Fix permanent Lucene index corruption under concurrent indexing requests

AzureSearchEmulator/Indexing/LuceneNetSearchIndexer.cs

@@ -1,4 +1,5 @@
-using AzureSearchEmulator.Models;
+using System.Collections.Concurrent;
+using AzureSearchEmulator.Models;
 using AzureSearchEmulator.SearchData;
 using Lucene.Net.Index;
 using Lucene.Net.Util;
@@ -10,41 +11,62 @@ public class LuceneNetSearchIndexer(
     ILuceneIndexReaderFactory luceneIndexReaderFactory)
     : ISearchIndexer
 {
+    // One writer gate per index. Lucene allows only a single IndexWriter per
+    // directory; this class creates a fresh writer per request, so two
+    // concurrent indexing requests against the same index would race — the
+    // loser either fails with LockObtainFailedException or, worse, observes
+    // a torn commit and permanently corrupts the segment files
+    // (FileNotFoundException: .../_N.si on every subsequent write until the
+    // index directory is deleted). Serializing IndexDocuments per index name
+    // removes both failure modes; requests against different indexes still
+    // run in parallel.
+    private static readonly ConcurrentDictionary<string, SemaphoreSlim> IndexWriteGates = new();
+
     public IndexDocumentsResult IndexDocuments(SearchIndex index, IList<IndexDocumentAction> actions)
     {
-        var analyzer = AnalyzerHelper.GetPerFieldIndexAnalyzer(index.Fields);
+        var gate = IndexWriteGates.GetOrAdd(index.Name, static _ => new SemaphoreSlim(1, 1));
+        gate.Wait();
 
-        var config = new IndexWriterConfig(LuceneVersion.LUCENE_48, analyzer);
+        try
+        {
+            var analyzer = AnalyzerHelper.GetPerFieldIndexAnalyzer(index.Fields);
 
-        var directory = luceneDirectoryFactory.GetDirectory(index.Name);
-        using var writer = new IndexWriter(directory, config);
+            var config = new IndexWriterConfig(LuceneVersion.LUCENE_48, analyzer);
 
-        var key = index.GetKeyField();
+            var directory = luceneDirectoryFactory.GetDirectory(index.Name);
+            using var writer = new IndexWriter(directory, config);
 
-        var results = new IndexDocumentsResult();
+            var key = index.GetKeyField();
 
-        // ReSharper disable once AccessToDisposedClosure
-        var readerLazy = new Lazy<IndexReader>(() => writer.GetReader(true));
+            var results = new IndexDocumentsResult();
 
-        var context = new IndexingContext(index, key, writer, readerLazy);
+            // ReSharper disable once AccessToDisposedClosure
+            var readerLazy = new Lazy<IndexReader>(() => writer.GetReader(true));
 
-        foreach (var action in actions)
-        {
-            var result = action.PerformIndexingAsync(context);
-            results.Value.Add(result);
-        }
+            var context = new IndexingContext(index, key, writer, readerLazy);
 
-        if (readerLazy.IsValueCreated)
-        {
-            var reader = readerLazy.Value;
-            reader.Dispose();
-        }
+            foreach (var action in actions)
+            {
+                var result = action.PerformIndexingAsync(context);
+                results.Value.Add(result);
+            }
+
+            if (readerLazy.IsValueCreated)
+            {
+                var reader = readerLazy.Value;
+                reader.Dispose();
+            }
 
-        writer.Commit();
-        writer.Flush(true, true);
+            writer.Commit();
+            writer.Flush(true, true);
 
-        luceneIndexReaderFactory.RefreshReader(index.Name);
+            luceneIndexReaderFactory.RefreshReader(index.Name);
 
-        return results;
+            return results;
+        }
+        finally
+        {
+            gate.Release();
+        }
     }
 }