Fix related WSP folding bug

Author: robsdedude

Lib/email/_header_value_parser.py

@@ -3058,7 +3058,9 @@ def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset,
         remaining_space = maxlen - len(lines[-1])
         text_space = remaining_space - chrome_len
         if text_space <= 0:
-            lines.append(' ')
+            newline = _steal_trailing_WSP_if_exists(lines)
+            lines.append(newline or ' ')
+            new_last_ew = len(lines[-1])
             continue
 
         to_encode_word = to_encode[:text_space]

Lib/test/test_email/test_generator.py

@@ -413,6 +413,30 @@ def test_defaults_handle_spaces_at_start_of_continuation_line_2(self):
         g.flatten(msg)
         self.assertEqual(s.getvalue(), expected)
 
+    def test_ew_folding_round_trip_1(self):
+        print()
+        source = "aaaaaaaaa фффффффф "
+        msg = EmailMessage()
+        msg['Subject'] = source
+        s = io.BytesIO()
+        g = BytesGenerator(s, maxheaderlen=30)
+        g.flatten(msg)
+        flat = s.getvalue()
+        reparsed = message_from_bytes(flat, policy=policy.default)['Subject']
+        self.assertMultiLineEqual(reparsed, source)
+
+    def test_ew_folding_round_trip_2(self):
+        print()
+        source = "aaa aaaaaaa   aaa ффф фффф  "
+        msg = EmailMessage()
+        msg['Subject'] = source
+        s = io.BytesIO()
+        g = BytesGenerator(s, maxheaderlen=30)
+        g.flatten(msg)
+        flat = s.getvalue()
+        reparsed = message_from_bytes(flat, policy=policy.default)['Subject']
+        self.assertMultiLineEqual(reparsed, source)
+
     def test_cte_type_7bit_handles_unknown_8bit(self):
         source = ("Subject: Maintenant je vous présente mon "
                  "collègue\n\n").encode('utf-8')

Lib/test/test_email/test_headerregistry.py

@@ -1711,7 +1711,7 @@ def test_fold_unstructured_with_overlong_word(self):
             'singlewordthatwontfit')
         self.assertEqual(
             h.fold(policy=policy.default.clone(max_line_length=20)),
-            'Subject: \n'
+            'Subject:\n'
             ' =?utf-8?q?thisisa?=\n'
             ' =?utf-8?q?verylon?=\n'
             ' =?utf-8?q?glineco?=\n'
@@ -1727,7 +1727,7 @@ def test_fold_unstructured_with_two_overlong_words(self):
             'singlewordthatwontfit plusanotherverylongwordthatwontfit')
         self.assertEqual(
             h.fold(policy=policy.default.clone(max_line_length=20)),
-            'Subject: \n'
+            'Subject:\n'
             ' =?utf-8?q?thisisa?=\n'
             ' =?utf-8?q?verylon?=\n'
             ' =?utf-8?q?glineco?=\n'

Lib/test/test_email/test_policy.py

@@ -273,7 +273,7 @@ def test_non_ascii_chars_do_not_cause_inf_loop(self):
         actual = policy.fold('Subject', 'ą' * 12)
         self.assertEqual(
             actual,
-            'Subject: \n' +
+            'Subject:\n' +
             12 * ' =?utf-8?q?=C4=85?=\n')
 
     def test_short_maxlen_error(self):