gh-124452: Fix header mismatches when folding/unfolding with email message (#125919)

The header-folder of the new email API has a long standing known buglet where
if the first token is longer than max_line_length, it puts that token on the next
line.  It turns out there is also a *parsing* bug when parsing such a header:
the space prefixing that first, non-empty line gets preserved and tacked on to
the start of the header value, which is not the expected behavior per the RFCs.
The bug arises from the fact that the parser assumed that there would be at
least one token on the line with the header, which is going to be true for
probably every email producer other than the python email library with its
folding buglet.  Clearly, though, this is a case that needs to be handled
correctly.  The fix is simple: strip the blanks off the start of the whole
value, not just the first physical line of the value.

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Lib/email/_policybase.py

@@ -302,12 +302,12 @@ class Compat32(Policy):
         """+
         The name is parsed as everything up to the ':' and returned unmodified.
         The value is determined by stripping leading whitespace off the
-        remainder of the first line, joining all subsequent lines together, and
+        remainder of the first line joined with all subsequent lines, and
         stripping any trailing carriage return or linefeed characters.
 
         """
         name, value = sourcelines[0].split(':', 1)
-        value = value.lstrip(' \t') + ''.join(sourcelines[1:])
+        value = ''.join((value, *sourcelines[1:])).lstrip(' \t\r\n')
         return (name, value.rstrip('\r\n'))
 
     def header_store_parse(self, name, value):

Lib/email/policy.py

@@ -119,13 +119,13 @@ class EmailPolicy(Policy):
         """+
         The name is parsed as everything up to the ':' and returned unmodified.
         The value is determined by stripping leading whitespace off the
-        remainder of the first line, joining all subsequent lines together, and
+        remainder of the first line joined with all subsequent lines, and
         stripping any trailing carriage return or linefeed characters.  (This
         is the same as Compat32).
 
         """
         name, value = sourcelines[0].split(':', 1)
-        value = value.lstrip(' \t') + ''.join(sourcelines[1:])
+        value = ''.join((value, *sourcelines[1:])).lstrip(' \t\r\n')
         return (name, value.rstrip('\r\n'))
 
     def header_store_parse(self, name, value):

Lib/test/test_email/test_message.py

@@ -1,6 +1,6 @@
-import unittest
 import textwrap
-from email import policy, message_from_string
+import unittest
+from email import message_from_bytes, message_from_string, policy
 from email.message import EmailMessage, MIMEPart
 from test.test_email import TestEmailBase, parameterize
 
@@ -958,6 +958,52 @@ class TestEmailMessage(TestEmailMessageBase, TestEmailBase):
                          b'123456789-123456789\n 123456789 Hello '
                          b'=?utf-8?q?W=C3=B6rld!?= 123456789 123456789\n\n')
 
+    def test_folding_with_short_nospace_1(self):
+        # bpo-36520
+        #
+        # Fold a line that contains a long whitespace after
+        # the fold point.
+
+        m = EmailMessage(policy.default)
+        m['Message-ID'] = '123456789' * 3
+        parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
+        self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
+
+    def test_folding_with_long_nospace_default_policy_1(self):
+        # Fixed: https://github.com/python/cpython/issues/124452
+        #
+        # When the value is too long, it should be converted back
+        # to its original form without any modifications.
+
+        m = EmailMessage(policy.default)
+        message = '123456789' * 10
+        m['Message-ID'] = message
+        self.assertEqual(m.as_bytes(),
+                         f'Message-ID:\n {message}\n\n'.encode())
+        parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
+        self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
+
+    def test_folding_with_long_nospace_compat32_policy_1(self):
+        m = EmailMessage(policy.compat32)
+        message = '123456789' * 10
+        m['Message-ID'] = message
+        parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
+        self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
+
+    def test_folding_with_long_nospace_smtp_policy_1(self):
+        m = EmailMessage(policy.SMTP)
+        message = '123456789' * 10
+        m['Message-ID'] = message
+        parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
+        self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
+
+    def test_folding_with_long_nospace_http_policy_1(self):
+        m = EmailMessage(policy.HTTP)
+        message = '123456789' * 10
+        m['Message-ID'] = message
+        parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
+        self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
+
     def test_get_body_malformed(self):
         """test for bpo-42892"""
         msg = textwrap.dedent("""\

Misc/NEWS.d/next/Library/2024-10-24-10-49-47.gh-issue-124452.eqTRgx.rst

@@ -0,0 +1,4 @@
+Fix an issue in :meth:`email.policy.EmailPolicy.header_source_parse` and
+:meth:`email.policy.Compat32.header_source_parse` that introduced spurious
+leading whitespaces into header values when the header includes a newline
+character after the header name delimiter (``:``) and before the value.