Discussion:
Why is systemd-run --scope synchronous?
(too old to reply)
worz
2018-02-02 18:00:05 UTC
Permalink
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 ***@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.
--Jonathon Kowalski
Lennart Poettering
2018-02-05 13:59:23 UTC
Permalink
Post by worz
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
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
--
Lennart Poettering, Red Hat
Jérémy Rosen
2018-02-05 14:03:43 UTC
Permalink
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...
Post by Lennart Poettering
Post by worz
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
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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worz
2018-02-06 12:09:26 UTC
Permalink
Sorry for the badly formatted emails, I'll use a different provider thatactually allows sending plain-text emails next time. I could only noticeafter it was sent.

Yes, I meant it in terms of foreground vs background. For example, not specifying --scope just pushes it into a service, while with scope, whilethe process executed is inside the scope unit, the systemd-run process isconnected to the terminal (without the use of -t) and running in the foreground of the shell. I am not sure why things are different. I alsonotice the difference is that in case of --scope, the service manager isnot really the parent process, and it's just that systemd-run creates a transient scope and places the process inside it, which is ok, but not surewhy it is different than running it as a service, or even running the processin the scope unit as a child of the service manager. I guess I don't reallyunderstand the purpose of scope units properly, which is why this might be confusing (and this probably relates to how tmux could have properly moved itself out of the session scope by creating a new one transparently makinga bus call to systemd). I would appreciate a little explanation on whatI'm getting wrong and how it is supposed to work.
Post by Lennart Poettering
Post by worz
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
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
--
Lennart Poettering, Red Hat
Simon McVittie
2018-02-06 14:28:33 UTC
Permalink
Post by worz
I am not sure why things are different. I also
notice the difference is that in case of --scope, the service manager is
not really the parent process, and it's just that systemd-run creates a
transient scope and places the process inside it, which is ok, but not sure
why it is different than running it as a service, or even running the process
in the scope unit as a child of the service manager.
If I understand correctly, that's exactly the purpose of scope units:
they allow processes that are not a child of the service manager to be
made visible to the service manager as a unit.

For instance, when Flatpak is run on a system with `systemd --user`,
it puts each instance of a Flatpak app in a scope, even though those
processes need to be children of the Flatpak process so they can inherit
various fds and environment variables from it.

(One notable example of a fd that might be inherited by child processes
is "the controlling terminal".)

smcv

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