[New-bugs-announce] [issue47197] ctypes mishandles `void` return type

[Hood Chatham]

New submission from Hood Chatham <roberthoodchatham at gmail.com>:

On wasm targets, `test_code` fails. It's because of a bad function pointer cast. The problematic code is as follows:
```py
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`:
https://github.com/python/cpython/blob/b183f486493e7e4c332566392ef18c6b346a6561/Modules/_ctypes/callproc.c#L1212

----------
components: ctypes
messages: 416526
nosy: christian.heimes, hoodchatham
priority: normal
severity: normal
status: open
title: ctypes mishandles `void` return type

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue47197>
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