api: add getters for NameResolver.Args and NameResolverRegistry in LoadBalancer.Helper (#5685)

Hierarchical LoadBalancers such as xDS will need this to create NameResolvers to resolve locality target names.

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

@@ -861,6 +861,25 @@ public abstract class LoadBalancer {
     public ChannelLogger getChannelLogger() {
       throw new UnsupportedOperationException();
     }
+
+    /**
+     * Returns the {@link NameResolver.Args} that the Channel uses to create {@link NameResolver}s.
+     *
+     * @since 1.22.0
+     */
+    public NameResolver.Args getNameResolverArgs() {
+      throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Returns the {@link NameResolverRegistry} that the Channel uses to look for {@link
+     * NameResolver}s.
+     *
+     * @since 1.22.0
+     */
+    public NameResolverRegistry getNameResolverRegistry() {
+      throw new UnsupportedOperationException();
+    }
   }
 
   /**

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

@@ -84,9 +84,6 @@ public abstract class AbstractManagedChannelImplBuilder
   private static final ObjectPool<? extends Executor> DEFAULT_EXECUTOR_POOL =
       SharedResourcePool.forResource(GrpcUtil.SHARED_CHANNEL_EXECUTOR);
 
-  private static final NameResolver.Factory DEFAULT_NAME_RESOLVER_FACTORY =
-      NameResolverRegistry.getDefaultRegistry().asFactory();
-
   private static final DecompressorRegistry DEFAULT_DECOMPRESSOR_REGISTRY =
       DecompressorRegistry.getDefaultInstance();
 
@@ -99,9 +96,10 @@ public abstract class AbstractManagedChannelImplBuilder
   ObjectPool<? extends Executor> executorPool = DEFAULT_EXECUTOR_POOL;
 
   private final List<ClientInterceptor> interceptors = new ArrayList<>();
+  final NameResolverRegistry nameResolverRegistry = NameResolverRegistry.getDefaultRegistry();
 
   // Access via getter, which may perform authority override as needed
-  private NameResolver.Factory nameResolverFactory = DEFAULT_NAME_RESOLVER_FACTORY;
+  private NameResolver.Factory nameResolverFactory = nameResolverRegistry.asFactory();
 
   final String target;
 
@@ -238,7 +236,7 @@ public abstract class AbstractManagedChannelImplBuilder
     if (resolverFactory != null) {
       this.nameResolverFactory = resolverFactory;
     } else {
-      this.nameResolverFactory = DEFAULT_NAME_RESOLVER_FACTORY;
+      this.nameResolverFactory = nameResolverRegistry.asFactory();
     }
     return thisT();
   }

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

@@ -63,6 +63,7 @@ import io.grpc.MethodDescriptor;
 import io.grpc.NameResolver;
 import io.grpc.NameResolver.ConfigOrError;
 import io.grpc.NameResolver.ResolutionResult;
+import io.grpc.NameResolverRegistry;
 import io.grpc.ProxyDetector;
 import io.grpc.Status;
 import io.grpc.SynchronizationContext;
@@ -128,6 +129,7 @@ final class ManagedChannelImpl extends ManagedChannel implements
 
   private final InternalLogId logId;
   private final String target;
+  private final NameResolverRegistry nameResolverRegistry;
   private final NameResolver.Factory nameResolverFactory;
   private final NameResolver.Args nameResolverArgs;
   private final AutoConfiguredLoadBalancerFactory loadBalancerFactory;
@@ -559,6 +561,7 @@ final class ManagedChannelImpl extends ManagedChannel implements
         builder.proxyDetector != null ? builder.proxyDetector : GrpcUtil.getDefaultProxyDetector();
     this.retryEnabled = builder.retryEnabled && !builder.temporarilyDisableRetry;
     this.loadBalancerFactory = new AutoConfiguredLoadBalancerFactory(builder.defaultLbPolicy);
+    this.nameResolverRegistry = builder.nameResolverRegistry;
     this.nameResolverArgs = NameResolver.Args.newBuilder()
         .setDefaultPort(builder.getDefaultPort())
         .setProxyDetector(proxyDetector)
@@ -1294,6 +1297,16 @@ final class ManagedChannelImpl extends ManagedChannel implements
     public ChannelLogger getChannelLogger() {
       return channelLogger;
     }
+
+    @Override
+    public NameResolver.Args getNameResolverArgs() {
+      return nameResolverArgs;
+    }
+
+    @Override
+    public NameResolverRegistry getNameResolverRegistry() {
+      return nameResolverRegistry;
+    }
   }
 
   private final class NameResolverListener extends NameResolver.Listener2 {

core/src/main/java/io/grpc/util/ForwardingLoadBalancerHelper.java

@@ -27,6 +27,7 @@ import io.grpc.LoadBalancer.SubchannelPicker;
 import io.grpc.LoadBalancer;
 import io.grpc.ManagedChannel;
 import io.grpc.NameResolver;
+import io.grpc.NameResolverRegistry;
 import io.grpc.SynchronizationContext;
 import java.util.List;
 import java.util.concurrent.ScheduledExecutorService;
@@ -107,6 +108,16 @@ public abstract class ForwardingLoadBalancerHelper extends LoadBalancer.Helper {
     return delegate().getChannelLogger();
   }
 
+  @Override
+  public NameResolver.Args getNameResolverArgs() {
+    return delegate().getNameResolverArgs();
+  }
+
+  @Override
+  public NameResolverRegistry getNameResolverRegistry() {
+    return delegate().getNameResolverRegistry();
+  }
+
   @Override
   public String toString() {
     return MoreObjects.toStringHelper(this).add("delegate", delegate()).toString();

core/src/test/java/io/grpc/internal/ManagedChannelImplTest.java

@@ -93,6 +93,7 @@ import io.grpc.MethodDescriptor.MethodType;
 import io.grpc.NameResolver;
 import io.grpc.NameResolver.ConfigOrError;
 import io.grpc.NameResolver.ResolutionResult;
+import io.grpc.NameResolverRegistry;
 import io.grpc.ProxiedSocketAddress;
 import io.grpc.ProxyDetector;
 import io.grpc.SecurityLevel;
@@ -1602,6 +1603,26 @@ public class ManagedChannelImplTest {
     }
   }
 
+  @Test
+  public void lbHelper_getNameResolverArgs() {
+    createChannel();
+
+    NameResolver.Args args = helper.getNameResolverArgs();
+    assertThat(args.getDefaultPort()).isEqualTo(DEFAULT_PORT);
+    assertThat(args.getProxyDetector()).isSameInstanceAs(GrpcUtil.DEFAULT_PROXY_DETECTOR);
+    assertThat(args.getSynchronizationContext())
+        .isSameInstanceAs(helper.getSynchronizationContext());
+    assertThat(args.getServiceConfigParser()).isNotNull();
+  }
+
+  @Test
+  public void lbHelper_getNameResolverRegistry() {
+    createChannel();
+
+    assertThat(helper.getNameResolverRegistry())
+        .isSameInstanceAs(NameResolverRegistry.getDefaultRegistry());
+  }
+
   @Test
   public void refreshNameResolution_whenSubchannelConnectionFailed_notIdle() {
     subtestNameResolutionRefreshWhenConnectionFailed(false, false);