func CommandAsm(c *cli.Context) {
	if len(c.Args()) > 0 {
		logrus.Warnf("%d additional arguments passed are ignored", len(c.Args()))
	}
	if len(c.String("input")) == 0 {
		logrus.Fatalf("--input filename must be set")
	}
	if len(c.String("output")) == 0 {
		logrus.Fatalf("--output filename must be set ([FILENAME|-])")
	}
	if len(c.String("path")) == 0 {
		logrus.Fatalf("--path must be set")
	}

	var outputStream io.Writer
	if c.String("output") == "-" {
		outputStream = os.Stdout
	} else {
		fh, err := os.Create(c.String("output"))
		if err != nil {
			logrus.Fatal(err)
		}
		defer fh.Close()
		outputStream = fh
	}

	// Get the tar metadata reader
	mf, err := os.Open(c.String("input"))
	if err != nil {
		logrus.Fatal(err)
	}
	defer mf.Close()
	mfz, err := gzip.NewReader(mf)
	if err != nil {
		logrus.Fatal(err)
	}
	defer mfz.Close()

	metaUnpacker := storage.NewJSONUnpacker(mfz)
	// XXX maybe get the absolute path here
	fileGetter := storage.NewPathFileGetter(c.String("path"))

	ots := asm.NewOutputTarStream(fileGetter, metaUnpacker)
	defer ots.Close()
	i, err := io.Copy(outputStream, ots)
	if err != nil {
		logrus.Fatal(err)
	}

	logrus.Infof("created %s from %s and %s (wrote %d bytes)", c.String("output"), c.String("path"), c.String("input"), i)
}

func (graph *Graph) assembleTarLayer(img *image.Image) (io.ReadCloser, error) {
	root := graph.imageRoot(img.ID)
	mFileName := filepath.Join(root, tarDataFileName)
	mf, err := os.Open(mFileName)
	if err != nil {
		if !os.IsNotExist(err) {
			logrus.Errorf("failed to open %q: %s", mFileName, err)
		}
		return nil, err
	}
	pR, pW := io.Pipe()
	// this will need to be in a goroutine, as we are returning the stream of a
	// tar archive, but can not close the metadata reader early (when this
	// function returns)...
	go func() {
		defer mf.Close()
		// let's reassemble!
		logrus.Debugf("[graph] TarLayer with reassembly: %s", img.ID)
		mfz, err := gzip.NewReader(mf)
		if err != nil {
			pW.CloseWithError(fmt.Errorf("[graph] error with %s:  %s", mFileName, err))
			return
		}
		defer mfz.Close()

		// get our relative path to the container
		fsLayer, err := graph.driver.Get(img.ID, "")
		if err != nil {
			pW.CloseWithError(err)
			return
		}
		defer graph.driver.Put(img.ID)

		metaUnpacker := storage.NewJSONUnpacker(mfz)
		fileGetter := storage.NewPathFileGetter(fsLayer)
		logrus.Debugf("[graph] %s is at %q", img.ID, fsLayer)
		ots := asm.NewOutputTarStream(fileGetter, metaUnpacker)
		defer ots.Close()
		if _, err := io.Copy(pW, ots); err != nil {
			pW.CloseWithError(err)
			return
		}
		pW.Close()
	}()
	return pR, nil
}

func (ls *layerStore) assembleTar(graphID string, metadata io.ReadCloser, size *int64) (io.ReadCloser, error) {
	type diffPathDriver interface {
		DiffPath(string) (string, func() error, error)
	}

	diffDriver, ok := ls.driver.(diffPathDriver)
	if !ok {
		diffDriver = &naiveDiffPathDriver{ls.driver}
	}

	// get our relative path to the container
	fsPath, releasePath, err := diffDriver.DiffPath(graphID)
	if err != nil {
		metadata.Close()
		return nil, err
	}

	pR, pW := io.Pipe()
	// this will need to be in a goroutine, as we are returning the stream of a
	// tar archive, but can not close the metadata reader early (when this
	// function returns)...
	go func() {
		defer releasePath()
		defer metadata.Close()

		metaUnpacker := storage.NewJSONUnpacker(metadata)
		upackerCounter := &unpackSizeCounter{metaUnpacker, size}
		fileGetter := storage.NewPathFileGetter(fsPath)
		logrus.Debugf("Assembling tar data for %s from %s", graphID, fsPath)
		ots := asm.NewOutputTarStream(fileGetter, upackerCounter)
		defer ots.Close()
		if _, err := io.Copy(pW, ots); err != nil {
			pW.CloseWithError(err)
			return
		}
		pW.Close()
	}()
	return pR, nil
}