Adds wait for task registration to testCanFetchIndexStatus

In the testCanFetchIndexStatus the task check can occur before the indexing process is started making the test to fail. This commit adds an additional lock to make sure we check tasks only after at least one of the tasks is registered.

core/src/test/java/org/elasticsearch/action/admin/cluster/node/tasks/TasksIT.java

@@ -48,7 +48,9 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Function;
 
@@ -252,11 +254,14 @@ public class TasksIT extends ESIntegTestCase {
          */
         ReentrantLock taskFinishLock = new ReentrantLock();
         taskFinishLock.lock();
+        CountDownLatch taskRegistered = new CountDownLatch(1);
         for (ClusterService clusterService : internalCluster().getInstances(ClusterService.class)) {
             ((MockTaskManager)clusterService.getTaskManager()).addListener(new MockTaskManagerListener() {
                 @Override
                 public void onTaskRegistered(Task task) {
-                    // Intentional noop
+                    if (task.getAction().startsWith(IndexAction.NAME)) {
+                        taskRegistered.countDown();
+                    }
                 }
 
                 @Override
@@ -275,6 +280,7 @@ public class TasksIT extends ESIntegTestCase {
             });
         }
         ListenableActionFuture<?> indexFuture = client().prepareIndex("test", "test").setSource("test", "test").execute();
+        taskRegistered.await(10, TimeUnit.SECONDS); // waiting for at least one task to be registered
         ListTasksResponse tasks = client().admin().cluster().prepareListTasks().setActions("indices:data/write/index*").setDetailed(true)
                 .get();
         taskFinishLock.unlock();