[multicast_dns] MDnsClient::listen supports onError callback (#8888)

MDnsClient::Listen now supports an optional onError callback function,
called in case of a stream error. If omitted any errors on the stream
are considered unhandled, and will be passed to the current [Zone]'s
error handler. By default unhandled async errors are treated as if they
were uncaught top-level errors.

This fixes an unhandled exception occuring (tested on Android), when the
network is disconnected.

Fixes flutter/flutter#165482

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

How can a test be added? It would need  to disconnect the device from WLAN beforehand.

Author: alexander-irion

packages/multicast_dns/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.3
+
+* Adds an optional error callback for `MDnsClient::start` to prevent uncaught exceptions.
+
 ## 0.3.2+8
 
 * Fixes stack overflows ocurring during the parsing of domain names in MDNS messages.

packages/multicast_dns/lib/multicast_dns.dart

@@ -84,13 +84,18 @@ class MDnsClient {
   /// for the mDNS query. If not provided, defaults to either `224.0.0.251` or
   /// or `FF02::FB`.
   ///
+  /// If provided, [onError] will be called in case of a stream error. If
+  /// omitted any errors on the stream are considered unhandled, and will be
+  /// passed to the current [Zone]'s error handler.
+  ///
   /// Subsequent calls to this method are ignored while the mDNS client is in
   /// started state.
   Future<void> start({
     InternetAddress? listenAddress,
     NetworkInterfacesFactory? interfacesFactory,
     int mDnsPort = mDnsPort,
     InternetAddress? mDnsAddress,
+    Function? onError,
   }) async {
     listenAddress ??= InternetAddress.anyIPv4;
     interfacesFactory ??= allInterfacesFactory;
@@ -152,7 +157,10 @@ class MDnsClient {
       // Join multicast on this interface.
       incoming.joinMulticast(_mDnsAddress!, interface);
     }
-    incoming.listen((RawSocketEvent event) => _handleIncoming(event, incoming));
+    incoming.listen(
+      (RawSocketEvent event) => _handleIncoming(event, incoming),
+      onError: onError,
+    );
     _started = true;
     _starting = false;
   }

packages/multicast_dns/pubspec.yaml

@@ -2,7 +2,7 @@ name: multicast_dns
 description: Dart package for performing mDNS queries (e.g. Bonjour, Avahi).
 repository: https://github.com/flutter/packages/tree/main/packages/multicast_dns
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+multicast_dns%22
-version: 0.3.2+8
+version: 0.3.3
 
 environment:
   sdk: ^3.4.0

packages/multicast_dns/test/client_test.dart

@@ -158,6 +158,35 @@ void main() {
       });
     }
   });
+
+  test('Calls onError callback in case of socket error', () async {
+    final FakeRawDatagramSocketThatSendsError datagramSocket =
+        FakeRawDatagramSocketThatSendsError();
+    final MDnsClient client = MDnsClient(
+      rawDatagramSocketFactory: (
+        dynamic host,
+        int port, {
+        bool reuseAddress = true,
+        bool reusePort = true,
+        int ttl = 1,
+      }) async {
+        return datagramSocket;
+      },
+    );
+
+    final Completer<void> onErrorCalledCompleter = Completer<void>();
+    await client.start(
+      mDnsPort: 1234,
+      interfacesFactory: (InternetAddressType type) async =>
+          <NetworkInterface>[],
+      onError: (Object e) {
+        expect(e, 'Error');
+        onErrorCalledCompleter.complete();
+      },
+    );
+
+    await onErrorCalledCompleter.future.timeout(const Duration(seconds: 5));
+  });
 }
 
 class FakeRawDatagramSocket extends Fake implements RawDatagramSocket {
@@ -196,6 +225,27 @@ class FakeRawDatagramSocket extends Fake implements RawDatagramSocket {
   }
 }
 
+class FakeRawDatagramSocketThatSendsError extends Fake
+    implements RawDatagramSocket {
+  @override
+  InternetAddress address = InternetAddress.anyIPv4;
+
+  @override
+  StreamSubscription<RawSocketEvent> listen(
+    void Function(RawSocketEvent event)? onData, {
+    Function? onError,
+    void Function()? onDone,
+    bool? cancelOnError,
+  }) {
+    return Stream<RawSocketEvent>.error('Error').listen(
+      onData,
+      onError: onError,
+      cancelOnError: cancelOnError,
+      onDone: onDone,
+    );
+  }
+}
+
 class FakeNetworkInterface implements NetworkInterface {
   FakeNetworkInterface(this._name, this._addresses, this._index);