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Not all types have "trashcan" protection and tp_dealloc can overflow stack #124715
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Regarding untracking, when I was working on my "integrate trashcan PR", I started wondering if
_PyObject_GC_UNTRACK()should just be changed to check if the object is already untracked and not crash. I originally made it not safe to call twice because I wanted the tiny performance benefit (avoid the branch, I wanted GC support to have as little extra overhead as possible). However, in retrospect, I think it was a mistake. Knowing when it is safe to call_PyObject_GC_UNTRACK()in theory is possible but in practice it gets really painful. If you look at typeobject.c, it has to take great care to avoid_PyObject_GC_UNTRACK()from crashing.Maybe we should profile this and see if it has any performance impact at all? There has been many bugs related to this over the years.
We occasionally fix these kinds of bugs, like GH-102356. However, the fix by adding the "trashcan" macros to the tp_dealloc method only fixes that specific type. It would be better to have a more generic fix. A crash due to the C stack overflow leaves the user mystified as to what the problem is and these kinds of bugs could be hard to reproduce (requiring long chains of objects calling tp_dealloc recursively).
Linked PRs
Py_Dealloc#132280