Recently I started using the Sway window manager, with occasional fallback to XFCE. Having both mako and xfce4-notifyd installed causes a conflict over the org.freedesktop.Notifications D-Bus service name (see Red Hat Bug 484945). This post describes the workaround I am currently using, until dynamic activation directories or another solution is implemented.

Background

The Desktop Notifications Specification defines a protocol that applications can use to generate popups to notify the user of events. It requires a notification server to implement the org.freedesktop.Notifications service on the D-Bus session bus. The D-Bus Specification defines service activation based on information in service description files. This mechanism is used by both xfce4-notifyd and mako use to start on demand. However, this poses a problem. From Message Bus Starting Services (Activation):

On the well-known session bus, if two .service files in the same directory offer the same service name, the result is undefined. Distributors should avoid this situation, for instance by naming session services’ .service files according to their service name.

On my system, the result is that xfce4-notifyd is always started, which provides a poor experience on Sway.

A Daemon Workaround

If all notification services are daemons (i.e. they only exit at the end of the user session), an easy workaround is to avoid D-Bus Activation altogether by starting whichever daemon is desired at the beginning of the user session. This can be done from systemd units, Desktop Autostart, or traditional startup scripts (e.g. .xsessionrc). My understanding is that both GNOME and KDE notification services have moved to this approach.

Unfortunately, this approach forfeits the benefits of D-Bus activation, such as deferring the costs of running the notification service until/unless required and avoiding delay and contention at the start of the user session. It also doesn’t work for notification services like mako which exit after displaying a notification.

An Autostart Workaround

The next paragraph in the D-Bus Specification hints at a potential workaround:

If two .service files in different directories offer the same service name, the one in the higher-priority directory is used: for instance, on the system bus, .service files in /usr/local/share/dbus-1/system-services take precedence over those in /usr/share/dbus-1/system-services.

The service directories are defined by the dbus-daemon configuration, as described in dbus-daemon(1), with the standard session directories:

  • $XDG_RUNTIME_DIR/dbus-1/services
  • $XDG_DATA_HOME/dbus-1/services (default ~/.local/share)
  • $XDG_DATA_DIRS/dbus-1/services for each directory in $XDG_DATA_DIRS (default /usr/local/share:/usr/share)
  • ${datadir}/dbus-1/services compiled-in default (default /usr/share)

This behavior can be used to implement a workaround by placing a service definition file in $XDG_DATA_HOME/dbus-1/services for a service which activates the appropriate notification service for the current desktop environment.

systemd Activation

The D-Bus Specification also defines systemd Activation for starting a service via systemd instead of executing a binary. Although the specification does not describe the details, systemd activation in bus/activation.c is reasonably easy to follow: Send an ActivationRequest signal to org.freedesktop.systemd1.Activator at path /org/freedesktop/DBus on the session bus with a string argument containing the service name to activate.

Implementation

To implement the workaround, create a service definition file named ~/.local/share/dbus-1/services/org.freedesktop.Notifications.service with the following content:

[D-BUS Service]
Name=org.freedesktop.Notifications
Exec=/home/username/.local/lib/notify-dispatch

Create an executable script named ~/.local/lib/notify-dispatch which runs the preferred notification service based on the environment:

#!/bin/sh

if [ "$XDG_SESSION_DESKTOP" = sway ]; then
	exec /usr/bin/mako "$@"
fi

if [ "$XDG_SESSION_DESKTOP" = xfce ]; then
        exec dbus-send --session \
                --dest=org.freedesktop.systemd1 \
                /org/freedesktop/DBus \
                org.freedesktop.systemd1.Activator.ActivationRequest \
                string:xfce4-notifyd.service
fi

echo "Error: No notification service for $XDG_SESSION_DESKTOP in $0." >&2
exit 1

Then log out and back in (or killall -HUP dbus-daemon) to apply the changes. To test, send a desktop notification (e.g. notify-send Test).