Cover unchanged file drift for #1735

Author: Widthdom

changelog.d/unreleased/1735.fixed.md

@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 1735
+affected:
+  - src/CodeIndex/Database/DbWriter.cs
+  - tests/CodeIndex.Tests/PreparedCommandCacheTests.cs
+---
+
+## English
+
+- **Locked in atomic unchanged-file reuse (#1735)** — `GetUnchangedFileId` now has regression coverage ensuring checksum drift does not touch stale file metadata.
+
+## 日本語
+
+- **未変更ファイル再利用の atomic 契約を固定しました (#1735)** — `GetUnchangedFileId` に checksum drift 時に古い file metadata を touch しない regression coverage を追加しました。

src/CodeIndex/Database/DbWriter.cs

@@ -482,6 +482,8 @@ private void RunPassiveWalCheckpoint()
         if (!SymbolExtractorVersionMatchesCurrent(language))
             return null;
 
+        // Keep the unchanged check and timestamp touch in one SQLite statement so
+        // concurrent row drift cannot slip between a SELECT and a later UPDATE (#1735).
         var cmd = RentCommand(
             @"UPDATE files
               SET modified = CASE

tests/CodeIndex.Tests/PreparedCommandCacheTests.cs

@@ -346,6 +346,30 @@ public void DbWriter_WithCache_GetUnchangedFileIdTouchUpdatesTimestamp()
         Assert.Equal(touched, reader.GetDateTime(0));
     }
 
+    [Fact]
+    public void DbWriter_WithCache_GetUnchangedFileIdDoesNotTouchWhenChecksumDrifts_Issue1735()
+    {
+        var writer = new DbWriter(_db);
+        var initial = new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        var touched = new DateTime(2025, 6, 1, 0, 0, 0, DateTimeKind.Utc);
+
+        writer.UpsertFile(new FileRecord
+        {
+            Path = "src/drift.py", Lang = "python", Size = 1, Lines = 1,
+            Checksum = "old_checksum", Modified = initial,
+        });
+
+        Assert.Null(writer.GetUnchangedFileId("src/drift.py", touched, "new_checksum"));
+
+        using var cmd = _db.Connection.CreateCommand();
+        cmd.CommandText = "SELECT modified, checksum FROM files WHERE path = @p";
+        cmd.Parameters.AddWithValue("@p", "src/drift.py");
+        using var reader = cmd.ExecuteReader();
+        Assert.True(reader.Read());
+        Assert.Equal(initial, reader.GetDateTime(0));
+        Assert.Equal("old_checksum", reader.GetString(1));
+    }
+
     [Fact]
     public void DbReader_WithCache_ReusesCSharpResolutionCommandsAcrossReaders()
     {