// Register imports a pre-existing image into the graph.
// Returns nil if the image is already registered.
func (graph *Graph) Register(im image.Descriptor, layerData io.Reader) (err error) {
	imgID := im.ID()

	if err := image.ValidateID(imgID); err != nil {
		return err
	}

	// We need this entire operation to be atomic within the engine. Note that
	// this doesn't mean Register is fully safe yet.
	graph.imageMutex.Lock(imgID)
	defer graph.imageMutex.Unlock(imgID)

	return graph.register(im, layerData)
}

func (graph *Graph) register(im image.Descriptor, layerData io.Reader) (err error) {
	imgID := im.ID()

	// Skip register if image is already registered
	if graph.Exists(imgID) {
		return nil
	}

	// The returned `error` must be named in this function's signature so that
	// `err` is not shadowed in this deferred cleanup.
	defer func() {
		// If any error occurs, remove the new dir from the driver.
		// Don't check for errors since the dir might not have been created.
		if err != nil {
			graph.driver.Remove(imgID)
		}
	}()

	// Ensure that the image root does not exist on the filesystem
	// when it is not registered in the graph.
	// This is common when you switch from one graph driver to another
	if err := os.RemoveAll(graph.imageRoot(imgID)); err != nil && !os.IsNotExist(err) {
		return err
	}

	// If the driver has this ID but the graph doesn't, remove it from the driver to start fresh.
	// (the graph is the source of truth).
	// Ignore errors, since we don't know if the driver correctly returns ErrNotExist.
	// (FIXME: make that mandatory for drivers).
	graph.driver.Remove(imgID)

	tmp, err := graph.mktemp()
	defer os.RemoveAll(tmp)
	if err != nil {
		return fmt.Errorf("mktemp failed: %s", err)
	}

	parent := im.Parent()

	// Create root filesystem in the driver
	if err := createRootFilesystemInDriver(graph, imgID, parent, layerData); err != nil {
		return err
	}

	// Apply the diff/layer
	config, err := im.MarshalConfig()
	if err != nil {
		return err
	}
	if err := graph.storeImage(imgID, parent, config, layerData, tmp); err != nil {
		return err
	}
	// Commit
	if err := os.Rename(tmp, graph.imageRoot(imgID)); err != nil {
		return err
	}
	graph.idIndex.Add(imgID)
	return nil
}