[3.14] gh-142651: use NonCallableMock._lock for thread safety of call_count (GH-142922) (#145739)

miss-islington · kumaraditya303 · web-flow · commit a5ed66df8ac5 · 2026-03-10T12:39:45.000Z
gh-142651: use `NonCallableMock._lock` for thread safety of `call_count` (GH-142922) (cherry picked from commit 728e4a0) Co-authored-by: Kumar Aditya <kumaraditya@python.org>
diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
@@ -1184,10 +1184,16 @@ def _increment_mock_call(self, /, *args, **kwargs):
         # handle call_args
         # needs to be set here so assertions on call arguments pass before
         # execution in the case of awaited calls
-        _call = _Call((args, kwargs), two=True)
-        self.call_args = _call
-        self.call_args_list.append(_call)
-        self.call_count = len(self.call_args_list)
+        with NonCallableMock._lock:
+            # Lock is used here so that call_args_list and call_count are
+            # set atomically otherwise it is possible that by the time call_count
+            # is set another thread may have appended to call_args_list.
+            # The rest of this function relies on list.append being atomic and
+            # skips locking.
+            _call = _Call((args, kwargs), two=True)
+            self.call_args = _call
+            self.call_args_list.append(_call)
+            self.call_count = len(self.call_args_list)
 
         # initial stuff for method_calls:
         do_method_calls = self._mock_parent is not None
