track digest offset in blobwriter

Signed-off-by: David Wu <david.wu@docker.com>

registry/storage/blobwriter.go

@@ -33,7 +33,7 @@ type blobWriter struct {
 	id        string
 	startedAt time.Time
 	digester  digest.Digester
-	written   int64 // track the contiguous write
+	written   int64 // track the write to digester
 
 	fileWriter storagedriver.FileWriter
 	driver     storagedriver.StorageDriver
@@ -119,7 +119,12 @@ func (bw *blobWriter) Write(p []byte) (int, error) {
 		return 0, err
 	}
 
-	n, err := io.MultiWriter(bw.fileWriter, bw.digester.Hash()).Write(p)
+	_, err := bw.fileWriter.Write(p)
+	if err != nil {
+		return 0, err
+	}
+
+	n, err := bw.digester.Hash().Write(p)
 	bw.written += int64(n)
 
 	return n, err
@@ -133,7 +138,11 @@ func (bw *blobWriter) ReadFrom(r io.Reader) (n int64, err error) {
 		return 0, err
 	}
 
-	nn, err := io.Copy(io.MultiWriter(bw.fileWriter, bw.digester.Hash()), r)
+	// Using a TeeReader instead of MultiWriter ensures Copy returns
+	// the amount written to the digester as well as ensuring that we
+	// write to the fileWriter first
+	tee := io.TeeReader(r, bw.fileWriter)
+	nn, err := io.Copy(bw.digester.Hash(), tee)
 	bw.written += nn
 
 	return nn, err

registry/storage/blobwriter_resumable.go

@@ -21,18 +21,13 @@ func (bw *blobWriter) resumeDigest(ctx context.Context) error {
 		return errResumableDigestNotAvailable
 	}
 
-	h, ok := bw.digester.Hash().(encoding.BinaryMarshaler)
+	h, ok := bw.digester.Hash().(encoding.BinaryUnmarshaler)
 	if !ok {
 		return errResumableDigestNotAvailable
 	}
 
-	state, err := h.MarshalBinary()
-	if err != nil {
-		return err
-	}
-
 	offset := bw.fileWriter.Size()
-	if offset == int64(len(state)) {
+	if offset == bw.written {
 		// State of digester is already at the requested offset.
 		return nil
 	}
@@ -62,19 +57,14 @@ func (bw *blobWriter) resumeDigest(ctx context.Context) error {
 			return err
 		}
 
-		// This type assertion is safe since we already did an assertion at the beginning
-		if err = h.(encoding.BinaryUnmarshaler).UnmarshalBinary(storedState); err != nil {
-			return err
-		}
-
-		state, err = h.(encoding.BinaryMarshaler).MarshalBinary()
-		if err != nil {
+		if err = h.UnmarshalBinary(storedState); err != nil {
 			return err
 		}
+		bw.written = hashStateMatch.offset
 	}
 
 	// Mind the gap.
-	if gapLen := offset - int64(len(state)); gapLen > 0 {
+	if gapLen := offset - bw.written; gapLen > 0 {
 		return errResumableDigestNotAvailable
 	}
 
@@ -136,14 +126,14 @@ func (bw *blobWriter) storeHashState(ctx context.Context) error {
 
 	state, err := h.MarshalBinary()
 	if err != nil {
-		return fmt.Errorf("could not marshal: %v", err)
+		return err
 	}
 
 	uploadHashStatePath, err := pathFor(uploadHashStatePathSpec{
 		name:   bw.blobStore.repository.Named().String(),
 		id:     bw.id,
 		alg:    bw.digester.Digest().Algorithm(),
-		offset: int64(len(state)),
+		offset: bw.written,
 	})
 
 	if err != nil {