MDEV-13779 InnoDB fails to shut down purge, causing hang

thd_destructor_proxy(): Ensure that purge actually exits,
like the logic should have been ever since MDEV-14080.

srv_purge_shutdown(): A new function to wait for the
purge coordinator to exit. Before exiting, the
purge coordinator will ensure that all purge workers have exited.

storage/innobase/handler/ha_innodb.cc

@@ -354,7 +354,7 @@ thd_destructor_proxy(void *)
 	need to be purged, so they have to be shut down before purge
 	threads if slow shutdown is requested.  */
 	srv_shutdown_bg_undo_sources();
-	srv_purge_wakeup();
+	srv_purge_shutdown();
 
 	destroy_thd(thd);
 	mysql_cond_destroy(&thd_destructor_cond);

storage/innobase/include/srv0srv.h

@@ -3,7 +3,7 @@
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All rights reserved.
 Copyright (c) 2008, 2009, Google Inc.
 Copyright (c) 2009, Percona Inc.
-Copyright (c) 2013, 2017, MariaDB Corporation.
+Copyright (c) 2013, 2018, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -920,6 +920,9 @@ srv_release_threads(enum srv_thread_type type, ulint n);
 void
 srv_purge_wakeup();
 
+/** Shut down the purge threads. */
+void srv_purge_shutdown();
+
 /** Check if tablespace is being truncated.
 (Ignore system-tablespace as we don't re-create the tablespace
 and so some of the action that are suppressed by this function

storage/innobase/srv/srv0srv.cc

@@ -2963,6 +2963,15 @@ srv_purge_wakeup()
 		     || srv_sys.n_threads_active[SRV_PURGE]));
 }
 
+/** Shut down the purge threads. */
+void srv_purge_shutdown()
+{
+	do {
+		ut_ad(!srv_undo_sources);
+		srv_purge_wakeup();
+	} while (srv_sys.sys_threads[SRV_PURGE_SLOT].in_use);
+}
+
 /** Check if tablespace is being truncated.
 (Ignore system-tablespace as we don't re-create the tablespace
 and so some of the action that are suppressed by this function