[MCUT] Create context 0xdecaf
[MCUT] Create event (type=MC_COMMAND_DISPATCH, handle=0xdecb0)
[MCUT] Launch API thread 140511951611456 (0)

    std::unique_lock<std::mutex> wait_for_data()
    {
        std::unique_lock<std::mutex> head_lock(head_mutex);
        auto until = [&]()
        {
            bool done = m_done->load();
            bool have_items = head.get() != get_tail();
            printf("wait_for_data sleep 3\n");
            sleep(3);
            return done || have_items;
        };
        data_cond.wait(head_lock, until);
        return head_lock;
    }
...
    void push(T new_value)
    {
        std::shared_ptr<T> new_data(std::make_shared<T>(std::move(new_value)));
        printf("push sleep before push\n");
        sleep(1);
        std::unique_ptr<node> p(new node);
        {
            std::lock_guard<std::mutex> tail_lock(tail_mutex);
            tail->data = new_data;
            node* const new_tail = p.get();
            tail->next = std::move(p);
            tail = new_tail;
        }
        printf("push sleep 1\n");
        sleep(1);
        printf("push notify one\n");
        data_cond.notify_one();
    }

# ./HelloWorld 
push sleep before push
wait_for_data sleep 3
push sleep 1
push notify one
<hangs here>