Fix concurrent modification error in GrpcWebClientChannel.terminate

Fixes #331

Co-authored-by: Vyacheslav Egorov <vegorov@google.com>

CHANGELOG.md

@@ -7,10 +7,12 @@
   newer of protobuf compiler plugin.
 * `Client.$createCall` is deprecated because it does not invoke client
   interceptors.
-* Fix an issue [#380](https://github.com/grpc/grpc-dart/issues/380) causing
+* Fix issue [#380](https://github.com/grpc/grpc-dart/issues/380) causing
   incorrect duplicated headers in gRPC-Web requests.
 * Change minimum required Dart SDK to 2.8 to enable access to Unix domain sockets.
 * Add support for Unix domain sockets in `Socket.serve` and `ClientChannel`.
+* Fix issue [#331](https://github.com/grpc/grpc-dart/issues/331) causing
+  an exception in `GrpcWebClientChannel.terminate()`.
 
 ## 2.7.0
 

lib/src/client/transport/xhr_transport.dart

@@ -214,7 +214,7 @@ class XhrClientConnection extends ClientConnection {
 
   @override
   Future<void> terminate() async {
-    for (XhrTransportStream request in _requests) {
+    for (var request in List.of(_requests)) {
       request.terminate();
     }
   }

test/grpc_web_test.dart

@@ -11,12 +11,20 @@ import 'package:grpc/grpc_web.dart';
 import 'src/generated/echo.pbgrpc.dart';
 
 void main() {
+  GrpcWebServer server;
+
+  setUpAll(() async {
+    server = await GrpcWebServer.start();
+  });
+
+  tearDownAll(() async {
+    await server.shutdown();
+  });
+
   // Test verifies that gRPC-web echo example works by talking to a gRPC
   // server (written in Dart) via gRPC-web protocol through a third party
   // gRPC-web proxy.
   test('gRPC-web echo test', () async {
-    final server = await GrpcWebServer.start();
-    try {
     final channel = GrpcWebClientChannel.xhr(server.uri);
     final service = EchoServiceClient(channel);
 
@@ -44,9 +52,47 @@ void main() {
     }).toList();
     final maxDelay = timings.reduce(math.max);
     expect(maxDelay, lessThan(500));
-    } finally {
+  });
-      await server.shutdown();
+
-    }
+  // Verify that terminate does not cause an exception when terminating
+  // channel with multiple active requests.
+  test("terminate works", () async {
+    final channel = GrpcWebClientChannel.xhr(server.uri);
+    final service = EchoServiceClient(channel);
+
+    const testMessage = 'hello from gRPC-web';
+
+    // First test a simple echo request.
+    final response = await service.echo(EchoRequest()..message = testMessage);
+    expect(response.message, equals(testMessage));
+
+    var terminated = false;
+
+    service
+        .serverStreamingEcho(ServerStreamingEchoRequest()
+          ..message = testMessage
+          ..messageCount = 20
+          ..messageInterval = 100)
+        .listen((response) {
+      expect(response.message, equals(testMessage));
+    }, onError: (e) {
+      expect(terminated, isTrue);
+    });
+
+    service
+        .serverStreamingEcho(ServerStreamingEchoRequest()
+          ..message = testMessage
+          ..messageCount = 20
+          ..messageInterval = 100)
+        .listen((response) {
+      expect(response.message, equals(testMessage));
+    }, onError: (e) {
+      expect(terminated, isTrue);
+    });
+
+    await Future.delayed(Duration(milliseconds: 500));
+    terminated = true;
+    await channel.terminate();
   });
 }
 
@@ -72,7 +118,8 @@ class GrpcWebServer {
   static Future<GrpcWebServer> start() async {
     // Spawn the server code on the server side, it will send us back port
     // number we should be talking to.
-    final serverChannel = spawnHybridUri('grpc_web_server.dart');
+    final serverChannel =
+        spawnHybridUri('grpc_web_server.dart', stayAlive: true);
     final portCompleter = Completer<int>();
     final exitCompleter = Completer<void>();
     serverChannel.stream.listen((event) {