plumb child channel options through xDS server

Author: AgraVator

api/src/main/java/io/grpc/ForwardingChannelBuilder.java

@@ -248,6 +248,12 @@ public T configureChannel(ManagedChannel parentChannel) {
     return thisT();
   }
 
+  @Override
+  public T configureChannel(Server parentServer) {
+    delegate().configureChannel(parentServer);
+    return thisT();
+  }
+
   @Override
   public T childChannelConfigurer(ChildChannelConfigurer childChannelConfigurer) {
     delegate().childChannelConfigurer(childChannelConfigurer);

api/src/main/java/io/grpc/ForwardingChannelBuilder2.java

@@ -275,6 +275,12 @@ public T configureChannel(ManagedChannel parentChannel) {
     return thisT();
   }
 
+  @Override
+  public T configureChannel(Server parentServer) {
+    delegate().configureChannel(parentServer);
+    return thisT();
+  }
+
   @Override
   public T childChannelConfigurer(ChildChannelConfigurer childChannelConfigurer) {
     delegate().childChannelConfigurer(childChannelConfigurer);

api/src/main/java/io/grpc/ForwardingServerBuilder.java

@@ -192,6 +192,18 @@ public T setBinaryLog(BinaryLog binaryLog) {
     return thisT();
   }
 
+  @Override
+  public T configureChannel(Server parentServer) {
+    delegate().configureChannel(parentServer);
+    return thisT();
+  }
+
+  @Override
+  public T childChannelConfigurer(ChildChannelConfigurer childChannelConfigurer) {
+    delegate().childChannelConfigurer(childChannelConfigurer);
+    return thisT();
+  }
+
   /**
    * Returns the {@link Server} built by the delegate by default. Overriding method can return
    * different value.

api/src/main/java/io/grpc/ManagedChannel.java

@@ -92,14 +92,14 @@ public ConnectivityState getState(boolean requestConnection) {
    * load balancers and the channel builder) to propagate configuration to child channels.
    * Application code should not call this method.
    *
-   * @return the configurer, or {@code null} if none is set.
+   * @return the configurer, or {@code noOp()} if none is set.
    * @since 1.79.0
    */
   @Internal
   public ChildChannelConfigurer getChildChannelConfigurer() {
-    // Return null by default so we don't break existing custom ManagedChannel implementations
+    // Return noOP() by default so we don't break existing custom ManagedChannel implementations
     // (like wrappers or mocks) that don't override this method.
-    return null;
+    return ChildChannelConfigurers.noOp();
   }
 
   /**

api/src/main/java/io/grpc/ManagedChannelBuilder.java

@@ -668,9 +668,6 @@ public <X> T setNameResolverArg(NameResolver.Args.Key<X> key, X value) {
    * child channels to ensure they inherit relevant configuration (like the
    * {@link ChildChannelConfigurer}) from the parent.
    *
-   * <p>The specific settings copied are implementation dependent, but typically include
-   * the child channel configurer and potentially user agents or offload executors.
-   *
    * @param parentChannel the channel to inherit configuration from
    * @return this
    * @since 1.79.0
@@ -680,12 +677,27 @@ public T configureChannel(ManagedChannel parentChannel) {
     throw new UnsupportedOperationException();
   }
 
+  /**
+   * Configures this builder using settings derived from an existing parent server.
+   *
+   * <p>This method is typically used by internal components (like LoadBalancers) when creating
+   * child channels to ensure they inherit relevant configuration (like the
+   * {@link ChildChannelConfigurer}) from the parent.
+   *
+   * @param parentServer the server to inherit configuration from
+   * @return this
+   * @since 1.79.0
+   */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/12574")
+  public T configureChannel(Server parentServer) {
+    throw new UnsupportedOperationException();
+  }
+
   /**
    * Sets a configurer that will be applied to all internal child channels created by this channel.
    *
    * <p>This allows injecting configuration (like credentials, interceptors, or flow control)
-   * into auxiliary channels created by gRPC infrastructure, such as xDS control plane connections
-   * or OOB load balancing channels.
+   * into auxiliary channels created by gRPC infrastructure, such as xDS control plane connections.
    *
    * @param childChannelConfigurer the configurer to apply.
    * @return this

api/src/main/java/io/grpc/Server.java

@@ -178,4 +178,21 @@ public List<ServerServiceDefinition> getMutableServices() {
    * @since 1.0.0
    */
   public abstract void awaitTermination() throws InterruptedException;
+
+  /**
+   * Returns the configurer for child channels.
+   *
+   * <p>This method is intended for use by the internal gRPC infrastructure
+   * to propagate configuration to child channels.
+   * Application code should not call this method.
+   *
+   * @return the configurer, or {@code noOp()} if none is set.
+   * @since 1.79.0
+   */
+  @Internal
+  public ChildChannelConfigurer getChildChannelConfigurer() {
+    // Return noOp() by default so we don't break existing custom ManagedChannel implementations
+    // (like wrappers or mocks) that don't override this method.
+    return ChildChannelConfigurers.noOp();
+  }
 }

api/src/main/java/io/grpc/ServerBuilder.java

@@ -424,6 +424,39 @@ public T setBinaryLog(BinaryLog binaryLog) {
     throw new UnsupportedOperationException();
   }
 
+  /**
+   * Configures this builder using settings derived from an existing parent server.
+   *
+   * <p>This method is typically used by internal components when creating
+   * child channels to ensure they inherit relevant configuration (like the
+   * {@link ChildChannelConfigurer}) from the parent.
+   *
+   * @param parentServer the server to inherit configuration from
+   * @return this
+   * @since 1.79.0
+   */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/12574")
+  public T configureChannel(Server parentServer) {
+    throw new UnsupportedOperationException();
+  }
+
+  /**
+   * Sets a configurer that will be applied to all internal child channels created by this server.
+   *
+   * <p>This allows injecting configuration (like credentials, interceptors, or flow control)
+   * into auxiliary channels created by gRPC infrastructure, such as xDS control plane connections
+   * or OOB load balancing channels.
+   *
+   * @param childChannelConfigurer the configurer to apply.
+   * @return this
+   * @since 1.79.0
+   */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/12574")
+  public T childChannelConfigurer(ChildChannelConfigurer childChannelConfigurer) {
+    throw new UnsupportedOperationException("Not implemented");
+  }
+
+
   /**
    * Builds a server using the given parameters.
    *

core/src/main/java/io/grpc/internal/ManagedChannelImpl.java

@@ -159,8 +159,8 @@ public Result selectConfig(PickSubchannelArgs args) {
   /**
    * Stores the user-provided configuration function for internal child channels.
    *
-   * <p>This is intended for use by gRPC internal components (NameResolvers, LoadBalancers)
-   * that are responsible for creating auxillary {@code ManagedChannel} instances.
+   * <p>This is intended for use by gRPC internal components
+   * that are responsible for creating auxiliary {@code ManagedChannel} instances.
    * guaranteed to be not null (defaults to no-op).
    */
   private ChildChannelConfigurer childChannelConfigurer =  ChildChannelConfigurers.noOp();
@@ -693,7 +693,7 @@ public CallTracer create() {
   /**
    * Retrieves the user-provided configuration function for internal child channels.
    *
-   * <p>This method is intended for use by gRPC internal components (NameResolvers, LoadBalancers)
+   * <p>This method is intended for use by gRPC internal components
    * that are responsible for creating auxiliary {@code ManagedChannel} instances.
    *
    * @return the ChildChannelConfigurer, guaranteed to be not null (defaults to no-op).

core/src/main/java/io/grpc/internal/ManagedChannelImplBuilder.java

@@ -46,6 +46,7 @@
 import io.grpc.NameResolverProvider;
 import io.grpc.NameResolverRegistry;
 import io.grpc.ProxyDetector;
+import io.grpc.Server;
 import io.grpc.StatusOr;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
@@ -724,7 +725,8 @@ protected ManagedChannelImplBuilder addMetricSink(MetricSink metricSink) {
    * automatically from a parent channel to any child channels it creates
    * (e.g., Subchannels or OOB channels).
    *
-   * @param parentChannel the channel whose configuration logic should be applied to this builder.
+   * @param parentChannel the channel whose child's configuration logic
+   *                      should be applied to this builder.
    */
   @Override
   public ManagedChannelImplBuilder configureChannel(ManagedChannel parentChannel) {
@@ -737,6 +739,27 @@ public ManagedChannelImplBuilder configureChannel(ManagedChannel parentChannel)
     return this;
   }
 
+  /**
+   * Applies the configuration logic from the given parent server to this builder.
+   *
+   * <p>This mechanism allows properties (like metrics, tracing, or interceptors) to propagate
+   * automatically from a parent server to any child channels it creates
+   * (e.g., xDS).
+   *
+   * @param parentServer the server whose child's configuration logic
+   *                      should be applied to this builder.
+   */
+  @Override
+  public ManagedChannelImplBuilder configureChannel(Server parentServer) {
+    if (parentServer != null) {
+      ChildChannelConfigurer childChannelConfigurer = parentServer.getChildChannelConfigurer();
+      if (childChannelConfigurer != null) {
+        childChannelConfigurer.accept(this);
+      }
+    }
+    return this;
+  }
+
   /**
    * Sets the configurer that will be stored in the channel built by this builder.
    *

xds/src/main/java/io/grpc/xds/GrpcXdsTransportFactory.java

@@ -26,8 +26,10 @@
 import io.grpc.Context;
 import io.grpc.Grpc;
 import io.grpc.ManagedChannel;
+import io.grpc.ManagedChannelBuilder;
 import io.grpc.Metadata;
 import io.grpc.MethodDescriptor;
+import io.grpc.Server;
 import io.grpc.Status;
 import io.grpc.xds.client.Bootstrapper;
 import io.grpc.xds.client.XdsTransportFactory;
@@ -37,15 +39,19 @@ final class GrpcXdsTransportFactory implements XdsTransportFactory {
 
   private final CallCredentials callCredentials;
   private final ManagedChannel parentChannel;
+  private final Server parentServer;
 
-  GrpcXdsTransportFactory(CallCredentials callCredentials, ManagedChannel parentChannel) {
+
+  GrpcXdsTransportFactory(CallCredentials callCredentials, ManagedChannel parentChannel,
+                          Server parentServer) {
     this.callCredentials = callCredentials;
     this.parentChannel = parentChannel;
+    this.parentServer = parentServer;
   }
 
   @Override
   public XdsTransport create(Bootstrapper.ServerInfo serverInfo) {
-    return new GrpcXdsTransport(serverInfo, callCredentials, parentChannel);
+    return new GrpcXdsTransport(serverInfo, callCredentials, parentChannel, parentServer);
   }
 
   @VisibleForTesting
@@ -78,13 +84,17 @@ public GrpcXdsTransport(Bootstrapper.ServerInfo serverInfo, CallCredentials call
     }
 
     public GrpcXdsTransport(Bootstrapper.ServerInfo serverInfo, CallCredentials callCredentials,
-                            ManagedChannel parentChannel) {
+                            ManagedChannel parentChannel, Server parentServer) {
       String target = serverInfo.target();
       ChannelCredentials channelCredentials = (ChannelCredentials) serverInfo.implSpecificConfig();
-      this.channel = Grpc.newChannelBuilder(target, channelCredentials)
-          .keepAliveTime(5, TimeUnit.MINUTES)
-          .configureChannel(parentChannel)
-          .build();
+      ManagedChannelBuilder<?> channelBuilder = Grpc.newChannelBuilder(target, channelCredentials)
+          .keepAliveTime(5, TimeUnit.MINUTES);
+      if (parentChannel != null) {
+        channelBuilder.configureChannel(parentChannel);
+      } else if (parentServer != null) {
+        channelBuilder.configureChannel(parentServer);
+      }
+      this.channel = channelBuilder.build();
       this.callCredentials = callCredentials;
     }