Descriptor: do not implement Describable interface

Commit cb6f002350 implemented a generic
Manifest interface to represent manifests in the registry and remove
references to schema specific manifests.

As part of this refactor, the Describable interface was introduced,
which allowed for a single ManifestBuilder interface to handle both
schema1 and schema2 manifests. Implementations of Describable are
generally objects which can be described, not simply descriptors, but
for convenience, this interface was also implemented on Descriptor in
2ff77c00ba.

This interface served its purpose, but no longer needed for most cases;
schema2 (and OCI) descriptors do not need this method, making it only
needed for `schema1.Reference`, which is now deprecated.

Requiring this interface to be implemented limits interoperability
between distribution's Descriptor and the OCI Descriptor types, which
are identical in every other way, except for the presence of the
Describable interface.

This patch:

- Removes the `Descriptor.Descriptor()` method (no longer implementing
  the `Describable` interface).
- Updates ManifestBuilder interface and implementations to accept either
- Updates ManifestBuilder interface and implementations to accept a
  `Descriptor`.

After this patch, the caller is responsible for changing a describable
type into a descriptor;

    builder.AppendReference(describable.Descriptor())

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

blobs.go

@@ -85,15 +85,6 @@ type Descriptor struct {
 	// depend on the simplicity of this type.
 }
 
-// Descriptor returns the descriptor, to make it satisfy the Describable
-// interface. Note that implementations of Describable are generally objects
-// which can be described, not simply descriptors; this exception is in place
-// to make it more convenient to pass actual descriptors to functions that
-// expect Describable objects.
-func (d Descriptor) Descriptor() Descriptor {
-	return d
-}
-
 // BlobStatter makes blob descriptors available by digest. The service may
 // provide a descriptor of a different digest if the provided digest is not
 // canonical.

manifest/ocischema/builder.go

@@ -96,8 +96,8 @@ func (mb *Builder) Build(ctx context.Context) (distribution.Manifest, error) {
 }
 
 // AppendReference adds a reference to the current ManifestBuilder.
-func (mb *Builder) AppendReference(d distribution.Describable) error {
-	mb.layers = append(mb.layers, d.Descriptor())
+func (mb *Builder) AppendReference(ref distribution.Descriptor) error {
+	mb.layers = append(mb.layers, ref)
 	return nil
 }
 

manifest/schema2/builder.go

@@ -46,8 +46,8 @@ func (mb *Builder) Build(ctx context.Context) (distribution.Manifest, error) {
 }
 
 // AppendReference adds a reference to the current ManifestBuilder.
-func (mb *Builder) AppendReference(d distribution.Describable) error {
-	mb.dependencies = append(mb.dependencies, d.Descriptor())
+func (mb *Builder) AppendReference(ref distribution.Descriptor) error {
+	mb.dependencies = append(mb.dependencies, ref)
 	return nil
 }
 

manifests.go

@@ -48,8 +48,12 @@ type ManifestEnumerator interface {
 	Enumerate(ctx context.Context, ingester func(digest.Digest) error) error
 }
 
-// Describable is an interface for descriptors
+// Describable is an interface for descriptors.
+//
+// Implementations of Describable are generally objects which can be
+// described, not simply descriptors.
 type Describable interface {
+	// Descriptor returns the descriptor.
 	Descriptor() Descriptor
 }