Keruspe's blag


Various free software hacking stuff

systemd as a session manager

by Marc-Antoine Perennou on January 1, 2013

Tagged as: sysadmin, systemd, gnome.

As we are supposed to take good resolutions for 2013, mine has been to switch from gnome-session to systemd for managing my user session. Here is how I replaced gnome-session with systemd.

How did it come to my mind?

Since its beginning, systemd has been created for both system and session management. The default is obviously the system manager, aka systemd --system, but what less people know is that you also can run systemd --user.

Since I’ve been using systemd as my init system for a while and have been quite happy with it, I often thought of trying a migration for my session. Recently, I started playing with my mails, first with fetchmail and procmail, and then with offlineimap (I’ll blog about this later). With offlineimap, I needed a way to fetch periodically my emails.

I could have installed a cron system, but I don’t like installing such things to only use them for a single command. Since systemd handles natively cron jobs with its timers, I thought back to systemd --session.

One problem, three solutions, which one to take?

The first solution was to run systemd --session as an autostart app for my session (e.g. writing a .desktop file for it and putting it in /etc/xdg/autostart:

With this solution I would have ended up with a first session manager (gnome-session) launching a second one (systemd –user), and the latter would only have been used for offlineimap… Come on! We can do better.

The second solution was to replace my X session with a new one, which would launch offlineimap on one side and gnome-session on the other, gnome-session taking care of all gnome-related stuff. This is way better! That’s the solution I adopted for a couple a hours and tries. I was pretty happy with it, but was still not convinced by the fact that two different software were managing my session at the same time. Here is my X session file: It has to be placed in /usr/share/xsessions.

Then comes the third solution, the one I’m currently running. Since my session was now directly managed by systemd, I decided to migrate everything launched by gnome-session to systemd services, in order to remove totally gnome-session. I found some helpful basis here and there. I took some of them, modified them to fit my needs and wrote some myself, ending up with a nearly ready system.

Last problem, gnome-session runtime dependency

Some of the gnome parts such as gnome-shell require gnome-session to be available at runtime, in order to synchronize a few informations across the session, such as the presence status or whether you want notifications to be displayed or ignored. This is all done via DBus but gnome-session does not use a helper to do so, it does it itself. Since we cannot get totally rid of gnome-session, I decided to create a dummy gnome session that gnome-session would launch.

Next problem: gnome-session refuses to launch such a session… Yay! I thus patched it and opened a bug upstream to allow it and provide the dummy session I created. With this patch applied locally, I could run gnome-session --session=gnome-dummy in my systemd service to get a session which does not launch anything. And then I realized that it was still starting autostart applications, which I did not want since I only wanted it for its DBus interface. Passing -a /dev/null as an extra arg to gnome-session so that it looks for autostart applications nowhere instead of in /etx/xdg/autostart did this last trick.

My system user units are available there: This folder corresponds to your ${HOME}/.config/systemd/user/.