diet-guard/app/lib/services/firebase_backend.dart

/// Wiring for the Firebase backend, during and after the GitHub cutover.
///
/// Split by what is safe to publish, because this repo is public:
///
/// * [kProject] holds the Web API key and database URL. Both are public
///   identifiers that already ship inside the APK; the security rules, not
///   their secrecy, are what protect the data.
/// * The account email and password are entered once per device and kept in
///   the OS keystore, next to the GitHub token this app already stores there.
///
/// Nothing here reads `~/.config/crdt-sync/` — that is the desktop/Python
/// half. On Android there is no such file.
library;

import 'package:crdt_sync/crdt_sync.dart';
import 'package:flutter_secure_storage/flutter_secure_storage.dart';

/// The shared `kuhy-syncs` project.
///
/// `databaseUrl` is the **regional** host. The plain `*.firebaseio.com` form
/// answers 404 with a `correctUrl` body rather than an obvious error, which
/// reads like an auth failure and wastes a debugging session.
const kProject = FirebaseProject(
  apiKey: 'AIzaSyCF_sA3xCMehAYXK8eND-rAygb9NXXW_8E',
  databaseUrl:
      'https://kuhy-syncs-default-rtdb.europe-west1.firebasedatabase.app',
);

/// Same options the GitHub token uses: off Android's deprecated
/// `encryptedSharedPreferences` path, and libsecret on Linux.
const _secure = FlutterSecureStorage();

// Everything below reaches the OS keystore through a platform channel, which
// `flutter test` has no binding for -- the same reason `main.dart` and
// `openRepository()` are excluded. The logic these wrap (parsing, the
// public/private split, sign-in) lives in `crdt_sync` and is covered there at
// 100%; what is left here is the two-line adapter.
// coverage:ignore-start

/// The keystore-backed home for the Firebase refresh token.
SecureCredentialStore credentialStore() => SecureCredentialStore(
  read: (key) => _secure.read(key: key),
  write: (key, value) => _secure.write(key: key, value: value),
  delete: (key) => _secure.delete(key: key),
);

/// Reads the per-device account, or null when sync has not been set up.
Future<FirebaseAccount?> loadAccount() async {
  try {
    return FirebaseAccount.tryParse(
      await _secure.read(key: kFirebaseAccountKey),
    );
  } on Object {
    // No secret service available: behave as "not configured" rather than
    // crashing the settings screen.
    return null;
  }
}

/// Stores the per-device account. Keystore only — never prefs, never source.
Future<void> saveAccount(FirebaseAccount account) =>
    _secure.write(key: kFirebaseAccountKey, value: account.toJsonString());

/// Forgets the account and any cached session.
Future<void> clearAccount() async {
  await _secure.delete(key: kFirebaseAccountKey);
  await credentialStore().clear();
}

/// Returns a signed-in Firebase client, or null when not configured.
///
/// Signs in with the stored password only when there is no cached refresh
/// token, so the usual path costs no authentication round trip.
Future<FirebaseRestClient?> openFirebase() async {
  final account = await loadAccount();
  if (account == null) return null;
  try {
    return await firebaseClientFor(
      config: kProject.configFor(account.email),
      store: credentialStore(),
      password: account.password,
    );
  } on Object {
    // Unreachable keystore or a rejected sign-in must not fail the tick: the
    // app keeps syncing over GitHub, exactly as before the cutover.
    // Broader than Exception because an uninitialised platform binding
    // raises a FlutterError, which is an Error.
    return null;
  }
}

// coverage:ignore-end

/// Wraps [github] so Firebase is primary and GitHub is kept as a mirror.
///
/// Returns [github] unchanged when this device has not been set up for
/// Firebase — a normal state during the cutover, not an error, and also the
/// rollback. Reads union both, so a device that has not moved yet still
/// converges.
Future<RemoteStore> syncBackend(RemoteStore github) async {
  final firebase = await openFirebase();
  return firebase == null
      ? github
      : MirrorStore(primary: firebase, mirror: github);
}