gh-98624 Add mutex to unittest.mock.NonCallableMock (#98688)

* Added lock to NonCallableMock in unittest.mock

* Add blurb

* Nitpick blurb

* Edit comment based on @Jason-Y-Z's review

* Add link to GH issue

Lib/unittest/mock.py

@@ -35,6 +35,7 @@ from asyncio import iscoroutinefunction
 from types import CodeType, ModuleType, MethodType
 from unittest.util import safe_repr
 from functools import wraps, partial
+from threading import RLock
 
 
 class InvalidSpecError(Exception):
@@ -402,6 +403,14 @@ class Base(object):
 class NonCallableMock(Base):
     """A non-callable version of `Mock`"""
 
+    # Store a mutex as a class attribute in order to protect concurrent access
+    # to mock attributes. Using a class attribute allows all NonCallableMock
+    # instances to share the mutex for simplicity.
+    #
+    # See https://github.com/python/cpython/issues/98624 for why this is
+    # necessary.
+    _lock = RLock()
+
     def __new__(cls, /, *args, **kw):
         # every instance has its own class
         # so we can create magic methods on the
@@ -644,6 +653,7 @@ class NonCallableMock(Base):
                     f"{name!r} is not a valid assertion. Use a spec "
                     f"for the mock if {name!r} is meant to be an attribute.")
 
+        with NonCallableMock._lock:
             result = self._mock_children.get(name)
             if result is _deleted:
                 raise AttributeError(name)

Misc/NEWS.d/next/Library/2022-10-25-20-17-34.gh-issue-98624.YQUPFy.rst

@@ -0,0 +1,2 @@
+Add a mutex to unittest.mock.NonCallableMock to protect concurrent access
+to mock attributes.