wait-for-started.conf-- a generic job that waits for a job to start

[Clint Byrum]

So in the course of discussing the problem of synchronizing with
multiple events (aka "the state problem") with people, I've had to write
this example job a few times:

# foo-wait

start on X and Y
stop on started foo or stopped foo

instance $WAITER

normal exit 2
task

script
  status foo | grep -q "start/running" && exit 0
  start foo || true
  while sleep 3600 ; do : ; done
end script

#EOF

This type of job was used to fix statd's startup in Ubuntu so that it
blocks mountall from mounting NFS shares until statd is running, and
also waits for local filesystems so it can deal with sm-notify's
requirements, and also waits for portmap to be fully started.

I understand that, going forward, we'll have the state rewrite, and that
will handle these situations properly.

However, until then, I think we can actually condense this, into the
following upstart job:


# wait-for-state

stop on started $WAITFOR or stopped $WAITFOR

normal exit 2
task

env WAITER=
env GOAL="start"
env TIMEOUT=5
env WAITFOREVER=N

instance $WAITER$WAITFOR$GOAL

script
  # already running..
  if [ x$GOAL = xstop ] ; then
    target_state="waiting"
  else
    target_state="running"
  fi
  status $WAITFOR | grep -q "$GOAL/$target_state" && exit 0
  # Make sure WAITFOR is an actual job..
  status $WAITFOR | grep -q "^$WAITFOR" || exit 1
  # Ok, try to start/stop it
  $GOAL $WAITFOR || true
  if [ x$WAITFOREVER = xY ] ; then
    while sleep 3600 ; do : ; done
  else
    sleep $TIMEOUT && exit 1
  fi
end script

# EOF


So, if anything needs to happen after a job reaches start/running or
stop/waiting, one only needs to precede it with this:

start wait-for-state WAITFOR=foo WAITFOREVER=Y GOAL=start WAITER=$UPSTART_JOB

This, unfortunately, doesn't work for jobs that rely on variables being
passed in to get them started properly.

I added in the WAITFOREVER and TIMEOUT's because I figure it may be
useful for instances where its possible the state will never be reached.
Having the default *not* to wait forever should help a user makig use of
this to decide when they want to wait forever.

I wanted to solicit any opinions on this before submitting this for
inclusion in the Ubuntu upstart package (and possibly patching nfs-utils
and portmap to use this method).