[FIX] ir.mail.server: squash redundant CRs (bpo-34424)

Problem description:
Using the chatter, send an email to someone having non-ascii characters
in their name: the body of the received email looks like it contains
a mix of the original body and headers.

Python encodes the name into either base64 or quoted-printable but
leaves some redundant carriage returns at the end of the header. Those
redundant carriage returns are not RFC conformant but are interpreted
as newlines by some email clients, thus are read like a headers/body
separator and the next headers are considered part of the body,
corrupting the email structure.

This problem is internal to Python and has been fixed in 3.8, cfr
bpo-34424 [1], and later backported to 3.7.4. As we also support 3.6
and earlier 3.7 and it is not possible to easily monkey-patch the
function, we fixed it by squashing duplicate carriage returns.
(There is no case where a series of bare CRs can occur in a normal
RFC5322 email message)

Task: 2003936

[1]: https://bugs.python.org/issue34424

odoo/addons/base/models/ir_mail_server.py

@@ -9,6 +9,7 @@ import email.policy
 import logging
 import re
 import smtplib
+import sys
 import threading
 
 import html2text
@@ -414,7 +415,14 @@ class IrMailServer(models.Model):
             smtp = smtp or self.connect(
                 smtp_server, smtp_port, smtp_user, smtp_password,
                 smtp_encryption, smtp_debug, mail_server_id=mail_server_id)
-            smtp.sendmail(smtp_from, smtp_to_list, message.as_string())
+
+            message_str = message.as_string()
+            # header folding code is buggy and adds redundant carriage
+            # returns, it got fixed in 3.7.4 thanks to bpo-34424
+            if sys.version_info < (3, 7, 4):
+                message_str = re.sub('\r+', '\r', message_str)
+
+            smtp.sendmail(smtp_from, smtp_to_list, message_str)
             # do not quit() a pre-established smtp_session
             if not smtp_session:
                 smtp.quit()

odoo/addons/base/tests/test_mail.py

@@ -3,9 +3,11 @@
 
 
 from unittest.mock import patch
+import email.policy
+import email.message
+import threading
 
-from odoo.tests.common import BaseCase
-from odoo.tests.common import SavepointCase
+from odoo.tests.common import BaseCase, SavepointCase, TransactionCase
 from odoo.tools import (
     html_sanitize, append_content_to_html, plaintext2html, email_split,
     misc, formataddr,
@@ -374,3 +376,31 @@ class EmailConfigCase(SavepointCase):
             "The body of an email",
         )
         self.assertEqual(message["From"], "settings@example.com")
+
+
+class TestEmailMessage(TransactionCase):
+    def test_as_string(self):
+        """Ensure all email sent are bpo-34424 free"""
+
+        class FakeSMTP:
+            """SMTP stub"""
+            def __init__(this):
+                this.email_sent = False
+
+            def sendmail(this, smtp_from, smtp_to_list, message_str):
+                this.email_sent = True
+                message_truth = (
+                    r'From: .+? <joe@example\.com>\r\n'
+                    r'To: .+? <joe@example\.com>\r\n'
+                    r'\r\n'
+                )
+                self.assertRegex(message_str, message_truth)
+
+        msg = email.message.EmailMessage(policy=email.policy.SMTP)
+        msg['From'] = '"Joé Doe" <joe@example.com>'
+        msg['To'] = '"Joé Doe" <joe@example.com>'
+
+        smtp = FakeSMTP()
+        self.patch(threading.currentThread(), 'testing', False)
+        self.env['ir.mail_server'].send_email(msg, smtp_session=smtp)
+        self.assertTrue(smtp.email_sent)