libcamera: camera_manager: Run the camera manager in a thread

Relying on the application event loop to process all our internal events
is a bad idea for multiple reasons. In many cases the user of libcamera
can't provide an event loop, for instance when running through one of
the adaptation layers. The Android camera HAL and V4L2 compatibility
layer create a thread for this reason, and the GStreamer element would
need to do so as well. Furthermore, relying on the application event
loop pushes libcamera's realtime constraints to the application, which
isn't manageable.

For these reasons it's desirable to always run the camera manager, the
pipeline handlers and the cameras in a separate thread. Doing so isn't
too complicated, it only involves creating the thread internally when
starting the camera manager, and synchronizing a few methods of the
Camera class. Do so as a first step towards defining the threading model
of libcamera.

The event dispatcher interface is still exposed to applications, to
enable cross-thread signal delivery if desired.

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

src/libcamera/camera_manager.cpp

@@ -7,6 +7,7 @@
 
 #include <libcamera/camera_manager.h>
 
+#include <condition_variable>
 #include <map>
 
 #include <libcamera/camera.h>
@@ -28,33 +29,92 @@ namespace libcamera {
 
 LOG_DEFINE_CATEGORY(Camera)
 
-class CameraManager::Private
+class CameraManager::Private : public Thread
 {
 public:
 	Private(CameraManager *cm);
 
 	int start();
-	void stop();
-
 	void addCamera(std::shared_ptr<Camera> &camera, dev_t devnum);
 	void removeCamera(Camera *camera);
 
+	/*
+	 * This mutex protects
+	 *
+	 * - initialized_ and status_ during initialization
+	 * - cameras_ and camerasByDevnum_ after initialization
+	 */
+	Mutex mutex_;
 	std::vector<std::shared_ptr<Camera>> cameras_;
 	std::map<dev_t, std::weak_ptr<Camera>> camerasByDevnum_;
 
+protected:
+	void run() override;
+
 private:
+	int init();
+	void cleanup();
+
 	CameraManager *cm_;
 
+	std::condition_variable cv_;
+	bool initialized_;
+	int status_;
+
 	std::vector<std::shared_ptr<PipelineHandler>> pipes_;
 	std::unique_ptr<DeviceEnumerator> enumerator_;
 };
 
 CameraManager::Private::Private(CameraManager *cm)
-	: cm_(cm)
+	: cm_(cm), initialized_(false)
 {
 }
 
 int CameraManager::Private::start()
+{
+	int status;
+
+	/* Start the thread and wait for initialization to complete. */
+	Thread::start();
+
+	{
+		MutexLocker locker(mutex_);
+		cv_.wait(locker, [&] { return initialized_; });
+		status = status_;
+	}
+
+	/* If a failure happened during initialization, stop the thread. */
+	if (status < 0) {
+		exit();
+		wait();
+		return status;
+	}
+
+	return 0;
+}
+
+void CameraManager::Private::run()
+{
+	LOG(Camera, Debug) << "Starting camera manager";
+
+	int ret = init();
+
+	mutex_.lock();
+	status_ = ret;
+	initialized_ = true;
+	mutex_.unlock();
+	cv_.notify_one();
+
+	if (ret < 0)
+		return;
+
+	/* Now start processing events and messages. */
+	exec();
+
+	cleanup();
+}
+
+int CameraManager::Private::init()
 {
 	enumerator_ = DeviceEnumerator::create();
 	if (!enumerator_ || enumerator_->enumerate())
@@ -89,7 +149,7 @@ int CameraManager::Private::start()
 	return 0;
 }
 
-void CameraManager::Private::stop()
+void CameraManager::Private::cleanup()
 {
 	/* TODO: unregister hot-plug callback here */
 
@@ -107,6 +167,8 @@ void CameraManager::Private::stop()
 void CameraManager::Private::addCamera(std::shared_ptr<Camera> &camera,
 				       dev_t devnum)
 {
+	MutexLocker locker(mutex_);
+
 	for (std::shared_ptr<Camera> c : cameras_) {
 		if (c->name() == camera->name()) {
 			LOG(Camera, Warning)
@@ -126,6 +188,8 @@ void CameraManager::Private::addCamera(std::shared_ptr<Camera> &camera,
 
 void CameraManager::Private::removeCamera(Camera *camera)
 {
+	MutexLocker locker(mutex_);
+
 	auto iter = std::find_if(cameras_.begin(), cameras_.end(),
 				 [camera](std::shared_ptr<Camera> &c) {
 					 return c.get() == camera;
@@ -232,7 +296,8 @@ int CameraManager::start()
  */
 void CameraManager::stop()
 {
-	p_->stop();
+	p_->exit();
+	p_->wait();
 }
 
 /**
@@ -242,10 +307,14 @@ void CameraManager::stop()
  * Before calling this function the caller is responsible for ensuring that
  * the camera manager is running.
  *
+ * \context This function is \threadsafe.
+ *
  * \return List of all available cameras
  */
 std::vector<std::shared_ptr<Camera>> CameraManager::cameras() const
 {
+	MutexLocker locker(p_->mutex_);
+
 	return p_->cameras_;
 }
 
@@ -256,10 +325,14 @@ std::vector<std::shared_ptr<Camera>> CameraManager::cameras() const
  * Before calling this function the caller is responsible for ensuring that
  * the camera manager is running.
  *
+ * \context This function is \threadsafe.
+ *
  * \return Shared pointer to Camera object or nullptr if camera not found
  */
 std::shared_ptr<Camera> CameraManager::get(const std::string &name)
 {
+	MutexLocker locker(p_->mutex_);
+
 	for (std::shared_ptr<Camera> camera : p_->cameras_) {
 		if (camera->name() == name)
 			return camera;
@@ -279,11 +352,15 @@ std::shared_ptr<Camera> CameraManager::get(const std::string &name)
  * Before calling this function the caller is responsible for ensuring that
  * the camera manager is running.
  *
+ * \context This function is \threadsafe.
+ *
  * \return Shared pointer to Camera object, which is empty if the camera is
  * not found
  */
 std::shared_ptr<Camera> CameraManager::get(dev_t devnum)
 {
+	MutexLocker locker(p_->mutex_);
+
 	auto iter = p_->camerasByDevnum_.find(devnum);
 	if (iter == p_->camerasByDevnum_.end())
 		return nullptr;
@@ -302,9 +379,12 @@ std::shared_ptr<Camera> CameraManager::get(dev_t devnum)
  *
  * \a devnum is used by the V4L2 compatibility layer to map V4L2 device nodes
  * to Camera instances.
+ *
+ * \context This function shall be called from the CameraManager thread.
  */
 void CameraManager::addCamera(std::shared_ptr<Camera> camera, dev_t devnum)
 {
+	ASSERT(Thread::current() == p_.get());
 
 	p_->addCamera(camera, devnum);
 }
@@ -316,15 +396,20 @@ void CameraManager::addCamera(std::shared_ptr<Camera> camera, dev_t devnum)
  * This function is called by pipeline handlers to unregister cameras from the
  * camera manager. Unregistered cameras won't be reported anymore by the
  * cameras() and get() calls, but references may still exist in applications.
+ *
+ * \context This function shall be called from the CameraManager thread.
  */
 void CameraManager::removeCamera(Camera *camera)
 {
+	ASSERT(Thread::current() == p_.get());
+
 	p_->removeCamera(camera);
 }
 
 /**
  * \fn const std::string &CameraManager::version()
  * \brief Retrieve the libcamera version string
+ * \context This function is \a threadsafe.
  * \return The libcamera version string
  */