No empty layers

repack.go

@@ -65,16 +65,38 @@ func Repack(engineExt casext.Engine, tagName string, bundlePath string, meta Met
 		mtreefilter.MaskFilter(maskedPaths),
 		mtreefilter.SimplifyFilter(diffs))
 
-	reader, err := layer.GenerateLayer(fullRootfsPath, diffs, &meta.MapOptions)
-	if err != nil {
-		return errors.Wrap(err, "generate diff layer")
-	}
-	defer reader.Close()
+	if len(diffs) == 0 {
+		config, err := mutator.Config(context.Background())
+		if err != nil {
+			return err
+		}
 
-	// TODO: We should add a flag to allow for a new layer to be made
-	//       non-distributable.
-	if err := mutator.Add(context.Background(), reader, history); err != nil {
-		return errors.Wrap(err, "add diff layer")
+		imageMeta, err := mutator.Meta(context.Background())
+		if err != nil {
+			return err
+		}
+
+		annotations, err := mutator.Annotations(context.Background())
+		if err != nil {
+			return err
+		}
+
+		err = mutator.Set(context.Background(), config, imageMeta, annotations, history)
+		if err != nil {
+			return err
+		}
+	} else {
+		reader, err := layer.GenerateLayer(fullRootfsPath, diffs, &meta.MapOptions)
+		if err != nil {
+			return errors.Wrap(err, "generate diff layer")
+		}
+		defer reader.Close()
+
+		// TODO: We should add a flag to allow for a new layer to be made
+		//       non-distributable.
+		if err := mutator.Add(context.Background(), reader, history); err != nil {
+			return errors.Wrap(err, "add diff layer")
+		}
 	}
 
 	newDescriptorPath, err := mutator.Commit(context.Background())

test/repack.bats

@@ -231,7 +231,7 @@ function teardown() {
 	bundle-verify "$BUNDLE"
 
 	# Make some small change.
-	touch "$BUNDLE/a_small_change"
+	touch "$ROOTFS/a_small_change"
 	now="$(date --iso-8601=seconds --utc)"
 
 	# Repack the image, setting history values.
@@ -790,3 +790,24 @@ function teardown() {
 	[ "$numLinesB" -gt "$numLinesA" ]
 	[ "$numLinesC" -gt "$numLinesB" ]
 }
+
+@test "umoci repack (empty diff)" {
+	# Unpack the original image
+	new_bundle_rootfs
+	umoci unpack --image "${IMAGE}:${TAG}" "$BUNDLE"
+	[ "$status" -eq 0 ]
+	bundle-verify "$BUNDLE"
+
+	# Repack the image under a new tag.
+	umoci repack --image "${IMAGE}:${TAG}-new" "$BUNDLE"
+	[ "$status" -eq 0 ]
+	image-verify "${IMAGE}"
+
+	# The two manifests should have the same number of layers.
+	manifest0=$(cat "${IMAGE}/oci/index.json" | jq -r .manifests[0].digest | cut -f2 -d:)
+	manifest1=$(cat "${IMAGE}/oci/index.json" | jq -r .manifests[1].digest | cut -f2 -d:)
+
+	layers0=$(cat "${IMAGE}/oci/blobs/sha256/$manifest0" | jq -r .layers)
+	layers1=$(cat "${IMAGE}/oci/blobs/sha256/$manifest1" | jq -r .layers)
+	[ "$layers0" == "$layers1" ]
+}