#! /bin/sh

set -e
case $# in
  2) ;;
  *) echo >&2 "usage: $0 {start|stop|restart|status} HOST"; exit 1 ;;
esac
op=$1 host=$2

writefile () {
  file=$1; shift
  echo "$*" >"$file.new"
  mv "$file.new" "$file"
}

runssh () { ssh -T -oControlPath="./$host.ctrl" "$@"; }

clobber () {

  ## Shut down an existing connection if there is one.
  if [ -S "$host.ctrl" ]; then
    runssh -Oexit "$host" >/dev/null 2>/dev/null || :
  fi

  ## If there's still a socket, then work out what to do.
  if [ -e "$host.ctrl" ]; then

    ## If the connection's still running then we have a problem.
    if runssh -Ocheck "$host" >/dev/null 2>/dev/null; then
      echo >&2 "$0: failed to kill existing connection to $host"
      exit 2
    fi

    ## Remove the stale socket.
    rm -f "$host.ctrl"
  fi
}

stopit () {

  ## Initial shutdown protocol.
  writefile "$host.state" stopping
  if [ -f "$host.pid" ]; then kill $(cat "$host.pid") 2>/dev/null || :; fi
  rm -f "$host.pid"

  ## Clobber the existing connection, if there is one.
  clobber

  ## Update the state.
  rm -f "$host.state" "$host.pid"
}

daemon () {

  ## There doesn't seem to be a better way of getting this. :-(
  read pid <"$host.daemonpipe"
  rm -f "$host.daemonpipe"

  ## Set up shop.
  trap 'rm -f "$host.pid"; stopit' EXIT INT TERM
  writefile "$host.pid" "$pid"

  ## Initial delay.
  delay=0

  ## Not waiting on a pipe yet.
  kidcat=nil

  ## Keep the connection up for as long as we can.
  while [ -f "$host.pid" ]; do

    ## Maybe back off before trying another connection.
    case $delay in
      0)
	delay=1
	;;
      *)
	writefile "$host.state" \
	  "wait until $(date -d+${delay}sec +"%Y-%m-%d %H:%M:%S %z")"
	sleep $delay
	delay=$(( 2*$delay ))
	if [ $delay -gt 120 ]; then delay=120; fi
	;;
    esac

    ## Prepare a pipe so that we can wait for SSH to finish.  This is a
    ## rotten hack.
    case $kidcat in
      nil) ;;
      *) kill $kidcat >/dev/null 2>&1 || :; kidcat=nil ;;
    esac
    rm -f "$host.pipe"; mkfifo -m600 "$host.pipe"
    cat $host.pipe >/dev/null& kidcat=$!

    ## Start a new connection.
    writefile "$host.state" starting
    if ! runssh -MNnf "$host" >"$host.pipe"; then continue; fi
    if ! runssh -Ocheck "$host" >/dev/null 2>&1; then
      echo "connection to $host apparently stillborn"
      continue
    fi
    writefile "$host.state" connected
    delay=0

    ## Wait until it gets torn down.
    wait $kidcat >/dev/null 2>&1 || :
    rm -f "$host.pipe"
    clobber
    writefile "$host.state" disconnected
  done
}

startit () {

  ## If there's already a connection then we have nothing to do.
  if runssh -Ocheck "$host" >/dev/null 2>/dev/null; then
    echo >&2 "$0: already connected to $host"
    exit 0
  fi

  ## Start a daemon which makes connections for us.  This is remarkably
  ## tricky.
  rm -f "$host.daemonpipe"; mkfifo -m600 "$host.daemonpipe"
  { daemon& echo $! >"$host.daemonpipe"; } 2>&1 | logger -pdaemon.notice&
}

## Main dispatch.
case "$op" in
  start) startit ;;
  stop) stopit ;;
  restart) stopit; startit ;;
  status)
    if [ -f "$host.state" ]
    then cat "$host.state"
    else echo "down"
    fi
    ;;
  *)
    echo >&2 "usage: $0 {start|stop|restart|status} HOST"
    exit 1
    ;;
esac