Passing a FD to an unnamed pipe over DBus using Vala

Question

I'm trying to send a large block of data between applications by sending a control message over DBus from one to the other requesting a Unix file descriptor. I have it so that the client can request this, the server creates a DBus message that includes a UnixFDList, and the client receives a reply message but it doesn't contain anything. On the server side in Vala the DBusConnection object is setup using register_object, unfortunately the Vapi hides the DBusInterfaceVTable parameter that all the C examples use that would let me specify a delegate for method calls. I've tried to use register_object_with_closures instead but I can't seem to get that to work and the Closure object in Vala is woefully undocumented.

It seems to me that I need one of these methods in order to receive the message from the DBusMethodInvocation object that you get from a call to the DBusInterfaceMethodCallFunc delegate, with that you can create a reply message. Is there a way to either specify a closure class that works with register_object_with_closures, or a way to specify a DBusInterfaceVTable object as part of the service data?

I know that one option is to just create the service in C, but I'd rather figure out and understand how this works in Vala.

Answer 1

Vala uses UnixFDList internally for methods that contain a parameter of type GLib.UnixInputStream, GLib.UnixOutputStream, GLib.Socket, or GLib.FileDescriptorBased.

Example:

[DBus(name="eu.tiliado.Nuvola")]
public interface MasterDbusIfce: GLib.Object {
    public abstract void get_connection(
        string app_id,
        string dbus_id,
        out GLib.Socket? socket,
        out string? token) throws GLib.Error;
}