封装Socket.BeginReceive/EndReceive以支持Timeout

Socket

.NET中的Socket类提供了网络通信常用的方法，分别提供了同步和异步两个版本，其中异步的实现是基于APM异步模式实现，即BeginXXX/EndXXX的方式。异步方法由于其非阻塞的特性，在需考虑程序性能和伸缩性的情况下，一般会选择使用异步方法。但使用过Socket提供的异步方法的同学，应该都会注意到了Socket的异步方法是无法设置Timeout的。以Receive操作为例，Socket提供了一个ReceiveTimeout属性，但该属性设置的是同步版本的Socket.Receive()方法的Timeout值，该设置对异步的Socket.BeginReceive()无效：如果对方没有返回任何消息，则BeginReceive操作将无法完成，其中提供的回调函数也将不会调用。如下示例代码所示：

private static void TestSocketBeginReceive()
{
    Socket socket = new Socket(AddressFamily.InterNetwork,
        SocketType.Dgram, ProtocolType.Udp);
    byte[] content = Encoding.ASCII.GetBytes("Hello world");

    IPAddress ip = Dns.Resolve("www.google.com").AddressList[0];
    IPEndPoint receiver = new IPEndPoint(ip, 80);

    socket.BeginSendTo(content, 0, content.Length, SocketFlags.None, 
        receiver, SendToCb, socket);
    Console.WriteLine("Sent bytes: " + content.Length);
}

private static void SendToCb(IAsyncResult ar)
{
    var socket = ar.AsyncState as Socket;
    socket.EndSendTo(ar);
    byte[] buffer = new byte[1024];

    IAsyncResult receiveAr = socket.BeginReceive(buffer, 0, buffer.Length,
        SocketFlags.None, null, null);
    int received = socket.EndReceive(receiveAr);
    Console.WriteLine("Received bytes: " + received);
} 

由于接收方不会返回任何消息，Socket.BeginReceive将永远不会完成，SentToCb方法中的socket.EndReceive()调用将永远阻塞，应用程序也无法得知操作的状态。

支持Timeout

在个别的应用场景下，我们希望既能使用Socket的异步通信方法，保证程序的性能，同时又希望能指定Timeout值，当操作没有在指定的时间内完成时，应用程序能得到通知，以进行下一步的操作，如retry等。以下介绍的就是一种支持Timeout的Socket异步Receive操作的实现，方式如下：

1. 基于APM异步模式封装Socket.BeginReceive/EndReceive方法。
2. 使用ThreadPool提供的RegisterWaitForSingleObject()方法注册一个WaitOrTimerCallback，如果指定时间内操作未完成，则结束操作，并设置状态为Timeout。
3. 将上述封装实现为Socket的扩展方法方便调用。

以下代码简化了所有的参数检查和异常处理，实际使用中需添加相关逻辑。

AsyncResultWithTimeout

首先看一下IAsyncResult接口的实现：

public class AsyncResultWithTimeout : IAsyncResult
{
    private ManualResetEvent m_waitHandle = new ManualResetEvent(false);
    public AsyncResultWithTimeout(AsyncCallback cb, object state)
    {
        this.AsyncState = state;
        this.Callback = cb;
    }

    #region IAsyncResult

    public object AsyncState { get; private set; }
    public WaitHandle AsyncWaitHandle { get { return m_waitHandle; } }
    public bool CompletedSynchronously { get { return false; } }
    public bool IsCompleted { get; private set; }

    #endregion

    public AsyncCallback Callback { get; private set; }
    public int ReceivedCount { get; private set; }
    public bool TimedOut { get; private set; }
    public void SetResult(int count)
    {
        this.IsCompleted = true;
        this.ReceivedCount = count;
        this.m_waitHandle.Set();

        if (Callback != null) Callback(this);
    }

    public void SetTimeout()
    {
        this.TimedOut = true;
        this.IsCompleted = true;
        this.m_waitHandle.Set();
    }
}

AsyncResultWithTimeOut类中包含了IAsyncResult接口中4个属性的实现、用户传入的AsyncCallback委托、接收到的字节数ReceivedCount以及两个额外的方法：

1. SetResult(): 用于正常接收到消息时设置结果，标记操作完成以及执行回调。
2. SetTimeout()：当超时时，标记操作完成以及设置超时状态。

StateInfo

StateInfo类用于保存相关的状态信息，该对象会作为Socket.BeginReceive()的最后一个参数传入。当接收到消息时，接收到的字节数会保存到AsyncResult属性中，并设置操作完成。当超时时，WatchTimeOut方法会将AsyncResult设置为TimeOut状态，并通过RegisteredWaitHandle属性取消注册的WaitOrTimerCallback.

public class StateInfo
{
    public StateInfo(AsyncResultWithTimeout result, Socket socket)
    {
        this.AsycResult = result;
        this.Socket = socket;
    }

    public Socket Socket { get; private set; }
    public AsyncResultWithTimeout AsycResult { get; private set; }
    public RegisteredWaitHandle RegisteredWaitHandle { get; set; }
}

封装Socket.BeginReceive

与Socket.BeginReceive方法相比，BeginReceive2添加了一个参数timeout，可以设置该操作的超时时间，单位为毫秒。BeginReceive2中调用Socket.BeginReceive()方法，其中指定的ReceiveCb回调将在正常接收到消息后将结果保存在stateInfo对象的AsyncResult属性中，该属性中的值就是BeginReceive2()方法返回的IAsyncResult。BeginReceive2调用Socket.BeginReceive后，在ThreadPool中注册了一个WaitOrTimerCallback委托。ThreadPool将在Receive操作完成或者Timeout时调用该委托。

public static class SocketExtension
{

    public static int EndReceive2(IAsyncResult ar)
    {
        var result = ar as AsyncResultWithTimeout;
        result.AsyncWaitHandle.WaitOne();

        return result.ReceivedCount;
    }

    public static AsyncResultWithTimeout BeginReceive2
    (
        this Socket socket,
        int timeout,
        byte[] buffer,
        int offset,
        int size,
        SocketFlags flags,
        AsyncCallback callback,
        object state
    )
    {
        var result = new AsyncResultWithTimeout(callback, state);

        var stateInfo = new StateInfo(result, socket);

        socket.BeginReceive(buffer, offset, size, flags, ReceiveCb, state);

        var registeredWaitHandle =
            ThreadPool.RegisterWaitForSingleObject(
                result.AsyncWaitHandle,
                WatchTimeOut,
                stateInfo, // 作为state传递给WatchTimeOut
                timeout,
                true);

        // stateInfo中保存RegisteredWaitHandle，以方便在úWatchTimeOut
        // 中unregister.
        stateInfo.RegisteredWaitHandle = registeredWaitHandle;

        return result;
    }

    private static void WatchTimeOut(object state, bool timeout)
    {
        var stateInfo = state as StateInfo;
        // 设置的timeout前，操作未完成，则设置为操作Timeout
        if (timeout)
        {
            stateInfo.AsycResult.SetTimeout();
        }

        // 取消之前注册的WaitOrTimerCallback
        stateInfo.RegisteredWaitHandle.Unregister(
            stateInfo.AsycResult.AsyncWaitHandle);
    }

    private static void ReceiveCb(IAsyncResult result)
    {
        var state = result.AsyncState as StateInfo;
        var asyncResultWithTimeOut = state.AsycResult;
        var count = state.Socket.EndReceive(result);
        state.AsycResult.SetResult(count);
    }
}

试一下

以下代码演示了如何使用BeginReceive2：

private static void TestSocketBeginReceive2()
{
    Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
    byte[] content = Encoding.ASCII.GetBytes("Hello world");

    IPAddress ip = Dns.Resolve("www.google.com").AddressList[0];
    IPEndPoint receiver = new IPEndPoint(ip, 80);

    socket.BeginSendTo(content, 0, content.Length, SocketFlags.None, receiver, SendToCb2, socket);
    Console.WriteLine("Sent bytes: " + content.Length);
}

private static void SendToCb2(IAsyncResult ar)
{
    var socket = ar.AsyncState as Socket;
    socket.EndSendTo(ar);
    byte[] buffer = new byte[1024];

    AsyncResultWithTimeout receiveAr = socket.BeginReceive2(2000, buffer, 0, buffer.Length, SocketFlags.None, null, null);
    receiveAr.AsyncWaitHandle.WaitOne();
    if (receiveAr.TimedOut)
    {
        Console.WriteLine("Operation timed out.");
    }
    else
    {
        int received = socket.EndReceive(ar);
        Console.WriteLine("Received bytes: " + received);
    }
}

输出结果如下：

 

上述实现是针对BeginReceive的封装，还可以以相同的方式将Send/Receive封装以支持Timeout， 或者更进一步支持retry操作。

附示例代码：下载