deltas: Us a single tag with an index instead of tag per delta

Instead of using lots of tags with names like delta-${shortid} we
use a single tag `deltas` with a list of delta manifest, each
specifying a target manifest by a `com.redhat.deltaTarget` annotation.

This means deltas are a lot less visible with a single tag, and
it also means we can use the full target digest instead of a short id.

The only disadvantage is that updating the index could race between
multiple writers, losing the concurrent update.

Signed-off-by: Alexander Larsson <alexl@redhat.com>

docker/docker_image_src.go

@@ -201,32 +201,37 @@ func (s *dockerImageSource) fetchManifest(ctx context.Context, tagOrDigest strin
 	return manblob, simplifyContentType(res.Header.Get("Content-Type")), nil
 }
 
-func (s *dockerImageSource) getDeltaManifestTagName(ctx context.Context, instanceDigest *digest.Digest) (string, error) {
-	digest, err := s.manifestDigest(ctx, instanceDigest)
-	if err != nil {
-		return "", err
-	}
-
-	return "delta-" + digest.Encoded()[:12], nil
-}
-
 func (s *dockerImageSource) GetDeltaManifest(ctx context.Context, instanceDigest *digest.Digest) ([]byte, string, error) {
-	tagname, err := s.getDeltaManifestTagName(ctx, instanceDigest)
+	// Get the real manifest digest
+	srcManifestDigest, err := s.manifestDigest(ctx, instanceDigest)
 	if err != nil {
 		return nil, "", err
 	}
+
+	// Load the delta manifest index
+	ib, _, err := s.fetchManifest(ctx, "deltas")
 	// Don't return error if the  manifest doesn't exist, only for internal errors
 	// Deltas are an optional optimization anyway
-	mb, mt, _ := s.fetchManifest(ctx, tagname)
-	return mb, mt, nil
+	if err == nil {
+		index, err := manifest.OCI1IndexFromManifest(ib)
+		if err != nil {
+			return nil, "", err
+		}
+
+		// Look up the delta manifest in the index by the real manifest digest
+		for _, manifest := range index.Manifests {
+			if manifest.Annotations["com.redhat.deltaTarget"] == srcManifestDigest.String() {
+				return s.fetchManifest(ctx, manifest.Digest.String())
+			}
+		}
+	}
+
+	// No delta
+	return nil, "", nil
 }
 
 func (s *dockerImageSource) GetDeltaManifestDestination(ctx context.Context, instanceDigest *digest.Digest) (types.ImageDestination, error) {
-	tagname, err := s.getDeltaManifestTagName(ctx, instanceDigest)
-	if err != nil {
-		return nil, err
-	}
-	deltaRef, err := reference.WithTag(s.ref.ref, tagname)
+	deltaRef, err := reference.WithTag(s.ref.ref, "deltas")
 	if err != nil {
 		return nil, err
 	}