cmd/dist: show real image size in list

As a demonstration of the power of the visitor implementation, we now
report the image size in the `dist images` command. This is the size of
the packed resources as would be pushed into a remote. A similar method
could be added to calculate the unpacked size.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

images/image.go

@@ -86,3 +86,40 @@ func (image *Image) RootFS(ctx context.Context, provider content.Provider) ([]di
 
 	return diffIDs, nil
 }
+
+// Size returns the total size of an image's packed resources.
+func (image *Image) Size(ctx context.Context, provider content.Provider) (int64, error) {
+	var size int64
+	return size, Walk(ctx, HandlerFunc(func(ctx context.Context, desc ocispec.Descriptor) ([]ocispec.Descriptor, error) {
+		switch image.Descriptor.MediaType {
+		case MediaTypeDockerSchema2Manifest, ocispec.MediaTypeImageManifest:
+			size += desc.Size
+			rc, err := provider.Reader(ctx, image.Descriptor.Digest)
+			if err != nil {
+				return nil, err
+			}
+			defer rc.Close()
+
+			p, err := ioutil.ReadAll(rc)
+			if err != nil {
+				return nil, err
+			}
+
+			var manifest ocispec.Manifest
+			if err := json.Unmarshal(p, &manifest); err != nil {
+				return nil, err
+			}
+
+			size += manifest.Config.Size
+
+			for _, layer := range manifest.Layers {
+				size += layer.Size
+			}
+
+			return nil, nil
+		default:
+			return nil, errors.New("unsupported type")
+		}
+
+	}), image.Descriptor)
+}

images/storage.go

@@ -12,7 +12,6 @@ import (
 
 var (
 	errImageUnknown = fmt.Errorf("image: unknown")
-	errNoTx         = fmt.Errorf("no transaction available")
 )
 
 var (