Re: pidsig 0.11 - a fghack like de-daemonisation tool

[Charlie Brady <charlieb-supervision@budge.apana.org.au>]

On Fri, 4 Jun 2010, Wayne Marshall wrote:

> On Thu, 3 Jun 2010 21:25:30 +0200
> Laurent Bercot <ska-supervision@skarnet.org> wrote:
> 
> > > These kinds of problems are not that theoretical - just
> > > recently I saw svscan/svscanboot crashing on a >1y uptime
> > > box, taking many of the processes with it, including most of
> > > the supervise infrastructure, very likely not due to any
> > > fault in them - could be oom gone wild, cosmic rays hitting
> > > svscan memory, whatever).
> > 
> >  That's a typical case of "weak" supervision, as opposed to a
> > "strong" supervision chain. "Strong" supervision makes sure
> > that all the infrastructure is connected to init.
> > 
> >  * svscan achieves strong supervision *if* svscanboot is
> > flagged as "respawn" in /etc/inittab on System V-style inits,
> > in /etc/event.d/ with Upstart, or in /etc/gettys on BSD. It
> > does *not* achieve it if svscanboot is started via some
> > rc.local script (as the stock daemontools instructions tell
> > you to do, shame on DJB! :))
> >  * perp is in the same boat, depending on how you start
> > perpboot.
> > ...
> > Strong supervision makes sure that your supervisor process
> > tree is *always* alive and complete, unless process 1 itself
> > crashes, in which case you're doomed to reboot anyway.

There is a weakness in this "strong supervision" model. Any service with a 
'down' file will not be restarted if its supervise/runsv or 
svscan/runsvdir is replaced.