ignore path not found error when look up tags

Signed-off-by: Wenkai Yin <yinw@vmware.com>
(cherry picked from commit 005c6e0236)

registry/storage/tagstore.go

@@ -182,6 +182,10 @@ func (ts *tagStore) Lookup(ctx context.Context, desc distribution.Descriptor) ([
 		tagLinkPath, err := pathFor(tagLinkPathSpec)
 		tagDigest, err := ts.blobStore.readlink(ctx, tagLinkPath)
 		if err != nil {
+			switch err.(type) {
+			case storagedriver.PathNotFoundError:
+				continue
+			}
 			return nil, err
 		}
 

registry/storage/tagstore_test.go

@@ -84,8 +84,8 @@ func TestTagStoreUnTag(t *testing.T) {
 	desc := distribution.Descriptor{Digest: "sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"}
 
 	err := tags.Untag(ctx, "latest")
-	if err == nil {
-		t.Errorf("Expected error untagging non-existant tag")
+	if err != nil {
+		t.Error(err)
 	}
 
 	err = tags.Tag(ctx, "latest", desc)