minor cleanup and prevent multiple exit (#39751)

packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart

@@ -73,6 +73,7 @@ class ResidentWebRunner extends ResidentRunner {
   WebFs _webFs;
   DebugConnection _debugConnection;
   StreamSubscription<vmservice.Event> _stdOutSub;
+  bool _exited = false;
 
   vmservice.VmService get _vmService => _debugConnection.vmService;
 
@@ -101,9 +102,13 @@ class ResidentWebRunner extends ResidentRunner {
   }
 
   Future<void> _cleanup() async {
+    if (_exited) {
+      return;
+    }
     await _debugConnection?.close();
     await _stdOutSub?.cancel();
     await _webFs?.stop();
+    _exited = true;
   }
 
   @override

packages/flutter_tools/lib/src/web/chrome.dart

@@ -155,11 +155,11 @@ class ChromeLauncher {
   static Future<Chrome> get connectedInstance => _currentCompleter.future;
 
   /// Returns the full URL of the Chrome remote debugger for the main page.
-///
-/// This takes the [base] remote debugger URL (which points to a browser-wide
-/// page) and uses its JSON API to find the resolved URL for debugging the host
-/// page.
-Future<Uri> _getRemoteDebuggerUrl(Uri base) async {
+  ///
+  /// This takes the [base] remote debugger URL (which points to a browser-wide
+  /// page) and uses its JSON API to find the resolved URL for debugging the host
+  /// page.
+  Future<Uri> _getRemoteDebuggerUrl(Uri base) async {
     try {
       final HttpClient client = HttpClient();
       final HttpClientRequest request = await client.getUrl(base.resolve('/json/list'));
@@ -171,8 +171,7 @@ Future<Uri> _getRemoteDebuggerUrl(Uri base) async {
       // the raw URL rather than crashing.
       return base;
     }
-}
-
+  }
 }
 
 /// A class for managing an instance of Chrome.

packages/flutter_tools/test/general.shard/resident_web_runner_test.dart

@@ -338,6 +338,25 @@ void main() {
     verify(mockVmService.callServiceExtension('ext.flutter.profileWidgetBuilds',
         args: <String, Object>{'enabled': true})).called(1);
   }));
+
+  test('cleanup of resources is safe to call multiple times', () => testbed.run(() async {
+    _setupMocks();
+    bool debugClosed = false;
+    when(mockDebugConnection.close()).thenAnswer((Invocation invocation) async {
+      if (debugClosed) {
+        throw StateError('debug connection closed twice');
+      }
+      debugClosed = true;
+    });
+    final Completer<DebugConnectionInfo> connectionInfoCompleter = Completer<DebugConnectionInfo>();
+     unawaited(residentWebRunner.run(
+      connectionInfoCompleter: connectionInfoCompleter,
+    ));
+    await connectionInfoCompleter.future;
+
+    await residentWebRunner.exit();
+    await residentWebRunner.exit();
+  }));
 }