Segfault on GC cleanup of subclass

[omerbenamram]

I'm not eniertly sure that it's specific to only subclasses, I've noticed this bug while working on the tests in rustapi_module.

🌍 Environment

• Your operating system and version: osx 10.14
• Your python version: all cpython versions
• Your rust version (rustc --version): rustc 1.35.0-nightly (88f755f8a 2019-03-07)
• Are you using the latest pyo3 version? master

💥 Reproducing

current code in https://github.com/PyO3/pyo3/blob/master/examples/rustapi_module/tests/test_subclassing.py

from rustapi_module.subclassing import Subclassable


class SomeSubClass(Subclassable):
    pass


a = SomeSubClass()
_b = str(a) + repr(a)

changing it to:

from rustapi_module.subclassing import Subclassable


class SomeSubClass(Subclassable):
    pass

def test_f():
    a = SomeSubClass()
    _b = str(a) + repr(a)

causes it to crash, looking at the core dump, it seems to be during garbage collection.

changing it to:

from rustapi_module.subclassing import Subclassable


class SomeSubClass(Subclassable):
    pass


a = SomeSubClass()
a = SomeSubClass()
a = SomeSubClass()
a = SomeSubClass()
a = SomeSubClass()
_b = str(a) + repr(a)

makes this crash with GC is already tracked message.

[konstin]

That's a duplicate of #220 but with a new case and an interesting error message