Bug #48541 Deadlock between LOCK_open and LOCK_mdl

The reason for the deadlock was an improper exit from
MDL_context::wait_for_locks() which caused mysys_var->current_mutex to remain
LOCK_mdl even though LOCK_mdl was no longer held by that connection. 

This could for example lead to a deadlock in the following way:
1) INSERT DELAYED tries to open a table but fails, and trying to recover it
calls wait_for_locks().
2) Due to a pending exclusive request, wait_for_locks() fails and exits without
resetting mysys_var->current_mutex for the delayed insert handler thread. So it
continues to point to LOCK_mdl.
3) The handler thread manages to open a table.
4) A different connection takes LOCK_open and tries to take LOCK_mdl.
5) FLUSH TABLES from a third connection notices that the handler thread has a
table open, and tries to kill it. This involves locking mysys_var->current_mutex
while having LOCK_open locked. Since current_mutex mistakenly points to LOCK_mdl,
we have a deadlock.

This patch makes sure MDL_EXIT_COND() is called before exiting wait_for_locks().
This clears mysys->current_mutex which resolves the issue. 

An assert is added to recover_from_failed_open_table_attempt() after
wait_for_locks() is called, to check that current_mutex is indeed reset.
With this assert in place, existing tests in (e.g.) mdl_sync.test will fail
without this patch.

sql/mdl.cc

@@ -1254,7 +1254,8 @@ MDL_context::wait_for_locks(MDL_request_list *mdl_requests)
     }
     if (!mdl_request)
     {
-      pthread_mutex_unlock(&LOCK_mdl);
+      /* As a side-effect MDL_EXIT_COND() unlocks LOCK_mdl. */
+      MDL_EXIT_COND(m_thd, mysys_var, old_msg);
       break;
     }
     pthread_cond_wait(&COND_mdl, &LOCK_mdl);

sql/sql_base.cc

@@ -3693,6 +3693,7 @@ recover_from_failed_open_table_attempt(THD *thd, TABLE_LIST *table)
     case OT_WAIT:
       result= (thd->mdl_context.wait_for_locks(&m_mdl_requests) ||
                tdc_wait_for_old_versions(thd, &m_mdl_requests));
+      DBUG_ASSERT(thd->mysys_var->current_mutex == NULL);
       break;
     case OT_DISCOVER:
       {