TopFieldCollectorManager: Support thread safe

lucene/CHANGES.txt

@@ -130,6 +130,8 @@ Optimizations
 
 Bug Fixes
 ---------------------
+* GITHUB#15605: Make TopFieldCollectorManager thread-safe by using CopyOnWriteArrayList and deprecated getCollectors(). (Prudhvi Godithi)
+
 * GITHUB#14049: Randomize KNN codec params in RandomCodec. Fixes scalar quantization div-by-zero
   when all values are identical. (Mike Sokolov)
 

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

@@ -17,9 +17,9 @@
 package org.apache.lucene.search;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
  * Create a TopFieldCollectorManager which uses a shared hit counter to maintain number of hits and
@@ -111,7 +111,7 @@ public TopFieldCollectorManager(Sort sort, int numHits, FieldDoc after, int tota
     this.after = after;
     this.totalHitsThreshold = totalHitsThreshold;
     this.minScoreAcc = totalHitsThreshold != Integer.MAX_VALUE ? new MaxScoreAccumulator() : null;
-    this.collectors = new ArrayList<>();
+    this.collectors = new CopyOnWriteArrayList<>();
   }
 
   /**
@@ -182,6 +182,8 @@ public TopFieldDocs reduce(Collection<TopFieldCollector> collectors) throws IOEx
     return TopDocs.merge(sort, 0, numHits, topDocs);
   }
 
+  /** Returns the collectors created by this manager. */
+  @Deprecated
   public List<TopFieldCollector> getCollectors() {
     return collectors;
   }

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

@@ -22,9 +22,16 @@
 import static org.hamcrest.Matchers.sameInstance;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Comparator;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.atomic.AtomicReference;
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Field;
 import org.apache.lucene.document.Field.Store;
@@ -45,6 +52,7 @@
 import org.apache.lucene.tests.search.CheckHits;
 import org.apache.lucene.tests.util.LuceneTestCase;
 import org.apache.lucene.tests.util.TestUtil;
+import org.apache.lucene.util.NamedThreadFactory;
 
 public class TestTopFieldCollector extends LuceneTestCase {
   private IndexSearcher is;
@@ -703,6 +711,45 @@ public void testRandomMinCompetitiveScore() throws Exception {
     dir.close();
   }
 
+  /**
+   * Test that concurrent newCollector() calls are thread-safe. See <a
+   * href="https://github.com/apache/lucene/issues/15605">GitHub issue #15605</a>
+   */
+  @SuppressWarnings("deprecation")
+  public void testConcurrentNewCollector() throws Exception {
+    Sort sort = new Sort(SortField.FIELD_SCORE);
+    TopFieldCollectorManager manager = new TopFieldCollectorManager(sort, 10, null, 100);
+    int numThreads = 8;
+    int callsPerThread = 1000;
+    int expectedTotal = numThreads * callsPerThread;
+    ExecutorService executor =
+        Executors.newFixedThreadPool(
+            numThreads, new NamedThreadFactory("testConcurrentNewCollector"));
+    CountDownLatch startLatch = new CountDownLatch(1);
+    AtomicReference<Throwable> error = new AtomicReference<>();
+    List<Future<?>> futures = new ArrayList<>();
+    for (int i = 0; i < numThreads; i++) {
+      futures.add(
+          executor.submit(
+              () -> {
+                try {
+                  startLatch.await();
+                  for (int j = 0; j < callsPerThread; j++) {
+                    manager.newCollector();
+                  }
+                } catch (Throwable t) {
+                  error.compareAndSet(null, t);
+                }
+              }));
+    }
+    startLatch.countDown();
+    for (Future<?> f : futures) {
+      f.get();
+    }
+    executor.shutdown();
+    assertEquals(expectedTotal, manager.getCollectors().size());
+  }
+
   public void testRelationVsTopDocsCount() throws Exception {
     Sort sort = new Sort(SortField.FIELD_SCORE, SortField.FIELD_DOC);
     try (Directory dir = newDirectory();