(You are already holding the lock.)
        Wait for the send window to open up, and allocate up to ``size`` bytes
        for transmission.  If no space opens up before the timeout, a timeout
        exception is raised.  Returns the number of bytes available to send
        (may be less than requested).

def _wait_for_send_window(self, size):
        """
        (You are already holding the lock.)
        Wait for the send window to open up, and allocate up to ``size`` bytes
        for transmission.  If no space opens up before the timeout, a timeout
        exception is raised.  Returns the number of bytes available to send
        (may be less than requested).
        """
        # you are already holding the lock
        if self.closed or self.eof_sent:
            return 0
        if self.out_window_size == 0:
            # should we block?
            if self.timeout == 0.0:
                raise socket.timeout()
            # loop here in case we get woken up but a different thread has
            # filled the buffer
            timeout = self.timeout
            while self.out_window_size == 0:
                if self.closed or self.eof_sent:
                    return 0
                then = time.time()
                self.out_buffer_cv.wait(timeout)
                if timeout is not None:
                    timeout -= time.time() - then
                    if timeout <= 0.0:
                        raise socket.timeout()
        # we have some window to squeeze into
        if self.closed or self.eof_sent:
            return 0
        if self.out_window_size < size:
            size = self.out_window_size
        if self.out_max_packet_size - 64 < size:
            size = self.out_max_packet_size - 64
        self.out_window_size -= size
        if self.ultra_debug:
            self._log(DEBUG, "window down to {}".format(self.out_window_size))
        return size