ctypes mishandles `void` return type · Issue #91353

ctypes mishandles `void` return type · Issue #91353 · python/cpython

On wasm targets, test_code fails. It's because of a bad function pointer cast. The problematic code is as follows:

import ctypes

py = ctypes.pythonapi
freefunc = ctypes.CFUNCTYPE(None, ctypes.c_voidp)

RequestCodeExtraIndex = py._PyEval_RequestCodeExtraIndex
RequestCodeExtraIndex.argtypes = (freefunc,)
RequestCodeExtraIndex.restype = ctypes.c_ssize_t

SetExtra = py._PyCode_SetExtra
SetExtra.argtypes = (ctypes.py_object, ctypes.c_ssize_t, ctypes.c_voidp)
SetExtra.restype = ctypes.c_int

LAST_FREED = None
def myfree(ptr):
    global LAST_FREED
    LAST_FREED = ptr

FREE_FUNC = freefunc(myfree)
FREE_INDEX = RequestCodeExtraIndex(FREE_FUNC)


# Verify that the provided free function gets invoked
# when the code object is cleaned up.
f = eval('lambda:42')

SetExtra(f.__code__, FREE_INDEX, ctypes.c_voidp(100))
del f # crashes!!

The problem: freefunc = ctypes.CFUNCTYPE(None, ctypes.c_voidp) declares a function with signature int f(void*). However, the definition of freefunc is void f(void*). These function signatures do not agree, and trying to make the call crashes.

Due to a bug(?) ctypes can produce closures that have void return type but it can't produce simple functions with void return type.

Closures code: checks if restype == Py_None and if so gives the callback a void return type
https://github.com/python/cpython/blob/main/Modules/_ctypes/callbacks.c#L381

Direct call code: calls _ctypes_get_ffi_type which never returns ffi_type_void:

rtype = _ctypes_get_ffi_type(restype);