response: Replace response.asynchronous attribute with new class.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/tornado/handlers.py

@@ -14,7 +14,7 @@ from django.urls import set_script_prefix
 from django.utils.cache import patch_vary_headers
 from tornado.wsgi import WSGIContainer
 
-from zerver.lib.response import json_response
+from zerver.lib.response import AsynchronousResponse, json_response
 from zerver.tornado.descriptors import get_descriptor_by_handler_id
 
 current_handler_id = 0
@@ -153,7 +153,7 @@ class AsyncDjangoHandler(tornado.web.RequestHandler, base.BaseHandler):
         response = await sync_to_async(lambda: self.get_response(request), thread_sensitive=True)()
 
         try:
-            if hasattr(response, "asynchronous"):
+            if isinstance(response, AsynchronousResponse):
                 # We import async_request_timer_restart during runtime
                 # to avoid cyclic dependency with zerver.lib.request
                 from zerver.middleware import async_request_timer_stop

zerver/tornado/views.py

@@ -9,7 +9,7 @@ from django.utils.translation import gettext as _
 from zerver.decorator import internal_notify_view, process_client
 from zerver.lib.exceptions import JsonableError
 from zerver.lib.request import REQ, RequestNotes, has_request_variables
-from zerver.lib.response import json_success
+from zerver.lib.response import AsynchronousResponse, json_success
 from zerver.lib.validator import (
     check_bool,
     check_int,
@@ -170,13 +170,11 @@ def get_events_backend(
         log_data["extra"] = result["extra_log_data"]
 
     if result["type"] == "async":
-        # Mark this response with .asynchronous; this will result in
+        # Return an AsynchronousResponse; this will result in
         # Tornado discarding the response and instead long-polling the
         # request.  See zulip_finish for more design details.
         handler._request = request
-        response = json_success(request)
-        response.asynchronous = True
-        return response
+        return AsynchronousResponse()
     if result["type"] == "error":
         raise result["exception"]
     return json_success(request, data=result["response"])