gstreamer: Fix concurrent access issues to CameraManager

It's not allowed to have multiple instances of CameraManager. This
requirement is not easy for GStreamer were the device monitor and
the camerasrc, or two camerasrc instances don't usually have any
interaction between each other. Fix this by implementing a minimalist
singleton around CameraManager constructor and start()/stop()
operations.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/gstreamer/gstlibcamera-utils.cpp

@@ -178,3 +178,27 @@ gst_libcamera_resume_task(GstTask *task)
 		GST_TASK_SIGNAL(task);
 	}
 }
+
+G_LOCK_DEFINE_STATIC(cm_singleton_lock);
+std::weak_ptr<CameraManager> cm_singleton_ptr;
+
+std::shared_ptr<CameraManager>
+gst_libcamera_get_camera_mananger(int &ret)
+{
+	std::shared_ptr<CameraManager> cm;
+
+	G_LOCK(cm_singleton_lock);
+
+	cm = cm_singleton_ptr.lock();
+	if (!cm) {
+		cm = std::make_shared<CameraManager>();
+		cm_singleton_ptr = cm;
+		ret = cm->start();
+	} else {
+		ret = 0;
+	}
+
+	G_UNLOCK(cm_singleton_lock);
+
+	return cm;
+}