Use a UUID (rather than a sentinel object) for sentinel on Pub / Sub …

…`Future`.

pubsub/google/cloud/pubsub_v1/futures.py

@@ -15,6 +15,7 @@
 from __future__ import absolute_import
 
 import threading
+import uuid
 
 import google.api_core.future
 from google.cloud.pubsub_v1.publisher import exceptions
@@ -29,7 +30,11 @@ class Future(google.api_core.future.Future):
  This object should not be created directly, but is returned by other
  methods in this library.
  """
- _SENTINEL = object()
+
+ # This could be a sentinel object or None, but the sentinel object's ID
+ # can change if the process is forked, and None has the possibility of
+ # actually being a result.
+ _SENTINEL = uuid.uuid4()
 
  def __init__(self):
  self._result = self._SENTINEL