response: Replace response.asynchronous attribute with new class.

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

zerver/lib/response.py

@@ -80,3 +80,12 @@ def json_response_from_error(exception: JsonableError) -> HttpResponse:
         response[header] = value
 
     return response
+
+
+class AsynchronousResponse(HttpResponse):
+    """
+    This response is just a sentinel to be discarded by Tornado and replaced
+    with a real response later; see zulip_finish.
+    """
+
+    status_code = 399

zerver/middleware.py

@@ -42,7 +42,12 @@ from zerver.lib.html_to_text import get_content_description
 from zerver.lib.markdown import get_markdown_requests, get_markdown_time
 from zerver.lib.rate_limiter import RateLimitResult
 from zerver.lib.request import REQ, RequestNotes, has_request_variables, set_request, unset_request
-from zerver.lib.response import json_response, json_response_from_error, json_unauthorized
+from zerver.lib.response import (
+    AsynchronousResponse,
+    json_response,
+    json_response_from_error,
+    json_unauthorized,
+)
 from zerver.lib.subdomains import get_subdomain
 from zerver.lib.types import ViewFuncT
 from zerver.lib.user_agent import parse_user_agent
@@ -401,8 +406,8 @@ class LogRequests(MiddlewareMixin):
     def process_response(
         self, request: HttpRequest, response: HttpResponseBase
     ) -> HttpResponseBase:
-        if getattr(response, "asynchronous", False):
-            # This special Tornado "asynchronous" response is
+        if isinstance(response, AsynchronousResponse):
+            # This special AsynchronousResponse sentinel is
             # discarded after going through this code path as Tornado
             # intends to block, so we stop here to avoid unnecessary work.
             return response

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"])