Re: basebackup: do not verify checksums on pages written before enabling checksums

> I think we should still to doing it for every transition in 19, and if someone
> feels like optimizing it can be revisited for 20.

Agree, this is the simplest/safest choice for now.

> Another thing I am pondering is to place these tests under PG_TEST_EXTRA.
> While 128MB for shared_buffers is pretty moderate, I'm not sure we want to
> induce that on a normal BF run. 0002 does some backup cleaning as well as a
> pgperltidy and pgindent and some very minor fiddling with test code.

If we include 011, that definitely should go into extra as that's also
slow. 010 is at least relatively quick in its current form. Also 128
is just a quick "should be large enough" guess, 32mb also seems to be
enough for 010, it still triggers the failure without the fix.

> Users will obtain a backup with zero checksum validation, while believing checksum verification is enabled.

Why would they believe that it has checksums enabled, if they started
the backup before checksums completed?

> > but 010_backup_straddle.pl does not. Should 010_backup_straddle.pl also disable
> > bgwriter to preserve dirty pages?
>
> I'm not sure, does it need to?

I think the test should be safe as-is, since we have more than enough
shared buffers. Even with 32mb shared buffers, the test reliably
errors out without the fix.