Re: Minimal logical decoding on standbys

On Tue, 24 Dec 2019 at 14:02, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
>
> On Thu, 19 Dec 2019 at 01:02, Rahila Syed <rahila(dot)syed(at)2ndquadrant(dot)com> wrote:
> >
> > Hi,
> >
> >> Hi, do you consistently get this failure on your machine ? I am not
> >> able to get this failure, but I am going to analyze when/how this can
> >> fail. Thanks
> >>
> > Yes, I am getting it each time I run make -C src/test/recovery/ check PROVE_TESTS=t/018_standby_logical_decoding_xmins.pl
> > Also, there aren't any errors in logs indicating the cause.
>
> Thanks for the reproduction. Finally I could reproduce the behaviour.
> It occurs once in 7-8 runs of the test on my machine. The issue is :
> on master, the catalog_xmin does not immediately get updated. It
> happens only after the hot standby feedback reaches on master. And I
> haven't used wait_for_xmins() for these failing cases. I should use
> that. Working on the same ...

As mentioned above, I have used wait_for_xmins() so that we can wait
for the xmins to be updated after hot standby feedback is processed.
In one of the 3 scenarios where it failed for you, I removed the check
at the second place because it was redundant. At the 3rd place, I did
some appropriate changes with detailed comments. Please check.
Basically we are checking that the master's phys catalog_xmin has
advanced but not beyond standby's logical catalog_xmin. And for making
sure the master's xmins are updated, I call txid_current() and then
wait for the master's xmin to advance after hot-standby_feedback, and
in this way I make sure the xmin/catalog_xmins are now up-to-date
because of hot-standby-feedback, so that we can check whether the
master's physical slot catalog_xmin has reached the value of standby's
catalog_xmin but not gone past it.

I have also moved the "wal_receiver_status_interval = 1" setting from
master to standby. It was wrongly kept in master. This now reduces the
test time by half, on my machine.

Attached patch set v5 has only the test changes. Please check if now
the test fails for you.

>
> --
> Thanks,
> -Amit Khandekar
> EnterpriseDB Corporation
> The Postgres Database Company

--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company