Update asserts and comments

synapse/handlers/oidc.py

@@ -957,7 +957,10 @@ async def grandfather_existing_users() -> Optional[str]:
                 # and attempt to match it.
                 attributes = await oidc_response_to_user_attributes(failures=0)
 
-                assert attributes.localpart
+                if attributes.localpart is None:
+                    # If no localpart is returned then we will generate one, so
+                    # there is no need to search for existing users.
+                    return None
 
                 user_id = UserID(attributes.localpart, self._server_name).to_string()
                 users = await self._store.get_users_by_id_case_insensitive(user_id)

synapse/rest/synapse/client/new_user_consent.py

@@ -61,7 +61,14 @@ async def _async_render_GET(self, request: Request) -> None:
             self._sso_handler.render_error(request, "bad_session", e.msg, code=e.code)
             return
 
-        assert session.chosen_localpart
+        # It should be impossible to get here without having first been through
+        # the pick-a-username step, which ensures chosen_localpart gets set.
+        if not session.chosen_localpart:
+            logger.warning("Session has no user name selected")
+            self._sso_handler.render_error(
+                request, "no_user", "No user name has been selected.", code=400
+            )
+            return
 
         user_id = UserID(session.chosen_localpart, self._server_name)
         user_profile = {