Re-land "Extend the mDNS package with a native extension used on Mac OS"

pkg/mdns/lib/mdns.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2015, the Fletch project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
+// BSD-style license that can be found in the LICENSE.md file.
+
+library mdns;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:io';
 import 'dart:typed_data';
 
-import 'package:mdns/src/constants.dart';
-import 'package:mdns/src/lookup_resolver.dart';
-import 'package:mdns/src/packet.dart';
+import 'package:mdns/src/native_extension_client.dart';
+import 'package:mdns/src/native_protocol_client.dart';
 
 /// Client for DNS lookup using the mDNS protocol.
 ///
 /// This client only support "One-Shot Multicast DNS Queries" as described in
 /// section 5.1 of https://tools.ietf.org/html/rfc6762
-class MDnsClient {
-  bool _starting = false;
-  bool _started = false;
-  RawDatagramSocket _incoming;
-  final List<RawDatagramSocket> _sockets = <RawDatagramSocket>[];
-  final LookupResolver _resolver = new LookupResolver();
+abstract class MDnsClient {
+  // Instantiate Client for DNS lookup using the mDNS protocol.
+  //
+  // On Mac OS a native extension is used as the mDNSResponder opens the mDNS
+  // port in exclusive mode. To test the protocol implementation on Mac OS
+  // one can turn off mDNSResponder:
+  //
+  // sudo launchctl unload -w \
+  //     /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
+  //
+  // And turn it on again:
+  //
+  // sudo launchctl load -w \
+  //    /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
+  factory MDnsClient() {
+    if (Platform.isMacOS) {
+      return new NativeExtensionMDnsClient();
+    } else {
+      return new NativeProtocolMDnsClient();
+    }
+  }
 
   /// Start the mDNS client.
-  Future start() async {
-    if (_started && _starting) {
-      throw new StateError('mDNS client already started');
-    }
-    _starting = true;
-
-    // Listen on all addresses.
-    _incoming = await RawDatagramSocket.bind(
-        InternetAddress.ANY_IP_V4, mDnsPort, reuseAddress: true);
-
-    // Find all network interfaces with an IPv4 address.
-    var interfaces =
-        await NetworkInterface.list(type: InternetAddressType.IP_V4);
-    for (NetworkInterface interface in interfaces) {
-      // Create a socket for sending on each adapter.
-      var socket = await RawDatagramSocket.bind(
-          interface.addresses[0], mDnsPort, reuseAddress: true);
-      _sockets.add(socket);
-
-      // Join multicast on this interface.
-      _incoming.joinMulticast(mDnsAddress, interface);
-    }
-    _incoming.listen(_handleIncoming);
-
-    _starting = false;
-    _started = true;
-  }
+  Future start();
 
   /// Stop the mDNS client.
-  void stop() {
-    if (!_started) return;
-    if (_starting) {
-      throw new StateError('Cannot stop mDNS client wile it is starting');
-    }
-
-    _sockets.forEach((socket) => socket.close());
-    _incoming.close();
-
-    _started = false;
-  }
+  void stop();
 
   /// Lookup [hostname] using mDNS.
   ///
   /// The `hostname` must have the form `single-dns-label.local`,
   /// e.g. `printer.local`.
   ///
   /// If no answer has been received within the specified [timeout]
   /// this method will complete with the value `null`.
   Future<InternetAddress> lookup(
-      String hostname, {Duration timeout: const Duration(seconds: 5)}) {
-    if (!_started) {
-      throw new StateError('mDNS client is not started');
-    }
-
-    // Add the pending request before sending the query.
-    var future = _resolver.addPendingRequest(hostname, timeout);
-
-    // Send the request on all interfaces.
-    List<int> packet = encodeMDnsQuery(hostname);
-    for (int i = 0; i < _sockets.length; i++) {
-      _sockets[i].send(packet, mDnsAddress, mDnsPort);
-    }
-
-    return future;
-  }
-
-  // Process incoming datagrams.
-  _handleIncoming(event) {
-    if (event == RawSocketEvent.READ) {
-      var data = _incoming.receive();
-      var response = decodeMDnsResponse(data.data);
-      if (response != null) {
-        _resolver.handleResponse(response);
-      }
-    }
-  }
+      String hostname, {Duration timeout: const Duration(seconds: 5)});
 }
 
 // Simple standalone test.
-main() async {
+Future main(List<String> args) async {
   var client = new MDnsClient();
   await client.start();
-  var address = await client.lookup('raspberrypi.local');
+  var address = await client.lookup(args[0]);
   client.stop();
   print(address);
 }