rls: Avoid missed config update from reentrancy

Since ChildPolicyWrapper() called into the child before
childPolicyMap.put(), it is possible for that child to call back into
RLS and further update state without that child being known. When CDS
is_dynamic=true, it registers the cluster with XdsDependencyManager,
which adds a watch to XdsClient. If XdsClient already has the results
cached then the watch callback can be enqueued immediately onto the
syncContext and execute still within the constructor.

Calling into the child with the lock held isn't great, as it allows for
this type of reentrancy bug. But that'll take larger changes to fix.

b/464116731

Author: ejona86

rls/src/main/java/io/grpc/rls/LbPolicyConfiguration.java

@@ -245,9 +245,10 @@ ChildPolicyWrapper createOrGet(String target) {
       RefCountedChildPolicyWrapper pooledChildPolicyWrapper = childPolicyMap.get(target);
       if (pooledChildPolicyWrapper == null) {
         ChildPolicyWrapper childPolicyWrapper = new ChildPolicyWrapper(
-            target, childPolicy, childLbResolvedAddressFactory, childLbHelperProvider);
+            target, childPolicy, childLbHelperProvider);
         pooledChildPolicyWrapper = RefCountedChildPolicyWrapper.of(childPolicyWrapper);
         childPolicyMap.put(target, pooledChildPolicyWrapper);
+        childPolicyWrapper.start(childLbResolvedAddressFactory);
         return pooledChildPolicyWrapper.getObject();
       } else {
         ChildPolicyWrapper childPolicyWrapper = pooledChildPolicyWrapper.getObject();
@@ -294,7 +295,6 @@ static final class ChildPolicyWrapper {
     public ChildPolicyWrapper(
         String target,
         ChildLoadBalancingPolicy childPolicy,
-        final ResolvedAddressFactory childLbResolvedAddressFactory,
         ChildLoadBalancerHelperProvider childLbHelperProvider) {
       this.target = target;
       this.helper = new ChildPolicyReportingHelper(childLbHelperProvider);
@@ -307,6 +307,9 @@ public ChildPolicyWrapper(
       this.childLbConfig = lbConfig.getConfig();
       helper.getChannelLogger().log(
           ChannelLogLevel.DEBUG, "RLS child lb created. config: {0}", childLbConfig);
+    }
+
+    void start(ResolvedAddressFactory childLbResolvedAddressFactory) {
       helper.getSynchronizationContext().execute(
           new Runnable() {
             @Override

rls/src/test/java/io/grpc/rls/LbPolicyConfigurationTest.java

@@ -21,6 +21,7 @@
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
@@ -46,6 +47,7 @@
 import io.grpc.rls.LbPolicyConfiguration.RefCountedChildPolicyWrapperFactory;
 import java.lang.Thread.UncaughtExceptionHandler;
 import java.util.Map;
+import java.util.concurrent.atomic.AtomicBoolean;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -60,6 +62,9 @@ public class LbPolicyConfigurationTest {
   private final LoadBalancer lb = mock(LoadBalancer.class);
   private final SubchannelStateManager subchannelStateManager = new SubchannelStateManagerImpl();
   private final SubchannelPicker picker = mock(SubchannelPicker.class);
+  private final SynchronizationContext syncContext = new SynchronizationContext((t, e) -> {
+    throw new AssertionError(e);
+  });
   private final ResolvedAddressFactory resolvedAddressFactory =
       new ResolvedAddressFactory() {
         @Override
@@ -81,15 +86,7 @@ public ResolvedAddresses create(Object childLbConfig) {
   @Before
   public void setUp() {
     doReturn(mock(ChannelLogger.class)).when(helper).getChannelLogger();
-    doReturn(
-        new SynchronizationContext(
-            new UncaughtExceptionHandler() {
-              @Override
-              public void uncaughtException(Thread t, Throwable e) {
-                throw new AssertionError(e);
-              }
-            }))
-        .when(helper).getSynchronizationContext();
+    doReturn(syncContext).when(helper).getSynchronizationContext();
     doReturn(lb).when(lbProvider).newLoadBalancer(any(Helper.class));
     doReturn(ConfigOrError.fromConfig(new Object()))
         .when(lbProvider).parseLoadBalancingPolicyConfig(ArgumentMatchers.<Map<String, ?>>any());
@@ -186,4 +183,22 @@ public void updateBalancingState_triggersListener() {
     // picker governs childPickers will be reported to parent LB
     verify(helper).updateBalancingState(ConnectivityState.READY, picker);
   }
+
+  @Test
+  public void refCountedGetOrCreate_addsChildBeforeConfiguringChild() {
+    AtomicBoolean calledAlready = new AtomicBoolean();
+    when(lb.acceptResolvedAddresses(any(ResolvedAddresses.class))).thenAnswer(i -> {
+      if (!calledAlready.get()) {
+        calledAlready.set(true);
+        // Should end up calling this function again, as this child should already be added to the
+        // list of children. In practice, this can be caused by CDS is_dynamic=true starting a watch
+        // when XdsClient already has the cluster cached (e.g., from another channel).
+        syncContext.execute(() ->
+            factory.acceptResolvedAddressFactory(resolvedAddressFactory));
+      }
+      return Status.OK;
+    });
+    ChildPolicyWrapper unused = factory.createOrGet("foo.google.com");
+    verify(lb, times(2)).acceptResolvedAddresses(any(ResolvedAddresses.class));
+  }
 }