Do not wait on task's future if it was rejected

lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java

@@ -668,14 +668,20 @@ public class IndexSearcher {
           search(Arrays.asList(leaves), weight, collector);
           return collector;
         });
+        boolean executedOnCallerThread = false;
         try {
           executor.execute(task);
         } catch (RejectedExecutionException e) {
           // Execute on caller thread
           search(Arrays.asList(leaves), weight, collector);
+          topDocsFutures.add(CompletableFuture.completedFuture(collector));
+          executedOnCallerThread = true;
         }
 
-        topDocsFutures.add(task);
+        // Do not add the task's future if it was not used
+        if (executedOnCallerThread == false) {
+          topDocsFutures.add(task);
+        }
       }
       final LeafReaderContext[] leaves = leafSlices[leafSlices.length - 1].leaves;
       final C collector = collectors.get(leafSlices.length - 1);

lucene/core/src/test/org/apache/lucene/search/TestIndexSearcher.java

@@ -273,8 +273,6 @@ public class TestIndexSearcher extends LuceneTestCase {
   }
 
   public void testRejectedExecution() throws IOException {
-    List<LeafReaderContext> leaves = reader.leaves();
-    AtomicInteger numExecutions = new AtomicInteger(0);
     ExecutorService service = new RejectingMockExecutor();
 
     IndexSearcher searcher = new IndexSearcher(reader, service) {
@@ -290,12 +288,8 @@ public class TestIndexSearcher extends LuceneTestCase {
 
     // To ensure that failing ExecutorService still allows query to run
     // successfully
-    searcher.search(new MatchAllDocsQuery(), 10);
+    TopDocs topDocs = searcher.search(new MatchAllDocsQuery(), 10);
-    if (leaves.size() <= 1) {
+    assert topDocs.scoreDocs.length == 10;
-      assertEquals(0, numExecutions.get());
-    } else {
-      assertEquals(leaves.size() - 1, numExecutions.get());
-    }
 
     service.shutdown();
   }