webhook: Catch potential JsonableError when parsing widget_content.

The `widget_content` key is expected to contain a string which parses
as JSON; in the event that it does not, log the error and notify the
bot owner, instead of failing silently.

Fixes #16850.

zerver/lib/outgoing_webhook.py

@@ -316,7 +316,15 @@ def do_rest_call(
         if not response:
             return None
         if str(response.status_code).startswith("2"):
-            process_success_response(event, service_handler, response)
+            try:
+                process_success_response(event, service_handler, response)
+            except JsonableError as e:
+                response_message = e.msg
+                logging.info("Outhook trigger failed:", stack_info=True)
+                fail_with_message(event, response_message)
+                response_message = f"The outgoing webhook server attempted to send a message in Zulip, but that request resulted in the following error:\n> {e}"
+                notify_bot_owner(event, failure_message=response_message)
+                return None
         else:
             logging.warning(
                 "Message %(message_url)s triggered an outgoing webhook, returning status "

zerver/tests/test_outgoing_webhook_system.py

@@ -3,6 +3,7 @@ from unittest import mock
 
 import orjson
 import requests
+import responses
 
 from version import ZULIP_VERSION
 from zerver.lib.actions import do_create_user
@@ -231,6 +232,35 @@ I'm a generic exception :(
         assert bot_user.bot_owner is not None
         self.assertEqual(bot_owner_notification.recipient_id, bot_user.bot_owner.recipient_id)
 
+    def test_jsonable_exception(self) -> None:
+        bot_user = self.example_user("outgoing_webhook_bot")
+        mock_event = self.mock_event(bot_user)
+        service_handler = GenericOutgoingWebhookService("token", bot_user, "service")
+
+        # The "widget_content" key is required to be a string which is
+        # itself JSON-encoded; passing arbitrary text data in it will
+        # cause the hook to fail.
+        response = {"content": "whatever", "widget_content": "test"}
+        expect_logging_info = self.assertLogs(level="INFO")
+        expect_fail = mock.patch("zerver.lib.outgoing_webhook.fail_with_message")
+
+        with responses.RequestsMock(assert_all_requests_are_fired=True) as requests_mock:
+            requests_mock.add(
+                requests_mock.POST, "https://example.zulip.com", status=200, json=response
+            )
+            with expect_logging_info, expect_fail as mock_fail:
+                do_rest_call("https://example.zulip.com", mock_event, service_handler)
+            self.assertTrue(mock_fail.called)
+            bot_owner_notification = self.get_last_message()
+            self.assertEqual(
+                bot_owner_notification.content,
+                """[A message](http://zulip.testserver/#narrow/stream/999-Verona/topic/Foo/near/) to your bot @_**Outgoing Webhook** triggered an outgoing webhook.
+The outgoing webhook server attempted to send a message in Zulip, but that request resulted in the following error:
+> Widgets: API programmer sent invalid JSON content""",
+            )
+        assert bot_user.bot_owner is not None
+        self.assertEqual(bot_owner_notification.recipient_id, bot_user.bot_owner.recipient_id)
+
 
 class TestOutgoingWebhookMessaging(ZulipTestCase):
     def create_outgoing_bot(self, bot_owner: UserProfile) -> UserProfile: