Implementing pass_fds on Windows is a problem if Popen has to implement the undocumented use of the STARTUPINFO cbReserved2 and lpReserved2 fields to inherit CRT file descriptors. I suppose we could implement this ourselves in _winapi since it's unlikely that the data format will ever change. Just copy what the CRT's accumulate_inheritable_handles() function does, but constrained by an array of file descriptors.