fix(#150): close residual TOCTOU with LimitedReader at docmap open

cmd/review-bot/validatedocmap.go

@@ -50,8 +50,8 @@ func validateDocmapPath(localPath, resolvedRoot string) (string, error) {
 		return "", fmt.Errorf("cannot resolve path (symlink): %w", err)
 	}
 
-	// Lstat the resolved path — EvalSymlinks guarantees resolvedPath is
-	// symlink-free, so ModeSymlink can never be set here; this is unreachable.
+	// Lstat the resolved path for size and existence checks — EvalSymlinks
+	// guarantees no symlink components remain, so ModeSymlink can never be set.
 	fi, err := os.Lstat(resolvedPath)
 	if err != nil {
 		return "", fmt.Errorf("cannot stat file: %w", err)
@@ -152,11 +152,28 @@ func runValidateDocmap(args []string) int {
 		return 2
 	}
 
-	// Parse docmap YAML using the resolved path — eliminates any TOCTOU race
-	// between validation and use.
-	cfg, err := review.ParseDocMapConfig(resolvedDocmap)
+	// Open and read the docmap with a LimitedReader — closes the residual TOCTOU
+	// window between the Lstat size check in validateDocmapPath and the file open
+	// here. The limit is maxDocmapBytes+1 so we can detect a file that grew past
+	// the cap after the stat without reading unbounded bytes.
+	f, err := os.Open(resolvedDocmap)
 	if err != nil {
-		fmt.Fprintf(errWriter, "Error: failed to parse docmap %q: %v\n", resolvedDocmap, err)
+		fmt.Fprintf(errWriter, "Error: failed to open docmap %q: %v\n", *docmapFlag, err)
+		return 2
+	}
+	defer f.Close() // nolint:errcheck
+	docmapData, err := io.ReadAll(io.LimitReader(f, maxDocmapBytes+1))
+	if err != nil {
+		fmt.Fprintf(errWriter, "Error: failed to read docmap %q: %v\n", *docmapFlag, err)
+		return 2
+	}
+	if int64(len(docmapData)) > maxDocmapBytes {
+		fmt.Fprintf(errWriter, "Error: --docmap %q exceeded %d-byte limit after open\n", *docmapFlag, maxDocmapBytes)
+		return 2
+	}
+	cfg, err := review.ParseDocMapConfigContent(string(docmapData), *docmapFlag)
+	if err != nil {
+		fmt.Fprintf(errWriter, "Error: failed to parse docmap %q: %v\n", *docmapFlag, err)
 		return 2
 	}