MDEV-18603 Debug option to disable InnoDB monitor threads

If --debug=d,innodb_skip_monitors is specified, skip the creation
of srv_error_monitor_thread and srv_monitor_thread. In this way,
interactive debugging sessions will not be interfered by messages
about a mutex or rw_lock being held for a long time.

Closes #1192

storage/innobase/srv/srv0start.cc

@@ -3,7 +3,7 @@
 Copyright (c) 1996, 2017, Oracle and/or its affiliates. All rights reserved.
 Copyright (c) 2008, Google Inc.
 Copyright (c) 2009, Percona Inc.
-Copyright (c) 2013, 2018, MariaDB Corporation.
+Copyright (c) 2013, 2019, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -2323,6 +2323,7 @@ dberr_t srv_start(bool create_new_db)
 		thread_started[2 + SRV_MAX_N_IO_THREADS] = true;
 		lock_sys.timeout_thread_active = true;
 
+		DBUG_EXECUTE_IF("innodb_skip_monitors", goto skip_monitors;);
 		/* Create the thread which warns of long semaphore waits */
 		srv_error_monitor_active = true;
 		thread_handles[3 + SRV_MAX_N_IO_THREADS] = os_thread_create(
@@ -2339,6 +2340,9 @@ dberr_t srv_start(bool create_new_db)
 		srv_start_state |= SRV_START_STATE_LOCK_SYS
 			| SRV_START_STATE_MONITOR;
 
+#ifndef DBUG_OFF
+skip_monitors:
+#endif
 		ut_ad(srv_force_recovery >= SRV_FORCE_NO_UNDO_LOG_SCAN
 		      || !purge_sys.enabled());