Switch content retrieval in manifests to be behind an interface

This allows for easy separation of retrieval of config blobs vs manifests

image/docker/interfaces.py

@@ -71,14 +71,13 @@ class ManifestInterface(object):
     """
 
   @abstractmethod
-  def child_manifests(self, lookup_manifest_fn):
+  def child_manifests(self, content_retriever):
     """ Returns an iterator of all manifests that live under this manifest, if any or None if not
-        applicable. The lookup_manifest_fn is a function that, when given a blob content SHA,
-        returns the contents of that blob in storage if any or None if none.
+        applicable.
     """
 
   @abstractmethod
-  def get_manifest_labels(self, lookup_config_fn):
+  def get_manifest_labels(self, content_retriever):
     """ Returns a dictionary of all the labels defined inside this manifest or None if this kind
         of manifest does not support labels. """
     pass
@@ -88,7 +87,7 @@ class ManifestInterface(object):
     """ Returns an unsigned version of this manifest. """
 
   @abstractmethod
-  def generate_legacy_layers(self, images_map, lookup_config_fn):
+  def generate_legacy_layers(self, images_map, content_retriever):
     """
     Rewrites Docker v1 image IDs and returns a generator of DockerV1Metadata.
 
@@ -100,7 +99,19 @@ class ManifestInterface(object):
     """
 
   @abstractmethod
-  def get_v1_compatible_manifest(self, namespace_name, repo_name, tag_name, lookup_fn):
+  def get_v1_compatible_manifest(self, namespace_name, repo_name, tag_name, content_retriever):
     """ Returns the manifest that is compatible with V1, by virtue of being `amd64` and `linux`.
         If none, returns None.
     """
+
+
+@add_metaclass(ABCMeta)
+class ContentRetriever(object):
+  """ Defines the interface for retrieval of various content referneced by a manifest. """
+  @abstractmethod
+  def get_manifest_bytes_with_digest(self, digest):
+    """ Returns the bytes of the manifest with the given digest or None if none found. """
+
+  @abstractmethod
+  def get_blob_bytes_with_digest(self, digest):
+    """ Returns the bytes of the blob with the given digest or None if none found. """