Return JSON errors instead of HTML errors for unknown resources.

synapse/http/server.py

@@ -559,6 +559,17 @@ def getChildWithDefault(self, path: str, request: Request) -> resource.Resource:
         return super().getChildWithDefault(path, request)
 
 
+class MissingResource(resource.Resource):
+    """Similar to resource.NoResource, but returns a JSON error with proper CORS headers."""
+
+    def render(self, request: SynapseRequest) -> int:
+        return_json_error(failure.Failure(UnrecognizedRequestError()), request)
+        return NOT_DONE_YET
+
+    def getChild(self, name: str, request: Request) -> resource.Resource:
+        return self
+
+
 class RootOptionsRedirectResource(OptionsResource, RootRedirect):
     pass
 

synapse/util/httpresourcetree.py

@@ -15,7 +15,9 @@
 import logging
 from typing import Dict
 
-from twisted.web.resource import NoResource, Resource
+from twisted.web.resource import Resource
+
+from synapse.http.server import MissingResource
 
 logger = logging.getLogger(__name__)
 
@@ -49,7 +51,7 @@ def create_resource_tree(
         for path_seg in full_path.split(b"/")[1:-1]:
             if path_seg not in last_resource.listNames():
                 # resource doesn't exist, so make a "dummy resource"
-                child_resource: Resource = NoResource()
+                child_resource: Resource = MissingResource()
                 last_resource.putChild(path_seg, child_resource)
                 res_id = _resource_id(last_resource, path_seg)
                 resource_mappings[res_id] = child_resource