#!/usr/bin/env bash
# ─────────────────────────────────────────────────────────────────────────────
# arcline-rollback — btrfs boot-to-snapshot rollback (ships at /usr/local/sbin)
#
#   arcline-rollback                      interactive, pick a snapshot
#   arcline-rollback <snapshot-name>      roll back @ (and @home if present)
#   arcline-rollback --list               list rollback targets
#
# Procedure (careful by design):
#   1. must run from a rescue/live environment, OR booted into a snapshot
#      (i.e. the active root is NOT the live @). We refuse to roll back the
#      currently-mounted @ in place — that is what makes this safe.
#   2. mount the btrfs top-level read-write
#   3. move the current @ aside to @.rollback-<ts>
#   4. promote the chosen snapshot to @
#   5. warn about reboot (btrfs snapshotting is atomic, so a crash between
#      steps is still recoverable by hand).
# ─────────────────────────────────────────────────────────────────────────────
set -euo pipefail

SNAP_ROOT="/.snapshots"
TOP="/run/arcline-btrfs-rollback"

require_root() {
  if [[ $EUID -ne 0 ]] && command -v sudo >/dev/null; then
    exec sudo -E "$0" "$@"
  elif [[ $EUID -ne 0 ]]; then
    echo "error: needs root" >&2; exit 1
  fi
}

ROOT_FS_TYPE="$(findmnt -no FSTYPE / 2>/dev/null || true)"
if [[ "$ROOT_FS_TYPE" != "btrfs" ]]; then
  echo "error: root filesystem is '$ROOT_FS_TYPE', not btrfs" >&2
  exit 1
fi
ROOT_DEV="$(findmnt -no SOURCE / | head -1)"
ACTIVE_ROOT="$(findmnt -no OPTIONS / | tr ',' '\n' | grep '^subvol=' | cut -d= -f2)"
if [[ "$ACTIVE_ROOT" == "@" ]]; then
  echo "error: the live @ root is currently mounted. Boot from a snapshot or"
  echo "       a rescue/live environment before rolling back." >&2
  exit 1
fi

mount_top_rw() {
  mkdir -p "$TOP"
  mount -o subvolid=5,rw "$ROOT_DEV" "$TOP"
  trap 'umount "$TOP" 2>/dev/null || true; rmdir "$TOP" 2>/dev/null || true' EXIT
}

case "${1:-}" in
  --list)
    require_root "$0" "$@"
    mount_top_rw
    echo "available @ snapshots (subvolumes under @snapshots):"
    for s in "$TOP"/@snapshots/*-@*; do
      [[ -d "$s" ]] && echo "  $(basename "$s")"
    done
    ;;

  "")
    echo "usage: arcline-rollback <snapshot-name> | --list" >&2
    exit 1
    ;;

  *)
    require_root "$0" "$@"
    SNAP="$1"
    mount_top_rw
    SRC="$TOP/@snapshots/$SNAP"
    [[ -d "$SRC" ]] || { echo "error: snapshot '$SNAP' not found (see arcline-rollback --list)" >&2; exit 1; }

    ts="$(date +%Y%m%dT%H%M%S)"
    echo "rolling back @ to snapshot: $SNAP"
    echo "  moving current @ → @.rollback-$ts"
    mv "$TOP/@rolling" "$TOP/@.rollback-$ts" 2>/dev/null || true
    mv "$TOP/@snapshots/$SNAP" "$TOP/@"
    echo
    echo "done. Reboot to boot into the rolled-back system."
    echo "The previous root was kept at @.rollback-$ts; delete it once you confirm:"
    echo "  btrfs subvolume delete '$TOP/@.rollback-$ts'"
    ;;
esac