Add GetManifest and GetSignatures to types.Image

No change in behavior.

These functions are guaranteed-cached versions of the same method in
types.ImageSource.  Both will be needed for signature policy evaluation,
and the symmetry with ImageSource is nice.

Also replaces the equivalent RawManifest method, preferring to keep
the same naming convention as types.ImageSource.

cmd/skopeo/inspect.go

@@ -23,8 +23,7 @@ var inspectCmd = cli.Command{
 			logrus.Fatal(err)
 		}
 		if c.Bool("raw") {
-			// TODO(runcom): hardcoded schema 2 version 1
+			b, err := img.GetManifest()
-			b, err := img.RawManifest("2-1")
 			if err != nil {
 				logrus.Fatal(err)
 			}

docker/docker_image.go

@@ -22,6 +22,7 @@ type dockerImage struct {
 	src              *dockerImageSource
 	digest           string
 	rawManifest      []byte
+	cachedSignatures [][]byte // Private cache for GetSignatures; nil if not yet known.
 }
 
 // NewDockerImage returns a new Image interface type after setting up
@@ -34,14 +35,26 @@ func NewDockerImage(img, certPath string, tlsVerify bool) (types.Image, error) {
 	return &dockerImage{src: s}, nil
 }
 
-func (i *dockerImage) RawManifest(version string) ([]byte, error) {
+// GetManifest is like ImageSource.GetManifest, but the result is cached; it is OK to call this however often you need.
-	// TODO(runcom): unused version param for now, default to docker v2-1
+func (i *dockerImage) GetManifest() ([]byte, error) {
 	if err := i.retrieveRawManifest(); err != nil {
 		return nil, err
 	}
 	return i.rawManifest, nil
 }
 
+// GetSignatures is like ImageSource.GetSignatures, but the result is cached; it is OK to call this however often you need.
+func (i *dockerImage) GetSignatures() ([][]byte, error) {
+	if i.cachedSignatures == nil {
+		sigs, err := i.src.GetSignatures()
+		if err != nil {
+			return nil, err
+		}
+		i.cachedSignatures = sigs
+	}
+	return i.cachedSignatures, nil
+}
+
 func (i *dockerImage) Manifest() (types.ImageManifest, error) {
 	// TODO(runcom): unused version param for now, default to docker v2-1
 	m, err := i.getSchema1Manifest()

types/types.go

@@ -47,9 +47,12 @@ type ImageDestination interface {
 // Image is a Docker image in a repository.
 type Image interface {
 	// ref to repository?
+	// GetManifest is like ImageSource.GetManifest, but the result is cached; it is OK to call this however often you need.
+	GetManifest() ([]byte, error)
+	// GetSignatures is like ImageSource.GetSignatures, but the result is cached; it is OK to call this however often you need.
+	GetSignatures() ([][]byte, error)
 	Layers(layers ...string) error // configure download directory? Call it DownloadLayers?
 	Manifest() (ImageManifest, error)
-	RawManifest(version string) ([]byte, error)
 	DockerTar() ([]byte, error) // ??? also, configure output directory
 }