Fix exception raised for certain non-JSON strings given to is_json

This is breaking pushes in production for certain manifests

Fixes https://jira.prod.coreos.systems/browse/QS-60

test/registry_tests.py

@@ -1536,6 +1536,27 @@ class V2RegistryTests(V2RegistryPullMixin, V2RegistryPushMixin, RegistryTestsMix
     self.assertTrue('text/plain' in media_types)
     self.assertTrue('application/json' in media_types)
 
+  def test_not_json_labels(self):
+    # Push a new repo with the latest tag.
+    images = [{
+      'id': 'someid',
+      'config': {'Labels': {'foo': '[hello world]', 'bar': '{wassup?!}'}},
+      'contents': 'somecontent'
+    }]
+
+    (_, manifests) = self.do_push('devtable', 'newrepo', 'devtable', 'password', images=images)
+    digest = manifests['latest'].digest
+
+    self.conduct_api_login('devtable', 'password')
+    labels = self.conduct('GET', '/api/v1/repository/devtable/newrepo/manifest/' + digest + '/labels').json()
+    self.assertEquals(2, len(labels['labels']))
+
+    media_types = set([label['media_type'] for label in labels['labels']])
+
+    self.assertTrue('text/plain' in media_types)
+    self.assertFalse('application/json' in media_types)
+
+
   def test_expiration_label(self):
     # Push a new repo with the latest tag.
     images = [{

util/test/test_validation.py

@@ -0,0 +1,17 @@
+import pytest
+
+from util.validation import is_json
+
+@pytest.mark.parametrize('string_value,expected', [
+  ('{}', True),
+  ('[]', True),
+  ('[hello world]', False),
+  ('{hello world}', False),
+  ('[test] this is a test', False),
+  ('hello world', False),
+  ('{"hi": "there"}', True),
+  ('[1, 2, 3, 4]', True),
+  ('[1, 2, {"num": 3}, 4]', True),
+])
+def test_is_json(string_value, expected):
+  assert is_json(string_value) == expected

util/validation.py

@@ -86,6 +86,6 @@ def is_json(value):
     try:
       json.loads(value)
       return True
-    except TypeError:
+    except (TypeError, ValueError):
       return False
-
+  return False