RE: Random pg_upgrade test failure on drongo

Dear Amit, Alexander,

> > We get the effect discussed when the background writer process decides to
> > flush a file buffer for pg_largeobject during stage 1.
> > (Thus, if a checkpoint somehow happened to occur during CREATE DATABASE,
> > the result must be the same.)
> > And another important factor is shared_buffers = 1MB (set during the test).
> > With the default setting of 128MB I couldn't see the failure.
> >
> > It can be reproduced easily (on old Windows versions) just by running
> > pg_upgrade in a loop (I've got failures on iterations 22, 37, 17 (with the
> > default cluster)).
> > If an old cluster contains dozen of databases, this increases the failure
> > probability significantly (with 10 additional databases I've got failures
> > on iterations 4, 1, 6).
> >
> 
> I don't have an old Windows environment to test but I agree with your
> analysis and theory. The question is what should we do for these new
> random BF failures? I think we should set bgwriter_lru_maxpages to 0
> and checkpoint_timeout to 1hr for these new tests. Doing some invasive
> fix as part of this doesn't sound reasonable because this is an
> existing problem and there seems to be another patch by Thomas that
> probably deals with the root cause of the existing problem [1] as
> pointed out by you.
> 
> [1] - https://commitfest.postgresql.org/40/3951/

Based on the suggestion by Amit, I have created a patch with the alternative
approach. This just does GUC settings. The reported failure is only for
003_logical_slots, but the patch also includes changes for the recently added
test, 004_subscription. IIUC, there is a possibility that 004 would fail as well.

Per our understanding, this patch can stop random failures. Alexander, can you
test for the confirmation?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED