Why is Gio.Socket.create_source() returning null?

Question

I'm new to sockets and trying to work through some socket programming in GJS/Gio and hit a wall creating a GLib.Source to handle receiving from a socket. The relevant code (I think) is:

const DeviceChannel = new Lang.Class({
    Name: "DeviceChannel",

    _init: function (device) {
        this.device = device;

        this.connection = null;
        this.inStream = null;
        this.outStream = null;
        this.socket = null;
        this.sock_source = 0;
    },

    open: function () {
        let client = new Gio.SocketClient();

        this.addr = new Gio.InetSocketAddress({
            address: this.device.tcpHost,
            port: this.device.tcpPort
        });

        let conn = client.connect_async(
            this.addr,
            null,
            Lang.bind(this, this.opened)
        );
    },

    opened: function (client, res) {
        this.connection = client.connect_finish(res);

        // Streams
        this.inStream = new Gio.DataInputStream({
            base_stream: this.connection.get_input_stream()
        });

        this.outStream = new Gio.DataOutputStream({
            base_stream: this.connection.get_output_stream()
        });

        // Socket
        this.socket = this.connection.get_socket();
        this.socket.set_option(6, 4, 10);   // TCP_KEEPIDLE
        this.socket.set_option(6, 5, 5);    // TCP_KEEPINTVL
        this.socket.set_option(6, 6, 3);    // TCP_KEEPCNT
        this.socket.set_keepalive(true);

        this.sock_source = this.socket.create_source(GLib.IOCondition.IN, null);
        this.sock_source.set_callback(Lang.bind(this, this._io_ready));
        this.sock_source.attach(null);
    },

    _io_ready: function (condition) {
        return true;
    }
});

All goes well until I call this.sock_source.set_callback() when I get the error:

(JSConnect:15118): Gjs-WARNING **: JS ERROR: TypeError: this.sock_source is null
DeviceChannel<[email protected]:184:9
wrapper@resource:///org/gnome/gjs/modules/lang.js:178:22
@application.js:427:2

I have called Gio.Socket.create_source() on another socket (although UDP) in another part of the code which works fine. Calling create_source() itself doesn't throw any errors (even if I run my script with G_MESSAGES_DEBUG=all) and there is no mention of the function ever returning null in the documentation so I'm baffled as to what I'm doing wrong.

Edit:

There is 3 year old comment here that states:

This does not work since 1) Socket.create_source doesn't exist in typelib as it is marked with (skip) in Glib/gio/gsocket.c

But I assume this is no longer true since I have created a source on my UDP socket, although that socket is "hand-made" not built using Gio.SocketClient().

Answer 1

You are probably inadvertently calling the Gio.DatagramBased.create_source() method. Looking at the source code, this will call g_socket_create_source() eventually, but does some checks first and returns null if those fail. Here are the checks: https://github.com/GNOME/glib/blob/master/gio/gsocket.c#L1114

It looks to be a minor bug that the method will simply return null without even printing the error from check_datagram_based().