print reason why parent task was cancelled

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix race condition while parsing derived fields from search definition ([14445](https://github.com/opensearch-project/OpenSearch/pull/14445))
 - Add allowlist setting for ingest-common and search-pipeline-common processors ([#14439](https://github.com/opensearch-project/OpenSearch/issues/14439))
 - Create SystemIndexRegistry with helper method matchesSystemIndex ([#14415](https://github.com/opensearch-project/OpenSearch/pull/14415))
+- Print reason why parent task was cancelled ([#14604](https://github.com/opensearch-project/OpenSearch/issues/14604))
 
 ### Dependencies
 - Bump `org.gradle.test-retry` from 1.5.8 to 1.5.9 ([#13442](https://github.com/opensearch-project/OpenSearch/pull/13442))

server/src/internalClusterTest/java/org/opensearch/action/admin/cluster/node/tasks/CancellableTasksIT.java

@@ -327,7 +327,7 @@ public void testFailedToStartChildTaskAfterCancelled() throws Exception {
         mainAction.startSubTask(taskId, subRequest, future);
         TransportException te = expectThrows(TransportException.class, future::actionGet);
         assertThat(te.getCause(), instanceOf(TaskCancelledException.class));
-        assertThat(te.getCause().getMessage(), equalTo("The parent task was cancelled, shouldn't start any child tasks"));
+        assertThat(te.getCause().getMessage(), equalTo("The parent task was cancelled, shouldn't start any child tasks, by user request"));
         allowEntireRequest(rootRequest);
         waitForRootTask(rootTaskFuture);
         ensureAllBansRemoved();
@@ -386,7 +386,7 @@ static void waitForRootTask(ActionFuture<TestResponse> rootTask) {
             assertThat(
                 cause.getMessage(),
                 anyOf(
-                    equalTo("The parent task was cancelled, shouldn't start any child tasks"),
+                    equalTo("The parent task was cancelled, shouldn't start any child tasks, by user request"),
                     containsString("Task cancelled before it started:"),
                     equalTo("Task was cancelled while executing")
                 )

server/src/main/java/org/opensearch/tasks/TaskCancellationService.java

@@ -92,7 +92,7 @@ void cancelTaskAndDescendants(CancellableTask task, String reason, boolean waitF
             Collection<DiscoveryNode> childrenNodes = taskManager.startBanOnChildrenNodes(task.getId(), () -> {
                 logger.trace("child tasks of parent [{}] are completed", taskId);
                 groupedListener.onResponse(null);
-            });
+            }, reason);
             taskManager.cancel(task, reason, () -> {
                 logger.trace("task [{}] is cancelled", taskId);
                 groupedListener.onResponse(null);

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

@@ -510,17 +510,22 @@ public Set<TaskId> getBannedTaskIds() {
         return Collections.unmodifiableSet(banedParents.keySet());
     }
 
+    public Collection<DiscoveryNode> startBanOnChildrenNodes(long taskId, Runnable onChildTasksCompleted) {
+        return startBanOnChildrenNodes(taskId, onChildTasksCompleted, "unknown");
+    }
+
     /**
      * Start rejecting new child requests as the parent task was cancelled.
      *
      * @param taskId                the parent task id
      * @param onChildTasksCompleted called when all child tasks are completed or failed
+     * @param reason                the ban reason
      * @return the set of current nodes that have outstanding child tasks
      */
-    public Collection<DiscoveryNode> startBanOnChildrenNodes(long taskId, Runnable onChildTasksCompleted) {
+    public Collection<DiscoveryNode> startBanOnChildrenNodes(long taskId, Runnable onChildTasksCompleted, String reason) {
         final CancellableTaskHolder holder = cancellableTasks.get(taskId);
         if (holder != null) {
-            return holder.startBan(onChildTasksCompleted);
+            return holder.startBan(onChildTasksCompleted, reason);
         } else {
             onChildTasksCompleted.run();
             return Collections.emptySet();
@@ -585,6 +590,7 @@ private static class CancellableTaskHolder {
         private List<Runnable> cancellationListeners = null;
         private Map<DiscoveryNode, Integer> childTasksPerNode = null;
         private boolean banChildren = false;
+        private String banReason;
         private List<Runnable> childTaskCompletedListeners = null;
 
         CancellableTaskHolder(CancellableTask task) {
@@ -662,7 +668,7 @@ public CancellableTask getTask() {
 
         synchronized void registerChildNode(DiscoveryNode node) {
             if (banChildren) {
-                throw new TaskCancelledException("The parent task was cancelled, shouldn't start any child tasks");
+                throw new TaskCancelledException("The parent task was cancelled, shouldn't start any child tasks, " + banReason);
             }
             if (childTasksPerNode == null) {
                 childTasksPerNode = new HashMap<>();
@@ -686,11 +692,13 @@ void unregisterChildNode(DiscoveryNode node) {
             notifyListeners(listeners);
         }
 
-        Set<DiscoveryNode> startBan(Runnable onChildTasksCompleted) {
+        Set<DiscoveryNode> startBan(Runnable onChildTasksCompleted, String reason) {
             final Set<DiscoveryNode> pendingChildNodes;
             final Runnable toRun;
             synchronized (this) {
                 banChildren = true;
+                assert reason != null;
+                banReason = reason;
                 if (childTasksPerNode == null) {
                     pendingChildNodes = Collections.emptySet();
                 } else {

server/src/test/java/org/opensearch/action/admin/cluster/node/tasks/CancellableTasksTests.java

@@ -428,7 +428,7 @@ public void testRegisterAndExecuteChildTaskWhileParentTaskIsBeingCanceled() thro
         );
         assertThat(cancelledException.getMessage(), startsWith("Task cancelled before it started:"));
         CountDownLatch latch = new CountDownLatch(1);
-        taskManager.startBanOnChildrenNodes(parentTaskId.getId(), latch::countDown);
+        taskManager.startBanOnChildrenNodes(parentTaskId.getId(), latch::countDown, cancelledException.getMessage());
         assertTrue("onChildTasksCompleted() is not invoked", latch.await(1, TimeUnit.SECONDS));
     }