Aggs: Fix Filters agg cancellation flaky test (#130810)

Fixes https://github.com/elastic/elasticsearch/issues/130770

Both a bigger thread pool and not draining the semaphore permits were leading to failing tests sometimes because of blocked threads (Too many threads searching would end up draining all permits in parallel, and getting stuck).

The test was added in https://github.com/elastic/elasticsearch/pull/130452

server/src/internalClusterTest/java/org/elasticsearch/search/aggregations/bucket/FiltersCancellationIT.java

@@ -28,6 +28,7 @@ import org.elasticsearch.tasks.TaskInfo;
 import org.elasticsearch.test.ESIntegTestCase;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.json.JsonXContent;
+import org.junit.Before;
 
 import java.util.Collection;
 import java.util.List;
@@ -39,9 +40,22 @@ import static org.elasticsearch.index.query.QueryBuilders.termQuery;
 import static org.elasticsearch.search.aggregations.AggregationBuilders.filters;
 import static org.elasticsearch.search.aggregations.AggregationBuilders.terms;
 import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.not;
 
+/**
+ * Ensures the filters aggregation checks task cancellation, by ensuring it doesn't process all the docs.
+ * <p>
+ *   The CancellableBulkScorer we use to break the execution is called per search thread in the query.
+ *   It currently breaks the "for each doc" into blocks of 4096 docs (x2 every iteration), and checks for cancellation between blocks.
+ *   This test creates N docs and releases N - X permits, to ensure the search request gets cancelled before grabbing all the permits.
+ * </p>
+ * <p>
+ *   Also, if the search thread pool size is too high, it can lead to them trying to process too many documents anyway (pool size * 4096),
+ *   eventually blocking the threads (And failing the test). So it's explicitly set to a small number to avoid this.
+ * </p>
+ */
 @ESIntegTestCase.SuiteScopeTestCase
 public class FiltersCancellationIT extends ESIntegTestCase {
 
@@ -55,11 +69,12 @@ public class FiltersCancellationIT extends ESIntegTestCase {
 
     @Override
     protected Collection<Class<? extends Plugin>> nodePlugins() {
-        return CollectionUtils.appendToCopy(super.nodePlugins(), pausableFieldPluginClass());
+        return CollectionUtils.appendToCopy(super.nodePlugins(), PauseScriptPlugin.class);
     }
 
-    protected Class<? extends Plugin> pausableFieldPluginClass() {
-        return PauseScriptPlugin.class;
+    @Override
+    public Settings nodeSettings(int nodeOrdinal, Settings otherSettings) {
+        return Settings.builder().put(super.nodeSettings(nodeOrdinal, otherSettings)).put("thread_pool.search.size", 4).build();
     }
 
     @Override
@@ -99,6 +114,11 @@ public class FiltersCancellationIT extends ESIntegTestCase {
         client().admin().indices().prepareForceMerge(INDEX).setMaxNumSegments(1).get();
     }
 
+    @Before
+    public void reset() {
+        SCRIPT_SEMAPHORE.drainPermits();
+    }
+
     public void testFiltersCountCancellation() throws Exception {
         ensureProperCancellation(
             client().prepareSearch(INDEX)
@@ -129,14 +149,14 @@ public class FiltersCancellationIT extends ESIntegTestCase {
 
     private void ensureProperCancellation(SearchRequestBuilder searchRequestBuilder) throws Exception {
         var searchRequestFuture = searchRequestBuilder.setTimeout(TimeValue.timeValueSeconds(1)).execute();
-        assertFalse(searchRequestFuture.isCancelled());
-        assertFalse(searchRequestFuture.isDone());
+        assertThat(searchRequestFuture.isCancelled(), equalTo(false));
+        assertThat(searchRequestFuture.isDone(), equalTo(false));
 
         // Check that there are search tasks running
         assertThat(getSearchTasks(), not(empty()));
 
         // Wait for the script field to get blocked
-        assertBusy(() -> { assertThat(SCRIPT_SEMAPHORE.getQueueLength(), greaterThan(0)); });
+        assertBusy(() -> assertThat(SCRIPT_SEMAPHORE.getQueueLength(), greaterThan(0)));
 
         // Cancel the tasks
         // Warning: Adding a waitForCompletion(true)/execute() here sometimes causes tasks to not get canceled and threads to get stuck
@@ -146,8 +166,8 @@ public class FiltersCancellationIT extends ESIntegTestCase {
 
         // Ensure the search request finished and that there are no more search tasks
         assertBusy(() -> {
-            assertTrue(searchRequestFuture.isDone());
-            assertThat(getSearchTasks(), empty());
+            assertThat("Search request didn't finish", searchRequestFuture.isDone(), equalTo(true));
+            assertThat("There are dangling search tasks", getSearchTasks(), empty());
         });
     }