Don't generate stacktrace in TaskCancelledException (elastic#125002)

# Conflicts:
#	modules/aggregations/src/internalClusterTest/java/org/elasticsearch/aggregations/bucket/SearchCancellationIT.java

Author: iverase

docs/changelog/125002.yaml

@@ -0,0 +1,5 @@
+pr: 125002
+summary: Don't generate stacktrace in `TaskCancelledException`
+area: Search
+type: bug
+issues: []

modules/aggregations/src/internalClusterTest/java/org/elasticsearch/aggregations/bucket/SearchCancellationIT.java

@@ -42,6 +42,7 @@
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertNoFailures;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.not;
 
 public class SearchCancellationIT extends AbstractSearchCancellationTestCase {
 
@@ -101,8 +102,8 @@ public void testCancellationDuringTimeSeriesAggregation() throws Exception {
             .addAggregation(
                 timeSeriesAggregationBuilder.subAggregation(
                     new ScriptedMetricAggregationBuilder("sub_agg").initScript(
-                        new Script(ScriptType.INLINE, "mockscript", ScriptedBlockPlugin.INIT_SCRIPT_NAME, Collections.emptyMap())
-                    )
+                            new Script(ScriptType.INLINE, "mockscript", ScriptedBlockPlugin.INIT_SCRIPT_NAME, Collections.emptyMap())
+                        )
                         .mapScript(
                             new Script(ScriptType.INLINE, "mockscript", ScriptedBlockPlugin.MAP_BLOCK_SCRIPT_NAME, Collections.emptyMap())
                         )
@@ -124,7 +125,9 @@ public void testCancellationDuringTimeSeriesAggregation() throws Exception {
         logger.info("All shards failed with", ex);
         if (lowLevelCancellation) {
             // Ensure that we cancelled in TimeSeriesIndexSearcher and not in reduce phase
-            assertThat(ExceptionsHelper.stackTrace(ex), containsString("TimeSeriesIndexSearcher"));
+            assertThat(ExceptionsHelper.stackTrace(ex), not(containsString("not building sub-aggregations due to task cancellation")));
+        } else {
+            assertThat(ExceptionsHelper.stackTrace(ex), containsString("not building sub-aggregations due to task cancellation"));
         }
     }
 }

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

@@ -27,6 +27,11 @@ public TaskCancelledException(StreamInput in) throws IOException {
         super(in);
     }
 
+    @Override
+    public Throwable fillInStackTrace() {
+        return this;  // this exception doesn't imply a bug, no need for a stack trace
+    }
+
     @Override
     public RestStatus status() {
         // Tasks are typically cancelled at the request of the client, so a 4xx status code is more accurate than the default of 500 (and