PYTHON-1177 Fix race condition in PeriodicExecutor

Author: ShaneHarvey

pymongo/periodic_executor.py

@@ -47,12 +47,27 @@ def __init__(self, interval, min_interval, target, name=None):
  self._thread = None
  self._name = name
 
+ self._thread_will_exit = False
+ self._lock = threading.Lock()
+
  def open(self):
  """Start. Multiple calls have no effect.
 
  Not safe to call from multiple threads at once.
  """
- self._stopped = False
+ with self._lock:
+ if self._thread_will_exit:
+ # If the background thread has read self._stopped as True
+ # there is a chance that it has not yet exited. The call to
+ # join should not block indefinitely because there is no
+ # other work done outside the while loop in self._run.
+ try:
+ self._thread.join()
+ except ReferenceError:
+ # Thread terminated.
+ pass
+ self._thread_will_exit = False
+ self._stopped = False
  started = False
  try:
  started = self._thread and self._thread.is_alive()
@@ -87,8 +102,15 @@ def wake(self):
  """Execute the target function soon."""
  self._event = True
 
+ def __should_stop(self):
+ with self._lock:
+ if self._stopped:
+ self._thread_will_exit = True
+ return True
+ return False
+
  def _run(self):
- while not self._stopped:
+ while not self.__should_stop():
  try:
  if not self._target():
  self._stopped = True