ThreadGroup#list

static VALUE
thgroup_list(VALUE group)
{
    VALUE ary = rb_ary_new();
    rb_thread_t *th = 0;
    rb_ractor_t *r = GET_RACTOR();

    list_for_each(&r->threads.set, th, lt_node) {
        if (th->thgroup == group) {
            rb_ary_push(ary, th->self);
        }
    }
    return ary;
}