Major QDBus regression in Qt6 with potential impact on KDE

[Bartosz Golaszewski]

Hi!

I'm working on a piece of software that extensively uses QDBus. The
project is based on Qt5. I'm trying to upgrade it to Qt6 but have
encountered an issue with QDBus that's described in detail in my
official bug report[1]. The report includes a link to a github repo
with reproducible examples.

Short version is: it's impossible to connect to the InterfacesAdded or
InterfacesRemoved signals (and potentially to any signals that have
slightly more complex arguments) on the system bus with QDBus in Qt6
(unless one enables QDBUS_DEBUG=1 in which case it's possible). I
suspect some timing issue/race condition but don't know Qt enough to
debug it.

While my project is not related to KDE, I thought I'd bring it to your
attention (as KDE is scheduled to upgrade to Qt6). This bug definitely
affects KDE as for instance the networkmanager-qt project subscribes
for the InterfacesAdded signal of NetworkManager's paths and will no
longer work with Qt6.

Has anyone in the KDE community encountered this bug yet? How does KDE
deal with blockers like this? Seems like Qt is not eager to fix it on
their side.

Best regards,
Bartosz Golaszewski

[1] https://bugreports.qt.io/projects/QTBUG/issues/QTBUG-108822