Re: s6 service 'really up' clarification

From: Laurent Bercot <>
Date: Tue, 13 Jan 2015 00:38:52 +0100

On 12/01/2015 23:27, Patrick Mahoney wrote:

> So, speculating, it seems that 's6-svwait -U' on a service that is
> currently 'down' either waits forever, or returns when something sends a
> 'U' event. But when the service is 'up' (but no U event ever happened),
> 's6-svwait -U' returns immediately as if the service was 'really up'.

  Yes, and I agree that is counter-intuitive and problematic, but I
haven't come up with a good solution yet.

  The issue is that s6-supervise stores the service state in supervise/status
and has no notion of service readiness. For s6-supervise, either the service
is up (pid nonzero) or it is down (pid zero), it cannot know whether the
service is "really up" or not.
  And adding an understanding of service readiness to s6-supervise would:
  - make no sense: it's a process supervisor, no matter what the process does.
I use it for cron-like jobs (do the job then sleep for a while), for instance,
where there's no notion of service readiness.
  - be absolutely bloated and ugly, since there should be a feedback canal from
the service to s6-supervise, and I really don't want to go there.

  s6-svwait listens on the event/ fifodir then checks on supervise/status to
know the initial state of the service before any notification arrives. But
that initial state is only "down" or "up", as s6-supervise wrote it; it's never
"ready". So for now, s6-svwait -U assumes that "up" means "ready". Which
causes the behaviour you observed.

  To solve that, the up vs. ready state should be stored in another file, that
s6-svwait -U should check. The logical place to do that operation is in
s6-notifywhenup.But that raises a few other issues:

  - That would more or less enforce s6-notifywhenup as *the* readiness
notification tool to use with s6-svwait. If daemons want to use another
notification mechanism than the one supported by s6-notifywhenup, there will
need to be another compatibility tool to convert notification mechanisms.
This can become pretty ugly really fast.

  - That means s6-notifywhenup has to have write permissions to supervise/
instead of simply read permissions. So, basically, s6-notifywhenup has to
run as root. It's nothing complex, but it's still more root code that the
admin has to trust.

  - s6-notifywhenup isn't around when the daemon dies, so it cannot update
the new state file. Only s6-supervise is around, so it has to do the job.
That is an ad-hoc hook that needs to be added to s6-supervise. Again,
nothing complex, but by definition, ad-hoc is ugly.

  All this is probably still better than the current behaviour, so I may add
it in a future release.

Received on Mon Jan 12 2015 - 23:38:52 UTC

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