Re: S6 Queries

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

>I thought the way to do what the OP asked is:
>
>=============================
>#!/bin/sh
>s6-svc -u myrequirement || exit 1
>exec mydaemon -myarg1 -myarg2
>=============================

  This is not a good idea in a s6-rc installation, because it sends
raw s6 commands, which may mess with the service state as viewed by
s6-rc. Also, it sends control commands to a service from another
service's run script, which is bad form in general: it is
unintuitive that starting the mydaemon service also causes the
myrequirement service to be started. Dependencies should be handled at
the service manager level, not at the process supervision level.

  Of course, you can do that in a pinch for small s6-only installations,
where you have no proper dependency engine, but that does not seem to be
what the OP is asking.

  And even then, this does not implement After= because s6-svc -u
returns instantly. This only implements Wants=. To implement After=,
you would need something like s6-svc -uwU instead, which is not good
because it adds the myrequirement readiness delay to the mydaemon
readiness delay, so if mydaemon has strict readiness timeouts, it
can make it fail.

  All in all, it's better to avoid controlling another service in a run
script: there's always an annoying corner case somewhere. Dependency
management is best handled by the external tool that is explicitly
supposed to do it: the service manager.

--
  Laurent