Add a newAttachHeadersServerInterceptor() util

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

@@ -22,10 +22,15 @@
 import io.grpc.Channel;
 import io.grpc.ClientCall;
 import io.grpc.ClientInterceptor;
+import io.grpc.ExperimentalApi;
 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 +148,63 @@ public void onClose(Status status, Metadata trailers) {
       }
     }
   }
+
+  /**
+   * Returns a ServerInterceptor that adds the specified Metadata to every response stream, one way
+   * or another.
+   *
+   * <p>If, absent this interceptor, a stream would have headers, 'extras' will be added to those
+   * headers. Otherwise, 'extras' will be sent as trailers. This pattern is useful when you have
+   * some fixed information, server identity say, that should be included no matter how the call
+   * turns out. The fallback to trailers avoids artificially committing clients to error responses
+   * that could otherwise be retried (see https://grpc.io/docs/guides/retry/ for more).
+   *
+   * <p>For correct operation, be sure to arrange for this interceptor to run *before* any others
+   * that might add headers.
+   *
+   * @param extras the Metadata to be added to each stream. Caller gives up ownership.
+   */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/11462")
+  public static ServerInterceptor newAttachMetadataServerInterceptor(Metadata extras) {
+    return new MetadataAttachingServerInterceptor(extras);
+  }
+
+  private static final class MetadataAttachingServerInterceptor implements ServerInterceptor {
+
+    private final Metadata extras;
+
+    MetadataAttachingServerInterceptor(Metadata extras) {
+      this.extras = extras;
+    }
+
+    @Override
+    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(
+        ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
+      return next.startCall(new MetadataAttachingServerCall<>(call), headers);
+    }
+
+    final class MetadataAttachingServerCall<ReqT, RespT>
+        extends SimpleForwardingServerCall<ReqT, RespT> {
+      boolean headersSent;
+
+      MetadataAttachingServerCall(ServerCall<ReqT, RespT> delegate) {
+        super(delegate);
+      }
+
+      @Override
+      public void sendHeaders(Metadata headers) {
+        headers.merge(extras);
+        headersSent = true;
+        super.sendHeaders(headers);
+      }
+
+      @Override
+      public void close(Status status, Metadata trailers) {
+        if (!headersSent) {
+          trailers.merge(extras);
+        }
+        super.close(status, trailers);
+      }
+    }
+  }
 }

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

@@ -0,0 +1,175 @@
+/*
+ * 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 static io.grpc.stub.MetadataUtils.newAttachMetadataServerInterceptor;
+import static io.grpc.stub.MetadataUtils.newCaptureMetadataInterceptor;
+import static org.junit.Assert.fail;
+
+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.ServerInterceptors;
+import io.grpc.ServerMethodDefinition;
+import io.grpc.ServerServiceDefinition;
+import io.grpc.Status;
+import io.grpc.Status.Code;
+import io.grpc.StatusRuntimeException;
+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.Iterator;
+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 final String SERVER_NAME = "test";
+  private static final 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();
+          });
+
+  MethodDescriptor<String, String> echoServerStreamingMethod =
+      MethodDescriptor.newBuilder(StringMarshaller.INSTANCE, StringMarshaller.INSTANCE)
+          .setFullMethodName("test/echoStream")
+          .setType(MethodDescriptor.MethodType.SERVER_STREAMING)
+          .build();
+
+  private final AtomicReference<Metadata> trailersCapture = new AtomicReference<>();
+  private final AtomicReference<Metadata> headersCapture = new AtomicReference<>();
+
+  @Test
+  public void shouldAttachHeadersToResponse() throws IOException {
+    Metadata extras = new Metadata();
+    extras.put(FOO_KEY, "foo-value");
+
+    ServerServiceDefinition serviceDef =
+        ServerInterceptors.intercept(
+            ServerServiceDefinition.builder("test").addMethod(echoMethod, echoCallHandler).build(),
+            ImmutableList.of(newAttachMetadataServerInterceptor(extras)));
+
+    grpcCleanup.register(newInProcessServerBuilder().addService(serviceDef).build().start());
+    ManagedChannel channel =
+        grpcCleanup.register(
+            newInProcessChannelBuilder()
+                .intercept(newCaptureMetadataInterceptor(headersCapture, trailersCapture))
+                .build());
+
+    String response =
+        ClientCalls.blockingUnaryCall(channel, echoMethod, CallOptions.DEFAULT, "hello");
+    assertThat(response).isEqualTo("hello");
+    assertThat(trailersCapture.get() == null || !trailersCapture.get().containsKey(FOO_KEY))
+        .isTrue();
+    assertThat(headersCapture.get().get(FOO_KEY)).isEqualTo("foo-value");
+  }
+
+  @Test
+  public void shouldAttachTrailersWhenNoResponse() throws IOException {
+    Metadata extras = new Metadata();
+    extras.put(FOO_KEY, "foo-value");
+
+    ServerServiceDefinition serviceDef =
+        ServerInterceptors.intercept(
+            ServerServiceDefinition.builder("test")
+                .addMethod(
+                    ServerMethodDefinition.create(
+                        echoServerStreamingMethod,
+                        ServerCalls.asyncUnaryCall(
+                            (req, respObserver) -> respObserver.onCompleted())))
+                .build(),
+            ImmutableList.of(newAttachMetadataServerInterceptor(extras)));
+    grpcCleanup.register(newInProcessServerBuilder().addService(serviceDef).build().start());
+
+    ManagedChannel channel =
+        grpcCleanup.register(
+            newInProcessChannelBuilder()
+                .intercept(newCaptureMetadataInterceptor(headersCapture, trailersCapture))
+                .build());
+
+    Iterator<String> response =
+        ClientCalls.blockingServerStreamingCall(
+            channel, echoServerStreamingMethod, CallOptions.DEFAULT, "hello");
+    assertThat(response.hasNext()).isFalse();
+    assertThat(headersCapture.get() == null || !headersCapture.get().containsKey(FOO_KEY)).isTrue();
+    assertThat(trailersCapture.get().get(FOO_KEY)).isEqualTo("foo-value");
+  }
+
+  @Test
+  public void shouldAttachTrailersToErrorResponse() throws IOException {
+    Metadata extras = new Metadata();
+    extras.put(FOO_KEY, "foo-value");
+
+    ServerServiceDefinition serviceDef =
+        ServerInterceptors.intercept(
+            ServerServiceDefinition.builder("test")
+                .addMethod(
+                    echoMethod,
+                    ServerCalls.asyncUnaryCall(
+                        (req, respObserver) ->
+                            respObserver.onError(Status.INVALID_ARGUMENT.asRuntimeException())))
+                .build(),
+            ImmutableList.of(newAttachMetadataServerInterceptor(extras)));
+    grpcCleanup.register(newInProcessServerBuilder().addService(serviceDef).build().start());
+
+    ManagedChannel channel =
+        grpcCleanup.register(
+            newInProcessChannelBuilder()
+                .intercept(newCaptureMetadataInterceptor(headersCapture, trailersCapture))
+                .build());
+    try {
+      ClientCalls.blockingUnaryCall(channel, echoMethod, CallOptions.DEFAULT, "hello");
+      fail();
+    } catch (StatusRuntimeException e) {
+      assertThat(e.getStatus()).isNotNull();
+      assertThat(e.getStatus().getCode()).isEqualTo(Code.INVALID_ARGUMENT);
+    }
+    assertThat(headersCapture.get() == null || !headersCapture.get().containsKey(FOO_KEY)).isTrue();
+    assertThat(trailersCapture.get().get(FOO_KEY)).isEqualTo("foo-value");
+  }
+
+  private static InProcessServerBuilder newInProcessServerBuilder() {
+    return InProcessServerBuilder.forName(SERVER_NAME).directExecutor();
+  }
+
+  private static InProcessChannelBuilder newInProcessChannelBuilder() {
+    return InProcessChannelBuilder.forName(SERVER_NAME).directExecutor();
+  }
+}