Bug report

PyList_SetItem currently usesPy_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 ofPyList_SetItem #129644
• [3.13] gh-129643: fix thread safety ofPyList_SetItem (#129644) #129677
• gh-129643: FixPyList_Insert in free-threading builds #129680
• [3.13] gh-129643: FixPyList_Insert in free-threading builds (GH-129680) #129725