Remove content-length header in gRPC trailers-only responses (#6511)

Motivation:
When a `content-length` header is mistakenly set for a gRPC trailers-only response, it must be removed.

Modifications:
- Remove content-length header in `statusToTrailers` method.
  - A simple fix, like removing the header from all responses with an empty body, is not feasible because it would break valid use cases, such as responses to HEAD requests, which correctly include a `content-length` header but no body.

Result:
- The gRPC service now sends spec-compliant trailers-only responses without a `content-length` header.

Author: minwoox

grpc/src/main/java/com/linecorp/armeria/internal/server/grpc/AbstractServerCall.java

@@ -571,6 +571,7 @@ protected final HttpObject responseTrailers(ServiceRequestContext ctx, Status st
     public static HttpHeaders statusToTrailers(
             ServiceRequestContext ctx, HttpHeadersBuilder trailersBuilder,
             Status status, @Nullable Metadata metadata) {
+        trailersBuilder.endOfStream(true);
         try {
             MetadataUtil.fillHeaders(metadata, trailersBuilder);
         } catch (Exception e) {
@@ -594,6 +595,8 @@ public static HttpHeaders statusToTrailers(
         final HttpHeaders additionalTrailers = ctx.additionalResponseTrailers();
         ctx.mutateAdditionalResponseTrailers(HttpHeadersBuilder::clear);
         trailersBuilder.add(additionalTrailers);
+        // Remove content-length just in case it was set mistakenly.
+        trailersBuilder.remove(HttpHeaderNames.CONTENT_LENGTH);
         return trailersBuilder.build();
     }
 

grpc/src/test/java/com/linecorp/armeria/server/grpc/InvalidContentLengthHeaderTest.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright 2022 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.server.grpc;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import com.linecorp.armeria.client.grpc.GrpcClients;
+import com.linecorp.armeria.common.CommonPools;
+import com.linecorp.armeria.common.HttpHeaderNames;
+import com.linecorp.armeria.common.HttpResponse;
+import com.linecorp.armeria.common.ResponseHeaders;
+import com.linecorp.armeria.internal.common.grpc.TestServiceImpl;
+import com.linecorp.armeria.server.ServerBuilder;
+import com.linecorp.armeria.testing.junit5.server.ServerExtension;
+
+import io.grpc.Metadata;
+import io.grpc.ServerCall;
+import io.grpc.ServerCall.Listener;
+import io.grpc.ServerCallHandler;
+import io.grpc.ServerInterceptor;
+import io.grpc.Status;
+import io.grpc.StatusRuntimeException;
+import testing.grpc.Messages.SimpleRequest;
+import testing.grpc.TestServiceGrpc.TestServiceBlockingStub;
+
+class InvalidContentLengthHeaderTest {
+
+    private static final Metadata.Key<String> CONTENT_LENGTH =
+            Metadata.Key.of("content-length", Metadata.ASCII_STRING_MARSHALLER);
+
+    private static final AtomicReference<ResponseHeaders> responseHeadersCapture = new AtomicReference<>();
+
+    @RegisterExtension
+    static ServerExtension server = new ServerExtension() {
+        @Override
+        protected void configure(ServerBuilder sb) {
+            final ServerInterceptor interceptor = new ServerInterceptor() {
+                @Override
+                public <I, O> Listener<I> interceptCall(ServerCall<I, O> call,
+                                                        Metadata requestHeaders,
+                                                        ServerCallHandler<I, O> next) {
+                    final Metadata metadata = new Metadata();
+                    metadata.put(CONTENT_LENGTH, "555");
+                    call.close(Status.UNAUTHENTICATED, metadata);
+                    return new Listener<I>() {};
+                }
+            };
+
+            final GrpcService grpcService =
+                    GrpcService.builder()
+                               .addService(new TestServiceImpl(CommonPools.blockingTaskExecutor()))
+                               .intercept(interceptor)
+                               .build();
+
+            sb.service(grpcService);
+            sb.decorator((delegate, ctx, req) -> {
+                final HttpResponse res = delegate.serve(ctx, req);
+                // Capture response headers.
+                return res.mapHeaders(headers -> {
+                    responseHeadersCapture.set(headers);
+                    return headers;
+                });
+            });
+        }
+    };
+
+    @Test
+    void contentLengthRemovedFromAdditionalTrailers() {
+        final TestServiceBlockingStub client =
+                GrpcClients.newClient(server.httpUri(),
+                                     TestServiceBlockingStub.class);
+
+        assertThatThrownBy(() -> {
+            client.unaryCall(SimpleRequest.newBuilder().setResponseSize(100).build());
+        }).isInstanceOf(StatusRuntimeException.class)
+          .hasMessageContaining("UNAUTHENTICATED");
+
+        final ResponseHeaders serverResponseHeaders = responseHeadersCapture.get();
+        assertThat(serverResponseHeaders).isNotNull();
+
+        // The mistakenly set content-length (500) is removed by statusToTrailers() and is set to 0
+        // in ArmeriaHttpUtil.maybeUpdateContentLengthAndEndOfStream.
+        final String contentLength = serverResponseHeaders.get(HttpHeaderNames.CONTENT_LENGTH);
+        assertThat(contentLength).isEqualTo("0");
+        assertThat(serverResponseHeaders.isEndOfStream()).isTrue();
+    }
+}