#! /bin/sh
# Inspired by http://members.toast.net/art.ross/rute/node24.html

TAG=default
CMD=

help() {
    echo "USAGE: lock_file (lock|unlock) [<tag>]"
    echo
    echo "lock_file locks or unlocks a lock file, for synchronization"
    echo "across multiple processes. The lock command will block until"
    echo "the lock can be obtained, upon which it exits with code 0."
    echo "The exit code will be 1 on failures, and 2 on input error."
    echo "You can use different tags for different locks."
}

while test "x$1" != "x"; do
    case "$1" in 
	"-h"|"--help"|"-help"|"-?"|"help")
	    help
	    exit 0
	    ;;
	"lock")
	    CMD=lock
	    shift 1
	    ;;
	"unlock")
	    CMD=unlock
	    shift 1
	    ;;
	*)
	    TAG="$1"
	    shift 1
	    ;;
    esac
done

TEMPFILE="/tmp/lock_${TAG}.$$"
LOCKFILE="/tmp/lock_${TAG}.lock"

if test "${CMD}" = "lock"; then

    { echo $$ > $TEMPFILE; } >/dev/null 2>&1 || {
	echo "You don't have permission to access `dirname $TEMPFILE`"
	exit 1
    }

    while true; do
	ln $TEMPFILE $LOCKFILE >/dev/null 2>&1 && {
	    rm -f $TEMPFILE
	    exit 0;
	}

	if test -e "$LOCKFILE"; then
	    kill -0 `cat $LOCKFILE` >/dev/null 2>&1 || {
		echo "Removing stale lock file"
		rm -f $LOCKFILE
	    }
	fi
	
	sleep 1
    done
fi

if test "${CMD}" = "unlock"; then
    rm -f $LOCKFILE && exit 0
    exit 1
fi

exit 2