Fix IndexError in read_game on an unmatched ) after an illegal move

chess/pgn.py

@@ -1229,7 +1229,12 @@ def begin_variation(self) -> None:
 
     @override
     def end_variation(self) -> None:
-        self.variation_stack.pop()
+        # Keep the root game node on the stack. A malformed PGN (e.g. an error
+        # recovery that skips to an unmatched ")") can call end_variation more
+        # often than begin_variation; popping the root here would empty the
+        # stack and make the next visit_move raise IndexError.
+        if len(self.variation_stack) > 1:
+            self.variation_stack.pop()
 
     @override
     def visit_result(self, result: str) -> None:

test.py

@@ -2358,6 +2358,17 @@ def test_variation_stack(self):
         self.assertEqual(game[0].san(), "c4")
         self.assertEqual(len(game.errors), 0)
 
+        # Survive a closing bracket reached through error recovery. The illegal
+        # move sends the parser into skip mode, and the unmatched ")" used to
+        # pop the root off the variation stack, so the following move raised an
+        # IndexError instead of being parsed.
+        pgn = io.StringIO("1. e4 Nf3 ) e5 *")
+        logging.disable(logging.ERROR)
+        game = chess.pgn.read_game(pgn)
+        logging.disable(logging.NOTSET)
+        self.assertEqual([node.san() for node in game.mainline()], ["e4", "e5"])
+        self.assertEqual(len(game.errors), 1)
+
     def test_game_starting_comment(self):
         pgn = io.StringIO("{ Game starting comment } 1. d3")
         game = chess.pgn.read_game(pgn)