oci: Refine storing manifests in index.json.

Before this commit, all untagged manifests were stored in index.json.
Now, we only store manifests which are not tagged and which predecessors are not
tagged.

Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>

content/oci/oci.go

@@ -433,10 +433,23 @@ func (s *Store) saveIndex() error {
 			tagged.Add(desc.Digest)
 		}
 	}
-	// 2. Add descriptors that are not associated with any tag
+	// 2. Add descriptors that are not associated with any tag.
+	// We only add descriptors which are not tagged and which do not have a tagged
+	// predecessors.
+outer:
 	for ref, desc := range refMap {
 		if ref == desc.Digest.String() && !tagged.Contains(desc.Digest) {
-			// skip tagged ones since they have been added in step 1
+			predecessors, err := s.graph.Predecessors(nil, desc)
+			if err != nil {
+				continue
+			}
+
+			for _, predecessor := range predecessors {
+				if tagged.Contains(predecessor.Digest){
+					continue outer
+				}
+			}
+
 			manifests = append(manifests, deleteAnnotationRefName(desc))
 		}
 	}