Support sleep/hibernate integration (generally and with systemd in particular)

[jimklimov]

There were several discussions in the past about NUT daemons emitting messages like "Data stale" when a system wakes up after prolonged sleep. One particular use-case is putting a system into hibernation when an UPS goes depleted, or into sleep when on-battery (hoping the lone RAM's power draw can be kept up for hours). This is not a platform-specific issue, everybody might do it, but it may have platform-specific optimal solutions (and/or general ones).

The messages make sense technically (lots of time passed since last poll) but seem confusing to some users and pollute the console cluttered by various subsystems after wake-up.

This issue is about exploring ways to integrate with service management frameworks like systemd to "know" a sleep happened, e.g. deal with dependencies on/against https://www.freedesktop.org/software/systemd/man/systemd-suspend.service.html family of units, or deliver hooks for them (example in https://blog.christophersmart.com/2016/05/11/running-scripts-before-and-after-suspend-with-systemd/ but official doc warns not to - deemed local hacks), or explore its "Inhibitor interface" per https://blog.christophersmart.com/2016/05/11/running-scripts-before-and-after-suspend-with-systemd/ and possibly something is possible by listening to systemd dbus...

With such integration, NUT daemons would know to silently reestablish connections when the system wakes up, and maybe chirp a message about that specifically. One solution could be to use the pre/post-sleep hooks to completely stop and start NUT services and so avoid "staleness" (also possibly account for IP address changes after wake-up).

A general-purpose alternative could be to somehow watch for system clock jumps (e.g. we have a 5 or 30 second pollfreq, and suddenly last loop took tens of minutes or more), and/or possibly track "local" time (as opposed to monotonous kernel clock) to see and react to NTP/RTC time jumps - which along with loops taking too long can be due to sleep or winter/summer time changes. Again, the purpose would be to tailor the reaction (and noise made) to this situation.

Also useful would be some documentation notes (FAQ?) and/or contributed scripts/* for different platforms, specifically about requesting sleep/hibernation as a SHUTDOWNCMD. Notably, such a command could take care of stopping/starting units and init-script styled services, and/or leaving flag-files for daemons to know about the intentional situation. (Service starts could be scheduled by cron/at facility, e.g. roughly at now + 1min generally).

Enthusiasts welcome to take on this, especially those who can test it well (have systems that do go to sleep while monitoring)!