#!/bin/bash
#
# Control program for bespoke arbitrary services through unshared sub-hosts
#
# $1 = the command: start or stop
# $2 = the sub-host to start or stop
# $3... = the optional chroot-ed command (/startup by default)

set -x

SCRIPT=$0
CMD=$1
NAME=$2
shift 2

usage() {
    echo "$SCRIPT (start|stop) <subhost>" >&2
    exit 1
}

[ -z "$NAME" ] && usage

: ${SUBHOST=/opt/subhost}
: ${OSROOT=}
: ${TOP=$SUBHOST/$NAME}
: ${TARGET=$TOP/live}
: ${IMAGE=$TOP/$NAME.img}
: ${UPPER=$TOP/root}
: ${WORK=$TOP/work}
: ${MOUNT=$TOP/mnt}
: ${NSNAME=$NAME}
: ${BRIDGES=lan_br}
: ${CONFIG=$TOP/config}

[ -e "$CONFIG" ] && . "$CONFIG"

cd "$SUBHOST" || exit 1

# Create a simple overlay subhost without its own image file
create_subhost() {
    mkdir -p $TARGET $MOUNT $UPPER $WORK
    [ -d "$OSROOT" ] || OSROOT=$SUBHOST/chimaera/base
    [ -e $CONFIG ] || cat <<EOF > "$CONFIG"
# Subhost $NAME is an autogenerated overlay subhost with shared filesystem
OSROOT="$OSROOT"
BRIDGES="$BRIDGES"
EOF
}

# map a string into a hexadecimal number with 10-digits by folding the given string into 9-digit ascii-code numbers and adding them
hexfoldsum() {
    local V
    V="$(b2sum -l 40 <<< "$1" | sed 's/\(..\)/\1:/g')"
    echo "02:${V:0:14}"
}

# Generate a mac address for given interface by passing hostname,
# subhost name and interface through 40-bit b2sum and with 02: prefix.
macaddr() {
    local V="$(b2sum -l 40 <<< "$(hostname)$NAME$1" | sed 's/\(..\)/\1:/g')"
    echo "02:${V:0:14}"
}

# setup the subhost network namespace and link up the host side
setup_network() {
    E=0
    ip netns add $NSNAME
    for BRIDGE in ${BRIDGES[@]} ; do
	brctl show $BRIDGE >& /dev/null || brctl addbr $BRIDGE
	IF=$NAME$E
	MAC=$(macaddr $IF)
	ip link add $IF type veth peer name eth$E address $MAC netns $NSNAME 
	ip link set $IF up
	[ -n "$BRIDGE" ] && brctl addif $BRIDGE $IF
	E=$((E+1))
    done
}

# check if $1 is mounted
is_mounted() {
    grep -qE "^[^ ]+ $1 " /proc/mounts
}

# Setup or re-setup the subhost filesystem
setup_rootfs() {
    if is_mounted $TARGET ; then
	mount -oremount $TARGET
    else
	if [ -e "$IMAGE" ] ; then
	    # $IMAGE is either an image file or a link to a partition,
	    # with /root and /work in it.
	    is_mounted $MOUNT || mount $IMAGE $MOUNT || exit 1
	    UPPER=$MOUNT/root
	    WORK=$MOUNT/work
	fi
	if [ -z "$OSROOT" ] ; then
	    # No overlay
	    mount --rbind $UPPER $TARGET
	else
	    # overlay of $UPPER (+$WORK) over $OSROOT onto $TARGET
	    mount -t overlay $NAME \
		  -olowerdir=$OSROOT,upperdir=$UPPER,workdir=$WORK \
		  $TARGET
	fi
    fi
}

case "$CMD" in
    start)
	[ -e "/run/netns/$NSNAME" ] || setup_network
	[ -d "$MOUNT" ] || create_subhost
	setup_rootfs
	START=/bin/bash
	[ -x $TARGET/startup ] && START=/startup
	exec ip netns exec $NSNAME unshare \
	     --fork --pid --mount-proc --kill-child \
	     --uts --ipc --mount --cgroup \
	     chroot $TARGET $START
	;;
    stop)
	umount $TARGET
	[ -e $IMAGE ] && umount $MOUNT
	ip netns del $NSNAME
	;;
    *)
	usage
	;;
esac