gh-113594: Fix UnicodeEncodeError in TokenList.fold() (GH-113730)

It occurred when try to re-encode an unknown-8bit part combined with non-unknown-8bit part.

Lib/email/_header_value_parser.py

@@ -2766,6 +2766,7 @@ def _refold_parse_tree(parse_tree, *, policy):
     encoding = 'utf-8' if policy.utf8 else 'us-ascii'
     lines = ['']
     last_ew = None
+    last_charset = None
     wrap_as_ew_blocked = 0
     want_encoding = False
     end_ew_not_allowed = Terminal('', 'wrap_as_ew_blocked')
@@ -2820,8 +2821,14 @@ def _refold_parse_tree(parse_tree, *, policy):
             else:
                 # It's a terminal, wrap it as an encoded word, possibly
                 # combining it with previously encoded words if allowed.
+                if (last_ew is not None and
+                    charset != last_charset and
+                    (last_charset == 'unknown-8bit' or
+                     last_charset == 'utf-8' and charset != 'us-ascii')):
+                    last_ew = None
                 last_ew = _fold_as_ew(tstr, lines, maxlen, last_ew,
                                       part.ew_combine_allowed, charset)
+                last_charset = charset
             want_encoding = False
             continue
         if len(tstr) <= maxlen - len(lines[-1]):

Lib/test/test_email/test__header_value_parser.py

@@ -2915,6 +2915,45 @@ class TestFolding(TestEmailBase):
                         "mich.  And that's\n"
                    " all I'm sayin.\n")
 
+    def test_unicode_after_unknown_not_combined(self):
+        self._test(parser.get_unstructured("=?unknown-8bit?q?=A4?=\xa4"),
+                   "=?unknown-8bit?q?=A4?==?utf-8?q?=C2=A4?=\n")
+        prefix = "0123456789 "*5
+        self._test(parser.get_unstructured(prefix + "=?unknown-8bit?q?=A4?=\xa4"),
+                   prefix + "=?unknown-8bit?q?=A4?=\n =?utf-8?q?=C2=A4?=\n")
+
+    def test_ascii_after_unknown_not_combined(self):
+        self._test(parser.get_unstructured("=?unknown-8bit?q?=A4?=abc"),
+                   "=?unknown-8bit?q?=A4?=abc\n")
+        prefix = "0123456789 "*5
+        self._test(parser.get_unstructured(prefix + "=?unknown-8bit?q?=A4?=abc"),
+                   prefix + "=?unknown-8bit?q?=A4?=\n =?utf-8?q?abc?=\n")
+
+    def test_unknown_after_unicode_not_combined(self):
+        self._test(parser.get_unstructured("\xa4"
+                                           "=?unknown-8bit?q?=A4?="),
+                   "=?utf-8?q?=C2=A4?==?unknown-8bit?q?=A4?=\n")
+        prefix = "0123456789 "*5
+        self._test(parser.get_unstructured(prefix + "\xa4=?unknown-8bit?q?=A4?="),
+                   prefix + "=?utf-8?q?=C2=A4?=\n =?unknown-8bit?q?=A4?=\n")
+
+    def test_unknown_after_ascii_not_combined(self):
+        self._test(parser.get_unstructured("abc"
+                                           "=?unknown-8bit?q?=A4?="),
+                   "abc=?unknown-8bit?q?=A4?=\n")
+        prefix = "0123456789 "*5
+        self._test(parser.get_unstructured(prefix + "abcd=?unknown-8bit?q?=A4?="),
+                   prefix + "abcd\n =?unknown-8bit?q?=A4?=\n")
+
+    def test_unknown_after_unknown(self):
+        self._test(parser.get_unstructured("=?unknown-8bit?q?=C2?="
+                                           "=?unknown-8bit?q?=A4?="),
+                   "=?unknown-8bit?q?=C2=A4?=\n")
+        prefix = "0123456789 "*5
+        self._test(parser.get_unstructured(prefix + "=?unknown-8bit?q?=C2?="
+                                           "=?unknown-8bit?q?=A4?="),
+                   prefix + "=?unknown-8bit?q?=C2?=\n =?unknown-8bit?q?=A4?=\n")
+
     # XXX Need test of an encoded word so long that it needs to be wrapped
 
     def test_simple_address(self):

Misc/NEWS.d/next/Library/2024-01-05-12-42-07.gh-issue-113594.4t8HiR.rst

@@ -0,0 +1,2 @@
+Fix :exc:`UnicodeEncodeError` in :mod:`email` when re-fold lines that
+contain unknown-8bit encoded part followed by non-unknown-8bit encoded part.