Fix symlink traversal cycle guard (#1711)

Author: Widthdom

src/CodeIndex/Indexer/Scanning/FileIndexer.cs

@@ -1365,6 +1365,31 @@ private bool ShouldSkipDirectoryLink(string subDir, List<ScanError> errors, Hash
         return true;
     }
 
+    private static string GetDirectoryTraversalIdentity(string directory)
+    {
+        if (!IsReparsePoint(directory))
+            return directory;
+
+        try
+        {
+            DirectoryInfo info = new(LongPath.EnsureWindowsPrefix(directory));
+            var target = info.ResolveLinkTarget(returnFinalTarget: true);
+            if (target?.FullName is { Length: > 0 } targetPath)
+                return targetPath;
+        }
+        catch (FileNotFoundException)
+        {
+        }
+        catch (DirectoryNotFoundException)
+        {
+        }
+        catch (IOException)
+        {
+        }
+
+        return directory;
+    }
+
     internal static FileProbeStatus GetFileIndexability(string filePath)
     {
         if (OperatingSystem.IsWindows() && IsWindowsDevicePath(filePath))
@@ -2074,7 +2099,7 @@ private bool EnumerateDirectory(
                     continue;
                 }
 
-                var resolvedSubDir = NormalizePathForComparison(subDir);
+                var resolvedSubDir = NormalizePathForComparison(GetDirectoryTraversalIdentity(subDir));
                 if (!visitedDirectories.Add(resolvedSubDir))
                 {
                     var subRelative = ToRelativePath(subDir);

tests/CodeIndex.Tests/FileIndexerTests.cs

@@ -3136,6 +3136,44 @@ public void ScanFiles_FollowSymlinksInternal_FollowsInTreeDirectorySymlinkOnce()
         }
     }
 
+    [Fact]
+    public void ScanFiles_FollowSymlinksInternal_SkipsCycleToProjectRoot()
+    {
+        if (OperatingSystem.IsWindows())
+            return;
+
+        var tempDir = Path.Combine(Path.GetTempPath(), $"cdidx-symlink-cycle-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempDir);
+        try
+        {
+            File.WriteAllText(Path.Combine(tempDir, "app.py"), "print('app')\n");
+            Directory.CreateSymbolicLink(Path.Combine(tempDir, "self"), tempDir);
+
+            var indexer = new FileIndexer(
+                tempDir,
+                ignoreCase: false,
+                ignoreRuleRoot: null,
+                maxFileSizeBytes: null,
+                directoryIgnoreCaseProbe: null,
+                symlinkPolicy: FileIndexer.SymlinkPolicy.Internal);
+
+            var result = indexer.ScanFilesDetailed();
+
+            Assert.Single(result.Files);
+            Assert.Contains(
+                result.Errors,
+                error => error.Path == "self"
+                    && error.Severity == FileIndexer.ScanIssueSeverity.Warning
+                    && error.Message.Contains("already scanned", StringComparison.OrdinalIgnoreCase));
+            Assert.False(result.HadErrors);
+        }
+        finally
+        {
+            if (Directory.Exists(tempDir))
+                Directory.Delete(tempDir, true);
+        }
+    }
+
     [Fact]
     public void ScanFiles_DescendsIntoSubmoduleHostedUnderSkipDir()
     {