Allow AlreadyClosedException to be a possible outcome of injecting fake exceptions in TestIndexFileDeleter (#15545)

* Allow AlreadyClosedException to be a possible outcome of injecting fake exceptions in TestIndexFileDeleter

* Fix duplicate branches.

Author: dweiss

lucene/core/src/test/org/apache/lucene/index/TestIndexFileDeleter.java

@@ -30,6 +30,7 @@
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Field;
 import org.apache.lucene.index.IndexWriterConfig.OpenMode;
+import org.apache.lucene.store.AlreadyClosedException;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.IOContext;
 import org.apache.lucene.store.IndexInput;
@@ -41,11 +42,6 @@
 import org.apache.lucene.tests.util.TestUtil;
 import org.apache.lucene.util.InfoStream;
 
-/*
-  Verify we can read the pre-2.1 file format, do searches
-  against it, and add documents to it.
-*/
-
 public class TestIndexFileDeleter extends LuceneTestCase {
 
   public void testDeleteLeftoverFiles() throws IOException {
@@ -440,7 +436,6 @@ public void eval(MockDirectoryWrapper dir) throws IOException {
         });
 
     IndexWriterConfig iwc = newIndexWriterConfig(new MockAnalyzer(random()));
-    // iwc.setMergeScheduler(new SerialMergeScheduler());
     MergeScheduler ms = iwc.getMergeScheduler();
     if (ms instanceof ConcurrentMergeScheduler) {
       final ConcurrentMergeScheduler suppressFakeFail =
@@ -486,8 +481,12 @@ protected void handleMergeException(Throwable exc) {
         }
       } catch (Throwable t) {
         if (t.toString().contains("fake fail")
-            || (t.getCause() != null && t.getCause().toString().contains("fake fail"))) {
-          // ok
+            || (t.getCause() != null && t.getCause().toString().contains("fake fail"))
+            || t instanceof AlreadyClosedException) {
+          // All these conditions are fine.
+          // AlreadyClosedException can happen if the injected exception (RuntimeException("fake
+          // fail")) happened inside the concurrent merges and this closed the index writer's
+          // reader pool.
         } else {
           throw t;
         }