[systemd-devel] dbus and exit-on-idle

[Matthijs van Duin]

I've been pondering how to allow my bus-activated service to exit when
it's unneeded (which is 99% of the time), and in particular how to deal
with exit/activate races correctly...

In the systemd sources I stumbled across the helper function
bus_event_loop_with_idle() which does a clever trick by
 - using sd_notify("STOPPING=1") to make sure systemd will queue any
   activations from here on
 - releasing the dbus service name
 - handling any remaining requests until the name deregistration is
   confirmed by the dbus daemon

Thus, any incoming request will either be delivered to you, or trigger a
new activation and be delivered to the next instance of your service.

I see two problems here:
 1. sd_notify is asynchronous and nothing is done to ensure systemd has
    acted on it before the service name is released.
 2. If you get an incoming request that requires you to continue running
    after you've told systemd you're stopping, you're screwed.

The first problem could be solved by monitoring your own unit status I
suppose, although it would be more elegant if the sd_notify() could be
replaced by a dbus call.

The second problem is more annoying... although you could just register
your bus name again (which presumably suffices to get any queued calls
delivered to you?), once you've told systemd you're stopping there's no
way back currently, and it will SIGKILL you if you take too long.

Could this be solved by simply allowing a stopping service to change its
mind, e.g. with sd_notify("STOPPING=0") or perhaps another READY=1 ?

Matthijs van Duin