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/pipeline_handler.cpp

@@ -5,7 +5,11 @@
  * pipeline_handler.cpp - Pipeline handler infrastructure
  */
 
+#include <libcamera/camera.h>
+#include <libcamera/camera_manager.h>
+
 #include "log.h"
+#include "media_device.h"
 #include "pipeline_handler.h"
 
 /**
@@ -97,6 +101,73 @@ PipelineHandler::~PipelineHandler()
  * constant for the whole lifetime of the pipeline handler.
  */
 
+/**
+ * \brief Register a camera to the camera manager and pipeline handler
+ * \param[in] camera The camera to be added
+ *
+ * This function is called by pipeline handlers to register the cameras they
+ * handle with the camera manager.
+ */
+void PipelineHandler::registerCamera(std::shared_ptr<Camera> camera)
+{
+	cameras_.push_back(camera);
+	manager_->addCamera(std::move(camera));
+}
+
+/**
+ * \brief Enable hotplug handling for a media device
+ * \param[in] media The media device
+ *
+ * This function enables hotplug handling, and especially hot-unplug handling,
+ * of the \a media device. It shall be called by pipeline handlers for all the
+ * media devices that can be disconnected.
+ *
+ * When a media device passed to this function is later unplugged, the pipeline
+ * handler gets notified and automatically disconnects all the cameras it has
+ * registered without requiring any manual intervention.
+ */
+void PipelineHandler::hotplugMediaDevice(MediaDevice *media)
+{
+	media->disconnected.connect(this, &PipelineHandler::mediaDeviceDisconnected);
+}
+
+/**
+ * \brief Device disconnection handler
+ *
+ * This virtual function is called to notify the pipeline handler that the
+ * device it handles has been disconnected. It notifies all cameras created by
+ * the pipeline handler that they have been disconnected, and unregisters them
+ * from the camera manager.
+ *
+ * The function can be overloaded by pipeline handlers to perform custom
+ * operations at disconnection time. Any overloaded version shall call the
+ * PipelineHandler::disconnect() base function for proper hot-unplug operation.
+ */
+void PipelineHandler::disconnect()
+{
+	for (std::weak_ptr<Camera> ptr : cameras_) {
+		std::shared_ptr<Camera> camera = ptr.lock();
+		if (!camera)
+			continue;
+
+		camera->disconnect();
+		manager_->removeCamera(camera.get());
+	}
+
+	cameras_.clear();
+}
+
+/**
+ * \brief Slot for the MediaDevice disconnected signal
+ */
+void PipelineHandler::mediaDeviceDisconnected(MediaDevice *media)
+{
+	if (cameras_.empty())
+		return;
+
+	disconnect();
+}
+
 /**
  * \class PipelineHandlerFactory
  * \brief Registration of PipelineHandler classes and creation of instances