outgoing_webhook: Improve invalid json handling when parsing response.

It's better to just raise JsonableError here, as that makes this error
processed in the central place for this kind of thing in do_rest_call:
---------
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
----------

which does all the things that are supposed to happen -
fail_with_message, appropriate logging and notifying the bot owner.

zerver/lib/outgoing_webhook.py

@@ -288,8 +288,7 @@ def process_success_response(
     try:
         response_json = json.loads(response.text)
     except json.JSONDecodeError:
-        fail_with_message(event, "Invalid JSON in response")
-        return
+        raise JsonableError(_("Invalid JSON in response"))
 
     if not isinstance(response_json, dict):
         raise JsonableError(_("Invalid response format"))

zerver/tests/test_outgoing_webhook_interfaces.py

@@ -5,6 +5,7 @@ from unittest import mock
 import requests
 
 from zerver.lib.avatar import get_gravatar_url
+from zerver.lib.exceptions import JsonableError
 from zerver.lib.message import MessageDict
 from zerver.lib.outgoing_webhook import get_service_interface_class, process_success_response
 from zerver.lib.test_classes import ZulipTestCase
@@ -47,13 +48,12 @@ class TestGenericOutgoingWebhookService(ZulipTestCase):
         response.status_code = 200
         response.text = "unparsable text"
 
-        with mock.patch("zerver.lib.outgoing_webhook.fail_with_message") as m:
+        with self.assertRaisesRegex(JsonableError, "Invalid JSON in response"):
             process_success_response(
                 event=event,
                 service_handler=service_handler,
                 response=response,
             )
-        self.assertTrue(m.called)
 
     def test_make_request(self) -> None:
         othello = self.example_user("othello")

zerver/tests/test_outgoing_webhook_system.py

@@ -288,6 +288,35 @@ The outgoing webhook server attempted to send a message in Zulip, but that reque
         assert bot_user.bot_owner is not None
         self.assertEqual(bot_owner_notification.recipient_id, bot_user.bot_owner.recipient_id)
 
+    def test_invalid_json_in_response(self) -> None:
+        bot_user = self.example_user("outgoing_webhook_bot")
+        mock_event = self.mock_event(bot_user)
+        service_handler = GenericOutgoingWebhookService("token", bot_user, "service")
+
+        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:
+            # We mock the endpoint to return response with a body which isn't valid json.
+            requests_mock.add(
+                requests_mock.POST,
+                "https://example.zulip.com",
+                status=200,
+                body="this isn't valid json",
+            )
+            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:
+> Invalid JSON in response""",
+            )
+        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: