bug: Fix code that mis-identifies missed message formats.

In our email mirror, we have a special format for missed
message emails that uses a 32-bit randomly generated token
that we put into redis that is then prefixed with "mm" for
a total of 34 characters.

We had a bug where we would mis-classify emails like
mmcfoo@example.com as being these system-generated emails
that were part of the redis setup.

It's actually a little unclear how the bug in the library
function would have manifested from the user's point of view,
but it was definitely buggy code, and it's possibly related in
a subtle way to an error report we got from a customer where
only one of their users, who happened to have a name like
mmcfoo, was having problems with the mirror.

zerver/tests/test_email_mirror.py

@@ -20,6 +20,7 @@ from zerver.lib.actions import (
 from zerver.lib.email_mirror import (
     process_message, process_stream_message, ZulipEmailForwardError,
     create_missed_message_address,
+    get_missed_message_token_from_address,
 )
 
 from zerver.lib.digest import handle_digest_email
@@ -38,11 +39,31 @@ import mock
 import os
 import sys
 from os.path import dirname, abspath
+from six import text_type
 from six.moves import cStringIO as StringIO
 from django.conf import settings
 
 from typing import Any, Callable, Mapping, Union
 
+class TestEmailMirrorLibrary(ZulipTestCase):
+    def test_get_missed_message_token(self):
+        # type: () -> None
+
+        def get_token(address):
+            # type: (text_type) -> text_type
+            with self.settings(EMAIL_GATEWAY_PATTERN="%s@example.com"):
+                return get_missed_message_token_from_address(address)
+
+        address = 'mm' + ('x' * 32) + '@example.com'
+        token = get_token(address)
+        self.assertEqual(token, 'x' * 32)
+
+        # This next section was a bug at one point--we'd treat ordinary
+        # user addresses that happened to begin with "mm" as being
+        # the special mm+32chars tokens.
+        address = 'mmathers@example.com'
+        with self.assertRaises(ZulipEmailForwardError):
+            get_token(address)
 
 class TestStreamEmailMessagesSuccess(ZulipTestCase):
     def test_receive_stream_email_messages_success(self):