Ye olde drop-the-database-you-just-left problem

I just finished giving someone the standard advice to wait a bit before
trying to drop a database that'd just been accessed:
http://archives.postgresql.org/pgsql-general/2007-05/msg01505.php

AFAICT a "real" fix for this would involve making PQfinish() synchronous
(don't return till backend is dead), which doesn't seem like a great
idea. However, it suddenly struck me that we could probably make most
of the problem go away if we put that same wait into DROP DATABASE
itself --- that is, if we see other backends in the target DB, sleep
for a second or two and then recheck before erroring out.

This isn't bulletproof since under high load the other backend might
not get to quit, but it'd surely reduce the frequency of complaints
a great deal. And we could take out the ad-hoc sleeps that are done
in (eg) the contrib regression tests.

Thoughts?

regards, tom lane