email_mirror: Add email address parsing.

When trying to find the email gateway address, use the
`email.util.getaddresses` function to deal with cases
where multiple recipients are included in the email header
or the stream address appears as an angle-addr with a
name given (e.g. if someone added it to their address book).

Added some other headers where the required address may
appear: "Resent" headers are sometimes used for forwarding,
and streams may also be found in CC. There is no way to find
the address if the email was recieved as a BCC.

zerver/lib/email_mirror.py

@@ -4,6 +4,7 @@ import logging
 import re
 
 from email.header import decode_header, Header
+from email.utils import getaddresses
 import email.message as message
 
 from django.conf import settings
@@ -287,19 +288,19 @@ def find_emailgateway_recipient(message: message.Message) -> str:
     # We can't use Delivered-To; if there is a X-Gm-Original-To
     # it is more accurate, so try to find the most-accurate
     # recipient list in descending priority order
-    recipient_headers = ["X-Gm-Original-To", "Delivered-To", "To"]
-    recipients = []  # type: List[Union[str, Header]]
-    for recipient_header in recipient_headers:
-        r = message.get_all(recipient_header, None)
-        if r:
-            recipients = r
-            break
+    recipient_headers = ["X-Gm-Original-To", "Delivered-To",
+                         "Resent-To", "Resent-CC", "To", "CC"]
 
     pattern_parts = [re.escape(part) for part in settings.EMAIL_GATEWAY_PATTERN.split('%s')]
     match_email_re = re.compile(".*?".join(pattern_parts))
-    for recipient_email in [str(recipient) for recipient in recipients]:
-        if match_email_re.match(recipient_email):
-            return recipient_email
+
+    header_addresses = [str(addr)
+                        for recipient_header in recipient_headers
+                        for addr in message.get_all(recipient_header, [])]
+
+    for addr_tuple in getaddresses(header_addresses):
+        if match_email_re.match(addr_tuple[1]):
+            return addr_tuple[1]
 
     raise ZulipEmailForwardError("Missing recipient in mirror email")
 

zerver/tests/test_email_mirror.py

@@ -185,6 +185,33 @@ class TestStreamEmailMessagesSuccess(ZulipTestCase):
         self.assertEqual(get_display_recipient(message.recipient), stream.name)
         self.assertEqual(message.topic_name(), incoming_valid_message['Subject'])
 
+    def test_receive_stream_email_multiple_recipient_success(self) -> None:
+        user_profile = self.example_user('hamlet')
+        self.login(user_profile.email)
+        self.subscribe(user_profile, "Denmark")
+        stream = get_stream("Denmark", user_profile.realm)
+
+        # stream address is angle-addr within multiple addresses
+        stream_to_addresses = ["A.N. Other <another@example.org>",
+                               "Denmark <{}>".format(encode_email_address(stream))]
+
+        incoming_valid_message = MIMEText('TestStreamEmailMessages Body')  # type: Any # https://github.com/python/typeshed/issues/275
+
+        incoming_valid_message['Subject'] = 'TestStreamEmailMessages Subject'
+        incoming_valid_message['From'] = self.example_email('hamlet')
+        incoming_valid_message['To'] = ", ".join(stream_to_addresses)
+        incoming_valid_message['Reply-to'] = self.example_email('othello')
+
+        process_message(incoming_valid_message)
+
+        # Hamlet is subscribed to this stream so should see the email message from Othello.
+        message = most_recent_message(user_profile)
+
+        self.assertEqual(message.content, "TestStreamEmailMessages Body")
+        self.assertEqual(get_display_recipient(message.recipient), stream.name)
+        self.assertEqual(message.topic_name(), incoming_valid_message['Subject'])
+
+
 class TestStreamEmailMessagesEmptyBody(ZulipTestCase):
     def test_receive_stream_email_messages_empty_body(self) -> None: