> I did intend to switch multiprocessing over to using _overlapped
> but I did not get round to it.

Do you mean that _overlapped module is newer and should be used instead of _winapi? If multiprocssing is patched to use _overlapped, we can drop overlapped code from _winapi, should we keep functions like WriteFile() without overlapped support? (I think that we should keep these functions, it was discussed to support the native Windows API for files.)

IMO such change can be done in Python 3.5, it is risky and can wait.

But until that, I'm concerned by overlapped deallocator which is different in the two modules. Attached fixes _overlapped module to use the same logic than _winapi: give up on Windows XP during Python finalization if the overlapped is still pending, don't deallocate memory, exit immediatly. See issue #19565 for the rationale of this change.