Ensure we can access layer by diffID on a mutated image

pkg/v1/mutate/image.go

@@ -277,6 +277,9 @@ func (i *image) LayerByDigest(h v1.Hash) (v1.Layer, error) {
 // LayerByDiffID is an analog to LayerByDigest, looking up by "diff id"
 // (the uncompressed hash).
 func (i *image) LayerByDiffID(h v1.Hash) (v1.Layer, error) {
+	if err := i.compute(); err != nil {
+		return nil, err
+	}
 	if layer, ok := i.diffIDMap[h]; ok {
 		return layer, nil
 	}

pkg/v1/mutate/mutate_test.go

@@ -28,6 +28,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
+
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/empty"
 	"github.com/google/go-containerregistry/pkg/v1/match"
@@ -200,6 +201,10 @@ func TestAppendLayers(t *testing.T) {
 		t.Fatalf("failed to append a layer: %v", err)
 	}
 
+	if !canGetLayerByDiffID(t, layer, result) {
+		t.Fatal("could not get layer by diffID before fetching manifest, config, or layers")
+	}
+
 	if manifestsAreEqual(t, source, result) {
 		t.Fatal("appending a layer did not mutate the manifest")
 	}
@@ -228,6 +233,12 @@ func TestAppendLayers(t *testing.T) {
 	}
 }
 
+func canGetLayerByDiffID(t *testing.T, layer v1.Layer, result v1.Image) bool {
+	diffID := getDiffID(t, layer)
+	_, err := result.LayerByDiffID(diffID)
+	return err == nil
+}
+
 func TestMutateConfig(t *testing.T) {
 	source := sourceImage(t)
 	cfg, err := source.ConfigFile()
@@ -700,6 +711,17 @@ func getLayers(t *testing.T, i v1.Image) []v1.Layer {
 	return l
 }
 
+func getDiffID(t *testing.T, l v1.Layer) v1.Hash {
+	t.Helper()
+
+	diffID, err := l.DiffID()
+	if err != nil {
+		t.Fatalf("Error fetching layer diffID: %v", err)
+	}
+
+	return diffID
+}
+
 func getConfigFile(t *testing.T, i v1.Image) *v1.ConfigFile {
 	t.Helper()