Re: s6-svscan shutdown notification

["Laurent Bercot" <ska-supervision@skarnet.org>]

>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