[systemd-devel] systemd based gnome-session

[Léo Gillot-Lamure]

Hi.

I've read the unit files you provide and I'm wondering about a possible
race condition related to dbus activation.

For example let's see the unit gconf.service:

> [Unit]
> Description=settings daemon for gnome
> 
> 
> [Service]
> Type=dbus
> BusName=org.gnome.GConf
> ExecStart=/usr/libexec/gconfd-2
> 
> 
> [Install]
> WantedBy=gnome.target
> Alias=dbus-org.gnome.GConf.service

Why this line Alias= ? Creating this alias is only useful if you add a
line "SystemdService=dbus-org.gnome.GConf.service" to the
file /usr/share/dbus-1/services/org.gnome.GConf.service (this is a major
pain in the current dbus and systemd architecture…).
Moreover, even if you did, I thought that bus activation in the user
session was not implemented ? (I'd be very happy to be proved wrong
here :).

Now, in such a situation, this is how a race condition can appear:
Let's say that gnome-settings-daemon makes use of the well known name
org.gnome.GConf on the bus (it's probably the case).
Usually, gconf.service will be launched, will take the name on the bus,
then gnome-settings-daemon.service will be launched, will talk to this
name, gconf will answer and everything is fine. However gconf.service
and gnome-settings-daemon.service are actually launched *in parallel*.
Let's imagine than for any reason gconf.service takes a looong time
before taking the well-know name on the bus. During this time
gnome-settings-daemon launches and try to talk to org.gnome.GConf. Since
no one owns this name (remember, gconf.service is a snail today), the
dbus daemon will launch gconf according to the
file /usr/share/dbus-1/services/org.gnome.GConf.service.
This causes the first problem : the gconf daemon will be part of the
dbus.service cgroup, which is bad.
Then a second problem appears : gconf.service finally awakes from its
nap and try to take the name on the bus. Ooops : it's already taken by
the daemon launched by dbus-daemon… Thus gconf.service will fail. Race
condition, not good.

The exact same kind of stuff appears with pulseaudio (launched by
gnome-settings-daemon aswell) and virtually any bus-activated user
service that you'd like to be a service managed by systemd instead of
dbus-daemon.

Regards,

Léo Gillot-Lamure.