From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Peter Geoghegan <pg@bowt.ie>, Dmitry Dolgov <9erthalion6@gmail.com>,
        Alexander Lakhin <exclusion@gmail.com>,
        Matthias van de Meent <boekewurm+postgres@gmail.com>,
        PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #17255: Server crashes in index_delete_sort_cmp() due to race
 condition with vacuum
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 03 Feb 2022 20:19:35 -0800"
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Date: Fri, 04 Feb 2022 10:15:21 -0500
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Andres Freund <andres@anarazel.de> writes:
> On 2022-02-03 15:54:28 -0500, Tom Lane wrote:
>> I'm writing release notes and wondering what I can tell users about
>> how to detect or recover from this bug.  Is a REINDEX sufficient,
>> or is the presence of the bogus redirect item going to cause
>> persistent problems?

> Good questions.

> It's hard to answer whether there's any danger after a REINDEX. Afaics t=
he
> build scan would just pick the "lower offset" version of the root
> pointer. Which should be fine.

> It's possible there could be trouble down the line, e.g. heap pruning do=
ing
> something weird once starting in a corrupted state, that then leads REIN=
DEX to
> do something bogus. The simple cases look OK, because a second visit/act=
ion by
> heap_prune_chain for one tid from two different root pointers would see
> ->marked[offnum] as true. It gets more complicated once multiple interme=
diary
> row versions are involved, because the intermediary row versions won't b=
e in
> ->marked if an entire chain is pruned. But afaict that should still end =
up
> looking like a hot chain ending in an aborted tuple or such.

OK, I'll just recommend REINDEX.

> Except that it's not trivial to get right, I could see it being worthwhi=
le to
> add verification of hot chains to amcheck, and backpatch that to 14.

I'd have thought that'd be a fundamental component of a heap check
module, so +1 for adding it.  Dunno about the back-patch part though.
It seems like a new feature.

			regards, tom lane