Monitor#wait_for_cond

static VALUE
monitor_wait_for_cond(VALUE monitor, VALUE cond, VALUE timeout)
{
    VALUE count = monitor_exit_for_cond(monitor);
    struct wait_for_cond_data data = {
        monitor,
        cond,
        timeout,
        count,
    };

    return rb_ensure(monitor_wait_for_cond_body, (VALUE)&data,
                     monitor_enter_for_cond, (VALUE)&data);
}