[Nut-upsuser] Synthesize low batt (LB) fron SNMP UPS which does not support this?

[Greg Troxel]

Carsten Aulbert <carsten.aulbert at aei.mpg.de> writes:

> Hi all,
>
> On 5/19/23 15:11, Greg Troxel wrote:
>>    LB is baked in to nut behavior.   So if a UPS doesn't report LB, then
>>    it makes sense to synthesize it.  Synthetic LB is the cleanest fix at
>>    the earlier processing point.
>
> I fully agree and thus this ought to be done in/near uspd IMHO. I
> glanced over the server/ directory and was not sure where even to
> begin hooking this in. Should this be directly in upsd (upsd.c) or
> rather in/near the driver level (driver/main.c)?

I think it belongs in upsd someplace, as what I want is a way to make LB
appear based on a rule instead of (or in addition) to the LB reported by
the device.   The 50% vs 10% example captures the need for %-LB  on
devices that actually do report LB perfectly, and I think it's a desire
many will have.

>>    Yes, this could be in upsmon, but that's not synthetic LB, it's
>>    choosing to shutdown if battery is below X.
>
> Yeah, this should be independent and fully on the client side (and
> thus upsmon) where each client can choose how to react to a UPS on
> battery.

It could also make sense to make upsmon smarter, but if we are really
talking about defining "this UPS's battery is now low", that's what LB
is supposed to mean and we should make it work that way.

>>    We should be clear on "critical" vs "LB" and clean up the language to
>>    be consistent.
>
> For upsmon, I would stay with critical as this is already mentioned in
> upsmon(8) and I guess "LB" could stay for the server part as each user
> could decide whether she wants to rely on LB from the UPS itself or on
> the self-defined override.

I think the difference is:

  LB is a state reported by a UPS (perhaps synthetic in upsd) that says
  that the battery is nearing empty and remaining runtime is limited

  critical is an opinion by upsmon that the UPS has reached a state
  where it is prudent to do an immediate shutdown.  Typically LB implies
  critical but it could be more complicated.

>>    Beware that I'm a bit fuzzy on details beyond my comments and even
>>    some on things I commented on, so take them as such.
>
> Same here, quite at a loss what is handled where on the server and
> always low on time due to other projects/tasks.
>
> Cheers and any pointers appreciated!

I would look where input from drivers is processed, and basically add

  if (battery% <= 50 && ! set?(LB)) {
     log that we are doing synthetic LB
     set!(LB)
  }

to mix C and scheme :)   Then of course make it configurable.  But it
probably is that easy to just make it hard-coded 50.