#!/bin/bash

set -eu

function snap {
    unsnap

    mkdir -p /a

    cat /etc/fstab | awk '{print $5 " " $1 " " $2;}' | while read -r dump dev mnt; do
        if [ "$dump" = 1 ]; then
            echo "bind mount $mnt"
            mount --bind "${mnt}" "/a${mnt}"
        elif [ "$dump" = 2 ]; then
            read -r lvname vgname _ < <(lvdisplay --noheadings -C "$dev")
            echo "snapshot ${vgname}/${lvname}"
            lvcreate -L1G --snapshot --name "${lvname}snap" "${vgname}/${lvname}"
            mount -o ro "${dev}snap" "/a${mnt}"
        fi
    done
}

function unsnap {
    tac /etc/fstab | awk '{print $5 " " $1 " " $2;}' | while read -r dump dev mnt; do
        snapmnt="/a${mnt}"
        snapdev="${dev}snap"
        if [ "$dump" = 1 ]; then
            echo "umount $snapmnt"
            mountpoint -q "$snapmnt" && umount "$snapmnt"
        elif [ "$dump" = 2 ]; then
            echo "umount $snapmnt"
            mountpoint -q "$snapmnt" && umount "$snapmnt"

            echo "unsnapshot $snapdev"
            [ -b "$(readlink -f "$snapdev")" ] && echo y | lvremove "${dev}snap"
        fi
    done

    rmdir /a || true
}

if [ "$(id -u)" -ne 0 ]; then
    echo "Must be root"
    exit 1
fi

if [ "${1:-}" = "off" ]; then
    unsnap
elif [ "${1:-}" = "on" ]; then
    snap
else
    echo "Usage: $0 on|off"
    exit 1
fi