sporadic crashes of innodb.innodb_prefix_index_restart_server

in slow shutdown mode purge threads really must exit only when there is
nothing to purge. Restore the trx_commit_disallowed check and
don't stop purge threads until all connection thread transactions
are gone.

storage/innobase/handler/ha_innodb.cc

@@ -340,6 +340,12 @@ thd_destructor_proxy(void *)
 	mysql_mutex_unlock(&thd_destructor_mutex);
 	thd_destructor_myvar = NULL;
 
+	srv_fast_shutdown = (ulint) innobase_fast_shutdown;
+	if (srv_fast_shutdown == 0) {
+		while (trx_sys_any_active_transactions()) {
+			os_thread_sleep(1000);
+		}
+	}
 	srv_purge_wakeup();
 
 	destroy_thd(thd);

storage/innobase/srv/srv0srv.cc

@@ -3073,6 +3073,12 @@ DECLARE_THREAD(srv_purge_coordinator_thread)(
 		n_pages_purged = trx_purge(1, srv_purge_batch_size, false);
 	}
 
+#ifdef UNIV_DEBUG
+	if (srv_fast_shutdown == 0) {
+		trx_commit_disallowed = true;
+	}
+#endif /* UNIV_DEBUG */
+
 	/* This trx_purge is called to remove any undo records (added by
 	background threads) after completion of the above loop. When
 	srv_fast_shutdown != 0, a large batch size can cause significant