mobile-music/lib/remote/discovery.dart

import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'dart:typed_data';

import 'package:bonsoir/bonsoir.dart';
import 'package:flutter/foundation.dart' show kIsWeb;
import 'package:nsd/nsd.dart';

import 'messages.dart';

/// A host advertising itself on the LAN, resolved enough to connect to.
class DiscoveredDevice {
  const DiscoveredDevice({
    required this.name,
    required this.host,
    required this.port,
    required this.serverKey,
    required this.version,
  });

  final String name;
  final String host;
  final int port;

  /// The host's Subsonic [serverKey]; only matches are shown/controllable.
  final String serverKey;
  final int version;

  /// `ws://host:port` — the WebSocket endpoint `remote_session.dart` dials.
  Uri get wsUri => Uri(scheme: 'ws', host: host, port: port);

  /// Stable identity for list diffing / selection (mDNS instance names are
  /// unique per network).
  String get id => name;
}

/// mDNS registration (advertise) + discovery (browse) behind a small,
/// app-shaped API. One instance owns at most one active registration and one
/// active browse at a time.
///
/// Advertising and browsing are independent: a device that is playing
/// advertises so others can find it, and any device can browse to control one.
/// TXT records carry the [serverKey] and protocol version so the browser can
/// filter to same-server, same-version peers before ever opening a socket.
///
/// There are two backends behind this interface, chosen by [RemoteDiscovery]'s
/// factory: `nsd` (Bonjour/NSD) on Android/iOS/macOS/Windows, and `bonsoir`
/// (Avahi over D-Bus) on Linux — `nsd` has no Linux implementation. Both speak
/// the same DNS-SD wire format, so a Linux host and a phone controller
/// interoperate.
abstract class RemoteDiscovery {
  factory RemoteDiscovery() {
    if (!kIsWeb && Platform.isLinux) return _BonsoirDiscovery();
    return _NsdDiscovery();
  }

  /// Latest discovered, filtered device list (also replayed as a stream).
  List<DiscoveredDevice> get current;
  Stream<List<DiscoveredDevice>> get devices;

  bool get isAdvertising;
  bool get isBrowsing;

  /// Publish this device as a controllable player on [port]. Replaces any
  /// existing advertisement.
  Future<void> advertise({
    required String deviceName,
    required int port,
    required String serverKey,
  });

  Future<void> stopAdvertising();

  /// Start browsing for players on [serverKey]'s network. Emits filtered
  /// [DiscoveredDevice] lists on [devices]. Replaces any existing browse.
  Future<void> startBrowsing(String serverKey);

  Future<void> stopBrowsing();

  Future<void> dispose();
}

/// TXT-record keys shared by both backends' wire format.
const _kServerKey = 'sk';
const _kVersion = 'v';

/// Stream plumbing + peer filtering shared by both backends. Keeps the
/// same-server / same-version / not-ourselves rules in one place so the two
/// implementations only differ in how they talk to their platform.
mixin _DiscoveryState {
  final StreamController<List<DiscoveredDevice>> _devices =
      StreamController<List<DiscoveredDevice>>.broadcast();
  List<DiscoveredDevice> _current = const [];

  /// The final (possibly conflict-renamed) name of our own advertisement, so
  /// the browser can filter it out of its own results.
  String? _ownName;
  String? _browseServerKey;

  List<DiscoveredDevice> get current => _current;
  Stream<List<DiscoveredDevice>> get devices => _devices.stream;

  bool _accepts(DiscoveredDevice d) {
    if (_browseServerKey != null && d.serverKey != _browseServerKey) {
      return false; // different Subsonic server — its song ids won't resolve here
    }
    if (d.version != kRemoteProtocolVersion) return false;
    if (_ownName != null && d.name == _ownName) return false; // ourselves
    return true;
  }

  /// Filter [all] to controllable peers and publish the result.
  void _emit(Iterable<DiscoveredDevice> all) {
    _current = all.where(_accepts).toList();
    if (!_devices.isClosed) _devices.add(_current);
  }

  Future<void> _closeDevices() => _devices.close();
}

/// `nsd`-backed discovery for Android/iOS/macOS/Windows.
class _NsdDiscovery with _DiscoveryState implements RemoteDiscovery {
  Registration? _registration;
  Discovery? _discovery;
  ServiceListener? _listener;

  @override
  bool get isAdvertising => _registration != null;
  @override
  bool get isBrowsing => _discovery != null;

  // ---- Advertise --------------------------------------------------------

  @override
  Future<void> advertise({
    required String deviceName,
    required int port,
    required String serverKey,
  }) async {
    await stopAdvertising();
    final reg = await register(Service(
      name: deviceName,
      type: kRemoteServiceType,
      port: port,
      txt: {
        _kServerKey: _bytes(serverKey),
        _kVersion: _bytes('$kRemoteProtocolVersion'),
      },
    ));
    _registration = reg;
    _ownName = reg.service.name;
  }

  @override
  Future<void> stopAdvertising() async {
    final reg = _registration;
    _registration = null;
    _ownName = null;
    if (reg != null) {
      try {
        await unregister(reg);
      } catch (_) {
        // Already gone / platform hiccup — nothing to recover.
      }
    }
  }

  // ---- Browse -----------------------------------------------------------

  @override
  Future<void> startBrowsing(String serverKey) async {
    await stopBrowsing();
    _browseServerKey = serverKey;
    // v4 lookup so we get a routable address to dial rather than a bare
    // `.local` hostname the socket layer may not resolve.
    final discovery = await startDiscovery(
      kRemoteServiceType,
      ipLookupType: IpLookupType.v4,
    );
    _discovery = discovery;
    void listener(Service service, ServiceStatus status) => _rebuild();
    _listener = listener;
    discovery.addServiceListener(listener);
    _rebuild();
  }

  @override
  Future<void> stopBrowsing() async {
    final discovery = _discovery;
    final listener = _listener;
    _discovery = null;
    _listener = null;
    _browseServerKey = null;
    _current = const [];
    if (discovery != null) {
      if (listener != null) discovery.removeServiceListener(listener);
      try {
        await stopDiscovery(discovery);
      } catch (_) {
        // Already stopped.
      }
    }
  }

  @override
  Future<void> dispose() async {
    await stopAdvertising();
    await stopBrowsing();
    await _closeDevices();
  }

  void _rebuild() {
    final discovery = _discovery;
    if (discovery == null) return;
    _emit(discovery.services.map(_toDevice).whereType<DiscoveredDevice>());
  }

  DiscoveredDevice? _toDevice(Service s) {
    final port = s.port;
    if (port == null) return null;
    final host = _hostOf(s);
    if (host == null) return null;
    final txt = s.txt ?? const <String, Uint8List?>{};
    return DiscoveredDevice(
      name: s.name ?? host,
      host: host,
      port: port,
      serverKey: _text(txt, _kServerKey) ?? '',
      version: int.tryParse(_text(txt, _kVersion) ?? '') ?? 0,
    );
  }

  /// Prefer a resolved IPv4 address; fall back to the reported hostname.
  static String? _hostOf(Service s) {
    final addrs = s.addresses;
    if (addrs != null && addrs.isNotEmpty) {
      final v4 = addrs.firstWhere(
        (a) => a.type == InternetAddressType.IPv4,
        orElse: () => addrs.first,
      );
      return v4.address;
    }
    return s.host;
  }

  static Uint8List _bytes(String s) => Uint8List.fromList(utf8.encode(s));

  static String? _text(Map<String, Uint8List?> txt, String key) {
    final v = txt[key];
    if (v == null || v.isEmpty) return null;
    return utf8.decode(v, allowMalformed: true);
  }
}

/// `bonsoir`-backed discovery for Linux (Avahi over D-Bus). `nsd` ships no
/// Linux implementation, so this backend fills the gap while speaking the same
/// DNS-SD service type and TXT keys as [_NsdDiscovery].
///
/// Requires the Avahi daemon to be running; if it isn't, advertise/browse
/// simply surface no peers rather than crashing.
class _BonsoirDiscovery with _DiscoveryState implements RemoteDiscovery {
  BonsoirBroadcast? _broadcast;
  StreamSubscription<BonsoirBroadcastEvent>? _broadcastSub;

  BonsoirDiscovery? _discovery;
  StreamSubscription<BonsoirDiscoveryEvent>? _discoverySub;

  /// Resolved peers keyed by mDNS instance name; browsing emits found events
  /// (no address yet) followed by resolved/updated events that fill in the IP.
  final Map<String, DiscoveredDevice> _found = {};

  @override
  bool get isAdvertising => _broadcast != null;
  @override
  bool get isBrowsing => _discovery != null;

  // ---- Advertise --------------------------------------------------------

  @override
  Future<void> advertise({
    required String deviceName,
    required int port,
    required String serverKey,
  }) async {
    await stopAdvertising();
    final service = BonsoirService(
      name: deviceName,
      type: kRemoteServiceType,
      port: port,
      attributes: {
        _kServerKey: serverKey,
        _kVersion: '$kRemoteProtocolVersion',
      },
    );
    final broadcast = BonsoirBroadcast(service: service);
    await broadcast.initialize();
    // Avahi renames on conflict; track the name it actually published so the
    // browser can filter our own advertisement out.
    _ownName = service.name;
    _broadcastSub = broadcast.eventStream?.listen((event) {
      if (event is BonsoirBroadcastStartedEvent) {
        _ownName = event.service.name;
      }
    });
    await broadcast.start();
    _broadcast = broadcast;
  }

  @override
  Future<void> stopAdvertising() async {
    await _broadcastSub?.cancel();
    _broadcastSub = null;
    final broadcast = _broadcast;
    _broadcast = null;
    _ownName = null;
    if (broadcast != null) {
      try {
        await broadcast.stop();
      } catch (_) {
        // Already gone / Avahi hiccup — nothing to recover.
      }
    }
  }

  // ---- Browse -----------------------------------------------------------

  @override
  Future<void> startBrowsing(String serverKey) async {
    await stopBrowsing();
    _browseServerKey = serverKey;
    _found.clear();
    final BonsoirDiscovery discovery;
    try {
      discovery = BonsoirDiscovery(type: kRemoteServiceType);
      await discovery.initialize();
    } catch (_) {
      // Avahi unavailable (daemon not running / no D-Bus) — degrade to an
      // empty device list rather than surfacing an uncaught async error.
      _browseServerKey = null;
      return;
    }
    _discovery = discovery;
    _discoverySub = discovery.eventStream?.listen((event) {
      switch (event) {
        case BonsoirDiscoveryServiceFoundEvent():
          // Found but not yet resolved — ask for its address/TXT records.
          event.service.resolve(discovery.serviceResolver);
        case BonsoirDiscoveryServiceResolvedEvent(:final service) ||
              BonsoirDiscoveryServiceUpdatedEvent(:final service):
          final dev = _toDevice(service);
          if (dev != null) {
            _found[service.name] = dev;
            _emit(_found.values);
          }
        case BonsoirDiscoveryServiceLostEvent():
          _found.remove(event.service.name);
          _emit(_found.values);
        default:
          break;
      }
    });
    try {
      await discovery.start();
    } catch (_) {
      await stopBrowsing(); // tear down the half-started browse cleanly
      return;
    }
    _emit(_found.values);
  }

  @override
  Future<void> stopBrowsing() async {
    await _discoverySub?.cancel();
    _discoverySub = null;
    _browseServerKey = null;
    _found.clear();
    _current = const [];
    final discovery = _discovery;
    _discovery = null;
    if (discovery != null) {
      try {
        await discovery.stop();
      } catch (_) {
        // Already stopped.
      }
    }
  }

  @override
  Future<void> dispose() async {
    await stopAdvertising();
    await stopBrowsing();
    await _closeDevices();
  }

  DiscoveredDevice? _toDevice(BonsoirService s) {
    final host = _hostOf(s);
    if (host == null) return null; // unresolved (no address yet)
    return DiscoveredDevice(
      name: s.name,
      host: host,
      port: s.port,
      serverKey: s.attributes[_kServerKey] ?? '',
      version: int.tryParse(s.attributes[_kVersion] ?? '') ?? 0,
    );
  }

  /// Prefer a routable IPv4 address; Avahi may also report IPv6 (which contain
  /// colons) or link-local addresses we can't dial cleanly.
  static String? _hostOf(BonsoirService s) {
    final addrs = s.hostAddresses;
    if (addrs.isEmpty) return null;
    return addrs.firstWhere((a) => !a.contains(':'), orElse: () => addrs.first);
  }
}