserness, and surely these things are
less dangerous than most formerly-superuser-only functions.

			regards, tom lane