PYTHON-2433 Fix Python 3 ServerDescription/Exception memory leak

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.

Author: ShaneHarvey

pymongo/server_description.py

@@ -15,6 +15,7 @@
 """Represent one server the driver is connected to."""
 
 from bson import EPOCH_NAIVE
+from bson.py3compat import PY3
 from pymongo.server_type import SERVER_TYPE
 from pymongo.ismaster import IsMaster
 from pymongo.monotonic import time as _time
@@ -69,6 +70,11 @@ def __init__(
         self._me = ismaster.me
         self._last_update_time = _time()
         self._error = error
+        # PYTHON-2433 Clear error traceback info.
+        if error and PY3:
+            error.__traceback__ = None
+            error.__context__ = None
+            error.__cause__ = None
         self._topology_version = ismaster.topology_version
         if error:
             if hasattr(error, 'details') and isinstance(error.details, dict):

test/test_client.py

@@ -57,6 +57,7 @@
 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 @@ def test_direct_connection(self):
         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."""