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

On Mon, Sep 25, 2023 at 1:23 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Mon, Sep 25, 2023 at 12:32 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > > > Is there anything else that stops this patch from supporting migration
> > > > of logical replication slots from PG versions < 17?
> > >
> > > IMHO one of the main change we are doing in PG 17 is that on shutdown
> > > checkpoint we are ensuring that if the confirmed flush lsn is updated
> > > since the last checkpoint and that is not yet synched to the disk then
> > > we are doing so. I think this is the most important change otherwise
> > > many slots for which we have already streamed all the WAL might give
> > > an error assuming that there are pending WAL from the slots which are
> > > not yet confirmed.
> > >
> >
> > You might need to refer to [1] for the change I am talking about
> >
> > [1] https://www.postgresql.org/message-id/CAA4eK1%2BLtWDKXvxS7gnJ562VX%2Bs3C6%2B0uQWamqu%3DUuD8hMfORg%40mail.gmail.com
>
> I see. IIUC, without that commit e0b2eed [1], it may happen that the
> slot's on-disk confirmed_flush LSN value can be higher than the WAL
> LSN that's flushed to disk, no? If so, can't it be detected if the WAL
> at confirmed_flush LSN is valid or not when reading WAL with
> xlogreader machinery?

Actually, without this commit the slot's "confirmed_flush LSN" value
in memory can be higher than the disk because if you notice this
function LogicalConfirmReceivedLocation(), if we change only the
confirmed flush the slot is not marked dirty that means on shutdown
the slot will not be persisted to the disk. But logically this will
not cause any issue so we can not treat it as a bug it may cause us to
process some extra records after the restart but that is not really a
bug.

> What if the commit e0b2eed [1] is treated to be fixing a bug with the
> reasoning [2] and backpatch? When done so, it's easy to support
> upgradation/migration of logical replication slots from PG versions <
> 17, no?

Maybe this could be backpatched in order to support this upgrade from
the older version but not as a bug fix.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com