ReindexIT wait for task to really start (#73018)

Reindex and friends have tasks that start but are not ready to
rethrottle before they figured out if they are leader or worker
tasks. Now wait for the task to fully start before rethrottling.

Also added additional assertions to help see if the inability
to rethrottle is caused by some failure.

Closes #60811

client/rest-high-level/src/test/java/org/elasticsearch/client/ESRestHighLevelClientTestCase.java

@@ -35,6 +35,7 @@
 import org.elasticsearch.ingest.Pipeline;
 import org.elasticsearch.search.SearchHit;
 import org.elasticsearch.search.SearchModule;
+import org.elasticsearch.tasks.RawTaskStatus;
 import org.elasticsearch.tasks.TaskId;
 import org.elasticsearch.test.rest.ESRestTestCase;
 import org.junit.AfterClass;
@@ -324,7 +325,10 @@ protected static TaskId findTaskToRethrottle(String actionName, String descripti
             }
             TaskGroup taskGroup = taskGroups.get(0);
             assertThat(taskGroup.getChildTasks(), empty());
-            return taskGroup.getTaskInfo().getTaskId();
+            // check that the task initialized enough that it can rethrottle too.
+            if (((RawTaskStatus) taskGroup.getTaskInfo().getStatus()).toMap().containsKey("batches")) {
+                return taskGroup.getTaskInfo().getTaskId();
+            }
         } while (System.nanoTime() - start < TimeUnit.SECONDS.toNanos(10));
         throw new AssertionError("Couldn't find tasks to rethrottle. Here are the running tasks " +
             highLevelClient().tasks().list(request, RequestOptions.DEFAULT));

client/rest-high-level/src/test/java/org/elasticsearch/client/ReindexIT.java

@@ -263,6 +263,8 @@ public void onFailure(Exception e) {
             float requestsPerSecond = 1000f;
             ListTasksResponse response = execute(new RethrottleRequest(taskIdToRethrottle, requestsPerSecond),
                 highLevelClient()::deleteByQueryRethrottle, highLevelClient()::deleteByQueryRethrottleAsync);
+            assertThat(response.getTaskFailures(), empty());
+            assertThat(response.getNodeFailures(), empty());
             assertThat(response.getTasks(), hasSize(1));
             assertEquals(taskIdToRethrottle, response.getTasks().get(0).getTaskId());
             assertThat(response.getTasks().get(0).getStatus(), instanceOf(RawTaskStatus.class));