Neil Schemenauer <nas-pyt...@arctrix.com> added the comment:
Victor: > I'm surprised that the function would be seen as "unreachable" if > it's reference counter was equal to 135: If all those references are contained within that garbage set, it is possible. Pablo: > For the weakref to be handled correctly the ctypedescr needs to be > identified correctly as part of the isolated cycle. If is not in > the isolated cycle something may be missing tp_traverse or the GC > flags. I think this analysis is correct. I did some poking around with using 'rr'. It is very handy to run the program in reverse and use conditional breakpoints based on pointer addresses. The objects involved seem to be: CF: cfield CT: ctypedescr F: function W: weakref They have the following structure: CF -> CT -> W -> F The first link the chain is not found by the GC because CF does not implement tp_traverse. All those objects are part of garbage kept alive by the reference cycle and the GC calls delete_garbage() on it. CT is not part of 'unreachable' and handle_weakrefs() does not find W. What I previously said about tp_clear was incorrect. We take great pains to avoid running non-trivial code after calling tp_clear (no finalizers and no callbacks). So, the func_clear method should be safe. Even if tp_clear doesn't succeed in breaking the cycle, there should be no way for user Python code to access those objects that have had tp_clear called on them. Is gc.get_objects() a hole in this? Seems like you could cause crashes with it. This bug is surprising to me in another way. If the analysis is correct then fully implementing the GC protocols is no longer optional (as it was when cyclic GC was first introduced). If the GC cannot find garbage weakrefs handle_weakrefs() doesn't work and we end up with crashes like this. Maybe Pablos fix to not call the weakref callback if we are in delete_garbage() is a good enough band-aid. The alternative would seem to be ensuring that tp_traverse methods exist so that every single reference can be followed by the GC. I imagine that would be a huge task and there are many extension types that don't have them. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue38006> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
