In the 10.1 InnoDB Plugin, a call os_event_free(buf_flush_event) was
misplaced. The event could be signalled by rollback of resurrected
transactions while shutdown was in progress. This bug was caught
by cmake -DWITH_ASAN testing. This call was only present in the
10.1 InnoDB Plugin, not in other versions, or in XtraDB.

That said, the bug affects all InnoDB versions. Shutdown assumes the
cessation of any page-dirtying activity, including the activity of
the background rollback thread. InnoDB only waited for the background
rollback to finish as part of a slow shutdown (innodb_fast_shutdown=0).
The default is a clean shutdown (innodb_fast_shutdown=1). In a scenario
where InnoDB is killed, restarted, and shut down soon enough, the data
files could become corrupted.

logs_empty_and_mark_files_at_shutdown(): Wait for the
rollback to finish, except if innodb_fast_shutdown=2
(crash-like shutdown) was requested.

trx_rollback_or_clean_recovered(): Before choosing the next
recovered transaction to roll back, terminate early if non-slow
shutdown was initiated. Roll back everything on slow shutdown
(innodb_fast_shutdown=0).

srv_innodb_monitor_mutex: Declare as static, because the mutex
is only used within one module.

After each call to os_event_free(), ensure that the freed event
is not reachable via global variables, by setting the relevant
variables to NULL.