stage2 as a service [was: Some suggestions on old-fashioned usage with s6 2.10.x]

[s.karrmann@web.de]

Dear all,

after Laurents explanation about the supervision tree in stage 2 and 3,
I got the idea to put stage2 completely into a normal supervised service:

> 2021-01-29.16:48
> From: "Laurent Bercot" <ska-supervision@skarnet.org>
> To: "Casper Ti. Vector" <caspervector@gmail.com>, supervision@list.skarnet.org
> Subject: Re: Some suggestions on old-fashioned usage with s6 2.10.x
> 
> [...]
> >There is some non-trivial trade-off: in short, the existence of the
> >supervision tree after stage 2 is by itself a kind of "special case"
> >(eg. search for "careful handling" in [1]).
> 
> I feel like you misinterpreted my meaning.
> The *absence* of a supervision tree after stage 2 is precisely what
> requires careful handling, and runit only works because Linux has
> that peculiarity that kill -9 -1 does not kill the emitter!
> Having a supervision tree in stage 3 actually *helps* with the
> late shutdown procedure: shutdownd dies right after the kill (which
> would make it usable even on a system without the Linux specialcase)
> and is restarted by the supervisor for stage 4.
> [...] 

$ cat /etc/s6/services/s6-rc-up/run 
#! /usr/bin/execlineb -P

s6-envdir /etc/s6/init-env
multisubstitute {
  importas SCANDIR SCANDIR
  importas LIVEDIR LIVEDIR      
  importas COMPILED COMPILED     
  importas RCDEFAULT RCDEFAULT                              
  importas PATH PATH                          
  }
export PATH ${PATH}

# optional:  -- Question: Is this necessary?
  redirfd -w 0 ${SCANDIR}/service/s6-svscan-log/fifo
  # now the catch all logger runs
  fdclose 0

foreground { mkdir -p ${LIVEDIR} }
foreground { s6-rc-init -l ${LIVEDIR}/live -c ${COMPILED} ${SCAN} }
foreground { s6-svc -O . } # don't restart me
foreground { s6-rc -l ${LIVEDIR}/live -t 10000 change ${RCDEFAULT} }
# notify s6-supervise:
fdmove 1 3
foreground { echo "s6-rc ready, stage 2 is up." }
fdclose 1  # -- Question: Is this necessary?
# NB: shutdown should create ./down here, to avoid race conditions
# NB: init must ensure that there is no ./down here at startup.
# That is automatically fulfilled, if copied from a repo to /run/...
### THE END #####################################################################

and my init is:

$ cat /etc/s6/init
#! /usr/bin/execlineb -P
   
cd /
s6-setsid -qb                                         
envfile /etc/s6/init-envfile                                                      
multisubstitute {
  importas SCANDIR SCANDIR
  importas LIVEDIR LIVEDIR      
  importas COMPILED COMPILED     
  importas RCDEFAULT RCDEFAULT                              
  importas PATH PATH                          
  }
export PATH ${PATH}                                                  
             
# stage 1 init as PID=1
 
ifelse -nX                                   
  { # basic initialization
      foreground { # a hooks
        elglob -s locals /etc/s6/init.d/stage1a.d/*
          forx -E local { ${locals} }
            ${local}
      }

      foreground {
        # cf. https://code.dogmap.org./fs/
        elglob fss /fs/*
        forx -E -p fs { ${fss} }
          if { test -e ${fs}/mount-at-boot }
          mount ${fs}/mount # todo fsmount ${fs}
      }

      foreground { # b hooks
        elglob -s locals /etc/s6/init.d/stage1b.d/*
          forx -E local { ${locals} }
            ${local}
      }

    foreground { ln -s /fs/run-s6/mount/${SCANDIR} /run/s6 }
    foreground { mkdir -p /fs/run-s6/mount/${SCANDIR} }
    cp -a ${REPO} ${SCANDIR}
  }
  { # fallback login
    sulogin --force -t 600 # timeout 600 seconds, i.e. 10 minutes.
    # kernel panic
  }

# now the /dev must contain some files, i.e. /dev/null
redirfd -r 0 /dev/null  # useful for testing from a tty, i.e. don't consume input

execline-cd ${SCAN}
# catch all log also for stage 2
# s6-log duplicates it to console
redirfd -wnb 1 ./s6-svscan-log/fifo  # open fifo nonblocking
fdmove -c 2 1                        # copy it
emptyenv -P
exec -c
s6-svscan                         # start service scanner, i.e. PID=1 in stage 2
### THE END #####################################################################

I have three basic services:
- s6-linux-init-early-getty
- s6-rc-up
- s6-svscan-log

Everything else is up to s6-rc. Well, it will be. I'm still moving my Debian from systemd ("black box") to s6...
Also I may switch to s6-linux-init finally.

Kind regards,
Stefan