PyList_SetItem missing atomic store

[kumaraditya303]

Bug report

PyList_SetItem currently uses Py_XSETREF to set the item and decref the old one, however the store is not atomic as such it can race with a concurrent read. The fix is to use a atomic store with release order to correctly set the new item and then decref the old object.

cc @colesbury @Yhg1s

Linked PRs

• gh-129643: fix thread safety of PyList_SetItem #129644
• [3.13] gh-129643: fix thread safety of PyList_SetItem (#129644) #129677
• gh-129643: Fix PyList_Insert in free-threading builds #129680
• [3.13] gh-129643: Fix PyList_Insert in free-threading builds (GH-129680) #129725

[colesbury]

There is a similar issue in the function ins1:

cpython/Objects/listobject.c

Lines 467 to 470 in 979d766

	items = self->ob_item;
	for (i = n; --i >= where; )
	items[i+1] = items[i];
	items[where] = Py_NewRef(v);