'Check if all downloads are finished and if so, trigger reading.
Arguments:
download: The finished DownloadItem.'

def on_download_finished(self, download):

    self._in_progress.remove(download)
    if download.successful:
        self._done_count += 1
        try:
            self._merge_file(download.fileobj)
        finally:
            download.fileobj.close()
    if (not self._in_progress):
        try:
            self.on_lists_downloaded()
        except OSError:
            log.misc.exception('Failed    to    write    host    block    list!')
