presto-hive/src/main/java/io/prestosql/plugin/hive/metastore/file/FileHiveMetastore.java/FileHiveMetastore/listRoles
class FileHiveMetastore:    @Override
    public synchronized Set<String> listRoles()
    {
        return ImmutableSet.copyOf(readFile("roles", getRolesFile(), rolesCodec).orElse(ImmutableList.of()));
    }