Re: recovering from "found xmin ... from before relfrozenxid ..."

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com>, Mark Dilger <mark(dot)dilger(at)enterprisedb(dot)com>, Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com>, "Andrey M(dot) Borodin" <x4mmm(at)yandex-team(dot)ru>, MBeena Emerson <mbeena(dot)emerson(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Andres Freund <andres(at)anarazel(dot)de>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>
Subject: Re: recovering from "found xmin ... from before relfrozenxid ..."
Date: 2020-09-19 20:19:05
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I wrote:
> I was able to partially reproduce whelk's failure here. I got a
> couple of cases of "cannot freeze committed xmax", which then leads
> to the second NOTICE diff; but I couldn't reproduce the first
> NOTICE diff. That was out of about a thousand tries :-( so it's not
> looking like a promising thing to reproduce without modifying the test.

... however, it's trivial to reproduce via manual interference,
using the same strategy discussed recently for another case:
add a pg_sleep at the start of the heap_surgery.sql script,
run "make installcheck", and while that's running start another
session in which you begin a serializable transaction, execute
any old SELECT, and wait. AFAICT this reproduces all of whelk's
symptoms with 100% reliability.

With a little more effort, this could be automated by putting
some long-running transaction (likely, it needn't be any more
complicated than "select pg_sleep(10)") in a second test script
launched in parallel with heap_surgery.sql.

So this confirms the suspicion that the cause of the buildfarm
failures is a concurrently-open transaction, presumably from
autovacuum. I don't have time to poke further right now.

regards, tom lane

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