outgoing_webhook: Ignore the exception on failure if the stream is gone.

In the outgoing webhook handler, there is potentially several seconds
of trying between when a message triggering an outgoing webhook
arrives, and when it fails.  In the meantime, the stream the
triggering message was on may have been deleted, causing the
"Failure!" message to have no valid stream to be sent to.

Rather than raise an exception in the outgoing webhook worker, ignore
the exception and move on.

zerver/lib/outgoing_webhook.py

@@ -11,7 +11,7 @@ from requests import Response
 
 from version import ZULIP_VERSION
 from zerver.actions.message_send import check_send_message
-from zerver.lib.exceptions import JsonableError
+from zerver.lib.exceptions import JsonableError, StreamDoesNotExistError
 from zerver.lib.message import MessageDict
 from zerver.lib.outgoing_http import OutgoingSession
 from zerver.lib.queue import retry_event
@@ -232,7 +232,12 @@ def fail_with_message(event: Dict[str, Any], failure_message: str) -> None:
     message_info = event["message"]
     content = "Failure! " + failure_message
     response_data = dict(content=content)
+    try:
         send_response_message(bot_id=bot_id, message_info=message_info, response_data=response_data)
+    except StreamDoesNotExistError:
+        # If the stream has vanished while we were failing, there's no
+        # reasonable place to report the error.
+        pass
 
 
 def get_message_url(event: Dict[str, Any]) -> str:

zerver/tests/test_outgoing_webhook_system.py

@@ -7,11 +7,13 @@ import responses
 
 from version import ZULIP_VERSION
 from zerver.actions.create_user import do_create_user
+from zerver.actions.streams import do_deactivate_stream
 from zerver.lib.exceptions import JsonableError
 from zerver.lib.outgoing_webhook import (
     GenericOutgoingWebhookService,
     SlackOutgoingWebhookService,
     do_rest_call,
+    fail_with_message,
 )
 from zerver.lib.test_classes import ZulipTestCase
 from zerver.lib.topic import TOPIC_NAME
@@ -609,6 +611,40 @@ class TestOutgoingWebhookMessaging(ZulipTestCase):
         display_recipient = get_display_recipient(stream_message.recipient)
         self.assertEqual(display_recipient, "Denmark")
 
+    @responses.activate
+    def test_stream_message_failure_deactivated_to_outgoing_webhook_bot(self) -> None:
+        bot_owner = self.example_user("othello")
+        bot = self.create_outgoing_bot(bot_owner)
+
+        def wrapped(event: Dict[str, Any], failure_message: str) -> None:
+            do_deactivate_stream(get_stream("Denmark", get_realm("zulip")), acting_user=None)
+            fail_with_message(event, failure_message)
+
+        responses.add(
+            responses.POST,
+            "https://bot.example.com/",
+            body=requests.exceptions.Timeout("Time is up!"),
+        )
+        with mock.patch(
+            "zerver.lib.outgoing_webhook.fail_with_message", side_effect=wrapped
+        ) as fail:
+            with self.assertLogs(level="INFO") as logs:
+                self.send_stream_message(
+                    bot_owner, "Denmark", content=f"@**{bot.full_name}** foo", topic_name="bar"
+                )
+
+        self.assert_length(logs.output, 5)
+        fail.assert_called_once()
+
+        last_message = self.get_last_message()
+        self.assertIn("Request timed out after 10 seconds", last_message.content)
+
+        prev_message = self.get_second_to_last_message()
+        self.assertIn(
+            "tried to send a message to stream #**Denmark**, but that stream does not exist",
+            prev_message.content,
+        )
+
     @responses.activate
     def test_empty_string_json_as_response_to_outgoing_webhook_request(self) -> None:
         """