Implement immutable manifest reference support

This changeset implements immutable manifest references via the HTTP API. Most
of the changes follow from modifications to ManifestService. Once updates were
made across the repo to implement these changes, the http handlers were change
accordingly. The new methods on ManifestService will be broken out into a
tagging service in a later PR.

Unfortunately, due to complexities around managing the manifest tag index in an
eventually consistent manner, direct deletes of manifests have been disabled.

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

registry.go

@@ -34,38 +34,41 @@ type Repository interface {
 
 // ManifestService provides operations on image manifests.
 type ManifestService interface {
+	// Exists returns true if the manifest exists.
+	Exists(dgst digest.Digest) (bool, error)
+
+	// Get retrieves the identified by the digest, if it exists.
+	Get(dgst digest.Digest) (*manifest.SignedManifest, error)
+
+	// Delete removes the manifest, if it exists.
+	Delete(dgst digest.Digest) error
+
+	// Put creates or updates the manifest.
+	Put(manifest *manifest.SignedManifest) error
+
+	// TODO(stevvooe): The methods after this message should be moved to a
+	// discrete TagService, per active proposals.
+
 	// Tags lists the tags under the named repository.
 	Tags() ([]string, error)
 
-	// Exists returns true if the manifest exists.
-	Exists(tag string) (bool, error)
+	// ExistsByTag returns true if the manifest exists.
+	ExistsByTag(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
+	// GetByTag retrieves the named manifest, if it exists.
+	GetByTag(tag string) (*manifest.SignedManifest, 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
+	//	1. Allow explicit tagging with Tag(digest digest.Digest, tag string)
+	//	2. 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
+	//	3. 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
+	//	4. 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
+	//	5. Long-term: Manifest should be an interface. This code shouldn't
 	//       really be concerned with the storage format.
 }