#!/bin/sh

# process independent locking utility

# Author:
#    http://www.pixelbeat.org/
# Notes:
#    There is a C equivalent of this implemented using open(EXCL)
#    rather than mkdir for illustration, available from the above web site.
#    There is a lockf util in BSD that supports locks external to commands,
#    and is not susceptiple to stale locks being left on the filesystem.
# Changes:
#    V0.1, 02 Apr 2003, Initial release
#    V0.2, 23 Feb 2006, Add trap so signals don't mess up atomic operations
#    V0.3, 06 Jan 2010, Use shell arithmetic and faster polling
#    V0.4, 08 Jul 2010, Don't hang if can't create $workdir

trap "" HUP INT QUIT ABRT #try to be atomic

POLL_FREQ=10 #Hz Must be 1,10,100,...
DELAY=$(echo $POLL_FREQ | sed 's/1\(.*\)0/.\11/')

# $workdir must be writeable or be creatable
[ "$workdir" ] || workdir=/var/run/sematree

usage() {
    echo "\
Usage: $(basename $0) {acquire,release,inc,dec} lock_name [seconds to wait]\
" >&2
    exit 1
}

[ $# -lt 2 ] && usage
action=$1
name=$2
if [ $# -ge 3 ]; then
    count=$(($3 * $POLL_FREQ))
else
    count=-1 #forever
fi

if [ ! -d $workdir ]; then
    #race here handled by -p
    mkdir -p $workdir
fi
[ -d $workdir ] || exit 1
lockname=$workdir/$name
semaname=$workdir/$name.sem

case $action in
acquire)
    until mkdir $lockname 2>/dev/null; do
        [ $count -eq 0 ] && exit 1
        [ ! $count -eq "-1" ] && count=$(($count - 1))
        sleep $DELAY
    done ;;
inc|dec)
    if [ ! -d $lockname ]; then
        echo "can't increment or decrement without acquiring first" >&2
        exit 1
    fi
    [ -f $semaname ] && cur_val=$(cat $semaname) || cur_val=0
    [ $action = inc ] && op=+ || op=-
    cur_val=$(($cur_val $op 1))
    [ $cur_val -eq 0 ] && rm $semaname || echo $cur_val > $semaname
    echo $cur_val ;;
release)
    [ ! -d $lockname ] && exit 1 || rmdir $lockname ;;
*)
    usage ;;
esac