Clean up warnings when the camera is stopped

Previously other threads might be trying to queue new requests while
the camera is being stopped. This could produce warnings if
queueRequest fails. Now the two procedures are protected from one
another with the camera_stop_mutex_.

Signed-off-by: David Plowman <[email protected]>

libcamera_app.cpp

@@ -344,6 +344,17 @@ void LibcameraApp::StartCamera()
 
 void LibcameraApp::StopCamera()
 {
+	{
+		// We don't want QueueRequest to run asynchronously while we stop the camera.
+		std::lock_guard<std::mutex> lock(camera_stop_mutex_);
+		if (camera_started_)
+		{
+			if (camera_->stop())
+				throw std::runtime_error("failed to stop camera");
+			camera_started_ = false;
+		}
+	}
+
 	if (camera_started_)
 	{
 		if (camera_->stop())
@@ -377,6 +388,12 @@ LibcameraApp::Msg LibcameraApp::Wait()
 
 void LibcameraApp::QueueRequest(CompletedRequest const &completed_request)
 {
+	// This function may run asynchronously so needs protection from the
+	// camera stopping at the same time.
+	std::lock_guard<std::mutex> stop_lock(camera_stop_mutex_);
+	if (!camera_started_)
+		return;
+
 	Request *request = nullptr;
 	{
 		std::lock_guard<std::mutex> lock(free_requests_mutex_);
@@ -404,11 +421,7 @@ void LibcameraApp::QueueRequest(CompletedRequest const &completed_request)
 	}
 
 	if (camera_->queueRequest(request) < 0)
-	{
-		// Don't make this fatal, some apps may stop the camera while the preview
-		// might still call us. (Arguably we should fix this better...)
-		std::cout << "(failed to queue request)" << std::endl;
-	}
+		throw std::runtime_error("failed to queue request");
 }
 
 void LibcameraApp::PostMessage(MsgType &t, MsgPayload &p)

libcamera_app.hpp

@@ -202,6 +202,7 @@ class LibcameraApp
 	std::queue<Request *> free_requests_;
 	std::vector<std::unique_ptr<Request>> requests_;
 	bool camera_started_ = false;
+	std::mutex camera_stop_mutex_;
 	MessageQueue<Msg> msg_queue_;
 	// Related to the preview window.
 	std::unique_ptr<Preview> preview_;