BUG #12772: Unexpected autovacuum behavior

The following bug has been logged on the website:

Bug reference:      12772
Logged by:          Jason DiCioccio
Email address:      jd@ods.org
PostgreSQL version: 9.3.6
Operating system:   Ubuntu 14.04 (Linux 3.13.0-40-generic)
Description:

Here's the situation I ran into:

autovacuum was vacuuming a large table in database 'db1' (to prevent txid
wraparound).  It turns out that one of the indexes of this table was
corrupted from the 9.3.4 WAL issue.  So VACUUM failed repeatedly.
Nonetheless, autovacuum kept trying.

Meanwhile in database 'db2', there were a number of tables in serious need
of vacuuming.  However, autovacuum would never touch these, even though
there were far less than autovacuum_max_workers running.  Other tables in
database 'db1' WERE being vacuumed, however.

So it appears that the logic is that autovacuum operates solely on one
database at a time.  Even if there is only one table that needs vacuuming in
that first database, it will not spawn any workers to vacuum tables that do
need vacuuming in a second database until it has completed vacuuming the
necessary tables from the first database.

This, to me, is unexpected behavior.  I'd expect autovacuum to not act as if
there were a large barrier between databases and to vacuum any table that
need it and that the configuration permits.