Raise HeaderParseError instead of continuing when parsing a word.

Also, modify the testcase.

Author: maxking

Lib/email/_header_value_parser.py

@@ -1360,16 +1360,13 @@ def get_word(value):
         leader, value = get_cfws(value)
     else:
         leader = None
-    if value:
-        if value[0]=='"':
-            token, value = get_quoted_string(value)
-        elif value[0] in SPECIALS:
-            raise errors.HeaderParseError("Expected 'atom' or 'quoted-string' "
-                                          "but found '{}'".format(value))
-        else:
-            token, value = get_atom(value)
-        if leader is not None:
-            token[:0] = [leader]
+    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:
+        raise errors.HeaderParseError((value))
     else:
         token, value = get_atom(value)
     if leader is not None:

Lib/test/test_email/test__header_value_parser.py

@@ -931,18 +931,10 @@ def test_get_word_atom_yields_atom(self):
         self.assertEqual(word[0].token_type, 'cfws')
 
     def test_get_word_all_CFWS(self):
-        word = self._test_get_x(parser.get_word,
-                                '(Recipients list suppressed',
-                                str(parser.CFWSList([parser.Comment([
-                                    parser.WhiteSpaceTerminal('Recipients', 'ptext'),
-                                    parser.WhiteSpaceTerminal(' ', 'fws'),
-                                    parser.WhiteSpaceTerminal('list', 'ptext'),
-                                    parser.WhiteSpaceTerminal(' ', 'fws'),
-                                    parser.WhiteSpaceTerminal('suppressed', 'ptext')
-                                    ])])),
-                                    ' ', [], ''
-                                )
-        self.assertEqual(word.token_type, 'cfws')
+        # 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,

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 with only comments in some headers.
+Patch by Abhilash Raj.