Initial implementation of image manifest storage

This change implements the first pass at image manifest storage on top of the
storagedriver. Very similar to LayerService, its much simpler due to less
complexity of pushing and pulling images.

Various components are still missing, such as detailed error reporting on
missing layers during verification, but the base functionality is present.

storage/services.go

@@ -1,6 +1,7 @@
 package storage
 
 import (
+	"github.com/docker/docker-registry/digest"
 	"github.com/docker/docker-registry/storagedriver"
 )
 
@@ -41,3 +42,42 @@ func NewServices(driver storagedriver.StorageDriver) *Services {
 func (ss *Services) Layers() LayerService {
 	return &layerStore{driver: ss.driver, pathMapper: ss.pathMapper, uploadStore: ss.layerUploadStore}
 }
+
+// Manifests returns an instance of ManifestService. Instantiation is cheap and
+// may be context sensitive in the future. The instance should be used similar
+// to a request local.
+func (ss *Services) Manifests() ManifestService {
+	return &manifestStore{driver: ss.driver, pathMapper: ss.pathMapper, layerService: ss.Layers()}
+}
+
+// ManifestService provides operations on image manifests.
+type ManifestService interface {
+	// Exists returns true if the layer exists.
+	Exists(name, tag string) (bool, error)
+
+	// Get retrieves the named manifest, if it exists.
+	Get(name, tag string) (*SignedManifest, error)
+
+	// Put creates or updates the named manifest.
+	Put(name, tag string, manifest *SignedManifest) error
+
+	// Delete removes the named manifest, if it exists.
+	Delete(name, tag string) error
+}
+
+// LayerService provides operations on layer files in a backend storage.
+type LayerService interface {
+	// Exists returns true if the layer exists.
+	Exists(name string, digest digest.Digest) (bool, error)
+
+	// Fetch the layer identifed by TarSum.
+	Fetch(name string, digest digest.Digest) (Layer, error)
+
+	// Upload begins a layer upload to repository identified by name,
+	// returning a handle.
+	Upload(name string) (LayerUpload, error)
+
+	// Resume continues an in progress layer upload, returning the current
+	// state of the upload.
+	Resume(uuid string) (LayerUpload, error)
+}