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#! /bin/sh
# Shell-based mutex using mkdir. This script is used in make to prefer
# serialized execution to avoid consuming too much RAM. If reusing it,
# bear in mind that the lock-breaking logic is not race-free, so disable
# it in err() if concurrent execution could cause more serious problems.
self=`basename $0`
lockdir="$1" prog="$2"; shift 2 || exit 1
# Remember when we started trying to acquire the lock.
count=0
err () {
if test -f $lockdir/lock-$1.$$; then
rm -rf $lockdir
echo "$self: *** (PID $$) removed stale $lockdir" >&2
# Possible variant for uses where races are more problematic:
#echo "$self: *** (PID $$) giving up, maybe rm -r $lockdir" >&2
#exit 42
else
touch $lockdir/lock-$1.$$
fi
}
until mkdir "$lockdir" 2>/dev/null; do
# Say something periodically so the user knows what's up.
if [ `expr $count % 30` = 0 ]; then
# Check for valid lock.
if pid=`cat $lockdir/pid 2>/dev/null` && kill -0 $pid 2>/dev/null; then
echo "$self: (PID $$) waiting $count sec to acquire $lockdir from PID $pid" >&2
elif test -z "$pid"; then
echo "$self: (PID $$) cannot read $lockdir/pid" >&2
err nopid
else
echo "$self: (PID $$) cannot signal $lockdir owner PID $pid" >&2
err dead
fi
fi
sleep 1
count=`expr $count + 1`
done
trap 'rm -rf "$lockdir"' 0
echo $$ > $lockdir/pidT && mv $lockdir/pidT $lockdir/pid
echo "$self: (PID $$) acquired $lockdir after $count seconds" >&2
echo $prog "$@"
$prog "$@"
# The trap runs on exit.
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