Fix race condition in cached_method

Multiple weakrefs can be created when there are concurrent writes to a
"new cache". When they finalize, their attempts to clear the dict should
be kept safe by using a pop() default.

Author: sirosen

Lib/functools.py

@@ -1134,7 +1134,7 @@ def register(self):
 
 def _cached_method_weakref_callback(cache_dict, id_key):
     def callback(ref):
-        cache_dict.pop(id_key)
+        cache_dict.pop(id_key, None)
     return callback