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

export LC_ALL=C
export PATH=/usr/sbin:/usr/bin:/sbin:/bin

if [[ $# -ne 1 ]]; then
  printf 'Usage: %s <policy>\n' "$0" >&2
  exit 64
fi

readonly pool={{ atlas_zfs_pool | quote }}
readonly prefix={{ atlas_zfs_snapshot_prefix | quote }}
readonly period="$1"

case "$period" in
{% for policy in atlas_zfs_snapshot_policies %}
  {{ policy.name | quote }})
    keep={{ policy.keep | int }}
    ;;
{% endfor %}
  *)
    printf 'Unknown Atlas ZFS snapshot policy: %s\n' "$period" >&2
    exit 64
    ;;
esac
readonly keep

zpool list -H -o name "$pool" >/dev/null

exec 9>/run/lock/atlas-zfs-snapshot.lock
flock 9

timestamp="$(date -u +%Y%m%dT%H%M%SZ)"
readonly timestamp
readonly snapshot_prefix="${pool}@${prefix}-${period}-"
readonly snapshot="${snapshot_prefix}${timestamp}"

zfs snapshot -r "$snapshot"
printf 'Created recursive ZFS snapshot %s\n' "$snapshot"

snapshot_listing="$(zfs list -H -t snapshot -o name -s creation -r "$pool")"
managed_snapshots=()
while IFS= read -r snapshot_name; do
  if [[ "$snapshot_name" == "$snapshot_prefix"* ]]; then
    snapshot_suffix="${snapshot_name#"$snapshot_prefix"}"
    if [[ "$snapshot_suffix" =~ ^[0-9]{8}T[0-9]{6}Z$ ]]; then
      managed_snapshots+=("$snapshot_name")
    fi
  fi
done <<< "$snapshot_listing"

{% raw %}
managed_snapshot_count="${#managed_snapshots[@]}"
{% endraw %}
prune_count=$((managed_snapshot_count - keep))
if ((prune_count <= 0)); then
  printf 'Retaining %d of %d managed %s snapshots\n' \
    "$managed_snapshot_count" "$keep" "$period"
  exit 0
fi

for ((index = 0; index < prune_count; index++)); do
  candidate="${managed_snapshots[$index]}"
  if [[ "$candidate" != "$snapshot_prefix"* ]]; then
    printf 'Refusing to destroy unexpected snapshot: %s\n' "$candidate" >&2
    exit 65
  fi

  zfs destroy -r "$candidate"
  printf 'Pruned recursive ZFS snapshot %s\n' "$candidate"
done