Method test_bug1055820d

Lib/test/test_gc.py:1530–1599

(self)

    @gc_threshold(1000, 0, 0)
    @unittest.skipIf(Py_GIL_DISABLED, "requires GC generations or increments")
    def test_bug1055820d(self):
        # Corresponds to temp2d.py in the bug report.  This is very much like
        # test_bug1055820c, but uses a __del__ method instead of a weakref
        # callback to sneak in a resurrection of cyclic trash.

        ouch = []
        class D(C1055820):
            def __del__(self):
                ouch[:] = [c2wr()]

        d0 = D(0)
        # Move all the above into generation 2.
        gc.collect()

        c1 = C1055820(1)
        c1.keep_d0_alive = d0
        del d0.loop # now only c1 keeps d0 alive

        c2 = C1055820(2)
        c2wr = weakref.ref(c2) # no callback!

        d0 = c1 = c2 = None

        # What we've set up:  d0, c1, and c2 are all trash now.  d0 is in
        # generation 2.  The only thing keeping it alive is that c1 points to
        # it.  c1 and c2 are in generation 0, and are in self-loops.  There's
        # a global weakref to c2 (c2wr), but that weakref has no callback.
        # There are no other weakrefs.
        #
        #               d0 has a __del__ method that references c2wr
        #               ^
        #               |
        #               |     Generation 2 above dots
        #. . . . . . . .|. . . . . . . . . . . . . . . . . . . . . . . .
        #               |     Generation 0 below dots
        #               |
        #               |
        #            ^->c1   ^->c2 has a wr but no callback
        #            |  |    |  |
        #            <--v    <--v
        #
        # So this is the nightmare:  when generation 0 gets collected, we see
        # that c2 has a callback-free weakref, and c1 doesn't even have a
        # weakref.  Collecting generation 0 doesn't see d0 at all.  gc clears
        # c1 and c2.  Clearing c1 has the side effect of dropping the refcount
        # on d0 to 0, so d0 goes away (despite that it's in an older
        # generation) and d0's __del__ triggers.  That in turn materializes
        # a reference to c2 via c2wr(), but c2 gets cleared anyway by gc.

        # We want to let gc happen "naturally", to preserve the distinction
        # between generations.
        detector = GC_Detector()
        junk = []
        i = 0
        if Py_GIL_DISABLED:
            # The free-threaded build doesn't have multiple generations, so
            # just trigger a GC manually.
            gc.collect()