Cause of missing pg_clog files

Yesterday I reported a WAL problem that could lead to tuples not being
marked as committed-good or committed-dead after we'd already removed
the pg_clog segment that had their transaction's commit status.
I wasn't completely satisfied with that, though, because on further
reflection it seemed a very low-probability mechanism.  I kept digging,
and finally came to the kind of bug that qualifies as a big DOH :-(

If you run a database-wide VACUUM (one with no specific target table
mentioned) as a non-superuser, then the VACUUM doesn't process tables
that don't belong to you.  But it will advance pg_database.datvacuumxid
anyway, which means that pg_clog could be truncated while old transaction
references still remain unmarked in those other tables.

In words of one syllable: running VACUUM as a non-superuser can cause
irrecoverable data loss in any 7.2.* release.

I think this qualifies as a "must fix" bug.  I recommend we back-patch
a fix for this into the REL7_2 branch and put out a 7.2.3 release.
We should also fix the "can't wait without a PROC" bug that was solved
a few days ago.
        regards, tom lane