Re: [HACKERS] Cause of occasional buildfarm failures in sequence test

On Sun, 2008-08-31 at 13:17 -0400, Tom Lane wrote:
> The current result from buildfarm member pigeon
> http://www.pgbuildfarm.org/cgi-bin/show_log.pl?nm=pigeon&dt=2008-08-31%2008:04:54
> shows a symptom I've noticed a few times before: everything is fine
> except that one "select * from sequence" shows log_cnt = 31 instead
> of the expected 32. This is a bit odd since no other backend is
> touching that sequence, so you'd expect perfectly reproducible results.
>
> I finally got around to looking through the sequence code to see if I
> could figure out what was happening, and indeed I did. The relevant
> parts of the test are basically
> create sequence foo;
> select nextval('foo');
> select nextval('foo');
> select * from foo;
> and you can reproduce the unexpected result if you insert a checkpoint
> command between the create and the first nextval. So the buildfarm
> failures are due to chance occurrences of a bgwriter-driven checkpoint
> occurring just there during the test.
>
> In the normal execution of this test, the CREATE SEQUENCE leaves the
> sequence in a state where one nextval() can be done "for free", without
> emitting a new WAL record. So the second nextval() is the one that
> emits a WAL record and pushes log_cnt up to 32. But if a checkpoint
> intervenes, then the first nextval() decides it'd better emit the WAL
> record (cf lines 500ff in sequence.c as of CVS HEAD), so it pushes
> log_cnt up to 32, and then the second one decreases it to 31.
>
> So unless we want to just live with this test failing occasionally,
> it seems we have two choices: redesign the behavior of nextval()
> to be insensitive to checkpoint timing, or provide an alternate
> regression "expected" file that matches the result with log_cnt = 31.
> I favor the second answer --- I don't want to touch the nextval
> logic, which has been stable for over six years.
>
> Comments?

Maybe you get consistent result by just changing the test thus:

checkpoint;
create sequence foo;
select nextval('foo');
select nextval('foo');
select * from foo;

--------------
Hannu

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