QDbus: connection fails but lastError indicates no error

SGaist

To make things simple to test:

Can you provide a requirement.txt for your example script ?
Can you provide a minimal C++ example that uses your provider for testing as well ?
That way it will be easier to reproduce your issue in similar conditions.

brgl

Here's a repo with a functional example: https://github.com/brgl/qdbus-signal-example

Inside you'll find a requirements.txt file for the python scripts, a functional python signal provider and receiver and a qt6 receiver that doesn't work.

For simplicity it uses the session bus. You need to run python-provider/provider.py and then within 5 seconds run the python-receiver/receiver.py script and observe that it correctly receives the signal. You can then run qmake6 and make inside the qt6-receiver directory and do the same for the qt receiver program. It will fail to connect and not receive the signal as well as not indicate any error.

Let me know if that's clear and thanks for helping!

n1k0

Hi @SGaist, were you able to male any progress on this? Thank you for your help!

SGaist

I am suspecting that there's an issue with the signature of the slot but I haven't been able to pinpoint it.

The D-Bus type is a{sa{sv}} So it's a dictionary with string as key and as value a dictionary with string as key and variant as values. The variant is mapped to QDBusVariant based on D-Bus type system so I would check in that direction.

brgl

How would I debug any such issues with signal-to-slot signatures?

brgl

https://doc.qt.io/qt-6/qdbustypesystem.html#the-type-system-in-use Does this:

Warning: You may not use any type that is not on the list above, including typedefs to the types listed. This also includes QList<QVariant> and QMap<QString,QVariant>.

Mean I cannot receive signals passing dicts as arguments?

QDbusViewer is telling me:

Error: Unable to connect to service org.foobar.Provider1, path /org/foobar/Provider1, interface org.freedesktop.DBus.ObjectManager, signal InterfacesAdded

brgl

I've been browsing the KDE Udisks code that uses qt AND connects to the InterfacesAdded signal. I added some bits that were missing from my code:

typedef QMap<QString, QVariantMap> QVariantMapMap;
Q_DECLARE_METATYPE(QVariantMapMap)

and

qDBusRegisterMetaType<QVariantMapMap>();

but that didn't fix it.

SGaist

Can you point me to code in question ?

Did you try with QDBusVariant in place of QVariant ?

brgl

The code's here: https://src.fedoraproject.org/rpms/kdelibs/blob/f94c05b3732a163d5bf76a9dc52d51688339b812/f/kdelibs-udisks2-backend.patch

And yes, I did try to use QDBusVariant with no effect.

brgl

After some more trial and error I finally figured it out.

Basically I need to use QDBusVariant AND also register both the single map and the "mapmap" type:

typedef QMap<QString, QDBusVariant> QDBusVariantMap;
Q_DECLARE_METATYPE(QDBusVariantMap);
typedef QMap<QString, QDBusVariantMap> QDBusVariantMapMap;
Q_DECLARE_METATYPE(QDBusVariantMapMap);

and

qDBusRegisterMetaType<QDBusVariantMap>();
qDBusRegisterMetaType<QDBusVariantMapMap>();

and

	auto ret = bus.connect(
		"org.foobar.Provider1",
		"/org/foobar/Provider1",
		"org.freedesktop.DBus.ObjectManager",
		"InterfacesAdded",
		&receiver,
		SLOT(InterfacesAdded(QDBusObjectPath, QDBusVariantMapMap))
	);

Honestly the error reporting on this thing could use some more work. :)

SGaist

Nice !

I agree with your suggestion. Did you already check the bug report system about that matter ? If there's nothing there, you could open a feature request to improve that message.