@@ -34,6 +34,7 @@ import (

 	"syscall"

 	"github.com/Sirupsen/logrus"

+	"github.com/vbatts/tar-split/tar/storage"

 	"github.com/docker/docker/daemon/graphdriver"

 	"github.com/docker/docker/pkg/archive"

@@ -367,10 +368,19 @@ func (a *Driver) Diff(id, parent string) (archive.Archive, error) {

 	})

 }

-// DiffPath returns path to the directory that contains files for the layer

-// differences. Used for direct access for tar-split.

-func (a *Driver) DiffPath(id string) (string, func() error, error) {

-	return path.Join(a.rootPath(), "diff", id), func() error { return nil }, nil

+type fileGetNilCloser struct {

+	storage.FileGetter

+}

+

+func (f fileGetNilCloser) Close() error {

+	return nil

+}

+

+// DiffGetter returns a FileGetCloser that can read files from the directory that

+// contains files for the layer differences. Used for direct access for tar-split.

+func (a *Driver) DiffGetter(id string) (graphdriver.FileGetCloser, error) {

+	p := path.Join(a.rootPath(), "diff", id)

+	return fileGetNilCloser{storage.NewPathFileGetter(p)}, nil

 }

 func (a *Driver) applyDiff(id string, diff archive.Reader) error {

@@ -8,6 +8,7 @@ import (

 	"strings"

 	"github.com/Sirupsen/logrus"

+	"github.com/vbatts/tar-split/tar/storage"

 	"github.com/docker/docker/pkg/archive"

 	"github.com/docker/docker/pkg/idtools"

@@ -92,6 +93,23 @@ type Driver interface {

 	DiffSize(id, parent string) (size int64, err error)

 }

+// DiffGetterDriver is the interface for layered file system drivers that

+// provide a specialized function for getting file contents for tar-split.

+type DiffGetterDriver interface {

+	Driver

+	// DiffGetter returns an interface to efficiently retrieve the contents

+	// of files in a layer.

+	DiffGetter(id string) (FileGetCloser, error)

+}

+

+// FileGetCloser extends the storage.FileGetter interface with a Close method

+// for cleaning up.

+type FileGetCloser interface {

+	storage.FileGetter

+	// Close cleans up any resources associated with the FileGetCloser.

+	Close() error

+}

+

 func init() {

 	drivers = make(map[string]InitFunc)

 }

@@ -22,6 +22,7 @@ import (

 	"github.com/docker/docker/pkg/idtools"

 	"github.com/docker/docker/pkg/ioutils"

 	"github.com/docker/docker/pkg/random"

+	"github.com/vbatts/tar-split/tar/storage"

 )

 // init registers the windows graph drivers to the register.

@@ -47,6 +48,8 @@ type Driver struct {

 	active map[string]int

 }

+var _ graphdriver.DiffGetterDriver = &Driver{}

+

 // InitFilter returns a new Windows storage filter driver.

 func InitFilter(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (graphdriver.Driver, error) {

 	logrus.Debugf("WindowsGraphDriver InitFilter at %s", home)

@@ -564,8 +567,20 @@ func (d *Driver) setLayerChain(id string, chain []string) error {

 	return nil

 }

-// DiffPath returns a directory that contains files needed to construct layer diff.

-func (d *Driver) DiffPath(id string) (path string, release func() error, err error) {

+type fileGetDestroyCloser struct {

+	storage.FileGetter

+	d          *Driver

+	folderName string

+}

+

+func (f *fileGetDestroyCloser) Close() error {

+	// TODO: activate layers and release here?

+	return hcsshim.DestroyLayer(f.d.info, f.folderName)

+}

+

+// DiffGetter returns a FileGetCloser that can read files from the directory that

+// contains files for the layer differences. Used for direct access for tar-split.

+func (d *Driver) DiffGetter(id string) (fg graphdriver.FileGetCloser, err error) {

 	id, err = d.resolveID(id)

 	if err != nil {

 		return

@@ -597,9 +612,6 @@ func (d *Driver) DiffPath(id string) (path string, release func() error, err err

 		return

 	}

-	return tempFolder, func() error {

-		// TODO: activate layers and release here?

-		_, folderName := filepath.Split(tempFolder)

-		return hcsshim.DestroyLayer(d.info, folderName)

-	}, nil

+	_, folderName := filepath.Split(tempFolder)

+	return &fileGetDestroyCloser{storage.NewPathFileGetter(tempFolder), d, folderName}, nil

 }

@@ -577,11 +577,7 @@ func (ls *layerStore) initMount(graphID, parent, mountLabel string, initFunc Mou

 }

 func (ls *layerStore) assembleTarTo(graphID string, metadata io.ReadCloser, size *int64, w io.Writer) error {

-	type diffPathDriver interface {

-		DiffPath(string) (string, func() error, error)

-	}

-

-	diffDriver, ok := ls.driver.(diffPathDriver)

+	diffDriver, ok := ls.driver.(graphdriver.DiffGetterDriver)

 	if !ok {

 		diffDriver = &naiveDiffPathDriver{ls.driver}

 	}

@@ -589,17 +585,16 @@ func (ls *layerStore) assembleTarTo(graphID string, metadata io.ReadCloser, size

 	defer metadata.Close()

 	// get our relative path to the container

-	fsPath, releasePath, err := diffDriver.DiffPath(graphID)

+	fileGetCloser, err := diffDriver.DiffGetter(graphID)

 	if err != nil {

 		return err

 	}

-	defer releasePath()

+	defer fileGetCloser.Close()

 	metaUnpacker := storage.NewJSONUnpacker(metadata)

 	upackerCounter := &unpackSizeCounter{metaUnpacker, size}

-	fileGetter := storage.NewPathFileGetter(fsPath)

-	logrus.Debugf("Assembling tar data for %s from %s", graphID, fsPath)

-	return asm.WriteOutputTarStream(fileGetter, upackerCounter, w)

+	logrus.Debugf("Assembling tar data for %s", graphID)

+	return asm.WriteOutputTarStream(fileGetCloser, upackerCounter, w)

 }

 func (ls *layerStore) Cleanup() error {

@@ -618,12 +613,20 @@ type naiveDiffPathDriver struct {

 	graphdriver.Driver

 }

-func (n *naiveDiffPathDriver) DiffPath(id string) (string, func() error, error) {

+type fileGetPutter struct {

+	storage.FileGetter

+	driver graphdriver.Driver

+	id     string

+}

+

+func (w *fileGetPutter) Close() error {

+	return w.driver.Put(w.id)

+}

+

+func (n *naiveDiffPathDriver) DiffGetter(id string) (graphdriver.FileGetCloser, error) {

 	p, err := n.Driver.Get(id, "")

 	if err != nil {

-		return "", nil, err

+		return nil, err

 	}

-	return p, func() error {

-		return n.Driver.Put(id)

-	}, nil

+	return &fileGetPutter{storage.NewPathFileGetter(p), n.Driver, id}, nil

 }