正确取消 Socket.XXXAsync 调用

Question

我正在为个人项目开发基于 SocketAsyncEventArgs 的基于任务的包装器。到目前为止，除了任务取消之外，一切都正常。在实施过程中，我遇到了一个问题，即对如何以正确的方式实施取消缺乏了解。

假设我有任务返回的 ReceiveAsync 和 ReceiveFromAsync 方法，它们采用具有以下签名的 CancellationToken：

public Task<int> ReceiveAsync(SocketAsyncEventArgs e, CancellationToken cancellationToken) {
    cancellationToken.Register(state => {
        // do something to cancel the operation when the cancellationToken is signalled
    }, someState);

    bool completedAsync = socket.ReceiveAsync(e);
    // ...
}

public Task<int> ReceiveFromAsync(SocketAsyncEventArgs e, CancellationToken cancellationToken) {
    cancellationToken.Register(state => {
        // do something to cancel the operation when the cancellationToken is signalled
    }, someState);

    bool completedAsync = socket.ReceiveFromAsync(e);
    // ...
}

此外，假设我有 SendAsync 和 SendToAsync 方法，还具有具有以下签名的 CancellationToken：

public Task<int> SendAsync(SocketAsyncEventArgs e, CancellationToken cancellationToken) {
    cancellationToken.Register(state => {
        // do something to cancel the operation when the cancellationToken is signalled
    }, someState);

    bool completedAsync = socket.SendAsync(e);
    // ...
}

public Task<int> SendToAsync(SocketAsyncEventArgs e, CancellationToken cancellationToken) {
    cancellationToken.Register(state => {
        // do something to cancel the operation when the cancellationToken is signalled
    }, someState);

    bool completedAsync = socket.SendToAsync(e);
    // ...
}

SocketAsyncEventArgs.Completed 事件处理程序如下所示。假设到目前为止 SocketAsyncEventArgs 处理程序的实现正确。唯一缺少的是取消：

public void HandleIoCompleted(object sender, SocketAsyncEventArgs args) {
    switch (args.LastOperation) {
        case SocketAsyncOperation.Receive:
            switch (args.SocketError) {
                    case SocketError.Success:
                        // ...
                        break;
                    case SocketError.OperationAborted:
                        Debug.WriteLine("CompleteReceive experienced SocketError.OperationAborted!");
                        break;
                    default:
                        // ...
                        break;
            }

            break;

        case SocketAsyncOperation.ReceiveFrom:
            switch (args.SocketError) {
                    case SocketError.Success:
                        // ...
                        break;
                    case SocketError.OperationAborted:
                        Debug.WriteLine("CompleteReceiveFrom experienced SocketError.OperationAborted!");
                        break;
                    default:
                        // ...
                        break;
            }

            break;

        case SocketAsyncOperation.Send:
            switch (args.SocketError) {
                    case SocketError.Success:
                        // ...
                        break;
                    case SocketError.OperationAborted:
                        Debug.WriteLine("CompleteSend experienced SocketError.OperationAborted!");
                        break;
                    default:
                        // ...
                        break;
            }

            break;

        case SocketAsyncOperation.SendTo:
            switch (args.SocketError) {
                    case SocketError.Success:
                        // ...
                        break;
                    case SocketError.OperationAborted:
                        Debug.WriteLine("CompleteSendTo experienced SocketError.OperationAborted!");
                        break;
                    default:
                        // ...
                        break;
            }

            break;

        default:
            throw new NotSupportedException($"{nameof(HandleIoCompleted)} doesn't support {args.LastOperation}");
    }
}

通过查看有关此主题的其他问题，我了解到要取消 Socket.ReceiveAsync 或 Socket.ReceiveFromAsnc 操作，我需要处理底层套接字。然而，这会干扰任何正在进行的异步发送操作，当套接字被处置时，这些操作将以 SocketError.OperationAborted 结束。同样，取消任何 Socket.SendAsync 或 Socket.SendToAsync 操作都会干扰当前正在进行的接收操作，在处置套接字时也会遇到 SocketError.OperationAborted。

以安全方式取消操作的正确方法是什么，以便 UDP 和 TCP 流量可以在数据包丢失后恢复（如果没有取消，则 receiveFromAsync 和 receiveAsync 调用永远不会调用 e.Completed）？

如果需要额外的代码，只需添加一条注释，我会尽可能充实代码。

Answer 1

使用

Task.WaitAsync

我在正确取消时遇到问题

await listener.AcceptAsync()

。当我关闭侦听器时，它引发异常并且套接字未正确释放。我找到了一个使用两步任务初始化的有效解决方案，也许它会帮助处于类似情况的其他人，并且它适用于任何

Socket.xxxAsync()

方法：

//just for testing: the cancellation will occur after 2000 milliseconds
using var cst = new CancellationSourceToken(2000); 

try
{
    //listener is your Socket instance
    var handlerTask = listener.AcceptAsync();
    var handler = await handlerTask.WaitAsync(cst.Token);

    //then you can do the same with the handler
    while (true)
    {
        var bytes = new byte[1024];
        var receiverTask = handler.ReceiveAsync(bytes);
        var bytesCount = await receiverTask.WaitAsync(cst.Token);
        //then process the data batch before receiving the next one
        if (your_end_of_msg_predicate())
            break;
    }
}
catch (OperationCanceledException) { }

小旁注

由于我将所有这些以及其他方法都放在

Parallel.Invoke

中，并且我需要在计算继续之前完成所有这些，所以我最终将

await

切换为

.Result

以避免使用

 调用调用的方法async

：

var handler = handlerTask.WaitAsync(cst.Token).Result;
//and a few lines later
var bytesCount = receiverTask.WaitAsync(cst.Token).Result;