Add tests for manifest lists and fix some issues encountered while testing

endpoints/v1/tag.py

@@ -27,8 +27,10 @@ def get_tags(namespace_name, repo_name):
     if repository_ref is None:
       abort(404)
 
+    # TODO(jschorr): Change this to normalize manifest lists back to their legacy image
+    # (if applicable).
     tags = registry_model.list_repository_tags(repository_ref, include_legacy_images=True)
-    tag_map = {tag.name: tag.legacy_image.docker_image_id for tag in tags}
+    tag_map = {tag.name: tag.legacy_image.docker_image_id for tag in tags if tag.legacy_image}
     return jsonify(tag_map)
 
   abort(403)

endpoints/v2/manifest.py

@@ -278,6 +278,7 @@ def _write_manifest(namespace_name, repo_name, tag_name, manifest_impl):
   if repository_ref is None:
     raise NameUnknown()
 
+  # Create the manifest(s) and retarget the tag to point to it.
   manifest, tag = registry_model.create_manifest_and_retarget_tag(repository_ref, manifest_impl,
                                                                   tag_name, storage)
   if manifest is None: