#!/usr/bin/env bash
set -euo pipefail

# dataset to backup
export ZFS_DATASET="my-pool/data"

# destroy leftover snapshots, usually already cleaned up
if [ -z "${EXEC_UNSHARED:-}" ]; then
  for snap in $(zfs list -rt snap -Ho name "${ZFS_DATASET}"); do
    if [[ "${snap}" =~ @restic-([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})$ ]]; then
      zfs destroy "${snap}" && echo "leftover zfs snapshot '${snap}' destroyed"
    fi
  done
fi

# create temporary snapshot for backup
if [[ -z "${ZFS_SNAP:-}" ]]; then
  ZFS_SNAP="restic-$(cat /proc/sys/kernel/random/uuid)"
  zfs snap -r "${ZFS_DATASET}@${ZFS_SNAP}"

  # clean up temporary snapshot on exit
  _clean_snap() {
    zfs destroy -r "${ZFS_DATASET}@${ZFS_SNAP}" && echo "zfs snapshot '${ZFS_DATASET}@${ZFS_SNAP}' destroyed"
  }
  trap _clean_snap EXIT

  export ZFS_SNAP
fi

# mount snapshot and backup within private mount namespace
if [ -z "${EXEC_UNSHARED:-}" ]; then
  export EXEC_UNSHARED=1
  echo "re-executing '$0' in new private mount namespace.."
  unshare --mount --propagation private "$0"
  exit 0
fi

# use tmpfs for further mountpoints
root_mnt="${TMPDIR:-/var/tmp}/zsnapmounts"
mount -t tmpfs -o X-mount.mkdir tmpfs "${root_mnt}"

# mount snapshot of (sub) datasets
snap_mnts=()
for ds in $(zfs list -r -Ho name "${ZFS_DATASET}"); do
  if [[ "$(zfs get -Ho value mountpoint "$ds")" == "none" ]]; then
    continue
  fi
  mount -t zfs -o X-mount.mkdir "$ds@${ZFS_SNAP}" "${root_mnt}/$ds"
  snap_mnts+=( "$ds" )
  echo "mounted dataset '$ds@${ZFS_SNAP}'"
done

# backup temporary snapshot
echo -e "start restic backup\n"
pushd "${root_mnt}" > /dev/null
restic backup -v \
  --exclude-caches \
  "${snap_mnts[@]}"

echo -e "\nrestic backup done"