gh-116946: fully implement GC protocol for _functools._lru_list_elem

[picnixz]

This type is immutable but it is a true container type, so it needs the GC. However I need to measure performances.

• Issue: Heap types implemented in C must have Py_TPFLAGS_HAVE_GC and implement tp_traverse #116946

[picnixz]

Oh there is a previous discussion: #76603. I'll check this.

[picnixz]

Mmh, so this now boils down to whether we want to make sure that the GC protocol is correctly implemented or if we want to keep performances.

[rhettinger]

The lru_cache_type_spec already is marked with Py_TPFLAGS_HAVE_GC and Py_TPFLAGS_IMMUTABLETYPE. And the lru_cache_tp_traverse is already visiting all of the _lru_list_elem entries. ISTM that all parts that can hold an object are already visible to GC and would be collected at shutdown. This PR seems unnecessary and will just hurt performance for no real use benefit.

[serhiy-storchaka]

Concur with @rhettinger.

Note also that the cache dict is already visited, and it visits all its elements, so they will be visited twice with this PR.

Also, clearing the key in lru_list_elem_clear() can have bad consequences.

[picnixz]

I see. Ok I'll close this one and will just change the immutability bit when necessary in the other PRs.