#!/bin/bash

set -eu

function snap {
    unsnap

    mkdir -p /a

    dispatch snap < /etc/fstab
}

function unsnap {
    tac /etc/fstab | dispatch unsnap

    if [ -d "/a" ]; then
        rmdir /a
    fi
}

function dispatch {
    local action="" snaptype=""
    local dev="" mnt="" fstype="" opts="" dump=""

    while read -r dev mnt fstype opts dump pass; do
        snaptype="$(get_snaptype "$fstype" "$dump")"
        if [ -z "$snaptype" ]; then
            continue
        fi
        action="${1}_${snaptype}"
        echo "$action $mnt"
        "$action" "$dev" "$mnt" "$opts"
    done
}

function get_snaptype {
    local fstype="$1" dump="$2"
    if [ "$dump" = "1" ]; then
        echo bind
    elif [ "$dump" = "2" ] && [ "$fstype" = "btrfs" ]; then
        echo btrfs
    elif [ "$dump" = "2" ]; then
        echo lvm
    fi
}

function snap_bind {
    local mnt="$2"
    mount --bind "${mnt}" "/a${mnt}"
}

function unsnap_bind {
    local snapmnt="/a$2"
    if mountpoint -q "$snapmnt"; then
        umount "$snapmnt"
    fi
}

function snap_lvm {
    local dev="$1" mnt="$2"
    local lvname="" vgname=""
    read -r lvname vgname _ < <(lvdisplay --noheadings -C "$dev")
    echo "snapshot ${vgname}/${lvname}"
    lvcreate -L1G --snapshot --name "${lvname}snap" "${vgname}/${lvname}"
    mount -o ro "${dev}snap" "/a${mnt}"
}

function unsnap_lvm {
    local snapdev="${1}snap"
    local snapmnt="/a$2"
    if mountpoint -q "$snapmnt"; then
        umount "$snapmnt"
    fi
    if [ -b "$(readlink -f "$snapdev")" ]; then
        lvremove -f "${dev}snap"
    fi
}

function snap_btrfs {
    local mnt="$2" opts="$3" snapdir=""

    snapdir="$(get_btrfs_snapshot_dir "$opts" "$mnt")"
    btrfs subvolume snapshot "$mnt" "$snapdir"

    mount --bind "$snapdir" "/a${mnt}"
}

function unsnap_btrfs {
    local mnt="$2" opts="$3" snapdir=""

    local snapmnt="/a$2"
    if mountpoint -q "$snapmnt"; then
        umount "$snapmnt"
    fi

    snapdir="$(get_btrfs_snapshot_dir "$opts" "$mnt")"
    if [ -d "$snapdir" ]; then
        btrfs subvolume delete "$snapdir"
    fi
}

function get_btrfs_snapshot_dir {
    local opts="$1" mnt="$2" subvol=""
    subvol="$(echo "$opts" | sed -nE 's/^.*\bsubvol=([^,]+)\b.*$/\1/p')"
    if [ -z "$subvol" ]; then
        echo "Unknown subvol for mountpoint: $mnt"
        exit 1
    fi
    echo "/$(hostname -s)/${subvol}-snap"
}

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