Support pulling of schema2 manifests directly via a manifest list tag

This change ensures that if a manifest list is requested with an accepts header for a *schema 2* manifest, the legacy manifest (if any) is returned as schema 2 if it was pushed as a schema 2 manifest (rather than being auto-converted to schema 1)

image/docker/schema2/list.py

@@ -263,6 +263,29 @@ class DockerSchema2ManifestList(ManifestInterface):
     """ Returns the manifest that is compatible with V1, by virtue of being `amd64` and `linux`.
         If none, returns None.
     """
+    legacy_manifest = self._get_legacy_manifest(content_retriever)
+    if legacy_manifest is None:
+      return None
+
+    return legacy_manifest.get_schema1_manifest(namespace_name, repo_name, tag_name,
+                                                content_retriever)
+
+  def convert_manifest(self, allowed_mediatypes, namespace_name, repo_name, tag_name,
+                       content_retriever):
+    if self.media_type in allowed_mediatypes:
+      return self
+
+    legacy_manifest = self._get_legacy_manifest(content_retriever)
+    if legacy_manifest is None:
+      return None
+
+    return legacy_manifest.convert_manifest(allowed_mediatypes, namespace_name, repo_name,
+                                            tag_name, content_retriever)
+
+  def _get_legacy_manifest(self, content_retriever):
+    """ Returns the manifest under this list with architecture amd64 and os linux, if any, or None
+        if none or error.
+    """
     for manifest_ref in self.manifests(content_retriever):
       platform = manifest_ref._manifest_data[DOCKER_SCHEMA2_MANIFESTLIST_PLATFORM_KEY]
       architecture = platform[DOCKER_SCHEMA2_MANIFESTLIST_ARCHITECTURE_KEY]
@@ -271,13 +294,11 @@ class DockerSchema2ManifestList(ManifestInterface):
         continue
 
       try:
-        manifest = manifest_ref.manifest_obj
+        return manifest_ref.manifest_obj
       except (ManifestException, IOError):
         logger.exception('Could not load child manifest')
         return None
 
-      return manifest.get_schema1_manifest(namespace_name, repo_name, tag_name, content_retriever)
-
     return None
 
   def unsigned(self):