bpo-29412: Fix indexError when parsing a header value ending unexpectedly (GH-14387)

* patched string index out of range error in get_word function of _header_value_parser.py and created tests in test__header_value_parser.py for CFWS.
* Raise HeaderParseError instead of continuing when parsing a word.

Lib/email/_header_value_parser.py

@@ -1360,6 +1360,9 @@ def get_word(value):
         leader, value = get_cfws(value)
     else:
         leader = None
+    if not value:
+        raise errors.HeaderParseError(
+            "Expected 'atom' or 'quoted-string' but found nothing.")
     if value[0]=='"':
         token, value = get_quoted_string(value)
     elif value[0] in SPECIALS:

Lib/test/test_email/test__header_value_parser.py

@@ -930,6 +930,12 @@ class TestParser(TestParserMixin, TestEmailBase):
         self.assertEqual(word.token_type, 'atom')
         self.assertEqual(word[0].token_type, 'cfws')
 
+    def test_get_word_all_CFWS(self):
+        # bpo-29412: Test that we don't raise IndexError when parsing CFWS only
+        # token.
+        with self.assertRaises(errors.HeaderParseError):
+            parser.get_word('(Recipients list suppressed')
+
     def test_get_word_qs_yields_qs(self):
         word = self._test_get_x(parser.get_word,
             '"bar " (bang) ah', '"bar " (bang) ', 'bar  ', [], 'ah')
@@ -2343,6 +2349,17 @@ class TestParser(TestParserMixin, TestEmailBase):
 
     # get_address_list
 
+    def test_get_address_list_CFWS(self):
+        address_list = self._test_get_x(parser.get_address_list,
+                                        '(Recipient list suppressed)',
+                                        '(Recipient list suppressed)',
+                                        ' ',
+                                        [errors.ObsoleteHeaderDefect],  # no content in address list
+                                        '')
+        self.assertEqual(address_list.token_type, 'address-list')
+        self.assertEqual(len(address_list.mailboxes), 0)
+        self.assertEqual(address_list.mailboxes, address_list.all_mailboxes)
+
     def test_get_address_list_mailboxes_simple(self):
         address_list = self._test_get_x(parser.get_address_list,
             'dinsdale@example.com',

Misc/NEWS.d/next/Library/2019-06-25-19-27-25.bpo-29412.n4Zqdh.rst

@@ -0,0 +1,2 @@
+Fix IndexError in parsing a header value ending unexpectedly. Patch by Abhilash
+Raj.