xds: fix bug for name resolution error propagation in prioirty_lb

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

@@ -101,11 +101,15 @@ final class PriorityLoadBalancer extends LoadBalancer {
   @Override
   public void handleNameResolutionError(Status error) {
     logger.log(XdsLogLevel.WARNING, "Received name resolution error: {0}", error);
-    if (children.isEmpty()) {
+    boolean gotoTransientFailure = true;
-      updateOverallState(TRANSIENT_FAILURE, new ErrorPicker(error));
-    }
     for (ChildLbState child : children.values()) {
-      child.lb.handleNameResolutionError(error);
+      if (priorityNames.contains(child.priority)) {
+        child.lb.handleNameResolutionError(error);
+        gotoTransientFailure = false;
+      }
+    }
+    if (gotoTransientFailure) {
+      updateOverallState(TRANSIENT_FAILURE, new ErrorPicker(error));
     }
   }
 

xds/src/test/java/io/grpc/xds/PriorityLoadBalancerTest.java

@@ -214,6 +214,43 @@ public class PriorityLoadBalancerTest {
     verify(barBalancer0, never()).shutdown();
   }
 
+  @Test
+  public void handleNameResolutionError() {
+    Object fooConfig0 = new Object();
+    PolicySelection fooPolicy0 = new PolicySelection(fooLbProvider, null, fooConfig0);
+    Object fooConfig1 = new Object();
+    PolicySelection fooPolicy1 = new PolicySelection(fooLbProvider, null, fooConfig1);
+
+    PriorityLbConfig priorityLbConfig =
+        new PriorityLbConfig(ImmutableMap.of("p0", fooPolicy0), ImmutableList.of("p0"));
+    priorityLb.handleResolvedAddresses(
+        ResolvedAddresses.newBuilder()
+            .setAddresses(ImmutableList.<EquivalentAddressGroup>of())
+            .setLoadBalancingPolicyConfig(priorityLbConfig)
+            .build());
+    LoadBalancer fooLb0 = Iterables.getOnlyElement(fooBalancers);
+    Status status = Status.DATA_LOSS.withDescription("fake error");
+    priorityLb.handleNameResolutionError(status);
+    verify(fooLb0).handleNameResolutionError(status);
+
+    priorityLbConfig =
+        new PriorityLbConfig(ImmutableMap.of("p1", fooPolicy1), ImmutableList.of("p1"));
+    priorityLb.handleResolvedAddresses(
+        ResolvedAddresses.newBuilder()
+            .setAddresses(ImmutableList.<EquivalentAddressGroup>of())
+            .setLoadBalancingPolicyConfig(priorityLbConfig)
+            .build());
+    assertThat(fooBalancers).hasSize(2);
+    LoadBalancer fooLb1 = Iterables.getLast(fooBalancers);
+    status = Status.UNAVAILABLE.withDescription("fake error");
+    priorityLb.handleNameResolutionError(status);
+    // fooLb0 is deactivated but not yet deleted. However, because it is delisted by the latest
+    // address update, name resolution error will not be propagated to it.
+    verify(fooLb0, never()).shutdown();
+    verify(fooLb0, never()).handleNameResolutionError(status);
+    verify(fooLb1).handleNameResolutionError(status);
+  }
+
   @Test
   public void typicalPriorityFailOverFlow() {
     PolicySelection policy0 = new PolicySelection(fooLbProvider, null, new Object());