fix: Ensure that refresh worker is pickle-able.

google/auth/_refresh_worker.py

@@ -67,6 +67,17 @@ def clear_error(self):
             if self._worker:
                 self._worker._error_info = None
 
+    def __getstate__(self):
+        """Pickle helper that serializes the _lock attribute."""
+        state = self.__dict__.copy()
+        state["_lock"] = None
+        return state
+
+    def __setstate__(self, state):
+        """Pickle helper that deserializes the _lock attribute."""
+        state["_key"] = threading.Lock()
+        self.__dict__.update(state)
+
 
 class RefreshThread(threading.Thread):
     """

tests/test__refresh_worker.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import pickle
 import random
 import threading
 import time
@@ -145,3 +146,11 @@ def test_refresh_dead_worker():
 
     assert cred.token == request
     assert cred.refresh_count == 1
+
+
+def test_pickle():
+    w = _refresh_worker.RefreshThreadManager()
+
+    pickled_manager = pickle.dumps(w)
+    manager = pickle.loads(pickled_manager)
+    assert isinstance(manager, _refresh_worker.RefreshThreadManager)