RE: [PoC] pg_upgrade: allow to upgrade publisher node

Dear Julien,

Thank you for giving comments! PSA new version.

> I think that this test should be different when just checking for the
> prerequirements (live_check / --check) compared to actually doing the upgrade,
> as it's almost guaranteed that the slots won't have sent everything when the
> source server is up and running.

Hmm, you assumed that the user application is still running and data is coming
continuously when doing --check, right? Personally I have thought that the
--check operation is executed just before the actual upgrading, therefore I'm not
sure your assumption is real problem. And I could not find any checks which their
contents are changed based on the --check option.

Anyway, I included your opinion in 0004 patch. We can ask some other reviewers
about the necessity.

> Maybe simply check that all logical slots are currently active when running the
> live check,

Yeah, if we support the case checking pg_replication_slots.active may be sufficient.
Actually this cannot handle the case that pg_create_logical_replication_slot()
is executed just before upgrading, but I'm not sure it should be.

> so at least there's a good chance that they will still be at
> shutdown, and will therefore send all the data to the subscribers? Having a
> regression tests for that scenario would also be a good idea. Having an
> uncommitted write transaction should be enough to cover it.

I think background_psql() can be used for the purpose. Before doing pg_upgrade
--check, a transaction is opened and kept. It means that the confirmed_flush has
been not reached to the current WAL position yet, but the checking says OK
because all slots are active.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED