[systemd-devel] Question about changing systemd target during boot

[Andrei Borzenkov]

On Mon, Aug 1, 2016 at 2:23 PM, Michael Chapman <mike at very.puzzling.org> wrote:
> On Mon, 1 Aug 2016, Andrei Borzenkov wrote:
> [...]
>>>
>>> So here goes what I've done:
>>>
>>> 1. Create a service and put it in the network-online.target:
>>>
>>> /etc/systemd/system/change-target.service:
>>> [Unit]
>>> Description=Change Target
>>> Wants=network-online.target
>>> After=network-online.target
>>>
>>> [Service]
>>> Type=oneshot
>>> ExecStart=/tmp/script.sh
>>> TimeoutSec=60s
>>>
>>> [Install]
>>> WantedBy=network-online.target
>>>
>>
>> This unit have conflicting requirements - on one hand it is
>> After=network-online.target, OTOH WantedBy=network-online.target
>> implies Before=network-online.target.
>
>
> I've seen this asserted on this list a few times, but as far as I can tell
> it isn't actually correct. After/Before are meant to be completely
> orthogonal to Wants/Requires/etc., according to the documentation.
>

Unless DefaultDependencies= is set to no in either of releated units
or an explicit ordering dependency is already defined, target units
will implicitly complement all configured dependencies of type Wants=
or Requires= with dependencies of type After=.

man systemd.target

> If A.service has:
>
>   Wants=B.service
>   Before=B.service
>
> then any time you ask A.service to be started, systemd will add a job to
> start B.service... but it still ensures that A.service is started *before*
> B.service.
>
> Is my understanding on this incorrect? I have been making use of this
> behaviour for quite some time, and as far as I can tell it has never failed.
>
> - Michael