Fix manifest UI display for manifests with empty layers

Also fixes the command for schema 2 manifests

endpoints/api/manifest.py

@@ -1,5 +1,6 @@
 """ Manage the manifests of a repository. """
 import json
+import logging
 
 from flask import request
 
@@ -20,6 +21,9 @@ BASE_MANIFEST_ROUTE = '/v1/repository/<apirepopath:repository>/manifest/<regex("
 MANIFEST_DIGEST_ROUTE = BASE_MANIFEST_ROUTE.format(digest_tools.DIGEST_PATTERN)
 ALLOWED_LABEL_MEDIA_TYPES = ['text/plain', 'application/json']
 
+
+logger = logging.getLogger(__name__)
+
 def _label_dict(label):
   return {
     'id': label.uuid,
@@ -31,10 +35,12 @@ def _label_dict(label):
 
 
 def _layer_dict(manifest_layer, index):
+  # NOTE: The `command` in the layer is either a JSON string of an array (schema 1) or
+  # a single string (schema 2). The block below normalizes it to have the same format.
   try:
     command = json.loads(manifest_layer.command)
   except (TypeError, ValueError):
-    command = []
+    command = [manifest_layer.command]
 
   return {
     'index': index,
@@ -53,6 +59,10 @@ def _manifest_dict(manifest):
     image = image_dict(manifest.legacy_image, with_history=True)
 
   layers = registry_model.list_manifest_layers(manifest, storage)
+  if layers is None and not manifest.is_manifest_list:
+    logger.debug('Missing layers for manifest `%s`', manifest.digest)
+    abort(404)
+
   return {
     'digest': manifest.digest,
     'is_manifest_list': manifest.is_manifest_list,