Re: Improving connection scalability: GetSnapshotData()

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Robert Haas <robertmhaas(at)gmail(dot)com>, "Jonathan S(dot) Katz" <jkatz(at)postgresql(dot)org>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Peter Geoghegan <pg(at)bowt(dot)ie>, Bruce Momjian <bruce(at)momjian(dot)us>, David Rowley <dgrowleyml(at)gmail(dot)com>
Subject: Re: Improving connection scalability: GetSnapshotData()
Date: 2020-08-15 15:10:51
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We have two essentially identical buildfarm failures since these patches
went in:

They're both in the same place in the freeze-the-dead isolation test:

TRAP: FailedAssertion("!TransactionIdPrecedes(members[i].xid, cutoff_xid)", File: "heapam.c", Line: 6051)
0x9613eb <ExceptionalCondition+0x5b> at /home/pgbuildfarm/buildroot/HEAD/inst/bin/postgres
0x52d586 <heap_prepare_freeze_tuple+0x926> at /home/pgbuildfarm/buildroot/HEAD/inst/bin/postgres
0x53bc7e <heap_vacuum_rel+0x100e> at /home/pgbuildfarm/buildroot/HEAD/inst/bin/postgres
0x6949bb <vacuum_rel+0x25b> at /home/pgbuildfarm/buildroot/HEAD/inst/bin/postgres
0x694532 <vacuum+0x602> at /home/pgbuildfarm/buildroot/HEAD/inst/bin/postgres
0x693d1c <ExecVacuum+0x37c> at /home/pgbuildfarm/buildroot/HEAD/inst/bin/postgres
2020-08-14 22:16:41.783 CDT [78410:4] LOG: server process (PID 80395) was terminated by signal 6: Abort trap
2020-08-14 22:16:41.783 CDT [78410:5] DETAIL: Failed process was running: VACUUM FREEZE tab_freeze;

peripatus has successes since this failure, so it's not fully reproducible
on that machine. I'm suspicious of a timing problem in computing vacuum's

(I'm also wondering why the failing check is an Assert rather than a real
test-and-elog. Assert doesn't seem like an appropriate way to check for
plausible data corruption cases.)

regards, tom lane

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