bpo-38698: Prevent UnboundLocalError to pop up in parse_message_id (GH-17277)

parse_message_id() was improperly using a token defined inside an exception
handler, which was raising `UnboundLocalError` on parsing an invalid value.

https://bugs.python.org/issue38698
(cherry picked from commit bb81549)

Co-authored-by: Claudiu Popa <[email protected]>

Author: miss-islington

Lib/email/_header_value_parser.py

@@ -2113,7 +2113,8 @@ def parse_message_id(value):
     except errors.HeaderParseError:
         message_id.defects.append(errors.InvalidHeaderDefect(
             "Expected msg-id but found {!r}".format(value)))
-    message_id.append(token)
+    else:
+        message_id.append(token)
     return message_id
 
 #

Lib/test/test_email/test__header_value_parser.py

@@ -2638,6 +2638,12 @@ def test_get_msg_id_no_id_right_part(self):
         )
         self.assertEqual(msg_id.token_type, 'msg-id')
 
+    def test_get_msg_id_invalid_expected_msg_id_not_found(self):
+        text = "Message-Id: 935-XPB-567:0:86089:180874:0:45327:9:90305:[email protected]"
+        msg_id = parser.parse_message_id(text)
+        self.assertDefectsEqual(msg_id.all_defects,
+                                [errors.InvalidHeaderDefect])
+
     def test_get_msg_id_no_angle_start(self):
         with self.assertRaises(errors.HeaderParseError):
             parser.get_msg_id("msgwithnoankle")

Misc/NEWS.d/next/Library/2019-12-02-10-35-19.bpo-38698.WZnAPQ.rst

@@ -0,0 +1,5 @@
+Prevent UnboundLocalError to pop up in parse_message_id
+
+parse_message_id() was improperly using a token defined inside an exception
+handler, which was raising `UnboundLocalError` on parsing an invalid value.
+Patch by Claudiu Popa.