[systemd-devel] Ordering (apt) timer services to not run at the same time

[Michal Sekletar]

On Fri, Apr 28, 2017 at 11:05 AM, Julian Andres Klode <jak at debian.org> wrote:
> From my testing, if B has After=A, and A is already started, the
> startup of B is delayed until A has completed - do you mean that
> with run queue, or is that merely by accident somehow?

Like I said, we can't do anything about job that is already dispatched.

Consider, following simple units:

# /etc/systemd/system/a.service
[Service]
ExecStart=/bin/sleep 60

# /etc/systemd/system/b.service
[Unit]
After=a.service

[Service]
ExecStart=/bin/sleep 60

When I start a.service first and then I invoke systemctl start second
time for b.service, it will start b.service immediately. Since start
job for a.service is already gone and a.service is already running.

Now consider second scenario. b.service stays unchanged and a.service
looks like this,

# /etc/systemd/system/a.service
[Service]
Type=oneshot
ExecStart=/bin/sleep 60

When I start a.service in non-blocking mode (systemctl by default
blocks until job it queued finishes) by "systemctl --no-block start
a.service" and then I list jobs via "systemctl list-jobs" I can see
following jobs,

JOB UNIT      TYPE  STATE
441 a.service start running

Then I start b.service. This time systemctl will block because start
job for b.service is waiting on start job for a.service and that is
still in the job queue. List of jobs then looks like this,

-bash-4.4# systemctl list-jobs
JOB UNIT      TYPE  STATE
631 a.service start running
667 b.service start waiting

Job for b.service can't run because there is running job for a.service
and a.service is still in activating state. That is not the case in
first scenario, due to different type of service a.

I looked over your unit files once again and I see your services are
indeed oneshots. So this should actually work for you. I don't know
why it doesn't.

>> Indeed, seems like lockfile + condition in other unit is the simplest way out.
>
> How unfortunate.

Scratch that, I've missed that you are using oneshot services.

Michal