Define size -1 as "unknown" in GetBlob

Use -1 because 0 is, technically, a valid size of a blob.

Also remove a FIXME about (docker save) tarballs, now we know that we
will need to make a temporary, seekable, copy.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

directory/directory_src.go

@@ -37,6 +37,7 @@ func (s *dirImageSource) GetManifest() ([]byte, string, error) {
 	return m, "", err
 }
 
+// GetBlob returns a stream for the specified blob, and the blob’s size (or -1 if unknown).
 func (s *dirImageSource) GetBlob(digest string) (io.ReadCloser, int64, error) {
 	r, err := os.Open(s.ref.layerPath(digest))
 	if err != nil {

docker/docker_image_src.go

@@ -96,6 +96,7 @@ func (s *dockerImageSource) GetManifest() ([]byte, string, error) {
 	return manblob, simplifyContentType(res.Header.Get("Content-Type")), nil
 }
 
+// GetBlob returns a stream for the specified blob, and the blob’s size (or -1 if unknown).
 func (s *dockerImageSource) GetBlob(digest string) (io.ReadCloser, int64, error) {
 	url := fmt.Sprintf(blobsURL, s.ref.ref.RemoteName(), digest)
 	logrus.Debugf("Downloading %s", url)
@@ -109,7 +110,7 @@ func (s *dockerImageSource) GetBlob(digest string) (io.ReadCloser, int64, error)
 	}
 	size, err := strconv.ParseInt(res.Header.Get("Content-Length"), 10, 64)
 	if err != nil {
-		size = 0
+		size = -1
 	}
 	return res.Body, size, nil
 }

openshift/openshift.go

@@ -216,6 +216,7 @@ func (s *openshiftImageSource) GetManifest() ([]byte, string, error) {
 	return s.docker.GetManifest()
 }
 
+// GetBlob returns a stream for the specified blob, and the blob’s size (or -1 if unknown).
 func (s *openshiftImageSource) GetBlob(digest string) (io.ReadCloser, int64, error) {
 	if err := s.ensureImageIsResolved(); err != nil {
 		return nil, 0, err

types/types.go

@@ -98,8 +98,7 @@ type ImageSource interface {
 	// GetManifest returns the image's manifest along with its MIME type. The empty string is returned if the MIME type is unknown.
 	// It may use a remote (= slow) service.
 	GetManifest() ([]byte, string, error)
-	// Note: Calling GetBlob() may have ordering dependencies WRT other methods of this type. FIXME: How does this work with (docker save) on stdin?
-	// the second return value is the size of the blob. If not known 0 is returned
+	// GetBlob returns a stream for the specified blob, and the blob’s size (or -1 if unknown).
 	GetBlob(digest string) (io.ReadCloser, int64, error)
 	// GetSignatures returns the image's signatures.  It may use a remote (= slow) service.
 	GetSignatures() ([][]byte, error)