the #GAsyncResult that was provided to the callback.
a #gsize to get the length of the data read in.
a string with the line that was read in (without the newlines). Set length to a #gsize to get the length of the read line. On an error, it will return null and error will be set. For UTF-8 conversion errors, the set error domain is G_CONVERT_ERROR. If there's no content to read, it will still return null, but error won't be set.
Finish an asynchronous call started by gio.data_input_stream.DataInputStream.readLineAsync.