Fix unencrypted delivery in case of message generation failure

When we fail to produce byte representation of the email message being
processed, we may end up bouncing a message.  An example of such case would be
a message with a Message-Id header that Python's email parser library cannot
process.

In such cases, just take whatever original content we have received and pass
it to the destination without touching it to minimise any chances of breaking
the overall flow.

lacre/config.py

@@ -143,6 +143,11 @@ def strict_mode():
     return ("default" in cfg and cfg["default"]["enc_keymap_only"] == "yes")
 
 
+def should_log_headers() -> bool:
+    """Check if Lacre should log message headers."""
+    return flag_enabled('daemon', 'log_headers')
+
+
 class FromStrMixin:
     """Additional operations for configuration enums."""
 

lacre/daemon.py

@@ -42,9 +42,6 @@ class MailEncryptionProxy:
                 if message.defects:
                     LOG.warning("Issues found: %d; %s", len(message.defects), repr(message.defects))
 
-                if conf.flag_enabled('daemon', 'log_headers'):
-                    LOG.info('Message headers: %s', self._extract_headers(message))
-
                 send = xport.SendFrom(envelope.mail_from)
                 for operation in gate.delivery_plan(envelope.rcpt_tos, message, keys):
                     LOG.debug(f"Sending mail via {operation!r}")
@@ -55,21 +52,22 @@ class MailEncryptionProxy:
                         # If the message can't be encrypted, deliver cleartext.
                         LOG.error('Unable to encrypt message, delivering in cleartext: %s', e)
                         if not isinstance(operation, KeepIntact):
-                            self._send_unencrypted(operation, message, envelope, send)
+                            self._send_unencrypted(operation, envelope, send)
                         else:
                             LOG.exception('Cannot perform: %s', operation)
                             raise
 
             except:
                 LOG.exception('Unexpected exception caught, bouncing message')
-                return xport.RESULT_ERROR
+                if conf.should_log_headers():
+                    LOG.info('Message headers: %s', self._extract_headers(message))
+                return xport.RESULT_ERRORR
 
         return xport.RESULT_OK
 
-    def _send_unencrypted(self, operation, message, envelope, send: xport.SendFrom):
-        keep = KeepIntact(operation.recipients())
-        new_message = keep.perform(message)
-        send(new_message, operation.recipients())
+    def _send_unencrypted(self, operation, envelope, send: xport.SendFrom):
+        # Do not parse and re-generate the message, just send it as it is.
+        send(envelope.original_content, operation.recipients())
 
     def _beginning(self, e: Envelope) -> bytes:
         double_eol_pos = e.original_content.find(DOUBLE_EOL_BYTES)

test/modules/test_contracts.py

@@ -27,7 +27,8 @@ documentation.
 import email
 import email.mime.multipart
 from email.message import EmailMessage
-from email.policy import SMTP
+from email.policy import SMTP, SMTPUTF8
+from email.errors import HeaderParseError
 
 import unittest
 from configparser import RawConfigParser
@@ -165,6 +166,26 @@ class EmailParsingTest(unittest.TestCase):
         self.assertIsInstance(payload, str)
         self.assertTrue(message_boundary in payload)
 
+    def test_fail_if_message_id_parsing_is_fixed(self):
+        # Unfortunately, Microsoft sends messages with Message-Id header values
+        # that email parser can't process.
+        #
+        # Bug: https://github.com/python/cpython/issues/105802
+        # Fix: https://github.com/python/cpython/pull/108133
+
+        rawmsg = b"From: alice@lacre.io\r\n" \
+          + b"To: bob@lacre.io\r\n" \
+          + b"Subject: Test message\r\n" \
+          + b"Content-Type: text/plain\r\n" \
+          + b"Content-Transfer-Encoding: base64\r\n" \
+          + b"Message-Id: <[yada-yada-yada@microsoft.com]>\r\n" \
+          + b"\r\n" \
+          + b"SGVsbG8sIFdvcmxkIQo=\r\n"
+
+        msg = email.message_from_bytes(rawmsg, policy=SMTPUTF8)
+        self.assertEqual(len(msg.defects), 0)
+        self.assertRaises(IndexError, lambda: msg['Message-Id'])
+
 
 class EmailTest(unittest.TestCase):
     def test_boundary_generated_after_as_string_call(self):