@@ -27,11 +27,7 @@ const (

 // 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) (TarSum, error) {

-	headerSelector, err := getTarHeaderSelector(v)

-	if err != nil {

-		return nil, err

-	}

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

+	return NewTarSumHash(r, dc, v, DefaultTHash)

 }

 // Create a new TarSum, providing a THash to use rather than the DefaultTHash

@@ -40,7 +36,9 @@ func NewTarSumHash(r io.Reader, dc bool, v Version, tHash THash) (TarSum, error)

 	if err != nil {

 		return nil, err

 	}

-	return &tarSum{Reader: r, DisableCompression: dc, tarSumVersion: v, headerSelector: headerSelector, tHash: tHash}, nil

+	ts := &tarSum{Reader: r, DisableCompression: dc, tarSumVersion: v, headerSelector: headerSelector, tHash: tHash}

+	err = ts.initTarSum()

+	return ts, err

 }

 // TarSum is the generic interface for calculating fixed time

@@ -134,12 +132,6 @@ func (ts *tarSum) initTarSum() error {

 }

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

-	if ts.writer == nil {

-		if err := ts.initTarSum(); err != nil {

-			return 0, err

-		}

-	}

-

 	if ts.finished {

 		return ts.bufWriter.Read(buf)

 	}

@@ -230,6 +230,17 @@ func TestEmptyTar(t *testing.T) {

 	if resultSum != expectedSum {

 		t.Fatalf("expected [%s] but got [%s]", expectedSum, resultSum)

 	}

+

+	// Test without ever actually writing anything.

+	if ts, err = NewTarSum(bytes.NewReader([]byte{}), true, Version0); err != nil {

+		t.Fatal(err)

+	}

+

+	resultSum = ts.Sum(nil)

+

+	if resultSum != expectedSum {

+		t.Fatalf("expected [%s] but got [%s]", expectedSum, resultSum)

+	}

 }

 var (