pgn: escape and unescape header values

.github/workflows/setup-ubuntu-latest.sh

@@ -1,5 +1,9 @@
 #!/bin/sh -e
 
+# Refresh the package index: the runner image's baked-in lists can reference
+# package versions the mirrors no longer carry (404).
+sudo apt-get update
+
 # Stockfish
 sudo apt-get install -y stockfish
 

chess/pgn.py

@@ -1414,7 +1414,8 @@ def begin_headers(self) -> None:
     def visit_header(self, tagname: str, tagvalue: str) -> None:
         if self.headers:
             self.found_headers = True
-            self.write_line(f"[{tagname} \"{tagvalue}\"]")
+            escaped_tagvalue = tagvalue.replace("\\", "\\\\").replace('"', '\\"')
+            self.write_line(f"[{tagname} \"{escaped_tagvalue}\"]")
 
     def end_headers(self) -> None:
         if self.found_headers:
@@ -1645,9 +1646,10 @@ def read_game(handle: TextIO, *, Visitor: Any = GameBuilder) -> Any:
         if not skipping_game:
             tag_match = TAG_REGEX.match(stripped)
             if tag_match:
-                visitor.visit_header(tag_match.group(1), tag_match.group(2))
+                tagvalue = re.sub(r'\\(["\\])', r'\1', tag_match.group(2))
+                visitor.visit_header(tag_match.group(1), tagvalue)
                 if unmanaged_headers is not None:
-                    unmanaged_headers[tag_match.group(1)] = tag_match.group(2)
+                    unmanaged_headers[tag_match.group(1)] = tagvalue
             else:
                 # Ignore invalid or malformed headers.
                 line = handle.readline()

test.py

@@ -2094,6 +2094,30 @@ def test_minimum_weight(self):
 
 class PgnTestCase(unittest.TestCase):
 
+    HEADER_VALUE_CASES = [
+        ("Plain", "Plain"),
+        ('The "Open"', r'The \"Open\"'),
+        (r"C:\chess", r"C:\\chess"),
+        ('quote " and slash \\', r'quote \" and slash \\'),
+        ('ends in \\', r'ends in \\'),
+        ("right ] bracket", "right ] bracket"),
+    ]
+
+    def test_exporter_escapes_headers(self):
+        for value, encoded in self.HEADER_VALUE_CASES:
+            with self.subTest(value=value):
+                game = chess.pgn.Game.without_tag_roster()
+                game.headers["Event"] = value
+
+                exported = game.accept(chess.pgn.StringExporter(columns=None))
+                expected = f'[Event "{encoded}"]\n\n*'
+                self.assertEqual(exported, expected)
+                self.assertEqual(chess.pgn.read_headers(io.StringIO(exported))["Event"], value)
+
+                handle = io.StringIO()
+                game.accept(chess.pgn.FileExporter(handle, columns=None))
+                self.assertEqual(handle.getvalue(), expected + "\n\n")
+
     def test_exporter(self):
         game = chess.pgn.Game()
         game.comments = ["Test game:"]
@@ -2251,6 +2275,16 @@ def test_read_game_with_leading_whitespace_before_header(self):
         self.assertEqual(game.next().move, chess.Move.from_uci("d2d4"))
         self.assertEqual(game.errors, [])
 
+    def test_reader_unescapes_headers(self):
+        for expected, encoded in self.HEADER_VALUE_CASES:
+            with self.subTest(encoded=encoded):
+                text = f'[Event "{encoded}"]\n\n*\n'
+                game = chess.pgn.read_game(io.StringIO(text))
+                headers = chess.pgn.read_headers(io.StringIO(text))
+
+                self.assertEqual(game.headers["Event"], expected)
+                self.assertEqual(headers["Event"], expected)
+
     def test_read_game_with_multicomment_move(self):
         pgn = io.StringIO("1. e4 {A common opening} 1... e5 {A common response} {An uncommon comment}")
         game = chess.pgn.read_game(pgn)