Add server interceptor acting as a middleware

lib/grpc.dart

@@ -42,7 +42,8 @@ export 'src/client/proxy.dart' show Proxy;
 export 'src/client/transport/http2_credentials.dart'
     show BadCertificateHandler, allowBadCertificates, ChannelCredentials;
 export 'src/server/call.dart' show ServiceCall;
-export 'src/server/interceptor.dart' show Interceptor;
+export 'src/server/interceptor.dart'
+    show Interceptor, ServerInterceptor, ServerStreamingInvoker;
 export 'src/server/server.dart'
     show
         ServerCredentials,

lib/src/server/handler.dart

@@ -37,6 +37,7 @@ class ServerHandler extends ServiceCall {
   final ServerTransportStream _stream;
   final ServiceLookup _serviceLookup;
   final List<Interceptor> _interceptors;
+  final List<ServerInterceptor> _serverInterceptors;
   final CodecRegistry? _codecRegistry;
   final GrpcErrorHandler? _errorHandler;
 
@@ -83,6 +84,7 @@ class ServerHandler extends ServiceCall {
     required ServerTransportStream stream,
     required ServiceLookup serviceLookup,
     required List<Interceptor> interceptors,
+    required List<ServerInterceptor> serverInterceptors,
     required CodecRegistry? codecRegistry,
     X509Certificate? clientCertificate,
     InternetAddress? remoteAddress,
@@ -94,7 +96,8 @@ class ServerHandler extends ServiceCall {
         _codecRegistry = codecRegistry,
         _clientCertificate = clientCertificate,
         _remoteAddress = remoteAddress,
-        _errorHandler = errorHandler;
+        _errorHandler = errorHandler,
+        _serverInterceptors = serverInterceptors;
 
   @override
   DateTime? get deadline => _deadline;
@@ -239,7 +242,7 @@ class ServerHandler extends ServiceCall {
       return;
     }
 
-    _responses = _descriptor.handle(this, requests.stream);
+    _responses = _descriptor.handle(this, requests.stream, _serverInterceptors);
 
     _responseSubscription = _responses.listen(_onResponse,
         onError: _onResponseError,

lib/src/server/interceptor.dart

@@ -27,3 +27,18 @@ import 'service.dart';
 /// If the interceptor returns null, the corresponding [ServiceMethod] of [Service] will be called.
 typedef Interceptor = FutureOr<GrpcError?> Function(
     ServiceCall call, ServiceMethod method);
+
+typedef ServerStreamingInvoker<Q, R> = Stream<R> Function(
+    ServiceCall call, ServiceMethod<Q, R> method, Stream<Q> requests);
+
+/// A gRPC Interceptor.
+///
+/// An interceptor is called around the corresponding [ServiceMethod] invocation.
+/// If the interceptor throws [GrpcError], the error will be returned as a response. [ServiceMethod] wouldn't be called if the error is thrown before calling the invoker.
+/// If the interceptor modifies the provided stream, the invocation will continue with the provided stream.
+abstract class ServerInterceptor {
+  Stream<R> intercept<Q, R>(ServiceCall call, ServiceMethod<Q, R> method,
+      Stream<Q> requests, ServerStreamingInvoker<Q, R> invoker) {
+    return invoker(call, method, requests);
+  }
+}

lib/src/server/server.dart

@@ -87,6 +87,7 @@ class ServerTlsCredentials extends ServerCredentials {
 class ConnectionServer {
   final Map<String, Service> _services = {};
   final List<Interceptor> _interceptors;
+  final List<ServerInterceptor> _serverInterceptors;
   final CodecRegistry? _codecRegistry;
   final GrpcErrorHandler? _errorHandler;
   final ServerKeepAliveOptions _keepAliveOptions;
@@ -100,11 +101,13 @@ class ConnectionServer {
   ConnectionServer(
     List<Service> services, [
     List<Interceptor> interceptors = const <Interceptor>[],
+    List<ServerInterceptor> serverInterceptors = const <ServerInterceptor>[],
     CodecRegistry? codecRegistry,
     GrpcErrorHandler? errorHandler,
     this._keepAliveOptions = const ServerKeepAliveOptions(),
   ])  : _codecRegistry = codecRegistry,
         _interceptors = interceptors,
+        _serverInterceptors = serverInterceptors,
         _errorHandler = errorHandler {
     for (final service in services) {
       _services[service.$name] = service;
@@ -168,6 +171,7 @@ class ConnectionServer {
         stream: stream,
         serviceLookup: lookupService,
         interceptors: _interceptors,
+        serverInterceptors: _serverInterceptors,
         codecRegistry: _codecRegistry,
         // ignore: unnecessary_cast
         clientCertificate: clientCertificate as io_bits.X509Certificate?,
@@ -201,11 +205,13 @@ class Server extends ConnectionServer {
     required List<Service> services,
     ServerKeepAliveOptions keepAliveOptions = const ServerKeepAliveOptions(),
     List<Interceptor> interceptors = const <Interceptor>[],
+    List<ServerInterceptor> serverInterceptors = const <ServerInterceptor>[],
     CodecRegistry? codecRegistry,
     GrpcErrorHandler? errorHandler,
   }) : super(
           services,
           interceptors,
+          serverInterceptors,
           codecRegistry,
           errorHandler,
           keepAliveOptions,
@@ -308,6 +314,7 @@ class Server extends ConnectionServer {
       stream: stream,
       serviceLookup: lookupService,
       interceptors: _interceptors,
+      serverInterceptors: _serverInterceptors,
       codecRegistry: _codecRegistry,
       // ignore: unnecessary_cast
       clientCertificate: clientCertificate as io_bits.X509Certificate?,

lib/src/server/service.dart

@@ -17,6 +17,7 @@ import 'dart:async';
 
 import '../shared/status.dart';
 import 'call.dart';
+import 'interceptor.dart';
 
 /// Definition of a gRPC service method.
 class ServiceMethod<Q, R> {
@@ -48,19 +49,39 @@ class ServiceMethod<Q, R> {
 
   List<int> serialize(dynamic response) => responseSerializer(response as R);
 
-  Stream<R> handle(ServiceCall call, Stream<Q> requests) {
+  ServerStreamingInvoker<Q, R> _createCall() => ((
-    if (streamingResponse) {
+        ServiceCall call,
-      if (streamingRequest) {
+        ServiceMethod<Q, R> method,
-        return handler(call, requests);
+        Stream<Q> requests,
-      } else {
+      ) {
-        return handler(call, _toSingleFuture(requests));
+        if (streamingResponse) {
-      }
+          if (streamingRequest) {
-    } else {
+            return handler(call, requests);
-      final response = streamingRequest
+          } else {
-          ? handler(call, requests)
+            return handler(call, _toSingleFuture(requests));
-          : handler(call, _toSingleFuture(requests));
+          }
-      return response.asStream();
+        } else {
+          final response = streamingRequest
+              ? handler(call, requests)
+              : handler(call, _toSingleFuture(requests));
+          return response.asStream();
+        }
+      });
+
+  Stream<R> handle(
+    ServiceCall call,
+    Stream<Q> requests,
+    List<ServerInterceptor> interceptors,
+  ) {
+    var invoker = _createCall();
+
+    for (final interceptor in interceptors.reversed) {
+      final delegate = invoker;
+      invoker = (call, method, requests) =>
+          interceptor.intercept<Q, R>(call, method, requests, delegate);
     }
+
+    return invoker(call, this, requests);
   }
 
   Future<Q> _toSingleFuture(Stream<Q> stream) {

test/server_test.dart

@@ -384,4 +384,221 @@ void main() {
       await harness.fromServer.done;
     });
   });
+
+  group('Server with server interceptor', () {
+    group('processes calls if interceptor allows request', () {
+      const expectedRequest = 5;
+      const expectedResponse = 7;
+      Future<int> methodHandler(ServiceCall call, Future<int> request) async {
+        expect(await request, expectedRequest);
+        return expectedResponse;
+      }
+
+      Null interceptor(call, method, requests) {
+        if (method.name == 'Unary') {
+          return null;
+        }
+        throw GrpcError.unauthenticated('Request is unauthenticated');
+      }
+
+      Future<void> doTest(TestServerInterceptorOnStart? handler) async {
+        harness
+          ..serverInterceptor.onStart = handler
+          ..service.unaryHandler = methodHandler
+          ..runTest('/Test/Unary', [expectedRequest], [expectedResponse]);
+
+        await harness.fromServer.done;
+      }
+
+      test('with sync interceptor', () => doTest(interceptor));
+      test(
+          'with async interceptor',
+          () => doTest((call, method, requests) async =>
+              interceptor(call, method, requests)));
+    });
+
+    group('returns error if interceptor blocks request', () {
+      Null interceptor(call, method, requests) {
+        if (method.name == 'Unary') {
+          throw GrpcError.unauthenticated('Request is unauthenticated');
+        }
+        return null;
+      }
+
+      Future<void> doTest(TestServerInterceptorOnStart handler) async {
+        harness
+          ..serverInterceptor.onStart = handler
+          ..expectErrorResponse(
+              StatusCode.unauthenticated, 'Request is unauthenticated')
+          ..sendRequestHeader('/Test/Unary');
+
+        await harness.fromServer.done;
+      }
+
+      test('with sync interceptor', () => doTest(interceptor));
+      test(
+          'with async interceptor',
+          () => doTest((call, method, request) async =>
+              interceptor(call, method, request)));
+    });
+
+    test("don't fail if interceptor await 2 times", () async {
+      Future<Null> interceptor(call, method, requests) async {
+        await Future.value();
+        await Future.value();
+        throw GrpcError.internal('Reason is unknown');
+      }
+
+      harness
+        ..serverInterceptor.onStart = interceptor
+        ..expectErrorResponse(StatusCode.internal, 'Reason is unknown')
+        ..sendRequestHeader('/Test/Unary')
+        ..sendData(1);
+
+      await harness.fromServer.done;
+    });
+
+    group('serviceInterceptors are invoked', () {
+      const expectedRequest = 5;
+      const expectedResponse = 7;
+      Future<int> methodHandler(ServiceCall call, Future<int> request) async {
+        expect(await request, expectedRequest);
+        return expectedResponse;
+      }
+
+      Future<void> doTest(List<TestServerInterceptor> interceptors) async {
+        harness
+          // ↓ mutation: Server is already built
+          ..serverInterceptors.addAll(interceptors)
+          ..service.unaryHandler = methodHandler
+          ..runTest('/Test/Unary', [expectedRequest], [expectedResponse]);
+
+        await harness.fromServer.done;
+      }
+
+      test('single serviceInterceptor is invoked', () async {
+        final invocationsOrderRecords = [];
+
+        await doTest([
+          TestServerInterceptor(
+            onStart: (call, method, requests) {
+              invocationsOrderRecords.add('Start');
+            },
+            onData: (call, method, requests, data) {
+              invocationsOrderRecords.add('Data [$data]');
+            },
+            onFinish: (call, method, requests) {
+              invocationsOrderRecords.add('Done');
+            },
+          )
+        ]);
+
+        expect(invocationsOrderRecords, equals(['Start', 'Data [7]', 'Done']));
+      });
+
+      test('multiple serviceInterceptors are invoked', () async {
+        final invocationsOrderRecords = [];
+
+        await doTest([
+          TestServerInterceptor(
+            onStart: (call, method, requests) {
+              invocationsOrderRecords.add('Start 1');
+            },
+            onData: (call, method, requests, data) {
+              invocationsOrderRecords.add('Data 1 [$data]');
+            },
+            onFinish: (call, method, requests) {
+              invocationsOrderRecords.add('Done 1');
+            },
+          ),
+          TestServerInterceptor(
+            onStart: (call, method, requests) {
+              invocationsOrderRecords.add('Start 2');
+            },
+            onData: (call, method, requests, data) {
+              invocationsOrderRecords.add('Data 2 [$data]');
+            },
+            onFinish: (call, method, requests) {
+              invocationsOrderRecords.add('Done 2');
+            },
+          )
+        ]);
+
+        expect(
+            invocationsOrderRecords,
+            equals([
+              'Start 1',
+              'Start 2',
+              'Data 2 [7]',
+              'Data 1 [7]',
+              'Done 2',
+              'Done 1',
+            ]));
+      });
+    });
+
+    test('can modify response', () async {
+      const expectedRequest = 5;
+      const baseResponse = 7;
+      const expectedResponse = 14;
+
+      final invocationsOrderRecords = [];
+
+      final interceptors = [
+        TestServerInterceptor(
+          onStart: (call, method, requests) {
+            invocationsOrderRecords.add('Start 1');
+          },
+          onData: (call, method, requests, data) {
+            invocationsOrderRecords.add('Data 1 [$data]');
+          },
+          onFinish: (call, method, requests) {
+            invocationsOrderRecords.add('Done 1');
+          },
+        ),
+        TestServerInterruptingInterceptor(transform: <R>(value) {
+          if (value is int) {
+            return value * 2 as R;
+          }
+
+          return value;
+        }),
+        TestServerInterceptor(
+          onStart: (call, method, requests) {
+            invocationsOrderRecords.add('Start 2');
+          },
+          onData: (call, method, requests, data) {
+            invocationsOrderRecords.add('Data 2 [$data]');
+          },
+          onFinish: (call, method, requests) {
+            invocationsOrderRecords.add('Done 2');
+          },
+        )
+      ];
+
+      Future<int> methodHandler(ServiceCall call, Future<int> request) async {
+        expect(await request, expectedRequest);
+        return baseResponse;
+      }
+
+      harness
+        // ↓ mutation: Server is already built
+        ..serverInterceptors.addAll(interceptors)
+        ..service.unaryHandler = methodHandler
+        ..runTest('/Test/Unary', [expectedRequest], [expectedResponse]);
+
+      await harness.fromServer.done;
+
+      expect(
+          invocationsOrderRecords,
+          equals([
+            'Start 1',
+            'Start 2',
+            'Data 2 [7]',
+            'Data 1 [14]',
+            'Done 2',
+            'Done 1',
+          ]));
+    });
+  });
 }

test/src/server_utils.dart

@@ -17,6 +17,7 @@ import 'dart:async';
 
 import 'package:grpc/grpc.dart';
 import 'package:grpc/src/client/http2_connection.dart';
+import 'package:grpc/src/server/interceptor.dart';
 import 'package:grpc/src/shared/message.dart';
 import 'package:http2/transport.dart';
 import 'package:test/test.dart';
@@ -90,6 +91,47 @@ class TestInterceptor {
   }
 }
 
+typedef TestServerInterceptorOnStart = Function(
+    ServiceCall call, ServiceMethod method, Stream requests);
+typedef TestServerInterceptorOnData = Function(
+    ServiceCall call, ServiceMethod method, Stream requests, dynamic data);
+typedef TestServerInterceptorOnFinish = Function(
+    ServiceCall call, ServiceMethod method, Stream requests);
+
+class TestServerInterceptor extends ServerInterceptor {
+  TestServerInterceptorOnStart? onStart;
+  TestServerInterceptorOnData? onData;
+  TestServerInterceptorOnFinish? onFinish;
+
+  TestServerInterceptor({this.onStart, this.onData, this.onFinish});
+
+  @override
+  Stream<R> intercept<Q, R>(ServiceCall call, ServiceMethod<Q, R> method,
+      Stream<Q> requests, ServerStreamingInvoker<Q, R> invoker) async* {
+    await onStart?.call(call, method, requests);
+
+    await for (final chunk
+        in super.intercept(call, method, requests, invoker)) {
+      await onData?.call(call, method, requests, chunk);
+      yield chunk;
+    }
+
+    await onFinish?.call(call, method, requests);
+  }
+}
+
+class TestServerInterruptingInterceptor extends ServerInterceptor {
+  final R Function<R>(R) transform;
+
+  TestServerInterruptingInterceptor({required this.transform});
+
+  @override
+  Stream<R> intercept<Q, R>(ServiceCall call, ServiceMethod<Q, R> method,
+      Stream<Q> requests, ServerStreamingInvoker<Q, R> invoker) async* {
+    yield* super.intercept(call, method, requests, invoker).map(transform);
+  }
+}
+
 class TestServerStream extends ServerTransportStream {
   @override
   final Stream<StreamMessage> incomingMessages;
@@ -123,6 +165,7 @@ class ServerHarness extends _Harness {
   ConnectionServer createServer() => Server.create(
         services: <Service>[service],
         interceptors: <Interceptor>[interceptor.call],
+        serverInterceptors: serverInterceptors..insert(0, serverInterceptor),
       );
 
   static ServiceMethod<int, int> createMethod(String name,
@@ -161,6 +204,10 @@ abstract class _Harness {
   final fromServer = StreamController<StreamMessage>();
   final service = TestService();
   final interceptor = TestInterceptor();
+  final serverInterceptor = TestServerInterceptor();
+
+  final serverInterceptors = <ServerInterceptor>[];
+
   ConnectionServer? _server;
 
   ConnectionServer createServer();