87   std::mutex mutex_;  variable
114     std::lock_guard<std::mutex> lock(mutex_);  in Enqueue()
122     std::lock_guard<std::mutex> lock(mutex_);  in Clear()
135     std::lock_guard<std::mutex> lock(mutex_);  in NotifyOnEmpty()
142     std::lock_guard<std::mutex> lock(mutex_);  in enqueue_callback()
154   mutable std::mutex mutex_;  variable
172   std::lock_guard<std::mutex> lock(mutex_);  in RegisterEnqueue()
188     std::lock_guard<std::mutex> lock(mutex_);  in UnregisterEnqueue()
205   std::lock_guard<std::mutex> lock(mutex_);  in RegisterDequeue()
219     std::lock_guard<std::mutex> lock(mutex_);  in UnregisterDequeue()
236   std::lock_guard<std::mutex> lock(mutex_);  in TryDequeue()
256   std::lock_guard<std::mutex> lock(mutex_);  in EnqueueCallbackInternal()