Outlier detection load balancer (#9447)

New outlier detection load balancer.

Tracks all RPC results to the addresses it is configured with and periodically attempts
to detect outlier. It wraps a child load balancer from which it hides any addresses that
are deemed outliers.

As specified in gRFC A50: gRPC xDS Outlier Detection Support:
https://github.com/grpc/proposal/blob/master/A50-xds-outlier-detection.md

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

@@ -163,6 +163,38 @@ public String toString() {
     return new ScheduledHandle(runnable, future);
   }
 
+  /**
+   * Schedules a task to be added and run via {@link #execute} after an inital delay and then
+   * repeated after the delay until cancelled.
+   *
+   * @param task the task being scheduled
+   * @param initialDelay the delay before the first run
+   * @param delay the delay after the first run.
+   * @param unit the time unit for the delay
+   * @param timerService the {@code ScheduledExecutorService} that provides delayed execution
+   *
+   * @return an object for checking the status and/or cancel the scheduled task
+   */
+  public final ScheduledHandle scheduleWithFixedDelay(
+      final Runnable task, long initialDelay, long delay, TimeUnit unit,
+      ScheduledExecutorService timerService) {
+    final ManagedRunnable runnable = new ManagedRunnable(task);
+    ScheduledFuture<?> future = timerService.scheduleWithFixedDelay(new Runnable() {
+      @Override
+      public void run() {
+        execute(runnable);
+      }
+
+      @Override
+      public String toString() {
+        return task.toString() + "(scheduled in SynchronizationContext with delay of " + delay
+            + ")";
+      }
+    }, initialDelay, delay, unit);
+    return new ScheduledHandle(runnable, future);
+  }
+
+
   private static class ManagedRunnable implements Runnable {
     final Runnable task;
     boolean isCancelled;

api/src/test/java/io/grpc/LoadBalancerRegistryTest.java

@@ -41,7 +41,7 @@ public void getClassesViaHardcoded_classesPresent() throws Exception {
   @Test
   public void stockProviders() {
     LoadBalancerRegistry defaultRegistry = LoadBalancerRegistry.getDefaultRegistry();
-    assertThat(defaultRegistry.providers()).hasSize(3);
+    assertThat(defaultRegistry.providers()).hasSize(4);
 
     LoadBalancerProvider pickFirst = defaultRegistry.getProvider("pick_first");
     assertThat(pickFirst).isInstanceOf(PickFirstLoadBalancerProvider.class);
@@ -52,6 +52,12 @@ public void stockProviders() {
         "io.grpc.util.SecretRoundRobinLoadBalancerProvider$Provider");
     assertThat(roundRobin.getPriority()).isEqualTo(5);
 
+    LoadBalancerProvider outlierDetection = defaultRegistry.getProvider(
+        "outlier_detection_experimental");
+    assertThat(outlierDetection.getClass().getName()).isEqualTo(
+        "io.grpc.util.OutlierDetectionLoadBalancerProvider");
+    assertThat(roundRobin.getPriority()).isEqualTo(5);
+
     LoadBalancerProvider grpclb = defaultRegistry.getProvider("grpclb");
     assertThat(grpclb).isInstanceOf(GrpclbLoadBalancerProvider.class);
     assertThat(grpclb.getPriority()).isEqualTo(5);