Not an option to be its parent since there's no persistent supervisor of a
jail's root process, but that script using .s6-svscan/finish should do
nicely. Thanks for the suggestion!

- Jw


On Wed, Feb 23, 2022 at 9:04 PM Laurent Bercot <ska-supervision@skarnet.org>
wrote:

>
> >What's the cleanest way to wait on s6-svscan to shut down after issuing of
> >a SIGTERM (say s6 via-svscanctl -t)?
>
>   Be its parent, and wait for it. :)
>   On SIGTERM, s6-svscan will not exit until the supervision tree is
> entirely down, so that will work.
>   If you're not the parent, then you'll have to wait for a notification
> somehow, but that's easy:
>
>   When s6-svscan wants to exit, it doesn't exit right away, but
> tries to exec into the .s6-svscan/finish script. So you have a clear
> indicator here: when .s6-svscan/finish runs, it means the supervision
> tree is down.
>   So, for instance, make a finish script that writes a byte in a fifo,
> and have your jail shutdown script read on that fifo. Something like:
>
> .s6-svscan/finish:
> #!/bin/sh
> exec echo > /run/blah/fifo
>
> shutdown script:
> #!/bin/sh
> ...
> rm -f /run/blah/fifo
> mkfifo /run/blah/fifo
> read < /run/blah/fifo &
> s6-svscanctl -t /run/service
> wait
> ...
>
> (read on the fifo before running s6-svscanctl, to avoid the small
> race condition.)
>
>
> >Looking at the documentation, my only option appears to be to check if the
> >return code of s6-svscanctl is 100, or maybe to monitor for the existence
> >of .s6-svscan/control (not sure if it's removed on exit). Are there any
> >other ways to monitor s6-svscan?
>
>   Ew. Don't poll.
>   Use .s6-svscan/finish to do anything you want to do at s6-svscan
> death time.
>
> --
>   Laurent
>
>