Fri, Feb 28, 2020 at 07:39:47AM +0100, Jan Braun wrote: > The Linux kernel uses SIGPWR to tell init "there's a power failure > imminent". There's a difference to "please shutdown the system", even if > (current versions of) busybox and openrc choose to shutdown the system > when told about an imminent power failure.
The Linux kernel never sends SIGPWR to init on most platforms. Grepping the source, there are only about two places where SIGPWR can be sent to the init process from the kernel: machine check exception on S390, and some power-related hardware failure indication on SGI workstations. I'm not familiar with either of those but I suspect in both cases the signal is expected to initiate immediate shutdown. Otherwise, why would they signal init. Just for reference, I checked apcupsd sources. It does include an example of sending SIGPWR to init when some UPS event arrives, but it's just an example, and in that setting spawning some script would be a better approach anyway. The same applies to pretty much anything else in userspace that might be monitoring power supply. It should not be signalling init, it should have some sort of a configuration event handler. Unless it's matter of principle, hard-coding SIGPWR as a shutdown signal sounds like a decent idea to me, at least on Linux. LXD and similar containers might be the only class of applications for which signalling init like that actually makes sense. The distinction between SIGINT for reboot and SIGPWR for shutdown does not sound awful either.
