Starting with Python 3.8 certain ctypes callbacks fail to restore the stack pointer.

In the repo below, when the DLL is compiled with MSVC under default debug settings, running the Python script leads to a debug error dialog which says:


Run-Time Check Failure #0 - The value of ESP was not properly saved across a function call.  This is usually a result of calling a function declared with one calling convention with a function pointer declared with a different calling convention.


It appears that when the C code calls the callback function, the value of ESP is 4 greater than it should be.

This problem does not occur with older versions of Python.


**DLL code**

#include <Windows.h>

BOOL APIENTRY DllMain(HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved)
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}

typedef void (__stdcall *MYCALLBACK)(int, double);

extern "C"
{
    __declspec(dllexport) void __stdcall foo(MYCALLBACK callback)
    {
        callback(1, 11);
        callback(2, 21);
        callback(3, 31);
    }
}

**Python code**

import ctypes
import ctypes.wintypes

def CallbackType(restype, *argtypes):

    def from_param(cls, obj):
        if obj is None:
            return obj
        return ctypes._CFuncPtr.from_param(obj)

    result = ctypes.WINFUNCTYPE(restype, *argtypes)
    result.from_param = classmethod(from_param)
    return result

MYCALLBACK = CallbackType(
    None,
    ctypes.c_int,
    ctypes.c_double
)

def callback(handle, time):
    print(handle, time)
mycallback = MYCALLBACK(callback)

lib = ctypes.WinDLL(r'path\to\dll\foo.dll')
func = getattr(lib, '_foo@4')
func.restype = None
func.argtypes = MYCALLBACK,
func(mycallback)