#!/bin/sh

# %W% %@% Copyright (c) 1998 Larry McVoy.
#
# Usage: SYNC_PID=3 SYNC_MAX=20 synchronize /tmp/sync_dir
#
# Used to sync up a bunch of processes so that they can operate in lockstep
# as much as possible.
#
# The first process to try and sync will mkdir(pathname) and create a named
# pipe in the directory.  It also creates a file, pathname/$PID where pid
# is not the process id, it is the process number.  The group of processes
# must be numbered from 1..N and they must each know their number.  The Nth
# process is the master.  Whereas all the other processes block, opening the
# pipe, the master spins in a loop, waiting for pathname/1 .. pathname/N-1
# to show up in the directory.  When they are all there, the master opens
# the pipe for writing and all the other processes get woken up and leave.
#
# It is outside of this program, but the directory must not exist before the
# synchronization.  So you typically rm -rf it well before trying to sync.

if [ X$1 = X ]; then echo "Usage: $0 pathname"; exit 1; fi
if [ X$SYNC_PID = X ]; then echo "Must set SYNC_PID"; exit 1; fi
if [ X$SYNC_MAX = X ]; then echo "Must set SYNC_MAX"; exit 1; fi

DIR=$1
mkdir -p $DIR
if [ ! -e $DIR/fifo ]
then	mkfifo $DIR/fifo
	chmod 666 $DIR/fifo
fi

# slaves just read the pipe
if [ $SYNC_PID != $SYNC_MAX ]
then	touch $DIR/$SYNC_PID
	read x < $DIR/fifo
	exit 0
fi

# Master waits for all the other processes to get there
PIDS=""
I=1
while [ $I -lt $SYNC_MAX ]
do	PIDS=" $I$PIDS"
	I=`expr $I + 1`
done
while true
do	GO=Y
	for s in $PIDS
	do	if [ ! -e $DIR/$s ]
		then	GO=N
		fi
	done
	if [ $GO = Y ]
	then	# This assumes that all the processes will 
		echo sleep 2 > $DIR/fifo &
		exit 0
	fi
	msleep 250
done
