bug#55444: elogind startup race between shepherd and dbus-daemon

["Ludovic Courtès" <ludo@gnu.org>]

Hello!

Currently (40a729a0e6f1d660b942241416c1e2c567616d4d), shepherd and
dbus-daemon compete to start elogind: shepherd tries to start it
eagerly, and dbus-daemon starts it on-demand upon bus activation.

Sometimes dbus-daemon wins, and thus shepherd tries a few times to start
it anyway, leading to the infamous:

  elogind is already running as PID 123

(elogind checks whether its PID file exists.  Note that you may see that
message also when shepherd wins, because dbus-daemon tries to start it
anyway.)  Eventually, shepherd considers that elogind cannot be started
and disables it.

In addition to being ridiculous, it’s harmful: the ‘xorg-server’ service
(from ‘gdm-service-type’ and ‘sddm-service-type’ depends on ‘elogind’),
so if shepherd loses the race, Xorg isn’t started (on my laptop,
shepherd never loses the race it seems, but i’ve seen it lose half of
the time on a slower machine).

The reason elogind is started by shepherd is explained in this comment:

       ;; Start elogind from the Shepherd rather than waiting
       ;; for bus activation.  This ensures that it can handle
       ;; events like lid close, etc.

This comes from 94a881178af9a9a918ce6de55641daa245c92e73, which was a
fix for <http://issues.guix.gnu.org/27580>.  I believe the justification
still holds.

So it would seem that the solution to this is to prevent dbus-daemon
from starting elogind.  We can do that by changing
org.freedesktop.login1.service so that it has “Exec=true” instead of
“Exec=elogind --daemon”.

“Exec=true” is a bit crude because it doesn’t guarantee that elogind is
really started; if that isn’t good enough, we could instead wait for the
PID file or something (as of Shepherd 0.9.0, invoking ‘herd start
elogind’ potentially leads shepherd to start a second instance if the
first one is still being started, so we can’t really do that).

Depending on what we end up with, we might also revisit whether
xorg-server needs to explicitly depend on elogind.

Thoughts?

Ludo’.