libcamera: pipeline_handler: Add camera disconnection support

Pipeline handlers are responsible for creating camera instances, but
also for destroying them when devices are unplugged. As camera objects
are reference-counted this isn't a straightforward operation and
involves the camera manager and camera object itself. Add two helper
methods in the PipelineHandler base class to register a camera and to
register a media device with the pipeline handler.

When registering a camera, the registerCamera() helper method will add
it to the camera manager. When registering a media device, the
registerMediaDevice() helper method will listen to device disconnection
events, and disconnect all cameras created by the pipeline handler as a
response.

Under the hood the PipelineHandler class needs to keep track of
registered cameras in order to handle disconnection. They can't be
stored as shared pointers as this would create a circular dependency
(the Camera class owns a shared pointer to the pipeline handler). Store
them as weak pointers instead. This is safe as a reference to the camera
is stored in the camera manager, and doesn't get removed until the
camera is unregistered from the manager by the PipelineHandler.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/include/pipeline_handler.h

@@ -16,6 +16,7 @@ namespace libcamera {
 
 class CameraManager;
 class DeviceEnumerator;
+class MediaDevice;
 
 class PipelineHandler : public std::enable_shared_from_this<PipelineHandler>
 {
@@ -27,6 +28,15 @@ public:
 
 protected:
 	CameraManager *manager_;
+
+	void registerCamera(std::shared_ptr<Camera> camera);
+	void hotplugMediaDevice(MediaDevice *media);
+
+private:
+	virtual void disconnect();
+	void mediaDeviceDisconnected(MediaDevice *media);
+
+	std::vector<std::weak_ptr<Camera>> cameras_;
 };
 
 class PipelineHandlerFactory