"multiple backends attempting to wait for pincount 1"

Two different CLOBBER_CACHE_ALWAYS critters recently reported exactly
the same failure pattern on HEAD:

http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=markhor&dt=2015-02-06%2011%3A59%3A59
http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tick&dt=2015-02-12%2010%3A22%3A57

I'd say we have a problem.  I'd even go so far as to say that somebody has
completely broken locking, because this looks like autovacuum and manual
vacuuming are hitting the same table at the same time.
        regards, tom lane