From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Michael Paquier <michael@paquier.xyz>,
        Postgres hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: REINDEX INDEX results in a crash for an index of pg_class since
 9.6
In-reply-to: <20190430173457.pg6i6urvevse5jer@alap3.anarazel.de>
References: <20190426020248.iyev66bkpf2abxqd@alap3.anarazel.de>
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 <20190429220924.lfptluzkujb26y2n@alap3.anarazel.de>
 <20190430030342.jmbgnih4q3cdoufz@alap3.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Tue, 30 Apr 2019 10:34:57 -0700"
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Date: Tue, 30 Apr 2019 14:05:50 -0400
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Andres Freund <andres@anarazel.de> writes:
> It's the lock-upgrade problem I theorized about
> upthread. ReindexIndex(), via RangeVarCallbackForReindexIndex(), takes a
> ShareLock on pg_class, and then goes on to upgrade to RowExclusiveLock
> in RelationSetNewRelfilenode(). But at that time another session
> obviously can already have the ShareLock and would also want to upgrade.

Hmm.  Note that this is totally independent of the deadlock mechanism
I reported in my last message on this thread.

I also wonder whether clobber-cache testing would expose cases
we haven't seen that trace to the additional catalog accesses
caused by cache reloads.

> I'm not sure it's worth fixing this.

I am not sure it's even *possible* to fix all these cases.  Even
if we could, it's out of scope for v12 let alone the back branches.

I think the only practical solution is to remove those reindex tests.
Even if we ran them in a script with no concurrent scripts, there'd
be risk of failures against autovacuum, I'm afraid.  Not often, but
often enough to be annoying.

Possibly we could run them in a TAP test that configures a cluster
with autovac disabled?

			regards, tom lane