@@ -93,12 +93,12 @@ type Builder struct {

 	// both of these are controlled by the Remove and ForceRemove options in BuildOpts

 	TmpContainers map[string]struct{} // a map of containers used for removes

-	dockerfile  *parser.Node           // the syntax tree of the dockerfile

-	image       string                 // image name for commit processing

-	maintainer  string                 // maintainer name. could probably be removed.

-	cmdSet      bool                   // indicates is CMD was set in current Dockerfile

-	context     tarsum.TarSumInterface // the context is a tarball that is uploaded by the client

-	contextPath string                 // the path of the temporary directory the local context is unpacked to (server side)

+	dockerfile  *parser.Node  // the syntax tree of the dockerfile

+	image       string        // image name for commit processing

+	maintainer  string        // maintainer name. could probably be removed.

+	cmdSet      bool          // indicates is CMD was set in current Dockerfile

+	context     tarsum.TarSum // the context is a tarball that is uploaded by the client

+	contextPath string        // the path of the temporary directory the local context is unpacked to (server side)

 }

@@ -167,7 +167,10 @@ func (b *Builder) runContextCommand(args []string, allowRemote bool, allowDecomp

 		if err != nil {

 			return err

 		}

-		tarSum := &tarsum.TarSum{Reader: r, DisableCompression: true}

+		tarSum, err := tarsum.NewTarSum(r, true, tarsum.Version0)

+		if err != nil {

+			return err

+		}

 		if _, err := io.Copy(ioutil.Discard, tarSum); err != nil {

 			return err

 		}

@@ -28,24 +28,24 @@ const (

 // This is used for calculating checksums of layers of an image, in some cases

 // including the byte payload of the image's json metadata as well, and for

 // calculating the checksums for buildcache.

-func NewTarSum(r io.Reader, dc bool, v Version) (TarSumInterface, error) {

+func NewTarSum(r io.Reader, dc bool, v Version) (TarSum, error) {

 	if _, ok := tarSumVersions[v]; !ok {

 		return nil, ErrVersionNotImplemented

 	}

-	return &TarSum{Reader: r, DisableCompression: dc, tarSumVersion: v}, nil

+	return &tarSum{Reader: r, DisableCompression: dc, tarSumVersion: v}, nil

 }

-// TarSumInterface is the generic interface for calculating fixed time

+// TarSum is the generic interface for calculating fixed time

 // checksums of a tar archive

-type TarSumInterface interface {

+type TarSum interface {

 	io.Reader

 	GetSums() map[string]string

 	Sum([]byte) string

 	Version() Version

 }

-// TarSum struct is the structure for a Version0 checksum calculation

-type TarSum struct {

+// tarSum struct is the structure for a Version0 checksum calculation

+type tarSum struct {

 	io.Reader

 	tarR               *tar.Reader

 	tarW               *tar.Writer

@@ -62,11 +62,11 @@ type TarSum struct {

 	tarSumVersion      Version // this field is not exported so it can not be mutated during use

 }

-func (ts TarSum) Version() Version {

+func (ts tarSum) Version() Version {

 	return ts.tarSumVersion

 }

-func (ts TarSum) selectHeaders(h *tar.Header, v Version) (set [][2]string) {

+func (ts tarSum) selectHeaders(h *tar.Header, v Version) (set [][2]string) {

 	for _, elem := range [][2]string{

 		{"name", h.Name},

 		{"mode", strconv.Itoa(int(h.Mode))},

@@ -89,7 +89,7 @@ func (ts TarSum) selectHeaders(h *tar.Header, v Version) (set [][2]string) {

 	return

 }

-func (ts *TarSum) encodeHeader(h *tar.Header) error {

+func (ts *tarSum) encodeHeader(h *tar.Header) error {

 	for _, elem := range ts.selectHeaders(h, ts.Version()) {

 		if _, err := ts.h.Write([]byte(elem[0] + elem[1])); err != nil {

 			return err

@@ -98,7 +98,7 @@ func (ts *TarSum) encodeHeader(h *tar.Header) error {

 	return nil

 }

-func (ts *TarSum) Read(buf []byte) (int, error) {

+func (ts *tarSum) Read(buf []byte) (int, error) {

 	if ts.gz == nil {

 		ts.bufTar = bytes.NewBuffer([]byte{})

 		ts.bufGz = bytes.NewBuffer([]byte{})

@@ -197,7 +197,7 @@ func (ts *TarSum) Read(buf []byte) (int, error) {

 	return ts.bufGz.Read(buf)

 }

-func (ts *TarSum) Sum(extra []byte) string {

+func (ts *tarSum) Sum(extra []byte) string {

 	var sums []string

 	for _, sum := range ts.sums {

@@ -217,6 +217,6 @@ func (ts *TarSum) Sum(extra []byte) string {

 	return checksum

 }

-func (ts *TarSum) GetSums() map[string]string {

+func (ts *tarSum) GetSums() map[string]string {

 	return ts.sums

 }