xds: Delay priority LB updates from children (#9670)

If a child policy triggers an update to the parent priority policy
it will be ignored if an update is already in process.

This is the second attempt to make this change, the first one caused a
problem with the ring hash LB. A new test that uses actual control plane
and data plane servers is now included to prove the issue no longer
appears.

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

@@ -58,7 +58,7 @@ final class PriorityLoadBalancer extends LoadBalancer {
   private final XdsLogger logger;
 
   // Includes all active and deactivated children. Mutable. New entries are only added from priority
-  // 0 up to the selected priority. An entry is only deleted 15 minutes after the its deactivation.
+  // 0 up to the selected priority. An entry is only deleted 15 minutes after its deactivation.
   private final Map<String, ChildLbState> children = new HashMap<>();
 
   // Following fields are only null initially.
@@ -70,6 +70,8 @@ final class PriorityLoadBalancer extends LoadBalancer {
   @Nullable private String currentPriority;
   private ConnectivityState currentConnectivityState;
   private SubchannelPicker currentPicker;
+  // Set to true if currently in the process of handling resolved addresses.
+  private boolean handlingResolvedAddresses;
 
   PriorityLoadBalancer(Helper helper) {
     this.helper = checkNotNull(helper, "helper");
@@ -94,11 +96,13 @@ final class PriorityLoadBalancer extends LoadBalancer {
         children.get(priority).deactivate();
       }
     }
+    handlingResolvedAddresses = true;
     for (String priority : priorityNames) {
       if (children.containsKey(priority)) {
         children.get(priority).updateResolvedAddresses();
       }
     }
+    handlingResolvedAddresses = false;
     tryNextPriority();
     return true;
   }
@@ -134,8 +138,11 @@ final class PriorityLoadBalancer extends LoadBalancer {
         ChildLbState child =
             new ChildLbState(priority, priorityConfigs.get(priority).ignoreReresolution);
         children.put(priority, child);
-        child.updateResolvedAddresses();
         updateOverallState(priority, CONNECTING, BUFFER_PICKER);
+        // Calling the child's updateResolvedAddresses() can result in tryNextPriority() being
+        // called recursively. We need to be sure to be done with processing here before it is
+        // called.
+        child.updateResolvedAddresses();
         return; // Give priority i time to connect.
       }
       ChildLbState child = children.get(priority);
@@ -298,18 +305,16 @@ final class PriorityLoadBalancer extends LoadBalancer {
       @Override
       public void updateBalancingState(final ConnectivityState newState,
           final SubchannelPicker newPicker) {
-        syncContext.execute(new Runnable() {
-          @Override
-          public void run() {
         if (!children.containsKey(priority)) {
           return;
         }
         connectivityState = newState;
         picker = newPicker;
+
         if (deletionTimer != null && deletionTimer.isPending()) {
           return;
         }
-            if (newState.equals(CONNECTING) ) {
+        if (newState.equals(CONNECTING)) {
           if (!failOverTimer.isPending() && seenReadyOrIdleSinceTransientFailure) {
             failOverTimer = syncContext.schedule(new FailOverTask(), 10, TimeUnit.SECONDS,
                 executor);
@@ -321,9 +326,12 @@ final class PriorityLoadBalancer extends LoadBalancer {
           seenReadyOrIdleSinceTransientFailure = false;
           failOverTimer.cancel();
         }
+
+        // If we are currently handling newly resolved addresses, let's not try to reconfigure as
+        // the address handling process will take care of that to provide an atomic config update.
+        if (!handlingResolvedAddresses) {
           tryNextPriority();
         }
-        });
       }
 
       @Override

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

@@ -24,6 +24,7 @@ import com.github.xds.type.v3.TypedStruct;
 import com.google.protobuf.Any;
 import com.google.protobuf.Struct;
 import com.google.protobuf.Value;
+import io.envoyproxy.envoy.config.cluster.v3.Cluster.LbPolicy;
 import io.envoyproxy.envoy.config.cluster.v3.LoadBalancingPolicy;
 import io.envoyproxy.envoy.config.cluster.v3.LoadBalancingPolicy.Policy;
 import io.envoyproxy.envoy.config.core.v3.TypedExtensionConfig;
@@ -151,4 +152,24 @@ public class FakeControlPlaneXdsIntegrationTest {
       };
     }
   }
+
+  /**
+   * Basic test to make sure RING_HASH configuration works.
+   */
+  @Test
+  public void pingPong_ringHash() {
+    controlPlane.setCdsConfig(
+        ControlPlaneRule.buildCluster().toBuilder()
+            .setLbPolicy(LbPolicy.RING_HASH).build());
+
+    ManagedChannel channel = dataPlane.getManagedChannel();
+    SimpleServiceGrpc.SimpleServiceBlockingStub blockingStub = SimpleServiceGrpc.newBlockingStub(
+        channel);
+    SimpleRequest request = SimpleRequest.newBuilder()
+        .build();
+    SimpleResponse goldenResponse = SimpleResponse.newBuilder()
+        .setResponseMessage("Hi, xDS!")
+        .build();
+    assertEquals(goldenResponse, blockingStub.unaryRpc(request));
+  }
 }

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

@@ -22,6 +22,7 @@ import static io.grpc.ConnectivityState.IDLE;
 import static io.grpc.ConnectivityState.READY;
 import static io.grpc.ConnectivityState.TRANSIENT_FAILURE;
 import static io.grpc.xds.XdsSubchannelPickers.BUFFER_PICKER;
+import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.ArgumentMatchers.isA;
 import static org.mockito.Mockito.atLeastOnce;
@@ -686,6 +687,37 @@ public class PriorityLoadBalancerTest {
     verifyNoMoreInteractions(helper);
   }
 
+  @Test
+  public void noDuplicateOverallBalancingStateUpdate() {
+    FakeLoadBalancerProvider fakeLbProvider = new FakeLoadBalancerProvider();
+
+    PriorityChildConfig priorityChildConfig0 =
+        new PriorityChildConfig(new PolicySelection(fakeLbProvider, new Object()), true);
+    PriorityChildConfig priorityChildConfig1 =
+        new PriorityChildConfig(new PolicySelection(fakeLbProvider, new Object()), false);
+    PriorityLbConfig priorityLbConfig =
+        new PriorityLbConfig(
+            ImmutableMap.of("p0", priorityChildConfig0),
+            ImmutableList.of("p0"));
+    priorityLb.handleResolvedAddresses(
+        ResolvedAddresses.newBuilder()
+            .setAddresses(ImmutableList.<EquivalentAddressGroup>of())
+            .setLoadBalancingPolicyConfig(priorityLbConfig)
+            .build());
+
+    priorityLbConfig =
+        new PriorityLbConfig(
+            ImmutableMap.of("p0", priorityChildConfig0, "p1", priorityChildConfig1),
+            ImmutableList.of("p0", "p1"));
+    priorityLb.handleResolvedAddresses(
+        ResolvedAddresses.newBuilder()
+            .setAddresses(ImmutableList.<EquivalentAddressGroup>of())
+            .setLoadBalancingPolicyConfig(priorityLbConfig)
+            .build());
+
+    verify(helper, times(6)).updateBalancingState(any(), any());
+  }
+
   private void assertLatestConnectivityState(ConnectivityState expectedState) {
     verify(helper, atLeastOnce())
         .updateBalancingState(connectivityStateCaptor.capture(), pickerCaptor.capture());
@@ -714,4 +746,49 @@ public class PriorityLoadBalancerTest {
     PickResult pickResult = pickerCaptor.getValue().pickSubchannel(mock(PickSubchannelArgs.class));
     assertThat(pickResult).isEqualTo(PickResult.withNoResult());
   }
+
+  private static class FakeLoadBalancerProvider extends LoadBalancerProvider {
+
+    @Override
+    public boolean isAvailable() {
+      return true;
+    }
+
+    @Override
+    public int getPriority() {
+      return 5;
+    }
+
+    @Override
+    public String getPolicyName() {
+      return "foo";
+    }
+
+    @Override
+    public LoadBalancer newLoadBalancer(Helper helper) {
+      return new FakeLoadBalancer(helper);
+    }
+  }
+
+  static class FakeLoadBalancer extends LoadBalancer {
+
+    private Helper helper;
+
+    FakeLoadBalancer(Helper helper) {
+      this.helper = helper;
+    }
+
+    @Override
+    public void handleResolvedAddresses(ResolvedAddresses resolvedAddresses) {
+      helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.INTERNAL));
+    }
+
+    @Override
+    public void handleNameResolutionError(Status error) {
+    }
+
+    @Override
+    public void shutdown() {
+    }
+  }
 }