_WaitCancelFuture in windows_events.py overrides _schedule_callbacks.
C implemented Future should allow overriding _schedule_callbacks.

Since `{"_future", PyInit__future},` is not in PC/config.c, _future is not registered as builtin module. So Python 3.6b2 doesn't use it.
Instead of registering it, we should try make it split extension module (.pyd file).

> C implemented Future should allow overriding _schedule_callbacks.

I'm not so sure about this.  Maybe we can just fix _WaitCancelFuture somehow?

https://github.com/search?p=3&q=_schedule_callbacks&type=Code&utf8=%E2%9C%93

At least, Future class in uvloop have same API.
Most of other results seems just copy of Python source tree.
(But I didn't check all search result)

> I'm not so sure about this.  Maybe we can just fix _WaitCancelFuture somehow?

@haypo, do you know why _WaitCancelFuture overrides _schedule_callbacks() instead
of self.add_done_callback(self._unregister_wait_cb)?
_unregister_wait_cb must be called after all callbacks are called?

INADA Naoki added the comment:
> @haypo, do you know why _WaitCancelFuture overrides _schedule_callbacks() instead
> of self.add_done_callback(self._unregister_wait_cb)?
> _unregister_wait_cb must be called after all callbacks are called?

Oh no. I tried to forget this mess :-( It took me 2 or 3 months to
understand and fix this complex issue of cancelling a wait on
Windows...

Hum, let me check.

I found this in IocpProactor:
---
    def _wait_cancel(self, event, done_callback):
        fut = self._wait_for_handle(event, None, True)
        # add_done_callback() cannot be used because the wait may only complete
        # in IocpProactor.close(), while the event loop is not running.
        fut._done_callback = done_callback
        return fut
---

I don't understand my comment anymore /o\

I just recall that it was complex to get this crap working in all
cases, especially in corner cases.

I think _WaitCancelFuture can do same thing by overriding
callers of _schedule_callbacks.

Attached patch does it, and make _schedule_callbacks private
by renaming it to __schedule_callbacks.

Latest patch looks good.

New changeset 6d20d6fe9b41 by INADA Naoki in branch '3.6':
Issue #28448: Fix C implemented asyncio.Future didn't work on Windows
https://hg.python.org/cpython/rev/6d20d6fe9b41

New changeset a9a136c9d857 by INADA Naoki in branch 'default':
Issue #28448: Fix C implemented asyncio.Future didn't work on Windows (merge 3.6)
https://hg.python.org/cpython/rev/a9a136c9d857