From: Tom Lane <tgl@sss.pgh.pa.us>
To: Smolkin Grigory <smallkeen@gmail.com>
cc: pgsql-hackers@postgresql.org
Subject: Re: race condition in pg_class
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Comments: In-reply-to Smolkin Grigory <smallkeen@gmail.com>
	message dated "Wed, 25 Oct 2023 13:39:41 +0300"
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Date: Wed, 25 Oct 2023 14:06:45 -0400
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Smolkin Grigory <smallkeen@gmail.com> writes:
> We are running PG13.10 and recently we have encountered what appears to be
> a bug due to some race condition between ALTER TABLE ... ADD CONSTRAINT and
> some other catalog-writer, possibly ANALYZE.
> The problem is that after successfully creating index on relation (which
> previosly didnt have any indexes), its pg_class.relhasindex remains set to
> "false", which is illegal, I think.
> Index was built using the following statement:
> ALTER TABLE "example" ADD constraint "example_pkey" PRIMARY KEY (id);

ALTER TABLE ADD CONSTRAINT would certainly have taken
AccessExclusiveLock on the "example" table, which should be sufficient
to prevent anything else from touching its pg_class row.  The only
mechanism I can think of that might bypass that is a manual UPDATE on
pg_class, which would just manipulate the row as a row without concern
for associated relation-level locks.  Any chance that somebody was
doing something like that?

			regards, tom lane