gh-56634: Gettext: handle gracefully empty plural-forms value

Author: m-aciek

Lib/gettext.py

@@ -410,7 +410,10 @@ def _parse(self, fp):
                         self._charset = v.split('charset=')[1]
                     elif k == 'plural-forms':
                         v = v.split(';')
-                        plural = v[1].split('plural=')[1]
+                        try:
+                            plural = v[1].split('plural=')[1]
+                        except IndexError as e:
+                            raise ValueError('invalid plural forms syntax') from e
                         self.plural = c2py(plural)
             # Note: we unconditionally convert both msgids and msgstrs to
             # Unicode using the character encoding specified in the charset

Lib/test/test_gettext.py

@@ -1,6 +1,6 @@
 import os
 import base64
-import gettext
+from Lib import gettext
 import unittest
 from functools import partial
 
@@ -624,6 +624,22 @@ def test_ignore_comments_in_headers_issue36239(self):
             t = gettext.GNUTranslations(fp)
             self.assertEqual(t.info()["plural-forms"], "nplurals=2; plural=(n != 1);")
 
+    def test_gracefully_handle_incorrect_plural_forms(self):
+        with open(MOFILE, 'wb') as fp:
+            # below is msgfmt run on such a PO file:
+            # msgid ""
+            # msgstr ""
+            # "Content-Type: text/plain; charset=UTF-8\n"
+            # "Plural-Forms: \n"
+            fp.write(
+                b'\xde\x12\x04\x95\x00\x00\x00\x00\x01\x00\x00\x00\x1c\x00\x00\x00$\x00\x00\x00\x03\x00\x00\x00,\x00'
+                b'\x00\x00\x00\x00\x00\x008\x00\x00\x007\x00\x00\x009\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00'
+                b'\x00\x00\x00\x00Content-Type: text/plain; charset=UTF-8\nPlural-Forms: \n\x00'
+            )
+        with self.assertRaisesRegex(ValueError, "invalid plural forms syntax"):
+            with open(MOFILE, 'rb') as fp:
+                gettext.GNUTranslations(fp)
+
 
 class UnicodeTranslationsTest(GettextBaseTest):
     def setUp(self):