Reintroduce canHandleEmptyAddressListFromNameResolution()

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

@@ -153,13 +153,20 @@ public void handleResolvedAddresses(ResolvedAddresses resolvedAddresses) {
    */
   @SuppressWarnings("deprecation")
   public boolean acceptResolvedAddresses(ResolvedAddresses resolvedAddresses) {
-    if (recursionCount++ == 0) {
-      handleResolvedAddresses(resolvedAddresses);
-    }
-    recursionCount = 0;
+    if (resolvedAddresses.getAddresses().isEmpty()
+        && !canHandleEmptyAddressListFromNameResolution()) {
+      handleNameResolutionError(Status.UNAVAILABLE.withDescription(
+          "NameResolver returned no usable address. addrs=" + resolvedAddresses.getAddresses()
+              + ", attrs=" + resolvedAddresses.getAttributes()));
+      return false;
+    } else {
+      if (recursionCount++ == 0) {
+        handleResolvedAddresses(resolvedAddresses);
+      }
+      recursionCount = 0;
 
-    // The legacy method does not tell us if the addresses were accepted, so we default to true.
-    return true;
+      return true;
+    }
   }
 
   /**
@@ -366,6 +373,20 @@ public void handleSubchannelState(
    */
   public abstract void shutdown();
 
+  /**
+   * Whether this LoadBalancer can handle empty address group list to be passed to {@link
+   * #handleResolvedAddresses(ResolvedAddresses)}.  The default implementation returns
+   * {@code false}, meaning that if the NameResolver returns an empty list, the Channel will turn
+   * that into an error and call {@link #handleNameResolutionError}.  LoadBalancers that want to
+   * accept empty lists should override this method and return {@code true}.
+   *
+   * <p>This method should always return a constant value.  It's not specified when this will be
+   * called.
+   */
+  public boolean canHandleEmptyAddressListFromNameResolution() {
+    return false;
+  }
+
   /**
    * The channel asks the LoadBalancer to establish connections now (if applicable) so that the
    * upcoming RPC may then just pick a ready connection without waiting for connections.  This

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

@@ -57,6 +57,11 @@ public void shutdown() {
     delegate().shutdown();
   }
 
+  @Override
+  public boolean canHandleEmptyAddressListFromNameResolution() {
+    return delegate().canHandleEmptyAddressListFromNameResolution();
+  }
+
   @Override
   public void requestConnection() {
     delegate().requestConnection();