Gio.DataInputStream.prototype.read_upto_finish

function read_upto_finish(result: Gio.AsyncResult): [return_value: String, length: Number(gsize)] {
    // Gjs wrapper for g_data_input_stream_read_upto_finish()
}

Finish an asynchronous call started by Gio.DataInputStream.prototype.read_upto_async.

Note that this function does not consume the stop character. You have to use Gio.DataInputStream.prototype.read_byte to get it before calling Gio.DataInputStream.prototype.read_upto_async again.

Since 2.24

result: the Gio.AsyncResult that was provided to the callback
return_value: a string with the data that was read before encountering any of the stop characters. Set length to a #gsize to get the length of the string. This function will return null on an error.
length: a #gsize to get the length of the data read in