BlockingList: fix unsafe getLockGuard() API

getLockGuard relies on copy elision to work correctly, which the compiler
is not required to do (only with C++17).
If no copy elision happens, the mutex ends up being unlocked twice, and the
CS is executed with the mutex unlocked.

The patch also ensures that the same pattern cannot be used again.

platforms/common/px4_work_queue/WorkQueueManager.cpp

@@ -70,7 +70,7 @@ FindWorkQueueByName(const char *name)
 		return nullptr;
 	}
 
-	auto lg = _wq_manager_wqs_list->getLockGuard();
+	LockGuard lg{_wq_manager_wqs_list->mutex()};
 
 	// search list
 	for (WorkQueue *wq : *_wq_manager_wqs_list) {
@@ -298,7 +298,7 @@ WorkQueueManagerStop()
 		// first ask all WQs to stop
 		if (_wq_manager_wqs_list != nullptr) {
 			{
-				auto lg = _wq_manager_wqs_list->getLockGuard();
+				LockGuard lg{_wq_manager_wqs_list->mutex()};
 
 				// ask all work queues (threads) to stop
 				// NOTE: not currently safe without all WorkItems stopping first
@@ -342,7 +342,7 @@ WorkQueueManagerStatus()
 		const size_t num_wqs = _wq_manager_wqs_list->size();
 		PX4_INFO_RAW("\nWork Queue: %-1zu threads                      RATE        INTERVAL\n", num_wqs);
 
-		auto lg = _wq_manager_wqs_list->getLockGuard();
+		LockGuard lg{_wq_manager_wqs_list->mutex()};
 		size_t i = 0;
 
 		for (WorkQueue *wq : *_wq_manager_wqs_list) {

src/include/containers/BlockingList.hpp

@@ -80,7 +80,7 @@ public:
 		List<T>::clear();
 	}
 
-	LockGuard getLockGuard() { return LockGuard{_mutex}; }
+	pthread_mutex_t &mutex() { return _mutex; }
 
 private:
 

src/include/containers/LockGuard.hpp

@@ -44,6 +44,9 @@ public:
 		pthread_mutex_lock(&_mutex);
 	}
 
+	LockGuard(const LockGuard &other) = delete;
+	LockGuard &operator=(const LockGuard &other) = delete;
+
 	~LockGuard()
 	{
 		pthread_mutex_unlock(&_mutex);