The master branch's role graph update logic has support for the following event on admin.system.roles:
BSON("createIndexes" << "system.roles" << "v" << 2 << "name" << "role_1_db_1" << "key" << BSON("role" << 1 << "db" << 1) << "unique" << true) {
We should ensure older stable branches can handle it as well. Without support, nodes may attempt to create a default index on the roles collection and trigger the following error message:
Unsupported modification to roles collection in oplog; restart this process to reenable user-defined roles; OplogOperationUnsupported: Unsupported oplog operation.