bpo-31061: fix crash in asyncio speedup module by thehesiod · Pull Request #2966 · python/cpython

CPython's circular reference GC uses doubly linked list to track object which can be member of circular reference.
PyObject_GC_UnTrack() removes object from the list, and PyObject_GC_Track() insert the object to the list.

Despite very undetarministic multithreading, I can cause SEGV quickly with this code:

import asyncio
import gc
gc.set_debug(gc.DEBUG_STATS)

class Evil:
    def __del__(self):
        gc.collect()

def main():
    f = asyncio.Future()
    f.set_result(Evil())

for i in range(100):
    main()