[systemd-devel] Antw: Re: Re: [EXT] Re: Q: Querying units for "what provides" a target

[Michael Biebl]

Am Fr., 9. Sept. 2022 um 14:01 Uhr schrieb Ulrich Windl
<Ulrich.Windl at rz.uni-regensburg.de>:
>
> >>> Andrei Borzenkov <arvidjaar at gmail.com> schrieb am 09.09.2022 um 13:41 in
> Nachricht
> <CAA91j0VQADw8g5mmoFUOvy=YLF666Z_Y9uqYDgTfnmhSBne3rw at mail.gmail.com>:
> > On Fri, Sep 9, 2022 at 2:13 PM Ulrich Windl
> > <Ulrich.Windl at rz.uni-regensburg.de> wrote:
> > ...
> >> >
> >> > If you are interested in services that pull in e.g. time-sync.target
> >> > via Wants (or Requires) and order themselves before the target, you
> >> > can use something like
> >> > $ systemctl show time-sync.target -p WantedBy -p RequiredBy -p After
> >> > RequiredBy=
> >> > WantedBy=chrony.service
> >> > After=chrony.service time-set.target
> >>
> >> It seems what I wanted to know is output by
> >> # systemctl show -p After time-set.target
> >> After=systemd-timesyncd.service
> >> # systemctl show -p After time-sync.target
> >> After=time-set.target ntp-wait.service
> >>
> >> However the "After=" is somewhat unexpected.
> >
> > This is exactly what targets are about. The only usage for targets is
> > to wait until something else becomes active and to do it they must
> > come After something.
> >
> >> And "-p WantedBy" is definitely
> >> wrong (it will output units that "require the target", not the units
> > "providing
> >> the target").
> >>
> >
> > There is no such thing as "units providing the target". Systemd
> > documentation makes distinction between targets that Want other units
> > ("active") and targets that are WantedBy other units ("passive"). It
> > is expected that services "providing" passive targets have
> > WantedBy=this-passive.target, otherwise passive targets will not be
> > activated at all. So WantedBy is exactly correct in this case.
>
> Hi!
>
> But when I include it (as suggested) I get:
> # systemctl show  -p WantedBy -p RequiredBy -p After time-sync.target
> RequiredBy=
> WantedBy=iotwatch at ROOT.service iotwatch at VPM.service iotwatch-generator.service ntp-wait.service systemd-timesyncd.service
> After=time-set.target ntp-wait.service
> ---
>
> Those iotwatch* units use "Before="; so is the WantedBy= incorrect for those?
>
> Those units use:
> Wants=nss-user-lookup.target time-sync.target paths.target
> After=nss-user-lookup.target time-sync.target paths.target


See man systemd.special, passive and active targets (as Andrei already
hinted at).
Quoting the relevant parts
"   Special Passive System Units
       A number of special system targets are defined that can be used
to properly order boot-up of optional services. These targets are
generally not part of the initial boot transaction, unless they are
explicitly pulled in by one of the implementing
       services. Note specifically that these passive target units are
generally not pulled in by the consumer of a service, but by the
provider of the service.
"
 iotwatch* does appear to be a consumer of the time-sync.target, so it
should merely have an ordering but not pull in the target.