Return host and port from devtools launcher (#63795)

packages/flutter_tools/doc/daemon.md

@@ -256,7 +256,11 @@ The returned `params` will contain:
 
 #### devtools.serve
 
-The `serve()` command starts a DevTools server if one isn't already running and returns the host and port of the server.
+The `serve()` command starts a DevTools server if one isn't already running. The return value will contain:
+
+- `success` - whether the server started.
+- `host` - the address host if the server successfully started.
+- `port` - the port if the server successfully started.
 
 ## 'flutter run --machine' and 'flutter attach --machine'
 

packages/flutter_tools/lib/src/commands/daemon.dart

@@ -897,11 +897,11 @@ class DevToolsDomain extends Domain {
 
   Future<Map<String, dynamic>> serve([ Map<String, dynamic> args ]) async {
     _devtoolsLauncher ??= DevtoolsLauncher.instance;
-    final HttpServer server = await _devtoolsLauncher.serve();
+    final DevToolsServerAddress server = await _devtoolsLauncher.serve();
 
     return<String, dynamic>{
-      'host': server.address.host,
-      'port': server.port,
+      'host': server?.host,
+      'port': server?.port,
     };
   }
 

packages/flutter_tools/lib/src/resident_runner.dart

@@ -1723,15 +1723,16 @@ class DevtoolsLauncher {
     }
   }
 
-  Future<io.HttpServer> serve() async {
+  Future<DevToolsServerAddress> serve() async {
     try {
       _devtoolsServer ??= await devtools_server.serveDevTools(
         enableStdinCommands: false,
       );
+      return DevToolsServerAddress(_devtoolsServer.address.host, _devtoolsServer.port);
     } on Exception catch (e, st) {
       globals.printTrace('Failed to serve DevTools: $e\n$st');
+      return null;
     }
-    return _devtoolsServer;
   }
 
   Future<void> close() async {
@@ -1741,3 +1742,10 @@ class DevtoolsLauncher {
 
   static DevtoolsLauncher get instance => context.get<DevtoolsLauncher>() ?? DevtoolsLauncher();
 }
+
+class DevToolsServerAddress {
+  DevToolsServerAddress(this.host, this.port);
+
+  final String host;
+  final int port;
+}

packages/flutter_tools/test/commands.shard/hermetic/daemon_test.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:io';
 
 import 'package:flutter_tools/src/android/android_workflow.dart';
 import 'package:flutter_tools/src/base/common.dart';
@@ -304,7 +303,7 @@ void main() {
       await input.close();
     });
 
-    testUsingContext('devtools.serve command should return host and port', () async {
+    testUsingContext('devtools.serve command should return host and port on success', () async {
       final StreamController<Map<String, dynamic>> commands = StreamController<Map<String, dynamic>>();
       final StreamController<Map<String, dynamic>> responses = StreamController<Map<String, dynamic>>();
       daemon = Daemon(
@@ -312,19 +311,34 @@ void main() {
         responses.add,
         notifyingLogger: notifyingLogger,
       );
-      final HttpServer mockDevToolsServer = MockDevToolsServer();
-      final InternetAddress mockInternetAddress = MockInternetAddress();
-      when(mockDevToolsServer.address).thenReturn(mockInternetAddress);
-      when(mockInternetAddress.host).thenReturn('127.0.0.1');
-      when(mockDevToolsServer.port).thenReturn(1234);
+      when(mockDevToolsLauncher.serve()).thenAnswer((_) async => DevToolsServerAddress('127.0.0.1', 1234));
 
-      when(mockDevToolsLauncher.serve()).thenAnswer((_) async => mockDevToolsServer);
+      commands.add(<String, dynamic>{'id': 0, 'method': 'devtools.serve'});
+      final Map<String, dynamic> response = await responses.stream.firstWhere((Map<String, dynamic> response) => response['id'] == 0);
+      expect(response['result'], isNotEmpty);
+      expect(response['result']['host'], '127.0.0.1');
+      expect(response['result']['port'], 1234);
+      await responses.close();
+      await commands.close();
+    }, overrides: <Type, Generator>{
+      DevtoolsLauncher: () => mockDevToolsLauncher,
+    });
+
+    testUsingContext('devtools.serve command should return null fields if null returned', () async {
+      final StreamController<Map<String, dynamic>> commands = StreamController<Map<String, dynamic>>();
+      final StreamController<Map<String, dynamic>> responses = StreamController<Map<String, dynamic>>();
+      daemon = Daemon(
+        commands.stream,
+        responses.add,
+        notifyingLogger: notifyingLogger,
+      );
+      when(mockDevToolsLauncher.serve()).thenAnswer((_) async => null);
 
       commands.add(<String, dynamic>{'id': 0, 'method': 'devtools.serve'});
       final Map<String, dynamic> response = await responses.stream.firstWhere((Map<String, dynamic> response) => response['id'] == 0);
       expect(response['result'], isNotEmpty);
-      expect(response['result']['host'], equals('127.0.0.1'));
-      expect(response['result']['port'], equals(1234));
+      expect(response['result']['host'], null);
+      expect(response['result']['port'], null);
       await responses.close();
       await commands.close();
     }, overrides: <Type, Generator>{