bpo-35682: Fix _ProactorBasePipeTransport._force_close() (GH-11462)

bpo-32622, bpo-35682: Fix asyncio.ProactorEventLoop.sendfile(): don't
attempt to set the result of an internal future if it's already done.

Fix asyncio _ProactorBasePipeTransport._force_close(): don't set the
result of _empty_waiter if it's already done.

Lib/asyncio/proactor_events.py

@@ -111,7 +111,7 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
             self._force_close(exc)
 
     def _force_close(self, exc):
-        if self._empty_waiter is not None:
+        if self._empty_waiter is not None and not self._empty_waiter.done():
             if exc is None:
                 self._empty_waiter.set_result(None)
             else:

Misc/NEWS.d/next/Library/2019-01-08-01-54-02.bpo-35682.KDM9lk.rst

+Fix ``asyncio.ProactorEventLoop.sendfile()``: don't attempt to set the result
+of an internal future if it's already done.