Get a reader or None if another reader is already reading.

def _get_reader(self, network_reader):
        """
        Get a reader or None if another reader is already reading.
        """
        with (yield from self._lock):
            if self._reader_process is None:
                self._reader_process = network_reader
            if self._reader:
                if self._reader_process == network_reader:
                    self._current_read = asyncio.async(self._reader.read(READ_SIZE))
                    return self._current_read
        return None