[systemd-devel] Why is systemd-run --scope synchronous?

[Jérémy Rosen]

I would understand the question as :

"why is the foreground/background concept linked to the concept of scope ?"

for most people --scope means "run in the background" which is only a 
side effect of what that command really does.

Maybe there is a need of some doc clarification, or get non-scope runs 
also be foreground and leave backgrounding to the shell...

Anyway, the first step is to understand why there is this difference of 
behaviour in the first place...


On 05/02/2018 14:59, Lennart Poettering wrote:
> On Fr, 02.02.18 19:00, worz (worz at tuta.io) wrote:
>
>> Hello, I was wondering why systemd-run --scope needs to be
>> synchronous, it would be nice if someone could explain what happens
>> under the hood, and what prevents it from just putting the process
>> in the scope, and maybe just call the Abandon() method on the slice
>> object's org.freedesktop.systemd1.Scope interface, when for example
>> it is used in combination with --user, the scope is under
>> user at 1000.service, so it should be cleaned up as soon as the service
>> manager goes away? Note that there's not a specific usecase to cover
>> here, I am just interested in knowing why it isn't the other way.
> Hmm, I am not sure what you precisely mean by "synchronous" in this
> context?
>
> Is this about the bus calls being issued synchronously? We do that so
> that at the time the user-specified command is executed we know for
> sure that the resource limits and other things are in effect on it. If
> we'd asynchronously register the scope and would already execute the
> user's command, then these resource limits would only be applied
> asynchronously, i.e. at some later point too, which is generally not
> what is intended.
>
> Or are you using the word "synchronous" in the context of foreground
> vs. background? I.e. you want your process to be forked into the
> background? That's actually done by default by systemd-run when you
> don't use "--scope", where the executed process is moved into into the
> background as a service.
>
> Note that "systemd-run --scope" is little more than an
> execve()-wrapper: if you want the whole thing to be running as a
> shell managed background process, then suffix the line in "&" like you
> would normally do too, when you don't use "systemd-run"...
>
> But then again, I am not sure what you actually are looking for so
> maybe my answers above completely miss the point...
>
> Lennart
>

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