to cancel a block

基本解釋

A call to cancel_work_sync will terminate the work in the queue or block until the callback has finished (if the work is already in progress in the handler).
調用 cancel_work_sync 將會終止隊列中的任務或者阻塞任務直到回調結束（如果處理程序已經在處理該任務）。
The recipient can ask the Future to block until a result is ready, poll to see whether it's complete, cancel the computation, or check whether another thread has canceled it.
接收者可以要求 Future阻止線程，直至一個結果就緒，檢測查看它是否完成并取消計算指令，或檢查是否有其他進程取消了它。