2012年8月14日星期二

封装Socket.BeginReceive/EndReceive以支持Timeout

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：

image_thumb4

 

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


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