Fix some races with threading.local

See #86

This does not fix races where the _localimpl is delete by another
thread... but I don't think the _threading_local implementation
is actually used.

Lib/_threading_local.py

@@ -171,7 +171,7 @@ def local_deleted(_, key=key):
             # When the localimpl is deleted, remove the thread attribute.
             thread = wrthread()
             if thread is not None:
-                del thread.__dict__[key]
+                del thread._locals[key]
         def thread_deleted(_, idt=idt):
             # When the thread is deleted, remove the local dict.
             # Note that this is suboptimal if the thread object gets
@@ -182,7 +182,7 @@ def thread_deleted(_, idt=idt):
                 dct = local.dicts.pop(idt)
         wrlocal = ref(self, local_deleted)
         wrthread = ref(thread, thread_deleted)
-        thread.__dict__[key] = wrlocal
+        thread._locals[key] = wrlocal
         self.dicts[idt] = wrthread, localdict
         return localdict
 

Lib/threading.py

@@ -808,6 +808,7 @@ class is implemented.
         # Copy of sys.stderr used by self._invoke_excepthook()
         self._stderr = _sys.stderr
         self._invoke_excepthook = _make_invoke_excepthook()
+        self._locals = {}
         # For debugging and _after_fork()
         _dangling.add(self)