gh-144054: shutdown fix for deferred ref counting

[nascheme]

When shutting down, disable deferred refcounting for all GC objects. It is important to do this also for untracked objects, which before this change were getting missed.

• Issue: Leak with untracked tuples and deferred ref counts #144054

[nascheme]

@colesbury I think this is a correct fix but some of the details are a bit tricky. It seems the !_Py_IsImmortal(op) test is required before calling disable_deferred_refcounting() but I'm not 100% sure why. I'd expect that immortal objects should not also have deferred ref counting enabled for them. The need to check for _PyObject_GC_IS_TRACKED() before calling frame_disable_deferred_refcounting() makes sense.

I split the "disable deferred" into a separate heap scan. Doing it inside scan_heap_visitor() would add extra branches to the hot executate path. So, I think a separate scan and keeping scan_heap_visitor() faster is better.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @nascheme for commit 296eba9 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F144055%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.

[encukou]

The refleak test failure in test_capi (test_replaced_interpreter) is unrelated (#142913)

[encukou]

Otherwise, buildbots are happy.
I'll merge to help fix the tier-1 failures. Any follow-ups can go in separate PRs.

[colesbury]

I don't think this is right. There shouldn't be any objects that are untracked and use deferred reference counting.

[colesbury]

If an object is untracked and and uses deferred reference counting, then it won't be freed when it's no longer referenced. Collecting those objects at shutdown will satisfy the leak tests, but that doesn't fix the actual problem.

[nascheme]

I'm working on a correct fix. If maybe_enable_deferred_ref_count() ensures that the object is tracked and gc_maybe_untrack() ensures the object does not have deferred refs enabled, I think that fixes it.