Some UPS notes (apcupsd et al)

If you didn’t notice, one of the packages I’ve been maintaining in Gentoo is apcupsd that is one of the daemons that can be used to control APC-produced UPS units. But for quite a while, my maintenance of the package was mostly limited to keeping it in a working state (with a wide range of different results, to be honest), since from the original (messy) ebuild I originally inherited, the current one is quite linear and, in my opinion, elegant.

But in the past two weeks or so, a few things happened that required me to look into the package more closely: a version bump, a customer having two UPSes connected to the same system, and the only remaining non-APC UPS in my home office declaring itself dead.

The version bump was the chance for me to finally fix the strict aliasing issue that is still present; this time, instead of simply disabling strict aliasing (quick, hacky way) I decided to look at the code, to make it actually strict aliasing compliant. This might not sound like much, but this kind of warnings is particularly nasty as you never know when it will cause an issue. Besides, it caused Portage to abort in stricter mode, that is what I use for packages I maintain myself.

Also, while my customer’s needs didn’t really influence my work on apcupsd itself, it caused me to look even more into munin’s apc_nis plugin as beforehand it was not configurable at all: it only ever used localhost:3551 to connect to APC NIS interface, which meant that if you wanted to change the port, or make it only listen on an external interface, you were out of luck. The patch to make this configurable is now part of Munin trunk, but I haven’t had time to ask Jeremy to add it to Gentoo as well (the few patches of mine to Munin are all merged upstream now, and Munin 2 will have those, and finally, native IPv6 transport, which means I probably won’t need to use ssh connections to fetch data over NAT, but just properly-configured firewalls).

There is another issue that comes up when having multiple UPS connected to the same box though: permanence of device names. While the daemon auto-discovers a single connected APC device, when you have multiple devices you need to explicitly tell it to access a given one. To do so, you could use the hiddev device paths, but the kernel does not make those persistent if you connect/disconnect the units. To solve this issue, the new ebuild for apcupsd that I committed today uses udev rules to provide /etc/apcups/usb-${SERIALNO} symlinks that you can use to provide stable reference to your apcupsd instances. I sent the rules upstream, hoping that they’ll be integrated in the next release.

A note here: while I’m a fan of autoconfiguration, I’m having trouble considering the idea of having apcupsd auto-started when an APC UPS is connected. The reason is not obvious though: while it would work fine if it was the only UPS and thus the only apcupsd instance to be present, if you had a second instance set up for a different UPS there would be no way to match the two together. This is at a minimum bothersome.

Speaking about init scripts, the powerfail init script currently only works in single-UPS configurations (whereas the main init script works fine in multiple UPS configurations), and even there it is a bit … broken. The powerfail flag can be written in a number of different places – the default and the Gentoo variants also point to different paths! – but this script does not take that into consideration at all. More to the point, the default, which uses /var/run might not be available at the shutdown init level since that would probably have been unmounted by that time. What I should do, I guess, is make it possible for the init script to fetch the configured value from the apcuspd configuration file, and move the default to use /run.

Next problem in my list is that apcaccess should not be among the superuser binaries, since it can be run from user just fine, but I’ll have to get that cleared with upstream first, it might break some scripts to move it in Gentoo only.

Finally, there is the problem that the sources of apcupsd are written with disregard for what many consider “library-only problems” – namely PIC – and has a very nasty copy-on-write scorecard. Unfortunately, some of the issues are so rooted into the design that I don’t feel up to fix the sources myself, but if somebody wanted a project to follow and optimise, that might be a good choice in this respect.

Sigh. I hope to find more time to fix the remaining issues with the scripts soon. For now if you have comments/notes that I might have missed, your feedback is welcome.

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