Re: refactoring basebackup.c

On Tue, Nov 2, 2021 at 10:32 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Meanwhile, I think it's probably OK for me to go ahead and commit
> 0001-0003 from my patches at this point, since it seems we have pretty
> good evidence that the abstraction basically works, and there doesn't
> seem to be any value in holding off and maybe having to do a bunch
> more rebasing.

I went ahead and committed 0001 and 0002, but got nervous about
proceeding with 0003. For those who may not have been following along
closely, what was 0003 and is now 0001 introduces a new COPY
subprotocol for taking backups. That probably needs to be documented
and as of now the patch does not do that, but the bigger question is
what to do about backward compatibility. I wrote the patch in such a
way that, post-patch, the server can do backups either the way that we
do them now, or the new way that it introduces, but I'm wondering if I
should rip that out and just support the new way only. If you run a
newer pg_basebackup against an older server, it will work, and still
does with the patch. If, however, you run an older pg_basebackup
against a newer server, it complains. For example running a pg13
pg_basebackup against a pg14 cluster produces this:

pg_basebackup: error: incompatible server version 14.0
pg_basebackup: removing data directory "pgstandby"

Now for all I know there is out-of-core software out there that speaks
the replication protocol and can take base backups using it and would
like it to continue working as it does today, and that's easy for me
to do, because that's the way the patch works. But on the other hand
since the patch adapts the in-core tools to use the new method when
talking to a new server, we wouldn't have test coverage for the old
method any more, which might possibly make it annoying to maintain.
But then again that is a problem we could leave for the future, and
rip it out then rather than now. I'm not sure which way to jump.
Anyone else have thoughts?

--
Robert Haas
EDB: http://www.enterprisedb.com