This is RFC because:

I'm not sure whether it's guaranteed that InterfacesAdded is emitted on service registration as well or both connections are necessary

I'm not sure about this either. I'll build it locally and try to use it for a while.

I don't know how it's possible to test this as org.kde.fakenetwork does not support the org.freedesktop.ObjectManager API

The fakenetwork stuff supports or simulates only a small portion of NM dbus, we don't currently test secret agent at all.

Is this supposed to fix also the issue where when you log into a Plasma session, you first get a notification that it failed to activate a connection and then it gets activated successfuly?

In D16471#354635, @jgrulich wrote:

Is this supposed to fix also the issue where when you log into a Plasma session, you first get a notification that it failed to activate a connection and then it gets activated successfuly?

I only saw that once and was unable to reproduce it, but someone else reported it downstream and posted an NM trace log: https://bugzilla.suse.com/attachment.cgi?id=777407

A more or less educated guess:

• User logs in
• NM tries autologin, but no secrets available
• NM tries WPS as fallback
• Agent registers
• WPS returns without success -> notification shown
• As the agent registered, it queued another autoactivate event

If that's true though it's a bug in NM and this won't fix it.