pypy: disable PyFunction

[davidhewitt]

Apparently this is an incomplete definition so it's not safe to use, see https://foss.heptapod.net/pypy/pypy/-/issues/3776

[mejrs]

This needs a rebase on the clippy fixes, I think.

[mattip]

Is there a downside to making this an opaque type? Will this limit what can be done with PyO3 and PyPy?

[davidhewitt]

Disabling this doesn't have a huge impact, it just means that users cannot check for this type using PyO3's safe APIs. They can still use the raw ffi call to do so.

The problem is that PyO3's type modelling assumes that a type_object() -> &PyType method can be implemented which hands out a valid PyType object to users. And because PyFunction::type_object() was handing out a &PyType with uninitialized fields, users hit crashes.

[mattip]

PyPy tries to implement the c-api emulation on a "need to have" basis. Do you know what project was hitting crashes, so I can interact with them and make the changes necessary to make the code work?

[davidhewitt]

Sure thing - it was this code in pydantic-core.

[mattip]

right, that is the PyObject_IsInstance(<something>, &PyFunction_Type) problem. Is that the only waythis type object is used?

[davidhewitt]

I think if PyO3 exposed the PyFunction type object as a high-level API then I wouldn't place a limit on how it might be used. Methods on the PyType object include getting the __qualname__ attribute or using it in PyObject_IsSubclass, however users could also just use it anywhere a PyAny is expected so unless this object is fully initialised I wouldn't be confident that users wouldn't produce crashes.