Complete all tasks.

def set_result(self, result):
        """Complete all tasks. """
        for future in self.traverse():
            # All cancelled futures should have callbacks to removed itself
            # from this linked list. However, these callbacks are scheduled in
            # an event loop, so we could still find them in our list.
            future.set_result(result)
        if not self.done():
            super().set_result(result)