Qt: Dbus Interface Errors

Qt DBus errors: “GLib-GIO-CRITICAL” and “Unable to find method”

Backstory

DBus in Qt is quirky

Problem

I received a couple errors when trying to access a Qt DBus service from external programs.

For instance, this python program:

import pydbus

bus = pydbus.SessionBus()
proxy = bus.get("org.veki.ui.DBusAgent", '/')
d = "1.5 2.0 3.1"
print(proxy.receiveMediaPipeData(d))

would return the error:

(process:35600): GLib-GIO-CRITICAL **: 17:28:39.935: g_dbus_connection_call_sync_internal: assertion 'interface_name != NULL && g_dbus_is_interface_name (interface_name)' failed
Traceback (most recent call last):
File "[dbusmessage.py](http://dbusmessage.py/)", line 6, in <module>
print(proxy.receiveMediaPipeData(d))
File "/home/robby/Projects/mp_env/lib/python3.8/site-packages/pydbus/proxy_method.py", line 72, in **call**
ret = instance._bus.con.call_sync(
AttributeError: 'NoneType' object has no attribute 'unpack'

and the Qt tool QDBusViewer would show:

Unable to find method receiveMediaPipeData on path / in interface local.veki-demo-qt5.DBusAgent

Context

I setup a DBus service under the name org.veki.ui.DBusAgent in Qt:

QDBusConnection::sessionBus().registerService(SERVICE_NAME)

And registered an object:

QDBusConnection::sessionBus().registerObject("/",&dbusagent, QDBusConnection::ExportAllSlots);

When I attempted to access this service from other programs, they would have trouble finding the service.

Fix

After some tinkering I discovered that by default the interface’s name would be the project’s .pro file name. So for instance a project with project.pro would translate to an interface named local.project.Object_class. This wouldn’t cause problems for Qt6 but breaks DBus in Qt 5. Curious.

One way to fix this is to use the overloaded registerObject() method:

bool QDBusConnection::registerObject(const QString &path, const QString &interface, QObject *object, QDBusConnection::RegisterOptions options = ExportAdaptors)

and specify the interface’s name. So now when registering the object, the code looks like this:

QDBusConnection::sessionBus().registerObject("/", "my.interface",&dbusagent, QDBusConnection::ExportAllSlots);

Now the external programs have no problem finding and accessing the DBus service.