[systemd-devel] Periodic tasks on battery-powered devices

[David Strauss]

I could be missing something, but I think there's a gap in the
functionality for timers for periodic tasks on battery-powered devices.
Basically, I want to generate a security report (SCAP style) approximately
weekly but avoid doing so on battery.

Here's what I've tried or looked at:

- If I use ConditionACPower=true on the Type=OneShot service performing the
task, then systemd will not run the task while on battery, but it still
resets the timer. With a longer OnCalendar= interval, there's a significant
chance that the infrequent check will overlap with battery-mode operation,
causing the task to never run.

- If I use ConditionACPower=true on the timer, then the timer just won't
start if the machine boots on battery. This is also undesirable, as a
machine may only boot every few weeks.

- OnActiveSec= doesn't support persistence, so intervals of approximately a
week may never fire on some laptops that shutdown or reboot frequently.

- OnBootSec= would be too frequent for laptops that shutdown or reboot
frequently.

It seems the best I can do right now is use a shorter timer interval with
no AC power condition but have the Type=OneShot service exit early if it
last ran too recently, especially if currently on battery power. I'm also
looking at just using anacron.

What I would prefer is a [Timer] option like AvoidOnBatterySec= that causes
the timer to avoid firing on battery yet fire on restoration of AC power if
overdue. If the AvoidOnBatterySec= duration has elapsed since the last
firing, the timer fires regardless of whether the device is on battery.
This would allow easy configuration of periodic tasks that are power-hungry
but need to run eventually. Some of our laptops rarely run on AC aside from
when they're closed at night.

I would even consider a directive like WakeSystemSec= that specifies a
maximum interval that must elapse since the timer last fired before waking
up the system to fire the timer. Similarly to AvoidOnBatterySec=, it would
serve as a way to get more aggressive if a periodic task is too far overdue.
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