PYTHON-799 Break ref cycle in Monitor.

Previous code to avoid the cycle was wrong.

pymongo/monitor.py

@@ -41,7 +41,7 @@ class Monitor(object):
         """
         self._server_description = server_description
 
-        # A weakref callback, takes ref to the dead topology as its parameter.
+        # A weakref callback, takes ref to the freed topology as its parameter.
         def close(dummy):
             self.close()
 
@@ -51,10 +51,15 @@ class Monitor(object):
         self._avg_round_trip_time = MovingAverage()
 
         # We strongly reference the executor and it weakly references us via
-        # this closure. When the monitor is freed, a call to target() raises
-        # ReferenceError and stops the executor.
+        # this closure. When the monitor is freed, stop the executor.
+        self_ref = weakref.ref(self)
+
         def target():
-            Monitor._run(weakref.proxy(self))
+            monitor = self_ref()
+            if monitor is None:
+                return False  # Stop the executor.
+            Monitor._run(monitor)
+            return True
 
         self._executor = periodic_executor.PeriodicExecutor(
             condition_class=self._settings.condition_class,

pymongo/periodic_executor.py

@@ -26,7 +26,7 @@ class PeriodicExecutor(object):
     def __init__(self, condition_class, interval, min_interval, target):
         """"Run a target function periodically on a background thread.
 
-        If the function raises an exception, the executor stops.
+        If the target's return value is false, the executor stops.
 
         :Parameters:
           - `condition_class`: A class like threading.Condition.
@@ -84,11 +84,12 @@ class PeriodicExecutor(object):
     def _run(self):
         while not self._stopped:
             try:
-                self._target()
-            except Exception:
-                # It's the target's responsibility to report errors.
+                if not self._target():
+                    self._stopped = True
+                    break
+            except:
                 self._stopped = True
-                break
+                raise
 
             # Avoid running too frequently if wake() is called very often.
             time.sleep(self._min_interval)