Add a newAttachHeadersServerInterceptor() util

stub/src/main/java/io/grpc/stub/MetadataUtils.java

@@ -24,8 +24,12 @@
 import io.grpc.ClientInterceptor;
 import io.grpc.ForwardingClientCall.SimpleForwardingClientCall;
 import io.grpc.ForwardingClientCallListener.SimpleForwardingClientCallListener;
+import io.grpc.ForwardingServerCall.SimpleForwardingServerCall;
 import io.grpc.Metadata;
 import io.grpc.MethodDescriptor;
+import io.grpc.ServerCall;
+import io.grpc.ServerCallHandler;
+import io.grpc.ServerInterceptor;
 import io.grpc.Status;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -143,4 +147,41 @@ public void onClose(Status status, Metadata trailers) {
       }
     }
   }
+
+  /**
+   * Returns a ServerInterceptor that attaches a given set of headers to every response.
+   *
+   * @param extraHeaders the headers to be added to each response. Caller gives up ownership.
+   */
+  public static ServerInterceptor newAttachHeadersServerInterceptor(Metadata extraHeaders) {
+    return new MetadataAttachingServerInterceptor(extraHeaders);
+  }
+
+  private static final class MetadataAttachingServerInterceptor implements ServerInterceptor {
+
+    private final Metadata extraHeaders;
+
+    MetadataAttachingServerInterceptor(Metadata extraHeaders) {
+      this.extraHeaders = extraHeaders;
+    }
+
+    @Override
+    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(
+        ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
+      return next.startCall(new HeaderAttachingServerCall<>(call), headers);
+    }
+
+    final class HeaderAttachingServerCall<ReqT, RespT>
+        extends SimpleForwardingServerCall<ReqT, RespT> {
+      HeaderAttachingServerCall(ServerCall<ReqT, RespT> delegate) {
+        super(delegate);
+      }
+
+      @Override
+      public void sendHeaders(Metadata headers) {
+        headers.merge(extraHeaders);
+        super.sendHeaders(headers);
+      }
+    }
+  }
 }

stub/src/test/java/io/grpc/stub/MetadataUtilsTest.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2024 The gRPC Authors
+ *
+ * Licensed 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
+ *
+ *     http://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 io.grpc.stub;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.collect.ImmutableList;
+import io.grpc.CallOptions;
+import io.grpc.ManagedChannel;
+import io.grpc.Metadata;
+import io.grpc.MethodDescriptor;
+import io.grpc.ServerCallHandler;
+import io.grpc.ServerInterceptor;
+import io.grpc.ServerInterceptors;
+import io.grpc.ServerServiceDefinition;
+import io.grpc.StringMarshaller;
+import io.grpc.inprocess.InProcessChannelBuilder;
+import io.grpc.inprocess.InProcessServerBuilder;
+import io.grpc.testing.GrpcCleanupRule;
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicReference;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class MetadataUtilsTest {
+
+  @Rule public final GrpcCleanupRule grpcCleanup = new GrpcCleanupRule();
+  private static Metadata.Key<String> FOO_KEY =
+      Metadata.Key.of("foo-key", Metadata.ASCII_STRING_MARSHALLER);
+
+  private final MethodDescriptor<String, String> echoMethod =
+      MethodDescriptor.newBuilder(StringMarshaller.INSTANCE, StringMarshaller.INSTANCE)
+          .setFullMethodName("test/echo")
+          .setType(MethodDescriptor.MethodType.UNARY)
+          .build();
+
+  private final ServerCallHandler<String, String> echoCallHandler =
+      ServerCalls.asyncUnaryCall(
+          (req, respObserver) -> {
+            respObserver.onNext(req);
+            respObserver.onCompleted();
+          });
+
+  @Test
+  public void testAttachHeadersServerInterceptor() throws IOException {
+    Metadata extraHeaders = new Metadata();
+    extraHeaders.put(FOO_KEY, "foo-value");
+
+    ImmutableList<ServerInterceptor> interceptors =
+        ImmutableList.of(MetadataUtils.newAttachHeadersServerInterceptor(extraHeaders));
+    ServerServiceDefinition serviceDef =
+        ServerInterceptors.intercept(
+            ServerServiceDefinition.builder("test").addMethod(echoMethod, echoCallHandler).build(),
+            interceptors);
+
+    InProcessServerBuilder server =
+        InProcessServerBuilder.forName("test").directExecutor().addService(serviceDef);
+    grpcCleanup.register(server.build().start());
+
+    AtomicReference<Metadata> trailersCapture = new AtomicReference<>();
+    AtomicReference<Metadata> headersCapture = new AtomicReference<>();
+    ManagedChannel channel =
+        InProcessChannelBuilder.forName("test")
+            .directExecutor()
+            .intercept(MetadataUtils.newCaptureMetadataInterceptor(headersCapture, trailersCapture))
+            .build();
+
+    String response =
+        ClientCalls.blockingUnaryCall(channel, echoMethod, CallOptions.DEFAULT, "hello");
+    assertThat(response).isEqualTo("hello");
+    Metadata headers = headersCapture.get();
+    assertThat(headers.get(FOO_KEY)).isEqualTo("foo-value");
+  }
+}