Horton

Re: Race condition in TransactionIdIsInProgress

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Simon Riggs <simon.riggs@enterprisedb.com>
Cc: Konstantin Knizhnik <knizhnik@garret.ru>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2022-02-12T00:41:24Z
Lists: pgsql-hackers 
Hi,

On 2022-02-10 22:11:38 -0800, Andres Freund wrote:
> Looks lik syncrep will make this a lot worse, because it can drastically
> increase the window between the TransactionIdCommitTree() and
> ProcArrayEndTransaction() due to the SyncRepWaitForLSN() inbetween.  But at
> least it might make it easier to write tests exercising this scenario...

FWIW, I've indeed reproduced this fairly easily with such a setup. A pgbench
r/w workload that's been modified to start 70 savepoints at the start shows

pgbench: error: client 22 script 0 aborted in command 12 query 0: ERROR:  t_xmin 3853739 is uncommitted in tuple (2,159) to be updated in table "pgbench_branches"
pgbench: error: client 13 script 0 aborted in command 12 query 0: ERROR:  t_xmin 3954305 is uncommitted in tuple (2,58) to be updated in table "pgbench_branches"
pgbench: error: client 7 script 0 aborted in command 12 query 0: ERROR:  t_xmin 4017908 is uncommitted in tuple (3,44) to be updated in table "pgbench_branches"

after a few minutes of running with a local, not slowed down, syncrep. Without
any other artifical slowdowns or such.

Greetings,

Andres Freund

Commits

  1. Fix visibility check when XID is committed in CLOG but not in procarray.

  2. Fix possible HOT corruption when RECENTLY_DEAD changes to DEAD while pruning.

  3. Make TransactionIdIsInProgress check transam.c's single-item XID status cache