#!/usr/bin/env bash
# Updates NixOS from a throwaway worktree of this repo, so the checkout you edit in
# is left alone until the update is done. Started from the "Desktop" half of the
# control centre's update tile in a kitty window (installed as `nixos-update`,
# modules/home/packages.nix, which sets DOT_REPO).
#
#   1. a temporary worktree of main
#   2. nix flake update, committed there
#   3. nixos-rebuild build, then nvd's list of what changed against the running system
#   4. asks before going on; then pushes the commit, installs it as the next boot
#      entry (nixos-rebuild boot), brings your checkout's main up to date and reboots
#
# If anything fails, the window stays open until you press Enter, and nothing is
# rebooted.
set -E

repo=${DOT_REPO:-$HOME/git/dot}
tmp=""

step() { printf '\n\e[1;34m==> %s\e[0m\n' "$*"; }
note() { printf '\e[1;33m%s\e[0m\n' "$*"; }
wait_close() { read -rp "Press Enter to close. " _ || true; }

die() {
  trap - ERR
  printf '\n\e[1;31m%s\e[0m\n' "$*"
  wait_close
  exit 1
}

cleanup() {
  if [ -n "$tmp" ]; then
    git -C "$repo" worktree remove --force "$tmp" 2>/dev/null || true
    rm -rf "$tmp"
    tmp=""
  fi
}

trap 'die "The update failed (see above). Nothing was rebooted."' ERR
trap cleanup EXIT

step "Fetching $repo"
git -C "$repo" worktree prune
git -C "$repo" fetch origin main
git -C "$repo" merge-base --is-ancestor origin/main main ||
  die "main in $repo is behind origin/main (or has diverged); pull first."
base=$(git -C "$repo" rev-parse main)
ahead=$(git -C "$repo" rev-list --count origin/main..main)
dirty=$(git -C "$repo" status --porcelain)

tmp=$(mktemp -d -t dot-update.XXXXXX)
step "Creating a worktree of main in $tmp"
git -C "$repo" worktree add --detach "$tmp" main
cd "$tmp" || die "Could not enter $tmp."

step "Updating flake inputs"
nix flake update --commit-lock-file
if [ "$(git rev-parse HEAD)" = "$base" ]; then
  printf '\nEverything is already up to date.\n'
  wait_close
  exit 0
fi

step "Building"
nixos-rebuild build --flake "$tmp"

step "What changes"
git --no-pager log -1 --format=%b
nvd diff /run/current-system "$tmp/result"

echo
[ -z "$dirty" ] || note "Uncommitted changes in $repo are not part of this update."
[ "$ahead" -eq 0 ] || note "main has $ahead commit(s) not on GitHub yet; they are pushed too."
read -rp "Commit, push, install and reboot? [y/N] " answer || answer=""
case $answer in
  [yY]*) ;;
  *)
    echo "Nothing was changed."
    sleep 1
    exit 0
    ;;
esac

step "Pushing"
git push origin HEAD:main

step "Installing as the next boot entry"
nixos-rebuild boot --sudo --flake "$tmp"

step "Updating main in $repo"
commit=$(git rev-parse HEAD)
if [ "$(git -C "$repo" symbolic-ref --quiet --short HEAD)" = main ]; then
  git -C "$repo" merge --ff-only "$commit"
else
  git -C "$repo" update-ref refs/heads/main "$commit" "$base"
fi
cd "$repo"
cleanup

trap 'printf "\nReboot cancelled. The update boots next time.\n"; wait_close; exit 0' INT
step "Rebooting in 5 seconds (Ctrl+C to cancel)"
sleep 5
systemctl reboot