bzr-dbus hacking

When working on my bzr-avahi plugin, Robert asked me about how it should fit in with his bzr-dbus plugin. The two plugins offer complementary features, and could share a fair bit of infrastructure code. Furthermore, by not cooperating, there is a risk that the two plugins could break when both installed together.

Given the dependencies of the two packages, it made more sense to put common infrastructure in bzr-dbus and have bzr-avahi depend on it. That said, bzr-dbus is a bit more difficult to install than bzr-avahi, since it requires installation of a D-Bus service activation file. After looking at the code, it seemed that there was room to simplify how bzr-dbus worked and improve its reliability at the same time.

The primary purpose of bzr-dbus is to send signals over the session bus whenever the head revision of a branch changes. This was implemented using a daemon that is started using D-Bus activation, and sends out the signals in response to method calls made by short lived bzr processes.

While this seems to be the design the dbus-python tutorial guides you to use, I don't think it is the best fit for bzr-dbus. The approach I took was to do away with the daemon altogether: the D-Bus session bus does a pretty good job of broadcasting the signals on its own.

The code that previously asked the broadcast daemon to send the revision signal was changed to simply send the signal. The following helper made this pretty easy to do without having to write any extra classes to emit the signals:

def send_signal(bus, dbus_interface, signal_name, signature, *args):
    """Send a signal on the bus."""
    message = dbus.lowlevel.SignalMessage('/', dbus_interface, signal_name)
    message.append(signature=signature, *args)
    bus.send_message(message)

With these changes, the commit hook now only needs to connect to the session bus and fire off the signal and return. Previously it was connecting to the bus, getting an the broadcast service (which might involve activating it), sending a method call message and waiting for a method return message. The new code is faster and if no one is listening for the signals, it only wakes the bus.

For code that was consuming the signals, they had to switch to the bus.add_signal_receiver() method to register the callbacks, which allows you to subscribe to a signal irrespective of its origin.

The only missing feature with these changes was annotating the signals with additional URLs when the branch was being shared over the network. As these additional URLs are only really interesting when accessing the branch remotely, I moved the functionality to the "bzr lan-notify" command so that it annotates the revision announcements just before broadcasting them to the local network.

With all the changes applied, the D-Bus API consists entirely of signal emissions, which gives a looser coupling between the various components: each component will happily function in the absence of the others, which is great for reliability.

Once the patches are merged, I'll have to look at porting bzr-avahi to this infrastructure. Together, these two plugins offer compelling features for local network collaboration.