message: Fix the KeyError in multi_worker_test_base.py caused by __iter__ returning (key, value) which gets interpreted as the key (and hence no value is found).
diff: @@ -347,9 +347,9 @@ class MockOsEnv(collections.Mapping):
  def __iter__(self):
  if not hasattr(self._thread_local, 'dict'):
  self._thread_local.dict = dict()
  - for x in self._thread_local.dict.items():
  + for x in self._thread_local.dict:
  yield x
  - for x in self._dict.items():
  + for x in self._dict:
  yield x
  def __len__(self):
  