Re: Flaky vacuum truncate test in reloptions.sql

On Fri, Apr 2, 2021 at 9:46 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:

> Okay, applied and back-patched down to 12 then.

Thank you both. Unfortunately and surprisingly, the test still fails
(perhaps even rarer, once in several hundred runs) under
multimaster. After scratching the head for some more time, it seems to
me the following happens: not only vacuum encounters locked page, but
also there exist a concurrent backend (as the parallel schedule is run)
who holds back oldestxmin keeping it less than xid of transaction which
did the insertion

INSERT INTO reloptions_test VALUES (1, NULL), (NULL, NULL);

FreezeLimit can't be higher than oldestxmin, so lazy_check_needs_freeze
decides there is nothing to freeze on the page. multimaster commits are
quite heavy, which apparently shifts the timings making the issue more
likely.

Currently we are testing the rather funny attached patch which forces
all such old-snapshot-holders to finish. It is crutchy, but I doubt we
want to change vacuum logic (e.g. checking tuple liveness in
lazy_check_needs_freeze) due to this issue. (it is especially crutchy in
xid::bigint casts, but wraparound is hardly expected in regression tests
run).

-- cheers, arseny