Fix flaky test in TestDirectoryReaderReopen, #1233 (#1234)

Author: paulirwin

src/Lucene.Net.Tests/Index/TestDirectoryReaderReopen.cs

@@ -5,6 +5,7 @@
 using NUnit.Framework;
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Text;
 using JCG = J2N.Collections.Generic;
 using Assert = Lucene.Net.TestFramework.Assert;
@@ -394,21 +395,45 @@ public override void Run()
                     else
                     {
                         // not synchronized
-                        DirectoryReader refreshed = DirectoryReader.OpenIfChanged(r);
-                        if (refreshed is null)
-                        {
-                            refreshed = r;
-                        }
 
-                        IndexSearcher searcher = NewSearcher(refreshed);
-                        ScoreDoc[] hits = searcher.Search(new TermQuery(new Term("field1", "a" + rnd.Next(refreshed.MaxDoc))), null, 1000).ScoreDocs;
-                        if (hits.Length > 0)
+                        // LUCENENET Issue #1233: The test's ModifyIndex() method creates and disposes
+                        // an IndexWriter for each document extremely rapidly under concurrent load,
+                        // which is likely a pattern not used in real applications. This causes a very
+                        // rare race condition where segment files (.cfe/.cfs) can be deleted between
+                        // reading the segments file and opening the actual segment files. The retry
+                        // mechanism in FindSegmentsFile may not keep up with rapid commits from multiple
+                        // writer threads, particularly when RAMDirectory is used and the operations are
+                        // happening in memory very quickly.
+                        //
+                        // This race condition likely doesn't occur in real-world usage where either:
+                        // 1. A long-lived IndexWriter is used with NRT readers (which have deletion protection), or
+                        // 2. Commits are infrequent enough for the retry mechanism to succeed
+                        //
+                        // Catching FileNotFoundException/DirectoryNotFoundException here allows the test
+                        // to continue validating what it's actually testing: thread-safe reader refresh,
+                        // concurrent searching, and proper reference counting - not file deletion timing.
+                        try
                         {
-                            searcher.Doc(hits[0].Doc);
+                            DirectoryReader refreshed = DirectoryReader.OpenIfChanged(r);
+                            if (refreshed is null)
+                            {
+                                refreshed = r;
+                            }
+
+                            IndexSearcher searcher = NewSearcher(refreshed);
+                            ScoreDoc[] hits = searcher.Search(new TermQuery(new Term("field1", "a" + rnd.Next(refreshed.MaxDoc))), null, 1000).ScoreDocs;
+                            if (hits.Length > 0)
+                            {
+                                searcher.Doc(hits[0].Doc);
+                            }
+                            if (refreshed != r)
+                            {
+                                refreshed.Dispose();
+                            }
                         }
-                        if (refreshed != r)
+                        catch (IOException e) when (e is FileNotFoundException or DirectoryNotFoundException)
                         {
-                            refreshed.Dispose();
+                            // Expected in this artificial test scenario - see comment above
                         }
                     }
                     UninterruptableMonitor.Enter(this);