Add error return to Repository method on Registry

The method (Registry).Repository may now return an error. This is too allow
certain implementationt to validate the name or opt to not return a repository
under certain conditions.

In conjunction with this change, error declarations have been moved into a
single file in the distribution package. Several error declarations that had
remained in the storage package have been moved into distribution, as well. The
declarations for Layer and LayerUpload have also been moved into the main
registry file, as a result.

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

registry.go

@@ -1,19 +1,20 @@
 package distribution
 
 import (
+	"io"
+	"time"
+
 	"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(ctx context.Context, name string) (Repository, error)
 }
 
 // Repository is a named collection of manifests and layers.
@@ -82,3 +83,42 @@ type LayerService interface {
 	// before proceeding.
 	Resume(uuid string) (LayerUpload, error)
 }
+
+// Layer provides a readable and seekable layer object. Typically,
+// implementations are *not* goroutine safe.
+type Layer interface {
+	// http.ServeContent requires an efficient implementation of
+	// ReadSeeker.Seek(0, os.SEEK_END).
+	io.ReadSeeker
+	io.Closer
+
+	// Digest returns the unique digest of the blob, which is the tarsum for
+	// layers.
+	Digest() digest.Digest
+
+	// CreatedAt returns the time this layer was created.
+	CreatedAt() time.Time
+}
+
+// LayerUpload provides a handle for working with in-progress uploads.
+// Instances can be obtained from the LayerService.Upload and
+// LayerService.Resume.
+type LayerUpload interface {
+	io.WriteSeeker
+	io.ReaderFrom
+	io.Closer
+
+	// UUID returns the identifier for this upload.
+	UUID() string
+
+	// StartedAt returns the time this layer upload was started.
+	StartedAt() time.Time
+
+	// Finish marks the upload as completed, returning a valid handle to the
+	// uploaded layer. The digest is validated against the contents of the
+	// uploaded layer.
+	Finish(digest digest.Digest) (Layer, error)
+
+	// Cancel the layer upload process.
+	Cancel() error
+}