Ensure tasks with banned parents always get cancelled (#90188) (#90248)

The check used to entirely skip parent lookup relies on
ConcurrentHashMap#isEmpty() which could return inconsistent results, and
potentially skip the cancellation of a task with a banned parent upon
registration, and it doesn't seem to have a benefit considering the hash
code computation.

Closes #88201

qa/smoke-test-http/src/javaRestTest/java/org/elasticsearch/http/IndicesSegmentsRestCancellationIT.java

@@ -11,9 +11,7 @@
 import org.apache.http.client.methods.HttpGet;
 import org.elasticsearch.action.admin.indices.segments.IndicesSegmentsAction;
 import org.elasticsearch.client.Request;
-import org.elasticsearch.test.junit.annotations.TestLogging;
 
-@TestLogging(value = "org.elasticsearch.tasks.TaskManager:TRACE,org.elasticsearch.test.TaskAssertions:TRACE", reason = "debugging")
 public class IndicesSegmentsRestCancellationIT extends BlockedSearcherRestCancellationTestCase {
     public void testIndicesSegmentsRestCancellation() throws Exception {
         runTest(new Request(HttpGet.METHOD_NAME, "/_segments"), IndicesSegmentsAction.NAME);

server/src/main/java/org/elasticsearch/tasks/TaskManager.java

@@ -148,9 +148,8 @@ private void registerCancellableTask(Task task) {
         CancellableTask cancellableTask = (CancellableTask) task;
         CancellableTaskHolder holder = new CancellableTaskHolder(cancellableTask);
         cancellableTasks.put(task, holder);
-        // Check if this task was banned before we start it. The empty check is used to avoid
-        // computing the hash code of the parent taskId as most of the time bannedParents is empty.
-        if (task.getParentTaskId().isSet() && bannedParents.isEmpty() == false) {
+        // Check if this task was banned before we start it.
+        if (task.getParentTaskId().isSet()) {
             final Ban ban = bannedParents.get(task.getParentTaskId());
             if (ban != null) {
                 try {