bug: sipWrapper_dealloc should save/restore Python exception state to avoid corrupting it

[Nathaniel Smith]

Hi all,

In the Python C API, tp_dealloc hooks can get called at all sorts of
weird times. For example, it can be called at a moment when there's a
live exception being propagated.

This means that if your tp_dealloc tries to naively run some Python
code, you might end up corrupting the interpreter's exception-tracking
state, since it doesn't expect to start running regular code with a
live exception. The solution is that tp_dealloc hooks should use
PyErr_Fetch/PyErr_Restore to bracket any arbitrary code execution. For
example, this is how the interpreter itself calls __del__ methods from
tp_dealloc:

   https://github.com/python/cpython/blob/5f4b229df7812f1788287095eb6b138bb21876a4/Objects/typeobject.c#L6952-L6967

Unfortunately, sip's tp_dealloc code doesn't seem to include a similar
save/restore. We encountered this while trying to use PyQt5 together
with Python's async/await support:

   https://github.com/python-trio/trio/issues/1576

The summary is: async functions are essentially generators, and so
when they return they raise a StopIteration exception to carry the
return value. So this exception is "live" when the function frame is
garbage collected, and all its locals are deallocated. This makes it
especially easy to provoke bugs in exception/tp_dealloc interactions.
In the link above we hit one case where sip's tp_dealloc ends up
running arbitrary Python code and then generating a bunch of confused
SystemError exceptions from the Python VM, but there are probably
others. The link also includes a gdb backtrace showing a path from
_Py_Dealloc -> sipWrapper_dealloc -> arbitrary Python.

Fortunately the solution is easy: just add PyErr_Fetch/PyErr_Restore
to sipWrapper_dealloc.

It would be great if this could be fixed, since it's probably a
blocker to using async/await with PyQt5.

Similar bug in PySide2:
https://bugreports.qt.io/projects/PYSIDE/issues/PYSIDE-1313

Thanks,
-n

-- 
Nathaniel J. Smith -- https://vorpus.org