request: Weaken ZulipRequestNotes.tornado_handler reference.

This prevents a memory leak arising from Python’s inability to collect
a reference cycle from a WeakKeyDictionary value to its key
(https://bugs.python.org/issue44680).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/test_helpers.py

@@ -3,6 +3,7 @@ import os
 import re
 import sys
 import time
+import weakref
 from contextlib import contextmanager
 from functools import wraps
 from typing import (
@@ -310,7 +311,6 @@ class HostRequestMock(HttpRequest):
             self.POST[key] = str(post_data[key])
             self.method = "POST"
 
-        self._tornado_handler = DummyHandler()
         self._log_data: Dict[str, Any] = {}
         if meta_data is None:
             self.META = {"PATH_INFO": "test"}
@@ -324,7 +324,7 @@ class HostRequestMock(HttpRequest):
         request_notes_map[self] = ZulipRequestNotes(
             client_name="",
             log_data={},
-            tornado_handler=tornado_handler,
+            tornado_handler=None if tornado_handler is None else weakref.ref(tornado_handler),
             client=get_client(client_name) if client_name is not None else None,
         )