notifications: Group messages by (recipient, sender) for PM's.

This fixes a issue with multiple PM's being clubbed into a single
missed message email.

Fixes #6224.

zerver/lib/notifications.py

@@ -367,6 +367,10 @@ def handle_missedmessage_emails(user_profile_id, missed_email_events):
 
     messages_by_recipient_subject = defaultdict(list)  # type: Dict[Tuple[int, Text], List[Message]]
     for msg in messages:
+        if msg.recipient.type == Recipient.PERSONAL:
+            # For PM's group using (recipient, sender).
+            messages_by_recipient_subject[(msg.recipient_id, msg.sender_id)].append(msg)
+        else:
             messages_by_recipient_subject[(msg.recipient_id, msg.topic_name())].append(msg)
 
     message_count_by_recipient_subject = {

zerver/tests/test_notifications.py

@@ -352,3 +352,29 @@ class TestMissedMessages(ZulipTestCase):
         body = '<a class="stream" data-stream-id="5" href="http://testserver/#narrow/stream/Verona">#Verona</a'
         subject = 'Othello, the Moor of Venice sent you a message'
         self._test_cases(tokens, msg_id, body, subject, send_as_user=False, verify_html_body=True)
+
+    @patch('zerver.lib.email_mirror.generate_random_token')
+    def test_multiple_missed_personal_messages(self, mock_random_token):
+        # type: (MagicMock) -> None
+        tokens = self._get_tokens()
+        mock_random_token.side_effect = tokens
+
+        hamlet = self.example_user('hamlet')
+        msg_id_1 = self.send_message(self.example_email('othello'),
+                                     hamlet.email,
+                                     Recipient.PERSONAL,
+                                     'Personal Message 1')
+        msg_id_2 = self.send_message(self.example_email('iago'),
+                                     hamlet.email,
+                                     Recipient.PERSONAL,
+                                     'Personal Message 2')
+
+        handle_missedmessage_emails(hamlet.id, [
+            {'message_id': msg_id_1},
+            {'message_id': msg_id_2},
+        ])
+        self.assertEqual(len(mail.outbox), 2)
+        subject = 'Iago sent you a message'
+        self.assertEqual(mail.outbox[0].subject, subject)
+        subject = 'Othello, the Moor of Venice sent you a message'
+        self.assertEqual(mail.outbox[1].subject, subject)