Inconsistency handling of immortal objects in gc

[sergey-miryanov]

Bug report

Bug description:

There is some inconsistency in handling of immortal objects in default-build's GC.

In update_refs immortal objects just untracked (all immortal objects that we face in GC is an accidentally immortalized).

cpython/Python/gc.c

Lines 495 to 500 in 1e69cd1

	if (_Py_IsImmortal(op)) {
	assert(!_Py_IsStaticImmortal(op));
	_PyObject_GC_UNTRACK(op);
	gc = next;
	continue;
	}

update_refs called for all collections (young, increment and full). But for incremental collection if increment_size < gcstate->work_to_do we steal some objects from neighbor gen and call expand_region_transitively_reachable where immortal objects moved to permanent generation:

cpython/Python/gc.c

Lines 1397 to 1402 in 1e69cd1

	if (_Py_IsImmortal(op)) {
	PyGC_Head *next = GC_NEXT(gc);
	gc_list_move(gc, &gcstate->permanent_generation.head);
	gc = next;
	continue;
	}

So, one part of immortal objects can be untracked, and other part can be moved to perm gen.

As I understand from #127519 it is preferring to untrack immortal objects in all cases.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-137110: Untrack immortal objects from expand_region_transitivity_reachable #137111