是否可以检测流是否已被客户端关闭？

Question

情况简介：

我有一个服务，它接收信息并通过套接字发送回复。连接不安全。我想设置另一个可以为这些连接提供 TLS 的服务——这个新服务将提供一个端口并根据提供的客户端证书分配连接。我不想使用 stunnel 有几个原因，一个是每个接收端口需要一个转发端口。

我目前正在尝试实施的解决方案：

本质上，我正在尝试将 SslStream（传入）与 NetworkStream（传出 - 可能是 Socket，但我将其放入 NetworkStream 以匹配传入）并将读/写操作链接到两者.此链接将提供客户端（通过 SSL/TLS）和服务（通过不安全的连接）之间的流。

这是我想出的用于链接这些流的类：

public class StreamConnector
{
    public StreamConnector(Stream s1, Stream s2)
    {
        StreamConnectorState state1 = new StreamConnectorState(s1, s2);
        StreamConnectorState state2 = new StreamConnectorState(s2, s1);
        s1.BeginRead(state1.Buffer, 0, state1.Buffer.Length, new AsyncCallback(ReadCallback), state1);
        s2.BeginRead(state2.Buffer, 0, state2.Buffer.Length, new AsyncCallback(ReadCallback), state2);
    }

    private void ReadCallback(IAsyncResult result)
    {
        // Get state object.
        StreamConnectorState state = (StreamConnectorState)result.AsyncState;

        // Finish reading data.
        int length = state.InStream.EndRead(result);

        // Write data.
        state.OutStream.Write(state.Buffer, 0, length);

        // Wait for new data.
        state.InStream.BeginRead(state.Buffer, 0, state.Buffer.Length, new AsyncCallback(ReadCallback), state);
    }
}

public class StreamConnectorState
{
    private const int BYTE_ARRAY_SIZE = 4096;

    public byte[] Buffer { get; set; }
    public Stream InStream { get; set; }
    public Stream OutStream { get; set; }

    public StreamConnectorState(Stream inStream, Stream outStream)
    {
        Buffer = new byte[BYTE_ARRAY_SIZE];
        InStream = inStream;
        OutStream = outStream;
    }
}

问题：

当客户端完成发送信息并处理 SslStream 时，服务器没有任何类型的指示表明这是否已经发生。这个 StreamConnector 类愉快地一直运行到永恒而没有抛出任何错误，而且我找不到任何指示它应该停止。 （当然，ReadCallback 每次都得到 0 的长度，但我需要能够提供长时间运行的连接，所以这不是一个好的判断方法。）

另一个潜在问题是即使没有可用数据也会调用 ReadCallback。不确定如果我直接使用 Socket 而不是流，这是否会有所不同，但一遍又一遍地运行该代码似乎效率低下。

我的问题：

1) 有没有办法判断 Stream 是否已从客户端关闭？

2) 有没有更好的方法来做我想做的事情？

2a) 有没有更高效的方式来运行异步读/写循环？

编辑：谢谢，罗伯特。原来循环一直被调用，因为我没有关闭 Streams（因为不知道如何判断 Streams 何时需要关闭）。我将包含完整的代码解决方案，以防其他人遇到此问题：

/// <summary>
/// Connects the read/write operations of two provided streams
/// so long as both of the streams remain open.
/// Disposes of both streams when either of them disconnect.
/// </summary>
public class StreamConnector
{
    public StreamConnector(Stream s1, Stream s2)
    {
        StreamConnectorState state1 = new StreamConnectorState(s1, s2);
        StreamConnectorState state2 = new StreamConnectorState(s2, s1);
        s1.BeginRead(state1.Buffer, 0, state1.Buffer.Length, new AsyncCallback(ReadCallback), state1);
        s2.BeginRead(state2.Buffer, 0, state2.Buffer.Length, new AsyncCallback(ReadCallback), state2);
    }

    private void ReadCallback(IAsyncResult result)
    {
        // Get state object.
        StreamConnectorState state = (StreamConnectorState)result.AsyncState;

        // Check to make sure Streams are still connected before processing.
        if (state.InStream.IsConnected() && state.OutStream.IsConnected())
        {
            // Finish reading data.
            int length = state.InStream.EndRead(result);

            // Write data.
            state.OutStream.Write(state.Buffer, 0, length);

            // Wait for new data.
            state.InStream.BeginRead(state.Buffer, 0, state.Buffer.Length, new AsyncCallback(ReadCallback), state);
        }
        else
        {
            // Dispose of both streams if either of them is no longer connected.
            state.InStream.Dispose();
            state.OutStream.Dispose();
        }
    }
}

public class StreamConnectorState
{
    private const int BYTE_ARRAY_SIZE = 4096;

    public byte[] Buffer { get; set; }
    public Stream InStream { get; set; }
    public Stream OutStream { get; set; }

    public StreamConnectorState(Stream inStream, Stream outStream)
    {
        Buffer = new byte[BYTE_ARRAY_SIZE];
        InStream = inStream;
        OutStream = outStream;
    }
}

public static class StreamExtensions
{
    private static readonly byte[] POLLING_BYTE_ARRAY = new byte[0];

    public static bool IsConnected(this Stream stream)
    {
        try
        {
            // Twice because the first time will return without issue but
            // cause the Stream to become closed (if the Stream is actually
            // closed.)
            stream.Write(POLLING_BYTE_ARRAY, 0, POLLING_BYTE_ARRAY.Length);
            stream.Write(POLLING_BYTE_ARRAY, 0, POLLING_BYTE_ARRAY.Length);
            return true;
        }
        catch (ObjectDisposedException)
        {
            // Since we're disposing of both Streams at the same time, one
            // of the streams will be checked after it is disposed.
            return false;
        }
        catch (IOException)
        {
            // This will be thrown on the second stream.Write when the Stream
            // is closed on the client side.
            return false;
        }
    }
}

Answer 1

您必须尝试读取或写入套接字（或基于套接字的任何内容）才能检测断开连接。

尝试写入将引发异常/返回错误（取决于您的语言范例）或者可能只写入 0 个字节。尝试读取将引发异常/返回错误（同样取决于您的语言范例）或返回 null。

值得注意的是，如果您使用的是基于选择的服务器模型，则断开连接的套接字会显示 - 即返回 select - 当它断开连接时可读，然后您尝试从中读取并得到错误或 @ 987654323@.

Answer 2

我不禁认为您的客户端应该在完成某种消息时通知服务器。最好为电线被切断或电源故障或插头被拔出做好准备，但通常您希望使用某种消息结束标记来终止连接。将例外保存为实际问题，而不是正常对话。