LP #1609364: --innodb_purge_threads=1 may result in incomplete purge with innodb_fast_shutdown=0

**Reported in Launchpad by Laurynas Biveinis last update 03-08-2016 17:17:03

A 5.5-only issue, the main loop of the separate 5.5 purge thread has the following break condition:

/* Check for shutdown and whether we should do purge at all. */
if (srv_force_recovery >= SRV_FORCE_NO_BACKGROUND

break;

}

Which means immediate exit on srv_shutdown_state != 0, even if the thread should continue to purge if srv_fast_shutdown == 0.

This is likely the cause of bug 756387. This is an upstream issue but the upstream defaults to --innodb-purge-threads=0 whereas the PS default is 1.