#!/bin/sh

# semaphore functions for locking

trap "" 1 2 3 6 #try to be atomic

# $workdir must be writeable or be creatable
if [ "$workdir" = "" ]; then
    workdir=/var/run/sematree
fi

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

if [ $# -lt 2 ]; then usage; fi
name=$2
action=$1
if [ $# -ge 3 ]; then
    seconds=$3
else
    seconds=-1 #forever
fi

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

case $action in

acquire)

    until mkdir $lockname 2>/dev/null; do
        if [ $seconds -eq 0 ]; then exit 1; fi
        if [ ! $seconds -eq "-1" ]; then seconds=`expr $seconds - 1`; fi
        sleep 1
    done ;;

inc|dec)

    if [ ! -d $lockname ]; then
        echo "can't increment or decrement without acquiring first"
        exit 1
    fi

    [ -f $semaname ] && cur_val=`cat $semaname` || cur_val=0
    [ $action = inc ] && op=+ || op=-
    cur_val=`expr $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