mklock

#!/bin/sh
# 
# Process-based file locking:
# Creates a file containing the PID of a process accessing a shared
# resource and prevents other processes _on the same host_ from
# accessing it.
#
# All this breaks down if the scripts are run on separate hosts
# since PID checking won't work
#
# Usage: mklock [-s sleeptime] [-r retries] [-d lockdir] lockfile pid
#

p=`basename $0`

die () {
    echo "$p: fatal error: $1 occurred at " `date` 1>&2
    exit 1
}
warn () {
    echo "$p: warning: $@" 1>&2
}
usage () {
    echo "$p: $@" 1>&2
    echo "Usage: $p [-s sleeptime] [-r retries] [-d lockdir] lockfile pid" 1>&2
    exit 1
}

dir=${ADS_TMP-/tmp}
try=0
sleeptime=10
retries=0

while [ $# -ne 0 ]; do
    case "$1" in
    -s)
	shift
	sleeptime="$1"
	;;
    -r)
	shift
	retries="$1"
	;;
    -d)
	shift
	dir="$1"
	;;
    -*)
        usage "$p: unknown option \"$1\"" 1>&2
        ;;
    *) 
	break
	;;
    esac
    shift
done

lf=`echo "$1" | sed -e "s/\//_/g"`
pid="$2"

[ "x$lf"  = "x" ] && usage "no lockfile specified"
[ "x$pid" = "x" ] && usage "no pid specified"
if [ ! -d $dir ] ; then
    warn "creating locking directory $dir"
    mkdir $dir || die "cannot create locking directory $dir"
fi

while [ -f "$dir/$lf" ] ; do
    oldpid=`cat $dir/$lf`
    if [ "x$oldpid" = "x" ] ; then
	warn "removing empty lock file $dir/$lf"
	/bin/rm -f "$dir/$lf" || die "cannot remove $dir/$lf"
    elif kill -0 $oldpid 2> /dev/null ; then
	if [ $sleeptime -gt 0 -a $try -lt $retries ] ; then
	    try=`expr $try + 1`
	    warn "[$try/$retries] locking process detected (pid=$oldpid), sleep $sleeptime sec..."
	    sleep $sleeptime
	else
	    warn "active locking process detected (pid=$oldpid), giving up"
	    exit 3
	fi
    else
	warn "removing stale lock file $dir/$lf"
	/bin/rm -f "$dir/$lf" || die "cannot remove $dir/$lf"
    fi
done

echo $pid > $dir/$lf || die "cannot create $dir/$lf"
exit 0