exceptions: RateLimited shouldn't inherit from PermissionDenied.

We will want to raise RateLimited in authenticate() in rate limiting
code - Django's authenticate() mechanism catches PermissionDenied, which
we don't want for RateLimited. We want RateLimited to propagate to our
code that called the authenticate() function.

zerver/lib/exceptions.py

@@ -2,7 +2,6 @@ from enum import Enum
 from typing import Any, Dict, List, Type, TypeVar, Optional
 from typing_extensions import NoReturn
 
-from django.core.exceptions import PermissionDenied
 from django.utils.translation import ugettext as _
 
 
@@ -179,7 +178,7 @@ class InvalidMarkdownIncludeStatement(JsonableError):
     def msg_format() -> str:
         return _("Invalid markdown include statement: {include_statement}")
 
-class RateLimited(PermissionDenied):
+class RateLimited(Exception):
     def __init__(self, msg: str="") -> None:
         super().__init__(msg)
 

zerver/middleware.py

@@ -3,8 +3,7 @@ import logging
 import time
 import traceback
 from typing import Any, AnyStr, Dict, \
-    Iterable, List, MutableMapping, Optional, \
-    Union
+    Iterable, List, MutableMapping, Optional
 
 from django.conf import settings
 from django.contrib.sessions.backends.base import UpdateError
@@ -350,12 +349,8 @@ class RateLimitMiddleware(MiddlewareMixin):
 
         return response
 
-    # TODO: When we have Django stubs, we should be able to fix the
-    # type of exception back to just Exception; the problem is without
-    # stubs, mypy doesn't know that RateLimited's superclass
-    # PermissionDenied inherits from Exception.
     def process_exception(self, request: HttpRequest,
-                          exception: Union[Exception, RateLimited]) -> Optional[HttpResponse]:
+                          exception: Exception) -> Optional[HttpResponse]:
         if isinstance(exception, RateLimited):
             entity_type = str(exception)  # entity type is passed to RateLimited when raising
             resp = json_error(