source: locker/cron/bin/heartbeat @ 1085

#!/bin/bash

CRONROOT=/afs/athena.mit.edu/contrib/scripts/cron

# Find our real hostname

# This big long mess just results in a list of ip/name.
for i in `/sbin/ip addr show dev eth0 | grep ' inet ' | cut -f 6 -d ' ' | cut -f 1 -d '/' | xargs -n 1 host | cut -f 1,5 -d ' ' | sed 'y/ /\//'`; do 
        hostip=`echo $i | cut -f 1 -d '.'`
        name=`echo $i | cut -f 2 -d '/'`
        case $name in
                SCRIPTS*) echo "$name";;
                *) echo "Heartbeat for $name ($hostip)"; HOSTNAME=$name; HOSTIP=$hostip;;
        esac;
done

# Tell everyone who's watching that we're alive
touch $CRONROOT/servers/$HOSTNAME

# Sleep based on our IP, in an attempt to not collide with another server also trying to gain control of the mirroring
sleep $(($HOSTIP - 50))

# Find the current master
MASTER="DOES-NOT-EXIST"
current_server () {
        for i in $CRONROOT/server-crontabs/*; do
                if [ -h $i ]; then
                        MASTER=`basename $i`
                        echo "Current master $MASTER"
                fi
        done
}
if lockfile -1 -r10 -l90 $CRONROOT/lock/heartbeat.lock; then
    current_server

        # The only way to compare times in bash is to compare the modtimes of two files.
        compare=`mktemp /tmp/heartbeat-compare.XXXXXXXXXX`
        touch -d '2 minutes ago' $compare
        
        if [[ $CRONROOT/servers/$MASTER -ot $compare ]]; then
                # Master died! Take over.
                echo "Master '$MASTER' died! Taking over."
                for i in $CRONROOT/server-crontabs/*; do
                        if [ -h $i ]; then
                                echo rm $i
                                rm $i
                        else
                                echo rmdir $i
                                rmdir $i
                        fi
                done
                for i in $CRONROOT/servers/*; do
                        server=`basename $i`
                        case $server in
                                $HOSTNAME)
                                        echo ln -s ../crontabs/ $CRONROOT/server-crontabs/$HOSTNAME
                                        ln -s ../crontabs/ $CRONROOT/server-crontabs/$HOSTNAME;;
                                *)
                                        echo mkdir $CRONROOT/server-crontabs/$server
                                        mkdir $CRONROOT/server-crontabs/$server;;
                        esac;
                done;
        fi

        rm $compare
    rm -f $CRONROOT/lock/heartbeat.lock
fi