Method test_function_tp_clear_leaves_consistent_state

Lib/test/test_gc.py:239–310

(self)

        self.assertIn(gc.collect(), (2, 3))

    def test_function_tp_clear_leaves_consistent_state(self):
        # https://github.com/python/cpython/issues/91636
        code = """if 1:

        import gc
        import weakref

        class LateFin:
            __slots__ = ('ref',)

            def __del__(self):

                # 8. Now `latefin`'s finalizer is called. Here we
                #    obtain a reference to `func`, which is currently
                #    undergoing `tp_clear`.
                global func
                func = self.ref()

        class Cyclic(tuple):
            __slots__ = ()

            # 4. The finalizers of all garbage objects are called. In
            #    this case this is only us as `func` doesn't have a
            #    finalizer.
            def __del__(self):

                # 5. Create a weakref to `func` now. In previous
                #    versions of Python, this would avoid having it
                #    cleared by the garbage collector before calling
                #    the finalizers.  Now, weakrefs get cleared after
                #    calling finalizers.
                self[1].ref = weakref.ref(self[0])

                # 6. Drop the global reference to `latefin`. The only
                #    remaining reference is the one we have.
                global latefin
                del latefin

            # 7. Now `func` is `tp_clear`-ed. This drops the last
            #    reference to `Cyclic`, which gets `tp_dealloc`-ed.
            #    This drops the last reference to `latefin`.

        latefin = LateFin()
        def func():
            pass
        cyc = tuple.__new__(Cyclic, (func, latefin))

        # 1. Create a reference cycle of `cyc` and `func`.
        func.__module__ = cyc

        # 2. Make the cycle unreachable, but keep the global reference
        #    to `latefin` so that it isn't detected as garbage. This
        #    way its finalizer will not be called immediately.
        del func, cyc

        # 3. Invoke garbage collection,
        #    which will find `cyc` and `func` as garbage.
        gc.collect()