RawDatagramSocket stopped working on iOS 17.4.1

[an-mediola]

The RawDatagramSocket stopped working on iOS 17.4.1.
To reproduce, I've made a simple app and run it on 4 different iphones.

import 'dart:io';

import 'package:flutter/material.dart';

void main() async {
  runApp(const MaterialApp(home: UdpTest()));
}


class UdpTest extends StatefulWidget {
  const UdpTest({super.key});

  @override
  State<UdpTest> createState() => _UdpTestState();
}

class _UdpTestState extends State<UdpTest> {
  late UdpCommunication _udpCommunication;

  final List _messages = [];

  @override
  void initState() {
    _udpCommunication = UdpCommunication();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: Column(
          children: [
            ElevatedButton(
                child: const Text('Init'),
                onPressed: () async {
                  await _udpCommunication.initializeSocket();
                  _udpCommunication.listenForMessages((message) => setState(() {
                        _messages.add(message);
                      }));
                }),
            ElevatedButton(
              child: const Text('Send broadcast message'),
              onPressed: () {
                _udpCommunication.sendMessage('GET\n', InternetAddress('239.255.255.250'), 1901);
                _udpCommunication.sendMessage('GET\n', InternetAddress('255.255.255.255'), 1901);
              },
            ),
            ListView.builder(
                shrinkWrap: true,
                itemBuilder: (context, index) => ListTile(
                      title: Text(_messages[index]),
                    ),
                itemCount: _messages.length),
          ],
        ),
      ),
    );
  }
}

class UdpCommunication {
  late RawDatagramSocket udpSocket;

  Future<void> initializeSocket() async {
    udpSocket = await RawDatagramSocket.bind(InternetAddress.anyIPv4, 0, reuseAddress: true, reusePort: true);
    print('socket bound to ${udpSocket.address}:${udpSocket.port}');

    udpSocket.broadcastEnabled = true;

    // join multicast group
    udpSocket.joinMulticast(InternetAddress('239.255.255.250'));

    // on error log it
    udpSocket.handleError((error) {
      print('Error: $error');
    });
  }

  void sendMessage(String message, InternetAddress address, int port) {
    udpSocket.send(message.codeUnits, address, port);
  }

  void listenForMessages(Function(String) onMessageReceived) {
    udpSocket.listen((RawSocketEvent event) {
      print(event);

      if (event == RawSocketEvent.read) {
        Datagram? datagram = udpSocket.receive();
        if (datagram != null) {
          String message = String.fromCharCodes(datagram.data);
          onMessageReceived(message);
        }
      }
    });
  }
}

Error on iphone 17.4.1:

flutter: socket bound to InternetAddress('0.0.0.0', IPv4):59136
flutter: RawSocketEvent.write
[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: SocketException: Send failed (OS Error: No route to host, errno = 65), address = 0.0.0.0, port = 59136
#0      _NativeSocket.send (dart:io-patch/socket_patch.dart:1269:34)
#1      _RawDatagramSocket.send (dart:io-patch/socket_patch.dart:2582:15)
#2      UdpCommunication.sendMessage (package:mediolaconnect/upd_socket_test.dart:76:15)
#3      _UdpTestState.build.<anonymous closure> (package:mediolaconnect/upd_socket_test.dart:40:35)
#4      _InkResponseState.handleTap (package:flutter/src/material/ink_well.dart:1183:21)
#5      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:315:24)
#6      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:652:11)
#7      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:309:5)
#8      BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:242:7)
#9      PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:670:9)
#10     PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:98:12)
#11     PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:143:9)
#12     _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:633:13)
#13     PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:141:18)
#14     PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:127:7)
#15     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:495:19)
#16     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:475:22)
#17     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:430:11)
#18     GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:420:7)
#19     GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:383:5)
#20     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:330:7)
#21     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:299:9)

Flutter doctor

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.19.6, on macOS 13.6 22G120 darwin-arm64 (Rosetta), locale en)
[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
[✓] Xcode - develop for iOS and macOS (Xcode 15.0)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2023.1)
[✓] VS Code (version 1.88.1)
[✓] Connected device (5 available)
[✓] Network resources

• No issues found!

I'd greatly appreciate assistance and guidance from someone on the core team in narrowing down the scope of debugging.

maybe related to #45824

[an-mediola]

Restarting the iPhone fixed it!
For now it isn't reproducible.

[an-mediola]

Have you found the cause of the problem? I can reproducible this issue.

unfortunately no, it seems related to OS not dart. restarting the phone didn't help?

[an-mediola]

Yes, restarting the phone socket to send data will return to normal (socket.send returns > 0), but I want to know what causes this so that we can prevent it from happening

Then if you know or you can find a reliable way to reproduce it, you can write it here for others and then we will reopen it.
I couldn't reproduce it after restarting the phone and then is not possible for others to investigate the reason.
Maybe somebody from core team can help us.