I think that signal.set_wakeup_fd(fd) must set the file descriptor to the non-blocking mode, instead of requiring the file descriptor mode to be non-blocking.

Atttached patch implements this idea.

See also the issue #22018 which proposes to support sockets in signal.set_wakeup_fd(fd) on Windows.

We have to decide if signal.set_wakeup_fd() is supposed to support files or not on Windows. Non-blocking mode does not exist for files on Windows, only for sockets.

I didn't test my patch on Windows yet.

Note: the new _Py_set_blocking() function tries to use ioctl() instead of fnctl() when available to only use one syscall instead of two. I used the same optimization in _Py_set_inheritable().