Leak with untracked tuples and deferred ref counts

[nascheme]

Bug report

Bug description:

With the merge of gh-142843, the test case test__interpchannels.ChannelTests.test_send_recv_different_interpreters_and_threads is now failing with reference leaks. A minimal reproducer is below. If the sys.flags object has deferred reference counting enabled in the sub-interpreter, it leaks.

import gc
import sys
import _interpreters


def test_leak():
 id1 = _interpreters.create()

 def f():
 # enable deferred ref counts for sys.flags, causes a leak, even with
 # an explicit gc.collect() call
 _interpreters.run_string(
 id1,
 'import sys, _testcapi; _testcapi.pyobject_enable_deferred_refcount(sys.flags)\n',
 )

 f()
 _interpreters.destroy(id1)


def main():
 for i in range(4):
 test_leak()
 gc.collect()
 old_refs = sys.gettotalrefcount()
 for i in range(10):
 test_leak()
 gc.collect()
 refs = sys.gettotalrefcount()
 print(f'refs {refs} diff {refs-old_refs}')
 old_refs = refs


if __name__ == '__main__':
 main()

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-144054: shutdown fix for deferred ref counting #144055
• gh-144054: no deferred refcount for untracked #144081

[nascheme]

I figured out what's special about sys.flags. It is a tuple and it is untracked from the GC. The free-threaded GC disables deferred refcounting while shutting down. However, it skips objects that are untracked. Making a patch to fix.