[systemd-devel] after=user.slice not enforced

[Michael Chapman]

On Thu, 24 Nov 2016, Kai Krakow wrote:
> Am Wed, 23 Nov 2016 09:14:34 +0100
> schrieb Cédric BRINER <cedric.briner at unige.ch>:
>
>> Hi,
>>
>> For the context, we are trying to stop a daemon launched by a user...
>>
>>>> Hi,
>>>>
>>>> sapRunning service contains a "After=user.slice". But at the
>>>> shutdown, a process (write-sysv-test.pl) running in user.slice is
>>>> killed before the end of the sapRunning's stop.
>>>
>>> Slices are a concept for resource management, and that's what they
>>> should be used for. Do not user them for anything else, such as
>>> ordering purposes.
>>>
>>> In systemd shutdown ordering is the inverse of start-up ordering,
>>> and After= and Before= declare the latter. This means that if your
>>> service has After=user.slice, this means at shutdown your service
>>> will be stopped first and user.slice second.
>> Thanks for the clarification.
>>
>> But this has not the expected impact. We were wishing with the
>> "After=user.slice", that the stop sapRunning will occur before any
>> user commands are stopped.
>>
>> Does using "After=user.slice" propagate also on all the *childs*. That
>> way we could ensure that our stop services' commmand is launched as
>> the first ever before any kill ?
>>
>> The question still remain for us, how can we do to have a daemon
>> launched by hand, that can be handled by systemd for its stopping.
>
> I think you could maybe use targets as synchronizations points... Maybe
> make a target that starts after multi-user.target and requires it. Then
> put your service as wanted by this new target (maybe also using after
> and requires), let's call it sap-started.target. Then make that the
> default target at boot.
>
> That way, on shutdown, it should first stop and wait for
> sap-started.target, and only then pull down the rest of the system.

Simply ordering target units won't help. At boot, systemd does not wait 
for the first target to be reached before considering starting the units 
wanted by the second target. Similarly, at shutdown it will not wait for 
the second group of units to be stopped before stopping the first group of 
units -- indeed, the target units themselves could be stopped almost 
immediately.

What you can do is have something like this:

   [Unit]
   After=multi-user.target

   [Install]
   WantedBy=multi-user.target

Normally, this unit would be Before=multi-user.target (this is an 
implicit dependency added when a target Wants= something).
By adding an explicit After=multi-user.target dependency, however, you can 
override this.

A unit with this dependency will be "started late" and "stopped early".

However, this doesn't really do anything to order it with respect to user 
sessions. As described elsewhere in this thread, there's no clean way to 
do that.

- Michael