xds: Allow child of cluster_impl LB to change

Under normal conditions the child LB of `ClusterImplLoadBalancer` does
not fluctuate, based on the field used to configure load balancing in
the xDS `Cluster` proto it is either:

1. `WrrLocalityLoadBalancer` if the newer `load_balancing_policy` field
   is used
2. `WeightedTargetLoadBalancer` if the legacy `lb_policy` field is used

`ClusterImplLoadBalancer` currently assumes that this child does not
change and so does not change the child LB when the name resolver sends an
update. If the control plane does switch to using a different field for
LB config, that update will have an LB config meant for the other child
LB type. This will result in a ClassCastException and a channel panic.

To address this, `ClusterImplLoadBalancer` will now use
`GracefulSwitchLoadBalancer` and makes sure if the child policy changes
the correct LB implementation is switched to.

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

@@ -35,6 +35,7 @@
 import io.grpc.internal.ObjectPool;
 import io.grpc.util.ForwardingLoadBalancerHelper;
 import io.grpc.util.ForwardingSubchannel;
+import io.grpc.util.GracefulSwitchLoadBalancer;
 import io.grpc.xds.Bootstrapper.ServerInfo;
 import io.grpc.xds.ClusterImplLoadBalancerProvider.ClusterImplConfig;
 import io.grpc.xds.Endpoints.DropOverload;
@@ -87,7 +88,7 @@ final class ClusterImplLoadBalancer extends LoadBalancer {
   private CallCounterProvider callCounterProvider;
   private ClusterDropStats dropStats;
   private ClusterImplLbHelper childLbHelper;
-  private LoadBalancer childLb;
+  private GracefulSwitchLoadBalancer childSwitchLb;
 
   ClusterImplLoadBalancer(Helper helper) {
     this(helper, ThreadSafeRandomImpl.instance);
@@ -120,7 +121,7 @@ public boolean acceptResolvedAddresses(ResolvedAddresses resolvedAddresses) {
       childLbHelper = new ClusterImplLbHelper(
           callCounterProvider.getOrCreate(config.cluster, config.edsServiceName),
           config.lrsServerInfo);
-      childLb = config.childPolicy.getProvider().newLoadBalancer(childLbHelper);
+      childSwitchLb = new GracefulSwitchLoadBalancer(childLbHelper);
       // Assume load report server does not change throughout cluster lifetime.
       if (config.lrsServerInfo != null) {
         dropStats = xdsClient.addClusterDropStats(config.lrsServerInfo, cluster, edsServiceName);
@@ -129,7 +130,8 @@ public boolean acceptResolvedAddresses(ResolvedAddresses resolvedAddresses) {
     childLbHelper.updateDropPolicies(config.dropCategories);
     childLbHelper.updateMaxConcurrentRequests(config.maxConcurrentRequests);
     childLbHelper.updateSslContextProviderSupplier(config.tlsContext);
-    childLb.handleResolvedAddresses(
+    childSwitchLb.switchTo(config.childPolicy.getProvider());
+    childSwitchLb.handleResolvedAddresses(
         resolvedAddresses.toBuilder()
             .setAttributes(attributes)
             .setLoadBalancingPolicyConfig(config.childPolicy.getConfig())
@@ -139,8 +141,8 @@ public boolean acceptResolvedAddresses(ResolvedAddresses resolvedAddresses) {
 
   @Override
   public void handleNameResolutionError(Status error) {
-    if (childLb != null) {
-      childLb.handleNameResolutionError(error);
+    if (childSwitchLb != null) {
+      childSwitchLb.handleNameResolutionError(error);
     } else {
       helper.updateBalancingState(ConnectivityState.TRANSIENT_FAILURE, new ErrorPicker(error));
     }
@@ -151,8 +153,8 @@ public void shutdown() {
     if (dropStats != null) {
       dropStats.release();
     }
-    if (childLb != null) {
-      childLb.shutdown();
+    if (childSwitchLb != null) {
+      childSwitchLb.shutdown();
       if (childLbHelper != null) {
         childLbHelper.updateSslContextProviderSupplier(null);
         childLbHelper = null;

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

@@ -165,6 +165,41 @@ public void handleResolvedAddresses_propagateToChildPolicy() {
         .isSameInstanceAs(xdsClientPool);
   }
 
+  /**
+   * If the control plane switches from using the legacy lb_policy field in the xDS Cluster proto
+   * to the newer load_balancing_policy then the child policy can switch from weighted_target to
+   * xds_wrr_locality (this could happen the opposite way as well). This test assures that this
+   * results in the child LB changing if this were to happen. If this is not done correctly the new
+   * configuration would be given to the old LB implementation which would cause a channel panic.
+   */
+  @Test
+  public void handleResolvedAddresses_childPolicyChanges() {
+    FakeLoadBalancerProvider weightedTargetProvider =
+        new FakeLoadBalancerProvider(XdsLbPolicies.WEIGHTED_TARGET_POLICY_NAME);
+    Object weightedTargetConfig = new Object();
+    ClusterImplConfig configWithWeightedTarget = new ClusterImplConfig(CLUSTER, EDS_SERVICE_NAME,
+        LRS_SERVER_INFO,
+        null, Collections.<DropOverload>emptyList(),
+        new PolicySelection(weightedTargetProvider, weightedTargetConfig), null);
+    EquivalentAddressGroup endpoint = makeAddress("endpoint-addr", locality);
+    deliverAddressesAndConfig(Collections.singletonList(endpoint), configWithWeightedTarget);
+    FakeLoadBalancer childBalancer = Iterables.getOnlyElement(downstreamBalancers);
+    assertThat(childBalancer.name).isEqualTo(XdsLbPolicies.WEIGHTED_TARGET_POLICY_NAME);
+    assertThat(childBalancer.config).isSameInstanceAs(weightedTargetConfig);
+
+    FakeLoadBalancerProvider wrrLocalityProvider =
+        new FakeLoadBalancerProvider(XdsLbPolicies.WRR_LOCALITY_POLICY_NAME);
+    Object wrrLocalityConfig = new Object();
+    ClusterImplConfig configWithWrrLocality = new ClusterImplConfig(CLUSTER, EDS_SERVICE_NAME,
+        LRS_SERVER_INFO,
+        null, Collections.<DropOverload>emptyList(),
+        new PolicySelection(wrrLocalityProvider, wrrLocalityConfig), null);
+    deliverAddressesAndConfig(Collections.singletonList(endpoint), configWithWrrLocality);
+    childBalancer = Iterables.getOnlyElement(downstreamBalancers);
+    assertThat(childBalancer.name).isEqualTo(XdsLbPolicies.WRR_LOCALITY_POLICY_NAME);
+    assertThat(childBalancer.config).isSameInstanceAs(wrrLocalityConfig);
+  }
+
   @Test
   public void nameResolutionError_beforeChildPolicyInstantiated_returnErrorPickerToUpstream() {
     loadBalancer.handleNameResolutionError(Status.UNIMPLEMENTED.withDescription("not found"));