services: cancel health-check when LoadBalancer.shutdown() is called. (#5051)

The health checking balancer won't receive an update about Subchannel
shutdown via handleSubchannelState(), because no more callback will be
called after LoadBalancer.shutdown() is called.

services/src/main/java/io/grpc/services/HealthCheckingLoadBalancerFactory.java

@@ -95,6 +95,7 @@ final class HealthCheckingLoadBalancerFactory extends Factory {
     
     private LoadBalancer delegateBalancer;
     @Nullable String healthCheckedService;
+    private boolean balancerShutdown;
 
     final HashSet<HealthCheckState> hcStates = new HashSet<HealthCheckState>();
 
@@ -119,7 +120,7 @@ final class HealthCheckingLoadBalancerFactory extends Factory {
       // createSubchannel() from the SynchronizationContext.
       syncContext.throwIfNotInThisSynchronizationContext();
       HealthCheckState hcState = new HealthCheckState(
-          delegateBalancer, syncContext, delegate.getScheduledExecutorService());
+          this, delegateBalancer, syncContext, delegate.getScheduledExecutorService());
       hcStates.add(hcState);
       Subchannel subchannel = super.createSubchannel(
           addrs, attrs.toBuilder().set(KEY_HEALTH_CHECK_STATE, hcState).build());
@@ -183,6 +184,19 @@ final class HealthCheckingLoadBalancerFactory extends Factory {
       }
     }
 
+    @Override
+    public void shutdown() {
+      super.shutdown();
+      helper.balancerShutdown = true;
+      for (HealthCheckState hcState : helper.hcStates) {
+        // ManagedChannel will stop calling handleSubchannelState() after shutdown() is called,
+        // which is required by LoadBalancer API semantics. We need to deliver the final SHUTDOWN
+        // signal to health checkers so that they can cancel the streams.
+        hcState.updateRawState(ConnectivityStateInfo.forNonError(SHUTDOWN));
+      }
+      helper.hcStates.clear();
+    }
+
     @Override
     public String toString() {
       return MoreObjects.toStringHelper(this).add("delegate", delegate()).toString();
@@ -202,6 +216,7 @@ final class HealthCheckingLoadBalancerFactory extends Factory {
     private final LoadBalancer delegate;
     private final SynchronizationContext syncContext;
     private final ScheduledExecutorService timerService;
+    private final HelperImpl helperImpl;
 
     private Subchannel subchannel;
     private ChannelLogger subchannelLogger;
@@ -225,8 +240,10 @@ final class HealthCheckingLoadBalancerFactory extends Factory {
     private ScheduledHandle retryTimer;
 
     HealthCheckState(
+        HelperImpl helperImpl,
         LoadBalancer delegate, SynchronizationContext syncContext,
         ScheduledExecutorService timerService) {
+      this.helperImpl = checkNotNull(helperImpl, "helperImpl");
       this.delegate = checkNotNull(delegate, "delegate");
       this.syncContext = checkNotNull(syncContext, "syncContext");
       this.timerService = checkNotNull(timerService, "timerService");
@@ -315,7 +332,7 @@ final class HealthCheckingLoadBalancerFactory extends Factory {
 
     private void gotoState(ConnectivityStateInfo newState) {
       checkState(subchannel != null, "init() not called");
-      if (!Objects.equal(concludedState, newState)) {
+      if (!helperImpl.balancerShutdown && !Objects.equal(concludedState, newState)) {
         concludedState = newState;
         delegate.handleSubchannelState(subchannel, concludedState);
       }

services/src/test/java/io/grpc/services/HealthCheckingLoadBalancerFactoryTest.java

@@ -183,14 +183,19 @@ public class HealthCheckingLoadBalancerFactoryTest {
     hcLb = hcLbFactory.newLoadBalancer(origHelper);
     // Make sure all calls into the hcLb is from the syncContext
     hcLbEventDelivery = new LoadBalancer() {
+        // Per LoadBalancer API, no more callbacks will be called after shutdown() is called.
+        boolean shutdown;
+
         @Override
         public void handleResolvedAddressGroups(
             final List<EquivalentAddressGroup> servers, final Attributes attributes) {
           syncContext.execute(new Runnable() {
               @Override
               public void run() {
+                if (!shutdown) {
                   hcLb.handleResolvedAddressGroups(servers, attributes);
                 }
+              }
             });
         }
 
@@ -200,8 +205,10 @@ public class HealthCheckingLoadBalancerFactoryTest {
           syncContext.execute(new Runnable() {
               @Override
               public void run() {
+                if (!shutdown) {
                   hcLb.handleSubchannelState(subchannel, stateInfo);
                 }
+              }
             });
         }
 
@@ -212,7 +219,15 @@ public class HealthCheckingLoadBalancerFactoryTest {
 
         @Override
         public void shutdown() {
-          throw new AssertionError("Not supposed to be called");
+          syncContext.execute(new Runnable() {
+              @Override
+              public void run() {
+                if (!shutdown) {
+                  shutdown = true;
+                  hcLb.shutdown();
+                }
+              }
+            });
         }
       };
     verify(origLbFactory).newLoadBalancer(any(Helper.class));
@@ -942,6 +957,43 @@ public class HealthCheckingLoadBalancerFactoryTest {
         .isEqualTo("FooService");
   }
 
+  @Test
+  public void balancerShutdown() {
+    Attributes resolutionAttrs = attrsWithHealthCheckService("TeeService");
+    hcLbEventDelivery.handleResolvedAddressGroups(resolvedAddressList, resolutionAttrs);
+
+    verify(origLb).handleResolvedAddressGroups(same(resolvedAddressList), same(resolutionAttrs));
+    verifyNoMoreInteractions(origLb);
+
+    Subchannel subchannel = createSubchannel(0, Attributes.EMPTY);
+    assertThat(subchannel).isSameAs(subchannels[0]);
+
+    // Trigger the health check
+    hcLbEventDelivery.handleSubchannelState(subchannel, ConnectivityStateInfo.forNonError(READY));
+
+    HealthImpl healthImpl = healthImpls[0];
+    assertThat(healthImpl.calls).hasSize(1);
+    ServerSideCall serverCall = healthImpl.calls.poll();
+    assertThat(serverCall.cancelled).isFalse();
+
+    verify(origLb).handleSubchannelState(
+        same(subchannel), eq(ConnectivityStateInfo.forNonError(CONNECTING)));
+
+    // Shut down the balancer
+    hcLbEventDelivery.shutdown();
+    verify(origLb).shutdown();
+
+    // Health check stream should be cancelled
+    assertThat(serverCall.cancelled).isTrue();
+
+    // LoadBalancer API requires no more callbacks on LoadBalancer after shutdown() is called.
+    verifyNoMoreInteractions(origLb);
+
+    // No more health check call is made or scheduled
+    assertThat(healthImpl.calls).isEmpty();
+    assertThat(clock.getPendingTasks()).isEmpty();
+  }
+
   @Test
   public void util_newHealthCheckingLoadBalancer() {
     Factory hcFactory =