有时，用户需要在流程中取消或退出请求。
Sometimes users need to cancel or withdraw the request during the process.
通道请求转送信息，比如服务器端流程的退出代码。
Channel requests relay information such as the exit code of a server-side process.
取消可能是特定的用户态请求的结果，或者在清理已经退出的进程时进行。
Cancellation may occur as the result of a specific user-mode request, or during the cleanup of a process which has exited.
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