[systemd-devel] Dependencies on DBus activated services during shutdown

[Michal Koutný]

On 01/08/2018 05:52 PM, Simon McVittie wrote:
> Does A.service even need B.service during shutdown if it has not
> previously interacted with (=> started) B.service?
That's not correct implication, due to concurrency, B.service may be
stopped before A.service.

> On Mon, 08 Jan 2018 at 16:53:04 +0100, Jérémy Rosen wrote:
>> That means that the only way to fix that without explicitely telling someone
>> about the dependency is to allow dbus to start units while its shutdown is
>> pending [...] this seems to be explicitely forbidden
> 
> I didn't write that special case, but I agree with it. Starting D-Bus
> services while the sword of Damocles is hanging over dbus-daemon's
> head does not sound like a route to guaranteed success. As soon as
> dbus-daemon gets SIGTERM, they'll find that their D-Bus AF_UNIX socket
> is rather less useful than it was a moment ago...
Nice explanation. I forgot to mention that A.service already has
dependency on dbus.service (which is unnecessary by the elegance
criterium). So there's a guarantee two sword will not fall down until
A.service terminates.

> If A.service can be made to shut down correctly without B.service,
> then that seems good in any case. (What happens if B.service crashes?)
The A's best effort would make sense in the crash case, however, I don't
think it's the right place .

> Failing that, if A.service genuinely needs D-Bus during its shutdown,
> it probably also makes sense to serialize it After=dbus.service (not
> just dbus.socket) so that dbus-daemon will be kept alive until A.service
> actually exits.
That's actually a valid idea (see above).

> To be honest that doesn't seem too bad to me.
Yes, the lesser viability of other options I see, the more I like it as
well.

The alternative of changing the condition from
manager_unit_inactive_or_pending to unit_inactive would AFAIU lead to a
conflict with the shutdown target if B.service was reactivated. (And if
not, it could potentially make shutdown transaction infinite.)

Thanks,
Michal

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