bots: Fix behavior for bots mentioned in PMs.

We've for a long time had the behavior that a bot mentioned in a
stream message receives the notification, regardless of whether the
bot was actually subscribed to the stream.

Apparently, this behavior also triggered if you mentioned a bot in a
private message (i.e. the bot would be delievered the private message
and would probably respond unhelpfully in a new group private message
thread with the PMs original recipients plus the bot).

The fix for this bug is simple: To exclude this feature for private
messages.

zerver/lib/actions.py

@@ -1078,8 +1078,8 @@ def get_service_bot_events(sender: UserProfile, service_bot_tuples: List[Tuple[i
         if user_profile_id not in mentioned_user_ids and user_profile_id not in active_user_ids:
             continue
 
-        # Mention triggers, primarily for stream messages
-        if user_profile_id in mentioned_user_ids:
+        # Mention triggers, for stream messages
+        if is_stream and user_profile_id in mentioned_user_ids:
             trigger = 'mention'
         # PM triggers for personal and huddle messsages
         elif (not is_stream) and (user_profile_id in active_user_ids):

zerver/tests/test_service_bot_system.py

@@ -91,6 +91,26 @@ class TestServiceBotBasics(ZulipTestCase):
 
         self.assertEqual(event_dict, expected)
 
+    def test_service_events_for_private_mentions(self) -> None:
+        """Service bots should not get access to mentions if they aren't a
+        direct recipient."""
+        sender = self.example_user('hamlet')
+        assert(not sender.is_bot)
+
+        outgoing_bot = self._get_outgoing_bot()
+
+        event_dict = get_service_bot_events(
+            sender=sender,
+            service_bot_tuples=[
+                (outgoing_bot.id, outgoing_bot.bot_type),
+            ],
+            active_user_ids=set(),
+            mentioned_user_ids={outgoing_bot.id},
+            recipient_type=Recipient.PERSONAL,
+        )
+
+        self.assertEqual(len(event_dict), 0)
+
 class TestServiceBotStateHandler(ZulipTestCase):
     def setUp(self) -> None:
         self.user_profile = self.example_user("othello")