MDEV-23996 Race conditions in SHOW ENGINE INNODB MUTEX

The function innodb_show_mutex_status() is the only ultimate caller of
LatchCounter::iterate() via MutexMonitor::iterate(). Because the call
is not protected by LatchCounter::m_mutex, any mutex_create() or
mutex_free() that is invoked concurrently during the execution, bad
things such as a crash could happen.

The most likely way for this to happen is buffer pool resizing,
which could cause buf_block_t::mutex (which existed before MDEV-15053)
to be created or freed. We could also register InnoDB mutexes in
TrxFactory::init() if trx_pools needs to grow.

The view INFORMATION_SCHEMA.INNODB_MUTEXES is not affected, because it
only displays information about rw-locks, not mutexes.

This commit intentionally touches also MutexMonitor::iterate()
and the only code that interfaces with LatchCounter::iterate()
to make it clearer for future readers that the scattered code
that is obfuscated by templates belongs together.

This is based on
mysql/mysql-server@273a93396f49c7e0a8b07b260128d9a990c2b154

storage/innobase/handler/ha_innodb.cc

@@ -16332,8 +16332,7 @@ struct ShowStatus {
 		/** Collect the latch metrics. Ignore entries where the
 		spins and waits are zero.
 		@param[in]	count		The latch metrics */
-		void operator()(Count* count)
-			UNIV_NOTHROW
+		void operator()(Count* count) const UNIV_NOTHROW
 		{
 			if (count->m_spins > 0 || count->m_waits > 0) {
 
@@ -16361,13 +16360,8 @@ struct ShowStatus {
 	bool operator()(latch_meta_t& latch_meta)
 		UNIV_NOTHROW
 	{
-		latch_meta_t::CounterType*	counter;
-
-		counter = latch_meta.get_counter();
-
-		GetCount	get_count(latch_meta.get_name(), &m_values);
-
-		counter->iterate(get_count);
+		latch_meta.get_counter()->iterate(
+			GetCount(latch_meta.get_name(), &m_values));
 
 		return(true);
 	}

storage/innobase/include/sync0types.h

@@ -659,10 +659,10 @@ class LatchCounter {
 	}
 
 	/** Iterate over the counters */
-	template <typename Callback>
-	void iterate(Callback& callback) const
-		UNIV_NOTHROW
+	template<typename C> void iterate(const C& callback) UNIV_NOTHROW
 	{
+		m_mutex.enter();
+
 		Counters::const_iterator	end = m_counters.end();
 
 		for (Counters::const_iterator it = m_counters.begin();
@@ -671,6 +671,8 @@ class LatchCounter {
 
 			callback(*it);
 		}
+
+		m_mutex.exit();
 	}
 
 	/** Disable the monitoring */

storage/innobase/include/ut0mutex.h

 /*****************************************************************************
 
 Copyright (c) 2012, 2015, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, MariaDB Corporation.
+Copyright (c) 2017, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -142,16 +142,10 @@ class MutexMonitor {
 
 			/* Some of the slots will be null in non-debug mode */
 
-			if (*it == NULL) {
-				continue;
-			}
-
-			latch_meta_t*	latch_meta = *it;
-
-			bool	ret = callback(*latch_meta);
-
-			if (!ret) {
-				return(ret);
+			if (latch_meta_t* l= *it) {
+				if (!callback(*l)) {
+					return false;
+				}
 			}
 		}