在.NET应用程序中检测重定向控制台输出中的已关闭管道

Question

当应用程序将其输出通过管道传送到另一个程序时,.NET Console类及其默认TextWriter实现(Console.Out在例如,可以隐含地显示Console.WriteLine())不会发出任何错误信号,而另一个程序在应用程序完成之前终止或关闭管道.这意味着应用程序可能会运行超过必要的时间,将输出写入黑洞.

如何检测重定向管道另一端的关闭？

更详细的解释如下:

以下是一对演示此问题的示例程序.Produce很慢地打印很多整数,以模拟计算的效果:

using System;
class Produce
{
    static void Main()
    {
        for (int i = 0; i < 10000; ++i)
        {
            System.Threading.Thread.Sleep(100); // added for effect
            Console.WriteLine(i);
        }
    }
}

Consume 只读取前10行输入然后退出:

using System;
class Consume
{
    static void Main()
    {
        for (int i = 0; i < 10; ++i)
            Console.ReadLine();
    }
}

如果编译了这两个程序,并将第一个管道输出到第二个,就像这样:

Produce | Consume

...可以观察到,Produce在Consume终止后长时间保持运行.

实际上,我的Consume程序是Unix风格的head,我的Produce程序打印的数据计算成本很高.我想在管道的另一端关闭连接时终止输出.

我怎么能在.NET中这样做？

(我知道一个显而易见的选择是传递一个命令行参数来限制输出,这确实是我现在正在做的事情,但我仍然想知道如何做到这一点,因为我希望能够做更多关于何时终止阅读的可配置判断;例如grep之前通过管道head.)

更新:看起来非常像System.IO.__ConsoleStream.NET中的实现是硬编码忽略错误0x6D(ERROR_BROKEN_PIPE)和0xE8(ERROR_NO_DATA).这可能意味着我需要重新实现控制台流.叹...)

Answer 1

要解决这个问题,我必须在Win32文件句柄上编写自己的基本流实现.这并不是非常困难,因为我不需要实现异步支持,缓冲或搜索.

遗憾的是,需要使用不安全的代码,但对于将在本地运行且完全信任的控制台应用程序而言,这通常不是问题.

这是核心流:

class HandleStream : Stream
{
    SafeHandle _handle;
    FileAccess _access;
    bool _eof;

    public HandleStream(SafeHandle handle, FileAccess access)
    {
        _handle = handle;
        _access = access;
    }

    public override bool CanRead
    {
        get { return (_access & FileAccess.Read) != 0; }
    }

    public override bool CanSeek
    {
        get { return false; }
    }

    public override bool CanWrite
    {
        get { return (_access & FileAccess.Write) != 0; }
    }

    public override void Flush()
    {
        // use external buffering if you need it.
    }

    public override long Length
    {
        get { throw new NotSupportedException(); }
    }

    public override long Position
    {
        get { throw new NotSupportedException(); }
        set { throw new NotSupportedException(); }
    }

    static void CheckRange(byte[] buffer, int offset, int count)
    {
        if (offset < 0 || count < 0 || (offset + count) < 0
            || (offset + count) > buffer.Length)
            throw new ArgumentOutOfRangeException();
    }

    public bool EndOfStream
    {
        get { return _eof; }
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        CheckRange(buffer, offset, count);
        int result = ReadFileNative(_handle, buffer, offset, count);
        _eof |= result == 0;
        return result;
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        int notUsed;
        Write(buffer, offset, count, out notUsed);
    }

    public void Write(byte[] buffer, int offset, int count, out int written)
    {
        CheckRange(buffer, offset, count);
        int result = WriteFileNative(_handle, buffer, offset, count);
        _eof |= result == 0;
        written = result;
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        throw new NotSupportedException();
    }

    public override void SetLength(long value)
    {
        throw new NotSupportedException();
    }

    [return: MarshalAs(UnmanagedType.Bool)]
    [DllImport("kernel32", SetLastError=true)]
    static extern unsafe bool ReadFile(
        SafeHandle hFile, byte* lpBuffer, int nNumberOfBytesToRead,
        out int lpNumberOfBytesRead, IntPtr lpOverlapped);

    [return: MarshalAs(UnmanagedType.Bool)]
    [DllImport("kernel32.dll", SetLastError=true)]
    static extern unsafe bool WriteFile(
        SafeHandle hFile, byte* lpBuffer, int nNumberOfBytesToWrite, 
        out int lpNumberOfBytesWritten, IntPtr lpOverlapped);

    unsafe static int WriteFileNative(SafeHandle hFile, byte[] buffer, int offset, int count)
    {
        if (buffer.Length == 0)
            return 0;

        fixed (byte* bufAddr = &buffer[0])
        {
            int result;
            if (!WriteFile(hFile, bufAddr + offset, count, out result, IntPtr.Zero))
            {
                // Using Win32Exception just to get message resource from OS.
                Win32Exception ex = new Win32Exception(Marshal.GetLastWin32Error());
                int hr = ex.NativeErrorCode | unchecked((int) 0x80000000);
                throw new IOException(ex.Message, hr);
            }

            return result;
        }
    }

    unsafe static int ReadFileNative(SafeHandle hFile, byte[] buffer, int offset, int count)
    {
        if (buffer.Length == 0)
            return 0;

        fixed (byte* bufAddr = &buffer[0])
        {
            int result;
            if (!ReadFile(hFile, bufAddr + offset, count, out result, IntPtr.Zero))
            {
                Win32Exception ex = new Win32Exception(Marshal.GetLastWin32Error());
                int hr = ex.NativeErrorCode | unchecked((int) 0x80000000);
                throw new IOException(ex.Message, hr);
            }
            return result;
        }
    }
}

BufferedStream如果需要,可以将它包裹起来进行缓冲,但是对于控制台输出,TextWriter无论如何都将进行字符级缓冲,并且只在新行上刷新.

流滥用Win32Exception以提取错误消息,而不是调用FormatMessage自身.

在这个流的基础上,我能够为控制台I/O编写一个简单的包装器:

static class ConsoleStreams
{
    enum StdHandle
    {
        Input = -10,
        Output = -11,
        Error = -12,
    }

    [DllImport("kernel32.dll", SetLastError = true)]
    static extern IntPtr GetStdHandle(int nStdHandle);  

    static SafeHandle GetStdHandle(StdHandle h)
    {
        return new SafeFileHandle(GetStdHandle((int) h), true);
    }

    public static HandleStream OpenStandardInput()
    {
        return new HandleStream(GetStdHandle(StdHandle.Input), FileAccess.Read);
    }

    public static HandleStream OpenStandardOutput()
    {
        return new HandleStream(GetStdHandle(StdHandle.Output), FileAccess.Write);
    }

    public static HandleStream OpenStandardError()
    {
        return new HandleStream(GetStdHandle(StdHandle.Error), FileAccess.Write);
    }

    static TextReader _in;
    static StreamWriter _out;
    static StreamWriter _error;

    public static TextWriter Out
    {
        get
        {
            if (_out == null)
            {
                _out = new StreamWriter(OpenStandardOutput());
                _out.AutoFlush = true;
            }
            return _out;
        }
    }

    public static TextWriter Error
    {
        get
        {
            if (_error == null)
            {
                _error = new StreamWriter(OpenStandardError());
                _error.AutoFlush = true;
            }
            return _error;
        }
    }

    public static TextReader In
    {
        get
        {
            if (_in == null)
                _in = new StreamReader(OpenStandardInput());
            return _in;
        }
    }
}

最后的结果是在管道另一端终止连接后写入控制台输出,导致消息出现一个很好的异常:

管道正在关闭

通过捕捉和忽略IOException最外层,看起来我很高兴.