Re: multi-install PostgresNode fails with older postgres versions

On Mon, 19 Apr 2021 12:37:08 -0400
Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:

>
> On 4/19/21 10:43 AM, Mark Dilger wrote:
> >
> >> On Apr 19, 2021, at 5:11 AM, Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:
> >>
> >> I think therefore I'm inclined for now to do nothing for old version
> >> compatibility.
> > I agree with waiting until the v15 development cycle.
> >
> >> I would commit the fix for the IPC::Run caching glitch,
> >> and version detection
> > Thank you.
> >
> >> I would add a warning if the module is used with
> >> a version <= 11.
> > Sounds fine for now.
> >
> >> The original goal of these changes was to allow testing of combinations
> >> of different builds with openssl and nss, which doesn't involve old
> >> version compatibility.
> > Hmm. I think different folks had different goals. My personal interest is
> > to write automated tests which spin up older servers, create data that
> > cannot be created on newer servers (such as heap tuples with HEAP_MOVED_IN
> > or HEAP_MOVED_OFF bits set), upgrade, and test that new code handles the
> > old data correctly. I think this is not only useful for our test suites as
> > a community, but is also useful for companies providing support services
> > who need to reproduce problems that customers are having on clusters that
> > have been pg_upgraded across large numbers of postgres versions.
> >
> >> As far as I know, without any compatibility changes the module is fully
> >> compatible with releases 13 and 12, and with releases 11 and 10 so long
> >> as you don't want a standby, and with releases 9.6 and 9.5 if you also
> >> don't want a backup. That makes it suitable for a lot of testing without
> >> any attempt at version compatibility.
> >>
> >> We can revisit compatibility further in the next release.
> > Sounds good.
>
>
> I'll work on this. Meanwhile FTR here's my latest revision - it's a lot
> less invasive of the main module, so it seems much more palatable to me,
> and still passes my test down to 7.2.

I spend a fair bit of time to wonder how useful it could be to either maintain
such a module in core, including for external needs, or creating a separate
external project with a different release/distribution/packaging policy.

Wherever the module is maintained, the goal would be to address broader
needs, eg. adding a switch_wal() method or wait_for_archive(), supporting
replication, backups, etc for multi-old-deprecated-PostgreSQL versions.

To be honest I have mixed feelings. I feel this burden shouldn't be carried
by the core, which has restricted needs compared to external projects. In the
opposite, maintaining an external project which shares 90% of the code seems to
be a useless duplicate and backport effort. Moreover Craig Ringer already opened
the door for external use of PostgresNode with his effort to install/package it,
see:
https://www.postgresql.org/message-id/CAGRY4nxxKSFJEgVAv5YAk%3DbqULtFmNw7gEJef0CCgzpNy6O%3D-w%40mail.gmail.com

Thoughts?