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

[Michael Chapman]

On Mon, 1 Aug 2016, Andrei Borzenkov wrote:
> 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

I just checked the code, and it looks like systemd explicitly *skips*
these default dependencies if they would create a loop. In 
target_add_default_dependencies:

         /* Imply ordering for requirement dependencies on target
          * units. Note that when the user created a contradicting
          * ordering manually we won't add anything in here to make
          * sure we don't create a loop. */

- Michael