Move layer interface definitions to distribution package

After consideration, it has been decided that the interfaces defined in the
storage package provide a good base for interacting with various registry
instances. Whether interacting with a remote API or a local, on-disk registry,
these types have proved flexible. By moving them here, they can become the
central components of interacting with distribution components.

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

registry.go

@@ -0,0 +1,84 @@
+package distribution
+
+import (
+	"github.com/docker/distribution/digest"
+	"github.com/docker/distribution/manifest"
+	"golang.org/x/net/context"
+)
+
+// TODO(stevvooe): These types need to be moved out of the storage package.
+
+// Registry represents a collection of repositories, addressable by name.
+type Registry interface {
+	// Repository should return a reference to the named repository. The
+	// registry may or may not have the repository but should always return a
+	// reference.
+	Repository(ctx context.Context, name string) Repository
+}
+
+// Repository is a named collection of manifests and layers.
+type Repository interface {
+	// Name returns the name of the repository.
+	Name() string
+
+	// Manifests returns a reference to this repository's manifest service.
+	Manifests() ManifestService
+
+	// Layers returns a reference to this repository's layers service.
+	Layers() LayerService
+}
+
+// ManifestService provides operations on image manifests.
+type ManifestService interface {
+	// Tags lists the tags under the named repository.
+	Tags() ([]string, error)
+
+	// Exists returns true if the manifest exists.
+	Exists(tag string) (bool, error)
+
+	// Get retrieves the named manifest, if it exists.
+	Get(tag string) (*manifest.SignedManifest, error)
+
+	// Put creates or updates the named manifest.
+	// Put(tag string, manifest *manifest.SignedManifest) (digest.Digest, error)
+	Put(tag string, manifest *manifest.SignedManifest) error
+
+	// Delete removes the named manifest, if it exists.
+	Delete(tag string) error
+
+	// TODO(stevvooe): There are several changes that need to be done to this
+	// interface:
+	//
+	//	1. Get(tag string) should be GetByTag(tag string)
+	//	2. Put(tag string, manifest *manifest.SignedManifest) should be
+	//       Put(manifest *manifest.SignedManifest). The method can read the
+	//       tag on manifest to automatically tag it in the repository.
+	//	3. Need a GetByDigest(dgst digest.Digest) method.
+	//	4. Allow explicit tagging with Tag(digest digest.Digest, tag string)
+	//	5. Support reading tags with a re-entrant reader to avoid large
+	//       allocations in the registry.
+	//	6. Long-term: Provide All() method that lets one scroll through all of
+	//       the manifest entries.
+	//	7. Long-term: break out concept of signing from manifests. This is
+	//       really a part of the distribution sprint.
+	//	8. Long-term: Manifest should be an interface. This code shouldn't
+	//       really be concerned with the storage format.
+}
+
+// LayerService provides operations on layer files in a backend storage.
+type LayerService interface {
+	// Exists returns true if the layer exists.
+	Exists(digest digest.Digest) (bool, error)
+
+	// Fetch the layer identifed by TarSum.
+	Fetch(digest digest.Digest) (Layer, error)
+
+	// Upload begins a layer upload to repository identified by name,
+	// returning a handle.
+	Upload() (LayerUpload, error)
+
+	// Resume continues an in progress layer upload, returning a handle to the
+	// upload. The caller should seek to the latest desired upload location
+	// before proceeding.
+	Resume(uuid string) (LayerUpload, error)
+}