As the maintainer of the NUT plugin for UNRAID (which basically acts as a frontend for NUT) I have two people with the APC series Back-UPS RS (models 900MI and 1350MS) reporting that the bi-monthly battery self-tests result in NUT setting FSD on their systems. This happens on the build from master (as of 06.10.2023), but does not happen on 2.8.0 or 2.7.4. stables respectively, so it's likely an issue with changes introduced after the 2.8.0 release - the driver affected seems to be usbhid-ups.

The problem seems to be that during the UPS self-testing and right before NUT sets FSD, the driver thinks to receive an OFF event from the UPS, resulting in the MinSupplies falling below 1 (on a home setup with one UPS configured) and NUT proceeding to set FSD because of that. This can be re-produced by triggering a manual self-test on the affected APC series. Setting the MinSupplies to 0 does not result in FSD being set by NUT.

I only have limited logs available so far and will try to get more detailed logs with an increased debug level, but this is what I have from the users affected so far. Note that NUT does not report anything else above the first line, so the problem starts with that.

The users reported to hear the UPS inverter clicking during self-testing, these logs appearing and the system shutting down. This can be re-produced by triggering a manual self-test on the affected APC series.

Oct  8 02:35:51 UNRAID upsmon[27897]: UPS [email protected]: administratively OFF or asleep
Oct  8 02:35:51 UNRAID upsd[27893]: Client [email protected] set FSD on UPS [qnapups]
Oct  8 02:35:51 UNRAID upsmon[27897]: Executing automatic power-fail shutdown
Oct  8 02:35:51 UNRAID upsmon[27897]: Auto logout and shutdown proceeding
Oct  8 02:35:56 UNRAID shutdown[3133]: shutting down for system halt

Here we can see the UPS being on OL CHRG OFF, as well as FSD being set right before the system goes down.

I'll try to get more diagnostic information from the users (hopefully, they've already downgraded to 2.8.0 stable), but I thought I'd report this here nonetheless, maybe other users have the same problem and I think unexpected shutdowns are a larger issue.