bpo-42972: Fully implement GC protocol for functools LRU cache by erlend-aasland · Pull Request #26423 · python/cpython
Oh. lru_list_elem_type was modified to not implemented the GC to optimize the code, I missed that:
Please explain it in a comment, it's non-obvious.
https://bugs.python.org/issue32422 seems to conflict with https://bugs.python.org/issue42972
Maybe it's acceptable to not implement the GC protocol if the link type is hidden well and it is immutable. In my experience, reference cycles are created in very surprising ways. I don't know about this one. I don't know if it's acceptable for now, if it's a trade-off, or if there is a risk of leaks.
An alternative would be to avoid completely Python objects in the cache. Replace lru_cache_object.cache Python dictionary with a _Py_hashtable_t. So the linked list items don't have to be Python objects anymore.
But this change would be a major change and should only be done in the main branch.
The question is what to do in the 3.10 branch:
- Implement again the GC protocol in the link type: revert https://bugs.python.org/issue32422
- Don't implement the GC protocol in the link type, make it immutable, and hide it to reduce the risk of reference cycles involing this type