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).

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:

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