File::Stat#writable_real?

static VALUE
rb_stat_W(VALUE obj)
{
    struct stat *st = get_stat(obj);

#ifdef USE_GETEUID
    if (getuid() == 0) return Qtrue;
#endif
#ifdef S_IWUSR
    if (rb_stat_rowned(obj))
        return RBOOL(st->st_mode & S_IWUSR);
#endif
#ifdef S_IWGRP
    if (rb_group_member(get_stat(obj)->st_gid))
        return RBOOL(st->st_mode & S_IWGRP);
#endif
#ifdef S_IWOTH
    if (!(st->st_mode & S_IWOTH)) return Qfalse;
#endif
    return Qtrue;
}