free-threading GC does not properly track references to a running function #160682

Bug report

def test_running_method_not_collected(self):
        # The class dict holds the only counted reference to the method.
        # The running frame's deferred reference must keep it alive.
        class C:
            def m(self):
                wr = weakref.ref(C.__dict__['m'])
                del C.m
                gc.collect()
                return wr() is not None

            def run(self):
                return self.m()

        self.assertTrue(C().run())

In the above gc.collect() cleans up C.m however m is still running in an instance so should not be cleaned up. The above is a bit of a contrived example but it shows the gap reliably.

In a more real world example this could show up in more common code with monkey patching, importlib.reload, repls. It should be a small fix to ensure frame->f_funcobj is properly accounted for in free threading GC.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-160682: Track running function objects in free-threaded GC #160707