Backward support for "websocket_handler_" prefix in view_name

sanic/app.py

@@ -12,6 +12,7 @@
 from traceback import format_exc
 from typing import Any, Dict, Optional, Type, Union
 from urllib.parse import urlencode, urlunparse
+from warnings import warn
 
 from sanic import reloader_helpers
 from sanic.asgi import ASGIApp
@@ -745,6 +746,23 @@ def url_for(self, view_name: str, **kwargs):
             kw.update(name=view_name)
 
         uri, route = self.router.find_route_by_view_name(view_name, **kw)
+
+        # TODO(laggardkernel): this fix should be removed in v21.3.
+        # Try again without the unnecessary prefix "websocket_handler_",
+        # which was added by accident on non-blueprint handlers. GH-2021
+        if not (uri and route) and view_name.startswith("websocket_handler_"):
+            view_name = view_name[18:]
+            uri, route = self.router.find_route_by_view_name(view_name, **kw)
+            if uri and route:
+                warn(
+                    "The bug of adding unnecessary `websocket_handler_` prefix "
+                    "in `view_name` param for non-blueprint handlers is fixed. "
+                    "This backward support will be removed in v21.3. Please "
+                    "update `Sanic.url_for()` callings in your code soon.",
+                    DeprecationWarning,
+                    stacklevel=2,
+                )
+
         if not (uri and route):
             raise URLBuildError(
                 f"Endpoint with name `{view_name}` was not found"