LP:489088 - On shutdown MySQL reports: [Warning] Plugin 'PBXT' will be forced to shutdown

As reported by Kristian:

MySQL reports: [Warning] Plugin 'PBXT' will be forced to shutdown

The bug has been repeated with MariaDB: lp:~maria-captains/maria/5.1-merge

Create a file "mysql-test/suite/pbxt/t/kn.test", with the following contents:

------------------------------ cut here ------------------------------
--skip "Test shutdown problem with skipped test"
------------------------------ cut here ------------------------------

Then enter:

cd mysql-test
perl mysql-test-run.pl --suite=pbxt pbxt.kn

Kristian says:

I get this in the mysqld.1.err:

CURRENT_TEST: pbxt.kn
091124 15:11:00 [Note] Plugin 'InnoDB' is disabled.
091124 15:11:00 [Note] PrimeBase XT (PBXT) Engine 1.0.08d RC loaded...
091124 15:11:00 [Note] Paul McCullagh, PrimeBase Technologies GmbH, http://www.primebase.org
091124 15:11:00 [Note] Event Scheduler: Loaded 0 events
091124 15:11:00 [Note] /home/knielsen/devel/maria/my/merge-5.1.41/sql/mysqld: ready for connections.
Version: '5.1.41-mariadb-beta-valgrind-max-debug-log' socket: '/home/knielsen/devel/maria/my/merge-5.1.41/mysql-test/var/tmp/mysqld.1.sock' port: 13000 Source distribution
091124 15:11:00 [Note] Got signal 15 to shutdown mysqld
091124 15:11:00 [Note] /home/knielsen/devel/maria/my/merge-5.1.41/sql/mysqld: Normal shutdown

091124 15:11:00 [Note] Event Scheduler: Purging the queue. 0 events
091124 15:11:00 [Warning] Plugin 'MyISAM' will be forced to shutdown
091124 15:11:00 [Warning] Plugin 'PBXT' will be forced to shutdown
091124 15:11:00 [Note] PrimeBase XT Engine shutdown...
091124 15:11:00 [Note] Debug sync points hit: 50
091124 15:11:00 [Note] Debug sync points executed: 0
091124 15:11:00 [Note] Debug sync points max active per thread: 0
091124 15:11:00 [Note] /home/knielsen/devel/maria/my/merge-5.1.41/sql/mysqld: Shutdown complete

This is from this tree on Launchpad containing the merge with MySQL 5.1.41:

lp:~maria-captains/maria/5.1-merge

The problem is that the ref_count of PBXT is 2 at shutdown time.

Maybe this is another problem with the way that PBXT has to do init in a
separate thread, I see you already had to solve some other possibly related
problems. Any ideas? It may be that this is something introduced in the merge
with MySQL 5.1.41, not 100% sure though as this particular warning was not
detected before in our Buildbot tests...