PYTHON-2433 Fix Python 3 ServerDescription/Exception memory leak (#520)

When the SDAM monitor check fails, a ServerDescription is created from
the exception. This exception is kept alive via the
ServerDescription.error field. Unfortunately, the exception's traceback
contains a reference to the previous ServerDescription. Altogether this
means that each consecutively failing check leaks memory by building an
ever growing chain of ServerDescription -> Exception -> Traceback ->
Frame -> ServerDescription -> ... objects.

This change breaks the chain and prevents the memory leak by clearing
the Exception's __traceback__, __context__, and __cause__ fields.

(cherry picked from commit 6c92e6c67e)

pymongo/monitor.py

@@ -18,6 +18,8 @@ import atexit
 import threading
 import weakref
 
+from bson.py3compat import PY3
+
 from pymongo import common, periodic_executor
 from pymongo.errors import (NotMasterError,
                             OperationFailure,
@@ -30,6 +32,14 @@ from pymongo.server_description import ServerDescription
 from pymongo.srv_resolver import _SrvResolver
 
 
+def _sanitize(error):
+    """PYTHON-2433 Clear error traceback info."""
+    if PY3:
+        error.__traceback__ = None
+        error.__context__ = None
+        error.__cause__ = None
+
+
 class MonitorBase(object):
     def __init__(self, topology, name, interval, min_interval):
         """Base class to do periodic work on a background thread.
@@ -169,6 +179,7 @@ class Monitor(MonitorBase):
             try:
                 self._server_description = self._check_server()
             except _OperationCancelled as exc:
+                _sanitize(exc)
                 # Already closed the connection, wait for the next check.
                 self._server_description = ServerDescription(
                     self._server_description.address, error=exc)
@@ -212,6 +223,7 @@ class Monitor(MonitorBase):
         except ReferenceError:
             raise
         except Exception as error:
+            _sanitize(error)
             sd = self._server_description
             address = sd.address
             duration = _time() - start

test/test_client.py

@@ -57,6 +57,7 @@ from pymongo.monotonic import time as monotonic_time
 from pymongo.driver_info import DriverInfo
 from pymongo.pool import SocketInfo, _METADATA
 from pymongo.read_preferences import ReadPreference
+from pymongo.server_description import ServerDescription
 from pymongo.server_selectors import (any_server_selector,
                                       writable_server_selector)
 from pymongo.server_type import SERVER_TYPE
@@ -1614,6 +1615,33 @@ class TestClient(IntegrationTest):
         with self.assertRaises(ConfigurationError):
             MongoClient(['host1', 'host2'], directConnection=True)
 
+    def test_continuous_network_errors(self):
+        def server_description_count():
+            i = 0
+            for obj in gc.get_objects():
+                try:
+                    if isinstance(obj, ServerDescription):
+                        i += 1
+                except ReferenceError:
+                    pass
+            return i
+        gc.collect()
+        with client_knobs(min_heartbeat_interval=0.003):
+            client = MongoClient(
+                'invalid:27017',
+                heartbeatFrequencyMS=3,
+                serverSelectionTimeoutMS=100)
+            initial_count = server_description_count()
+            self.addCleanup(client.close)
+            with self.assertRaises(ServerSelectionTimeoutError):
+                client.test.test.find_one()
+            gc.collect()
+            final_count = server_description_count()
+            # If a bug like PYTHON-2433 is reintroduced then too many
+            # ServerDescriptions will be kept alive and this test will fail:
+            # AssertionError: 4 != 22 within 5 delta (18 difference)
+            self.assertAlmostEqual(initial_count, final_count, delta=5)
+
 
 class TestExhaustCursor(IntegrationTest):
     """Test that clients properly handle errors from exhaust cursors."""