Allow config files to be mounted

Because we wanted to reuse the layer uploading code, we wrap the config
bytes up as a Layer. Unfortunately, the partial.ConfigLayer
implementation doesn't preserve the fact that for mountable images, we
can perform a cross-repo mount.

This change updates partial.ConfigLayer to look for a ConfigLayer
implementation on the given image to see if it can simply dispatch to
that, falling back to calculating it from the config bytes otherwise.

pkg/v1/partial/compressed_test.go

@@ -87,6 +87,14 @@ func TestRemote(t *testing.T) {
 	if got, want := ok, true; got != want {
 		t.Errorf("Exists() = %t != %t", got, want)
 	}
+
+	cl, err := partial.ConfigLayer(img)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := cl.(*remote.MountableLayer); !ok {
+		t.Errorf("ConfigLayer() expected to be MountableLayer, got %T", cl)
+	}
 }
 
 type noDiffID struct {

pkg/v1/partial/with.go

@@ -88,9 +88,22 @@ func (cl *configLayer) MediaType() (types.MediaType, error) {
 
 var _ v1.Layer = (*configLayer)(nil)
 
+// withConfigLayer allows partial image implementations to provide a layer
+// for their config file.
+type withConfigLayer interface {
+	ConfigLayer() (v1.Layer, error)
+}
+
 // ConfigLayer implements v1.Layer from the raw config bytes.
 // This is so that clients (e.g. remote) can access the config as a blob.
+//
+// Images that want to return a specific layer implementation can implement
+// withConfigLayer.
 func ConfigLayer(i WithRawConfigFile) (v1.Layer, error) {
+	if wcl, ok := unwrap(i).(withConfigLayer); ok {
+		return wcl.ConfigLayer()
+	}
+
 	h, err := ConfigName(i)
 	if err != nil {
 		return nil, err

pkg/v1/remote/mount.go

@@ -93,3 +93,16 @@ func (mi *mountableImage) LayerByDiffID(d v1.Hash) (v1.Layer, error) {
 func (mi *mountableImage) Descriptor() (*v1.Descriptor, error) {
 	return partial.Descriptor(mi.Image)
 }
+
+// ConfigLayer retains the original reference so that it can be mounted.
+// See partial.ConfigLayer.
+func (mi *mountableImage) ConfigLayer() (v1.Layer, error) {
+	l, err := partial.ConfigLayer(mi.Image)
+	if err != nil {
+		return nil, err
+	}
+	return &MountableLayer{
+		Layer:     l,
+		Reference: mi.Reference,
+	}, nil
+}