bpo-40597: email: Use CTE if lines are longer than max_line_length consistently (gh-20038)

raw_data_manager (default for EmailPolicy, EmailMessage)
does correct wrapping of 'text' parts as long as the message contains
characters outside of 7bit US-ASCII set: base64 or qp
Content-Transfer-Encoding is applied if the lines would be too long
without it.  It did not, however, do this for ascii-only text,
which could result in lines that were longer than
policy.max_line_length or even the rfc 998  maximum.

This changeset fixes the heuristic so that if lines are longer than
policy.max_line_length, it will always apply a
content-transfer-encoding so that the lines are wrapped correctly.

Lib/email/contentmanager.py

@@ -146,13 +146,13 @@ def _encode_text(string, charset, cte, policy):
     def normal_body(lines): return b'\n'.join(lines) + b'\n'
     if cte==None:
         # Use heuristics to decide on the "best" encoding.
-        try:
-            return '7bit', normal_body(lines).decode('ascii')
-        except UnicodeDecodeError:
-            pass
-        if (policy.cte_type == '8bit' and
-                max(len(x) for x in lines) <= policy.max_line_length):
-            return '8bit', normal_body(lines).decode('ascii', 'surrogateescape')
+        if max(len(x) for x in lines) <= policy.max_line_length:
+            try:
+                return '7bit', normal_body(lines).decode('ascii')
+            except UnicodeDecodeError:
+                pass
+            if policy.cte_type == '8bit':
+                return '8bit', normal_body(lines).decode('ascii', 'surrogateescape')
         sniff = embedded_body(lines[:10])
         sniff_qp = quoprimime.body_encode(sniff.decode('latin-1'),
                                           policy.max_line_length)

Lib/test/test_email/test_contentmanager.py

@@ -329,6 +329,21 @@ class TestRawDataManager(TestEmailBase):
         self.assertEqual(m.get_payload(decode=True).decode('utf-8'), content)
         self.assertEqual(m.get_content(), content)
 
+    def test_set_text_plain_long_line_heuristics(self):
+        m = self._make_message()
+        content = ("Simple but long message that is over 78 characters"
+                   " long to force transfer encoding.\n")
+        raw_data_manager.set_content(m, content)
+        self.assertEqual(str(m), textwrap.dedent("""\
+            Content-Type: text/plain; charset="utf-8"
+            Content-Transfer-Encoding: quoted-printable
+
+            Simple but long message that is over 78 characters long to =
+            force transfer encoding.
+            """))
+        self.assertEqual(m.get_payload(decode=True).decode('utf-8'), content)
+        self.assertEqual(m.get_content(), content)
+
     def test_set_text_short_line_minimal_non_ascii_heuristics(self):
         m = self._make_message()
         content = "et là il est monté sur moi et il commence à m'éto.\n"

Misc/NEWS.d/next/Library/2020-05-11-19-17-23.bpo-40597.4SGfgm.rst

@@ -0,0 +1 @@
+If text content lines are longer than policy.max_line_length, always use a content-encoding to make sure they are wrapped.