bot_lib: Raise exception when PM recipient list is empty.

We need to explicitly check for empty recipient lists in
send_message to ensure that internal_send_huddle_message doesn't
call Addressee.for_private with an empty recipient list.

zerver/lib/bot_lib.py

@@ -15,6 +15,8 @@ from zerver.lib.bot_config import get_bot_config, ConfigError
 from zerver.lib.integrations import EMBEDDED_BOTS
 from zerver.lib.topic import get_topic_from_message_info
 
+from django.utils.translation import ugettext as _
+
 import configparser
 
 from mypy_extensions import NoReturn
@@ -60,6 +62,9 @@ class StateHandler:
 class EmbeddedBotQuitException(Exception):
     pass
 
+class EmbeddedBotEmptyRecipientsList(Exception):
+    pass
+
 class EmbeddedBotHandler:
     def __init__(self, user_profile: UserProfile) -> None:
         # Only expose a subset of our UserProfile's functionality
@@ -84,7 +89,9 @@ class EmbeddedBotHandler:
         # usual 'to' field could be either a List[str] or a str.
         recipients = ','.join(message['to']).split(',')
 
-        if len(message['to']) == 1:
+        if len(message['to']) == 0:
+            raise EmbeddedBotEmptyRecipientsList(_('Message must have recipients!'))
+        elif len(message['to']) == 1:
             recipient_user = get_active_user(recipients[0], self.user_profile.realm)
             internal_send_private_message(self.user_profile.realm, self.user_profile,
                                           recipient_user, message['content'])

zerver/tests/test_service_bot_system.py

@@ -11,7 +11,8 @@ from zerver.lib.actions import (
     do_create_user,
     get_service_bot_events,
 )
-from zerver.lib.bot_lib import StateHandler, EmbeddedBotHandler
+from zerver.lib.bot_lib import StateHandler, EmbeddedBotHandler, \
+    EmbeddedBotEmptyRecipientsList
 from zerver.lib.bot_storage import StateError
 from zerver.lib.bot_config import set_bot_config, ConfigError, load_bot_config_template
 from zerver.lib.test_classes import ZulipTestCase
@@ -399,6 +400,10 @@ class TestServiceBotConfigHandler(ZulipTestCase):
         self.assertTrue(isinstance(bot_config, dict))
         self.assertEqual(len(bot_config), 0)
 
+    def test_bot_send_pm_with_empty_recipients_list(self) -> None:
+        with self.assertRaisesRegex(EmbeddedBotEmptyRecipientsList, 'Message must have recipients!'):
+            self.bot_handler.send_message(message={'type': 'private', 'to': []})
+
 
 class TestServiceBotEventTriggers(ZulipTestCase):