api: Replace ErrorPicker with FixedResultPicker

FixedResultPicker can be used in more situations. Note that
WrrLocalityLoadBalancerTest's test was changed non-trivially. The
noChildLb test was particularly nasty as it assumed
LoadBalancer.ErrorPicker had same toString() as
GracefulSwitchLoadBalancer's ErrorPicker.

api/src/main/java/io/grpc/LoadBalancer.java

@@ -1412,6 +1412,12 @@ public abstract class LoadBalancer {
     public abstract LoadBalancer newLoadBalancer(Helper helper);
   }
 
+  /**
+   * A picker that always returns an erring pick.
+   *
+   * @deprecated Use {@code new FixedResultPicker(PickResult.withError(error))} instead.
+   */
+  @Deprecated
   public static final class ErrorPicker extends SubchannelPicker {
 
     private final Status error;
@@ -1433,4 +1439,22 @@ public abstract class LoadBalancer {
     }
   }
 
+  /** A picker that always returns the same result. */
+  public static final class FixedResultPicker extends SubchannelPicker {
+    private final PickResult result;
+
+    public FixedResultPicker(PickResult result) {
+      this.result = Preconditions.checkNotNull(result, "result");
+    }
+
+    @Override
+    public PickResult pickSubchannel(PickSubchannelArgs args) {
+      return result;
+    }
+
+    @Override
+    public String toString() {
+      return "FixedResultPicker(" + result + ")";
+    }
+  }
 }

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

@@ -20,7 +20,6 @@ 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 io.grpc.ConnectivityState;
 import io.grpc.ConnectivityStateInfo;
 import io.grpc.ExperimentalApi;
@@ -57,21 +56,9 @@ public final class GracefulSwitchLoadBalancer extends ForwardingLoadBalancer {
 
     @Override
     public void handleNameResolutionError(final Status error) {
-      class ErrorPicker extends SubchannelPicker {
-        @Override
-        public PickResult pickSubchannel(PickSubchannelArgs args) {
-          return PickResult.withError(error);
-        }
-
-        @Override
-        public String toString() {
-          return MoreObjects.toStringHelper(ErrorPicker.class).add("error", error).toString();
-        }
-      }
-
       helper.updateBalancingState(
           ConnectivityState.TRANSIENT_FAILURE,
-          new ErrorPicker());
+          new FixedResultPicker(PickResult.withError(error)));
     }
 
     @Override

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

@@ -83,7 +83,7 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
   }
 
   protected SubchannelPicker getErrorPicker(Status error)  {
-    return new ErrorPicker(error);
+    return new FixedResultPicker(PickResult.withError(error));
   }
 
   @VisibleForTesting

xds/src/main/java/io/grpc/xds/CdsLoadBalancer2.java

@@ -103,7 +103,8 @@ final class CdsLoadBalancer2 extends LoadBalancer {
     if (cdsLbState != null && cdsLbState.childLb != null) {
       cdsLbState.childLb.handleNameResolutionError(error);
     } else {
-      helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(error));
+      helper.updateBalancingState(
+          TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
     }
   }
 
@@ -211,7 +212,8 @@ final class CdsLoadBalancer2 extends LoadBalancer {
       }
 
       if (loopStatus != null) {
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(loopStatus));
+        helper.updateBalancingState(
+            TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(loopStatus)));
         return;
       }
 
@@ -223,7 +225,8 @@ final class CdsLoadBalancer2 extends LoadBalancer {
         Status unavailable =
             Status.UNAVAILABLE.withDescription("CDS error: found 0 leaf (logical DNS or EDS) "
                 + "clusters for root cluster " + root.name);
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(unavailable));
+        helper.updateBalancingState(
+            TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(unavailable)));
         return;
       }
 
@@ -295,7 +298,8 @@ final class CdsLoadBalancer2 extends LoadBalancer {
       if (childLb != null) {
         childLb.handleNameResolutionError(error);
       } else {
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(error));
+        helper.updateBalancingState(
+            TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
       }
     }
 

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

@@ -143,7 +143,8 @@ final class ClusterImplLoadBalancer extends LoadBalancer {
     if (childSwitchLb != null) {
       childSwitchLb.handleNameResolutionError(error);
     } else {
-      helper.updateBalancingState(ConnectivityState.TRANSIENT_FAILURE, new ErrorPicker(error));
+      helper.updateBalancingState(
+          ConnectivityState.TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
     }
   }
 

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

@@ -197,7 +197,8 @@ final class ClusterResolverLoadBalancer extends LoadBalancer {
       if (childLb != null) {
         childLb.handleNameResolutionError(error);
       } else {
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(error));
+        helper.updateBalancingState(
+            TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
       }
     }
 
@@ -240,7 +241,8 @@ final class ClusterResolverLoadBalancer extends LoadBalancer {
               Status.UNAVAILABLE.withCause(endpointNotFound.getCause())
                   .withDescription(endpointNotFound.getDescription());
         }
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(endpointNotFound));
+        helper.updateBalancingState(
+            TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(endpointNotFound)));
         if (childLb != null) {
           childLb.shutdown();
           childLb = null;
@@ -275,7 +277,8 @@ final class ClusterResolverLoadBalancer extends LoadBalancer {
         if (childLb != null) {
           childLb.handleNameResolutionError(error);
         } else {
-          helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(error));
+          helper.updateBalancingState(
+              TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
         }
       }
     }

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

@@ -126,7 +126,8 @@ final class PriorityLoadBalancer extends LoadBalancer {
       }
     }
     if (gotoTransientFailure) {
-      updateOverallState(null, TRANSIENT_FAILURE, new ErrorPicker(error));
+      updateOverallState(
+          null, TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
     }
   }
 
@@ -225,8 +226,8 @@ final class PriorityLoadBalancer extends LoadBalancer {
           // The child is deactivated.
           return;
         }
-        picker = new ErrorPicker(
+        picker = new FixedResultPicker(PickResult.withError(
-            Status.UNAVAILABLE.withDescription("Connection timeout for priority " + priority));
+            Status.UNAVAILABLE.withDescription("Connection timeout for priority " + priority)));
         logger.log(XdsLogLevel.DEBUG, "Priority {0} failed over to next", priority);
         currentPriority = null; // reset currentPriority to guarantee failover happen
         tryNextPriority();

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

@@ -270,7 +270,8 @@ final class RingHashLoadBalancer extends LoadBalancer {
   @Override
   public void handleNameResolutionError(Status error) {
     if (currentState != READY) {
-      helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(error));
+      helper.updateBalancingState(
+          TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
     }
   }
 

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

@@ -114,7 +114,8 @@ final class WeightedTargetLoadBalancer extends LoadBalancer {
   public void handleNameResolutionError(Status error) {
     logger.log(XdsLogLevel.WARNING, "Received name resolution error: {0}", error);
     if (childBalancers.isEmpty()) {
-      helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(error));
+      helper.updateBalancingState(
+          TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
     }
     for (LoadBalancer childBalancer : childBalancers.values()) {
       childBalancer.handleNameResolutionError(error);

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

@@ -78,14 +78,14 @@ final class WrrLocalityLoadBalancer extends LoadBalancer {
       Integer localityWeight = eagAttrs.get(InternalXdsAttributes.ATTR_LOCALITY_WEIGHT);
 
       if (locality == null) {
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(
+        helper.updateBalancingState(TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(
-            Status.UNAVAILABLE.withDescription("wrr_locality error: no locality provided")));
+            Status.UNAVAILABLE.withDescription("wrr_locality error: no locality provided"))));
         return false;
       }
       if (localityWeight == null) {
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(
+        helper.updateBalancingState(TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(
             Status.UNAVAILABLE.withDescription(
-                "wrr_locality error: no weight provided for locality " + locality)));
+                "wrr_locality error: no weight provided for locality " + locality))));
         return false;
       }
 

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

@@ -365,7 +365,8 @@ public class ClusterManagerLoadBalancerTest {
       config = resolvedAddresses.getLoadBalancingPolicyConfig();
 
       if (failing) {
-        helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.INTERNAL));
+        helper.updateBalancingState(
+            TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.INTERNAL)));
       }
       return true;
     }

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

@@ -41,7 +41,7 @@ import io.grpc.Attributes;
 import io.grpc.ConnectivityState;
 import io.grpc.EquivalentAddressGroup;
 import io.grpc.LoadBalancer;
-import io.grpc.LoadBalancer.ErrorPicker;
+import io.grpc.LoadBalancer.FixedResultPicker;
 import io.grpc.LoadBalancer.Helper;
 import io.grpc.LoadBalancer.PickResult;
 import io.grpc.LoadBalancer.PickSubchannelArgs;
@@ -306,7 +306,8 @@ public class PriorityLoadBalancerTest {
     assertCurrentPickerPicksSubchannel(subchannel0);
 
     // p0 fails over to p1 immediately.
-    helper0.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.ABORTED));
+    helper0.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.ABORTED)));
     assertLatestConnectivityState(CONNECTING);
     assertThat(fooBalancers).hasSize(2);
     assertThat(fooHelpers).hasSize(2);
@@ -345,11 +346,13 @@ public class PriorityLoadBalancerTest {
     assertCurrentPickerPicksSubchannel(subchannel2);
 
     // p2 fails but does not affect overall picker
-    helper2.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.UNAVAILABLE));
+    helper2.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.UNAVAILABLE)));
     assertCurrentPickerPicksSubchannel(subchannel2);
 
     // p0 fails over to p3 immediately since p1 already timeout and p2 already in TRANSIENT_FAILURE.
-    helper0.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.UNAVAILABLE));
+    helper0.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.UNAVAILABLE)));
     assertLatestConnectivityState(CONNECTING);
     assertThat(fooBalancers).hasSize(4);
     assertThat(fooHelpers).hasSize(4);
@@ -362,7 +365,8 @@ public class PriorityLoadBalancerTest {
 
     // p3 fails then the picker should have error status updated
     helper3.updateBalancingState(
-        TRANSIENT_FAILURE, new ErrorPicker(Status.DATA_LOSS.withDescription("foo")));
+        TRANSIENT_FAILURE,
+        new FixedResultPicker(PickResult.withError(Status.DATA_LOSS.withDescription("foo"))));
     assertCurrentPickerReturnsError(Status.Code.DATA_LOSS, "foo");
 
     // p2 gets back to READY
@@ -390,7 +394,8 @@ public class PriorityLoadBalancerTest {
     assertCurrentPickerPicksSubchannel(subchannel4);
 
     // p0 fails over to p2 and picker is updated to p2's existing picker.
-    helper0.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.UNAVAILABLE));
+    helper0.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.UNAVAILABLE)));
     assertCurrentPickerPicksSubchannel(subchannel3);
 
     // Deactivate child balancer get deleted.
@@ -564,7 +569,8 @@ public class PriorityLoadBalancerTest {
     assertCurrentPickerIsBufferPicker();
 
     // p0 fails over to p1 immediately.
-    helper0.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.ABORTED));
+    helper0.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.ABORTED)));
     assertLatestConnectivityState(CONNECTING);
     assertThat(fooBalancers).hasSize(2);
     assertThat(fooHelpers).hasSize(2);
@@ -591,11 +597,13 @@ public class PriorityLoadBalancerTest {
     assertCurrentPickerIsBufferPicker();
 
     // p2 fails but does not affect overall picker
-    helper2.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.UNAVAILABLE));
+    helper2.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.UNAVAILABLE)));
     assertCurrentPickerIsBufferPicker();
 
     // p0 fails over to p3 immediately since p1 already timeout and p2 already in TRANSIENT_FAILURE.
-    helper0.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.UNAVAILABLE));
+    helper0.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.UNAVAILABLE)));
     assertLatestConnectivityState(CONNECTING);
     assertThat(fooBalancers).hasSize(4);
     assertThat(fooHelpers).hasSize(4);
@@ -608,7 +616,8 @@ public class PriorityLoadBalancerTest {
 
     // p3 fails then the picker should have error status updated
     helper3.updateBalancingState(
-        TRANSIENT_FAILURE, new ErrorPicker(Status.DATA_LOSS.withDescription("foo")));
+        TRANSIENT_FAILURE,
+        new FixedResultPicker(PickResult.withError(Status.DATA_LOSS.withDescription("foo"))));
     assertCurrentPickerReturnsError(Status.Code.DATA_LOSS, "foo");
 
     // p2 gets back to IDLE
@@ -624,7 +633,8 @@ public class PriorityLoadBalancerTest {
     assertCurrentPickerIsBufferPicker();
 
     // p0 fails over to p2 and picker is updated to p2's existing picker.
-    helper0.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.UNAVAILABLE));
+    helper0.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.UNAVAILABLE)));
     assertCurrentPickerIsBufferPicker();
 
     // Deactivate child balancer get deleted.
@@ -655,7 +665,8 @@ public class PriorityLoadBalancerTest {
     verify(helper, never()).refreshNameResolution();
 
     // Simulate fallback to priority p1.
-    priorityHelper0.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.UNAVAILABLE));
+    priorityHelper0.updateBalancingState(
+        TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.UNAVAILABLE)));
     assertThat(fooHelpers).hasSize(2);
     Helper priorityHelper1 = Iterables.getLast(fooHelpers);
     priorityHelper1.refreshNameResolution();
@@ -780,7 +791,8 @@ public class PriorityLoadBalancerTest {
 
     @Override
     public void handleResolvedAddresses(ResolvedAddresses resolvedAddresses) {
-      helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.INTERNAL));
+      helper.updateBalancingState(
+          TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.INTERNAL)));
     }
 
     @Override

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

@@ -39,7 +39,7 @@ import com.google.common.collect.Iterables;
 import io.grpc.Attributes;
 import io.grpc.EquivalentAddressGroup;
 import io.grpc.LoadBalancer;
-import io.grpc.LoadBalancer.ErrorPicker;
+import io.grpc.LoadBalancer.FixedResultPicker;
 import io.grpc.LoadBalancer.Helper;
 import io.grpc.LoadBalancer.PickResult;
 import io.grpc.LoadBalancer.PickSubchannelArgs;
@@ -324,10 +324,10 @@ public class WeightedTargetLoadBalancerTest {
         mock(SubchannelPicker.class),
         mock(SubchannelPicker.class)};
     final SubchannelPicker[] failurePickers = new SubchannelPicker[]{
-        new ErrorPicker(Status.CANCELLED),
+        new FixedResultPicker(PickResult.withError(Status.CANCELLED)),
-        new ErrorPicker(Status.ABORTED),
+        new FixedResultPicker(PickResult.withError(Status.ABORTED)),
-        new ErrorPicker(Status.DATA_LOSS),
+        new FixedResultPicker(PickResult.withError(Status.DATA_LOSS)),
-        new ErrorPicker(Status.DATA_LOSS)
+        new FixedResultPicker(PickResult.withError(Status.DATA_LOSS))
     };
     ArgumentCaptor<SubchannelPicker> pickerCaptor = ArgumentCaptor.forClass(SubchannelPicker.class);
 
@@ -463,7 +463,8 @@ public class WeightedTargetLoadBalancerTest {
 
     @Override
     public void handleResolvedAddresses(ResolvedAddresses resolvedAddresses) {
-      helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(Status.INTERNAL));
+      helper.updateBalancingState(
+          TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(Status.INTERNAL)));
     }
 
     @Override

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

@@ -20,6 +20,8 @@ import static com.google.common.truth.Truth.assertThat;
 import static io.grpc.xds.XdsLbPolicies.WEIGHTED_TARGET_POLICY_NAME;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.ArgumentMatchers.isA;
+import static org.mockito.Mockito.argThat;
+import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -30,8 +32,9 @@ import io.grpc.Attributes;
 import io.grpc.ConnectivityState;
 import io.grpc.EquivalentAddressGroup;
 import io.grpc.LoadBalancer;
-import io.grpc.LoadBalancer.ErrorPicker;
 import io.grpc.LoadBalancer.Helper;
+import io.grpc.LoadBalancer.PickResult;
+import io.grpc.LoadBalancer.PickSubchannelArgs;
 import io.grpc.LoadBalancer.ResolvedAddresses;
 import io.grpc.LoadBalancer.SubchannelPicker;
 import io.grpc.LoadBalancerProvider;
@@ -52,6 +55,7 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 import org.mockito.ArgumentCaptor;
+import org.mockito.ArgumentMatcher;
 import org.mockito.Captor;
 import org.mockito.Mock;
 import org.mockito.junit.MockitoJUnit;
@@ -78,10 +82,6 @@ public class WrrLocalityLoadBalancerTest {
 
   @Captor
   private ArgumentCaptor<ResolvedAddresses> resolvedAddressesCaptor;
-  @Captor
-  private ArgumentCaptor<ConnectivityState> connectivityStateCaptor;
-  @Captor
-  private ArgumentCaptor<SubchannelPicker> errorPickerCaptor;
 
   private WrrLocalityLoadBalancer loadBalancer;
   private LoadBalancerRegistry lbRegistry = new LoadBalancerRegistry();
@@ -153,18 +153,17 @@ public class WrrLocalityLoadBalancerTest {
     // With no locality weights, we should get a TRANSIENT_FAILURE.
     verify(mockHelper).getAuthority();
     verify(mockHelper).updateBalancingState(eq(ConnectivityState.TRANSIENT_FAILURE),
-        isA(ErrorPicker.class));
+        pickerReturns(Status.Code.UNAVAILABLE));
   }
 
   @Test
   public void handleNameResolutionError_noChildLb() {
-    loadBalancer.handleNameResolutionError(Status.DEADLINE_EXCEEDED);
+    Status status = Status.DEADLINE_EXCEEDED.withDescription("down low");
+    loadBalancer.handleNameResolutionError(status);
 
-    verify(mockHelper).updateBalancingState(connectivityStateCaptor.capture(),
+    verify(mockHelper).updateBalancingState(
-        errorPickerCaptor.capture());
+        eq(ConnectivityState.TRANSIENT_FAILURE),
-    assertThat(connectivityStateCaptor.getValue()).isEqualTo(ConnectivityState.TRANSIENT_FAILURE);
+        pickerReturns(PickResult.withError(status)));
-    assertThat(errorPickerCaptor.getValue().toString()).isEqualTo(
-        new ErrorPicker(Status.DEADLINE_EXCEEDED).toString());
   }
 
   @Test
@@ -172,11 +171,12 @@ public class WrrLocalityLoadBalancerTest {
     deliverAddresses(new WrrLocalityConfig(new PolicySelection(mockChildProvider, null)),
         ImmutableList.of(
             makeAddress("addr1", Locality.create("test-region1", "test-zone", "test-subzone"), 1)));
-    loadBalancer.handleNameResolutionError(Status.DEADLINE_EXCEEDED);
+    Status status = Status.DEADLINE_EXCEEDED.withDescription("too slow");
+    loadBalancer.handleNameResolutionError(status);
 
-    verify(mockHelper, never()).updateBalancingState(isA(ConnectivityState.class),
+    verify(mockHelper, never()).updateBalancingState(eq(ConnectivityState.TRANSIENT_FAILURE),
-        isA(ErrorPicker.class));
+        pickerReturns(PickResult.withError(status)));
-    verify(mockWeightedTargetLb).handleNameResolutionError(Status.DEADLINE_EXCEEDED);
+    verify(mockWeightedTargetLb).handleNameResolutionError(status);
   }
 
   @Test
@@ -224,6 +224,42 @@ public class WrrLocalityLoadBalancerTest {
             .build());
   }
 
+  private static SubchannelPicker pickerReturns(final PickResult result) {
+    return pickerReturns(new ArgumentMatcher<PickResult>() {
+      @Override public boolean matches(PickResult obj) {
+        return result.equals(obj);
+      }
+
+      @Override public String toString() {
+        return "[equals " + result + "]";
+      }
+    });
+  }
+
+  private static SubchannelPicker pickerReturns(Status.Code code) {
+    return pickerReturns(new ArgumentMatcher<PickResult>() {
+      @Override public boolean matches(PickResult obj) {
+        return obj.getStatus() != null && code.equals(obj.getStatus().getCode());
+      }
+
+      @Override public String toString() {
+        return "[with code " + code + "]";
+      }
+    });
+  }
+
+  private static SubchannelPicker pickerReturns(final ArgumentMatcher<PickResult> matcher) {
+    return argThat(new ArgumentMatcher<SubchannelPicker>() {
+      @Override public boolean matches(SubchannelPicker picker) {
+        return matcher.matches(picker.pickSubchannel(mock(PickSubchannelArgs.class)));
+      }
+
+      @Override public String toString() {
+        return "[picker returns: result " + matcher + "]";
+      }
+    });
+  }
+
   /**
    * Create a locality-labeled address.
    */