Merge pull request #576 from stevvooe/seek-reset-handling

content: allow reset via Truncate

content/client.go

@@ -199,6 +199,7 @@ func (rw *remoteWriter) Commit(size int64, expected digest.Digest) error {
 	resp, err := rw.send(&contentapi.WriteRequest{
 		Action:   contentapi.WriteActionCommit,
 		Total:    size,
+		Offset:   rw.offset,
 		Expected: expected,
 	})
 	if err != nil {
@@ -216,6 +217,12 @@ func (rw *remoteWriter) Commit(size int64, expected digest.Digest) error {
 	return nil
 }
 
+func (rw *remoteWriter) Truncate(size int64) error {
+	// This truncation won't actually be validated until a write is issued.
+	rw.offset = size
+	return nil
+}
+
 func (rw *remoteWriter) Close() error {
 	return rw.client.CloseSend()
 }

content/content.go

@@ -44,6 +44,7 @@ type Writer interface {
 	Status() (Status, error)
 	Digest() digest.Digest
 	Commit(size int64, expected digest.Digest) error
+	Truncate(size int64) error
 }
 
 type Ingester interface {

content/helpers.go

@@ -36,7 +36,14 @@ func WriteBlob(ctx context.Context, cs Ingester, ref string, r io.Reader, size i
 	if ws.Offset > 0 {
 		r, err = seekReader(r, ws.Offset, size)
 		if err != nil {
-			return errors.Wrapf(err, "unabled to resume write to %v", ref)
+			if !isUnseekable(err) {
+				return errors.Wrapf(err, "unabled to resume write to %v", ref)
+			}
+
+			// reader is unseekable, try to move the writer back to the start.
+			if err := cw.Truncate(0); err != nil {
+				return errors.Wrapf(err, "content writer truncate failed")
+			}
 		}
 	}
 
@@ -49,13 +56,19 @@ func WriteBlob(ctx context.Context, cs Ingester, ref string, r io.Reader, size i
 
 	if err := cw.Commit(size, expected); err != nil {
 		if !IsExists(err) {
-			return err
+			return errors.Wrapf(err, "failed commit on ref %q", ref)
 		}
 	}
 
 	return nil
 }
 
+var errUnseekable = errors.New("seek not supported")
+
+func isUnseekable(err error) bool {
+	return errors.Cause(err) == errUnseekable
+}
+
 // seekReader attempts to seek the reader to the given offset, either by
 // resolving `io.Seeker` or by detecting `io.ReaderAt`.
 func seekReader(r io.Reader, offset, size int64) (io.Reader, error) {
@@ -81,7 +94,7 @@ func seekReader(r io.Reader, offset, size int64) (io.Reader, error) {
 		return sr, nil
 	}
 
-	return nil, errors.Errorf("cannot seek to offset %v", offset)
+	return r, errors.Wrapf(errUnseekable, "seek to offset %v failed", offset)
 }
 
 func readFileString(path string) (string, error) {

content/writer.go

@@ -74,7 +74,7 @@ func (w *writer) Commit(size int64, expected digest.Digest) error {
 	}
 
 	if size > 0 && size != fi.Size() {
-		return errors.Errorf("failed size validation: %v != %v", fi.Size(), size)
+		return errors.Errorf("%q failed size validation: %v != %v", w.ref, fi.Size(), size)
 	}
 
 	if err := w.fp.Close(); err != nil {
@@ -133,3 +133,12 @@ func (cw *writer) Close() (err error) {
 
 	return nil
 }
+
+func (w *writer) Truncate(size int64) error {
+	if size != 0 {
+		return errors.New("Truncate: unsupported size")
+	}
+	w.offset = 0
+	w.digester.Hash().Reset()
+	return w.fp.Truncate(0)
+}