util: Replace BUFFER_PICKER with FixedResultPicker

I think at some point there were more usages in the tests. But now it
is pretty easy.

PriorityLb.ChildLbState.picker is initialized to
FixedResultPicker(NoResult). So now that GracefulSwitchLb is using the
same picker, equals() is able to de-dup an update.

util/src/main/java/io/grpc/util/GracefulSwitchLoadBalancer.java

@@ -19,7 +19,6 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Preconditions.checkState;
 
-import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.MoreObjects;
 import com.google.common.base.Objects;
 import io.grpc.ConnectivityState;
@@ -66,19 +65,6 @@ public void handleNameResolutionError(final Status error) {
     public void shutdown() {}
   };
 
-  @VisibleForTesting
-  static final SubchannelPicker BUFFER_PICKER = new SubchannelPicker() {
-    @Override
-    public PickResult pickSubchannel(PickSubchannelArgs args) {
-      return PickResult.withNoResult();
-    }
-
-    @Override
-    public String toString() {
-      return "BUFFER_PICKER";
-    }
-  };
-
   private final Helper helper;
 
   // While the new policy is not fully switched on, the pendingLb is handling new updates from name
@@ -128,7 +114,7 @@ private void switchToInternal(LoadBalancer.Factory newBalancerFactory) {
     pendingLb = defaultBalancer;
     pendingBalancerFactory = null;
     pendingState = ConnectivityState.CONNECTING;
-    pendingPicker = BUFFER_PICKER;
+    pendingPicker = new FixedResultPicker(PickResult.withNoResult());
 
     if (newBalancerFactory.equals(currentBalancerFactory)) {
       return;

util/src/test/java/io/grpc/util/GracefulSwitchLoadBalancerTest.java

@@ -21,7 +21,6 @@
 import static io.grpc.ConnectivityState.IDLE;
 import static io.grpc.ConnectivityState.READY;
 import static io.grpc.ConnectivityState.TRANSIENT_FAILURE;
-import static io.grpc.util.GracefulSwitchLoadBalancer.BUFFER_PICKER;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.inOrder;
@@ -521,7 +520,11 @@ public void switchWhileOldPolicyGoesFromReadyToNotReadyWhileNewPolicyStillIdle()
     helper0.updateBalancingState(CONNECTING, picker);
 
     verify(mockHelper, never()).updateBalancingState(CONNECTING, picker);
-    inOrder.verify(mockHelper).updateBalancingState(CONNECTING, BUFFER_PICKER);
+    ArgumentCaptor<SubchannelPicker> pickerCaptor = ArgumentCaptor.forClass(SubchannelPicker.class);
+    inOrder.verify(mockHelper).updateBalancingState(eq(CONNECTING), pickerCaptor.capture());
+    assertThat(pickerCaptor.getValue().pickSubchannel(mock(PickSubchannelArgs.class)).hasResult())
+        .isFalse();
+
     inOrder.verify(lb0).shutdown(); // shutdown after update
 
     picker = mock(SubchannelPicker.class);

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

@@ -522,8 +522,7 @@ public void connectingResetFailOverIfSeenReadyOrIdleSinceTransientFailure() {
             .setLoadBalancingPolicyConfig(priorityLbConfig)
             .build());
     // Nothing important about this verify, other than to provide a baseline
-    verify(helper, times(2))
-        .updateBalancingState(eq(CONNECTING), pickerReturns(PickResult.withNoResult()));
+    verify(helper).updateBalancingState(eq(CONNECTING), pickerReturns(PickResult.withNoResult()));
     assertThat(fooBalancers).hasSize(1);
     assertThat(fooHelpers).hasSize(1);
     Helper helper0 = Iterables.getOnlyElement(fooHelpers);
@@ -539,7 +538,7 @@ public void connectingResetFailOverIfSeenReadyOrIdleSinceTransientFailure() {
     helper0.updateBalancingState(
         CONNECTING,
         EMPTY_PICKER);
-    verify(helper, times(3))
+    verify(helper, times(2))
         .updateBalancingState(eq(CONNECTING), pickerReturns(PickResult.withNoResult()));
 
     // failover happens
@@ -805,7 +804,7 @@ public void raceBetweenShutdownAndChildLbBalancingStateUpdate() {
             .setAddresses(ImmutableList.<EquivalentAddressGroup>of())
             .setLoadBalancingPolicyConfig(priorityLbConfig)
             .build());
-    verify(helper, times(2)).updateBalancingState(eq(CONNECTING), isA(SubchannelPicker.class));
+    verify(helper).updateBalancingState(eq(CONNECTING), isA(SubchannelPicker.class));
 
     // LB shutdown and subchannel state change can happen simultaneously. If shutdown runs first,
     // any further balancing state update should be ignored.
@@ -843,7 +842,7 @@ public void noDuplicateOverallBalancingStateUpdate() {
             .setLoadBalancingPolicyConfig(priorityLbConfig)
             .build());
 
-    verify(helper, times(6)).updateBalancingState(any(), any());
+    verify(helper, times(4)).updateBalancingState(any(), any());
   }
 
   private void assertLatestConnectivityState(ConnectivityState expectedState) {