Re: xid_wraparound tests intermittent failure.

Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> I noticed this when working on the PostgreSQL::Test::Session project I
> have in hand. All the tests pass except occasionally the xid_wraparound
> tests fail. It's not always the same test script that fails either. I
> tried everything but couldn't make the failure stop. So then I switched
> out my patch so it's running on plain master and set things running in a
> loop. Lo and behold it can be relied on to fail after only a few
> iterations.

I have been noticing xid_wraparound failures in the buildfarm too.
They seemed quite infrequent, but it wasn't till just now that
I realized that xid_wraparound is not run by default. (You have to
put "xid_wraparound" in PG_TEST_EXTRA to enable it.) AFAICS the
only buildfarm animals that have enabled it are dodo and perentie.
dodo is failing this test fairly often:

https://buildfarm.postgresql.org/cgi-bin/show_history.pl?nm=dodo&br=HEAD

perentie doesn't seem to be having a problem, but I will bet that
part of the reason is it's running with cranked-up timeouts:

'build_env' => {
'PG_TEST_EXTRA' => 'xid_wraparound',
'PG_TEST_TIMEOUT_DEFAULT' => '360'
},

One thing that seems quite interesting is that the test seems to
take about 10 minutes when successful on dodo, but when it fails
it's twice that. Why the instability? (Perhaps dodo has highly
variable background load, and the thing simply times out in some
runs but not others?)

Locally, I've not managed to reproduce the failure yet; so perhaps
there is some platform dependency. What are you testing on?

regards, tom lane