使用TaskCompletionSource作为WaitHandle替代品是否可以接受？

Question

我的代码处理到远程主机的TCP连接,ConcurrentQueue用于存储传出消息.它打算在单个线程中运行.连接的生命周期包含在内,RunAsync而单独的对象包含连接的"公共状态":

class PublicState
{
    internal readonly ConcurrentQueue<Message> OutgoingMessageQueue = new ConcurrentQueue<Message>();
    internal TaskCompletionSource<Object> OutgoingMessageTcs = null;

    internal readonly TaskCompletionSource<Object> ConnectedTcs = new TaskCompletionSource<Object>();

    public void EnqueueMessages(IEnumerable<Message> messages)
    {
        foreach( Message m in messages ) this.OutgoingMessageQueue.Enqueue( m);
        if( this.OutgoingMessageTcs == null ) this.OutgoingMessageTcs = new TaskCompletionSource<Object>();
        this.OutgoingMessageTcs.SetResult( null );
    }
}

static async Task RunAsync(IPEndPoint endPoint, PublicState state)
{
    using( TcpClient tcp = new TcpClient() )
    {
        await tcp.ConnectAsync( endPoint.Address, endPoint.Port ).ConfigureAwait(false);

        Byte[] reusableBuffer = new Byte[ 4096 ];

        using( NetworkStream ns = tcp.GetStream() )
        {
            state.ConnectedTcs.SetResult( null );

            Task<Int32> nsReadTask = null;

            while( tcp.Connected )
            {
                if( !state.writeQueue.IsEmpty )
                {
                    await WriteMessagesAsync( ... ).ConfigureAwait( false );
                }

                if( ns.DataAvailable )
                {
                    await ReadMessagesAsync( ... ).ConfigureAwait( false );
                }

                // Wait for new data to arrive from remote host or for new messages to send:
                if( state.OutgoingMessageTcs == null ) state.OutgoingMessageTcs = new TaskCompletionSource<Object>();
                if( nsReadTask == null ) nsReadTask = ns.ReadAsync( reusableBuffer, 0, 0 ).ConfigureAwait( false );

                Task c = await Task.WhenAny( state.OutgoingMessageTcs,  nsReadTask ).ConfigureAwait( false );
                if( c == state.OutgoingMessageTcs.Task ) state.OutgoingMessageTcs = null;
                else if( c == nsReadTask ) nsReadTask = null;
            }
        }
    }
}

像这样使用:

public async Task Main(String[] args)
{
    PublicState state = new PublicState();
    Task clientTask = Client.RunAsync( new IPEndPoint(args[0]), state );

    await state.ConnectedTcs.Task; // awaits until TCP connection is established

    state.EnqueueMessage( new Message("foo") );
    state.EnqueueMessage( new Message("bar") );
    state.EnqueueMessage( new Message("baz") );

    await clientTask; // awaits until the TCP connection is closed
}

这段代码可以工作,但我不喜欢它:感觉就像我正在使用TaskCompletionSource它代表一个实际的任务或某种背景操作,而我真的使用它TaskCompletionSource作为一种廉价EventWaitHandle.我没有使用,EventWaitHandle因为它IDisposable(我不想冒泄漏原生资源的风险),它缺乏一个WaitAsync或WaitOneAsync方法.我可以使用SemaphoreSlim(这是等待的,但包装一个EventWaitHandle)但我的代码并不真正代表一个信号量的良好使用.

我是否使用了TaskCompletionSource<T>可接受的,或者是否有更好的方法在RunAsync添加项目时"取消等待"执行OutgoingMessageQueue？

我觉得它"错误"的另一个原因是TaskCompletionSource<T>只能使用一次,然后需要更换.我渴望避免无关的分配.

Answer 1

如果我理解正确的话 - TPLBufferBlock可能就是您所需要的。当前的模拟量Enqueue为Post，可以通过ReceiveAsync扩展方法接收下一个值。

所以BufferBlock你的代码变成这样：

class PublicState {
    internal readonly BufferBlock<Message> OutgoingMessageQueue = new BufferBlock<Message>();
    internal readonly TaskCompletionSource<Object> ConnectedTcs = new TaskCompletionSource<Object>();

    public void EnqueueMessage(Message message) {
        this.OutgoingMessageQueue.Post(message);
    }
}

static async Task RunAsync(IPEndPoint endPoint, PublicState state) {
    using (TcpClient tcp = new TcpClient()) {
        await tcp.ConnectAsync(endPoint.Address, endPoint.Port).ConfigureAwait(false);

        Byte[] reusableBuffer = new Byte[4096];

        using (NetworkStream ns = tcp.GetStream()) {
            state.ConnectedTcs.SetResult(null);

            Task<Int32> nsReadTask = null;
            Task<Message> newMessageTask = null;
            while (tcp.Connected) {
                // Wait for new data to arrive from remote host or for new messages to send:
                if (nsReadTask == null)
                    nsReadTask = ns.ReadAsync(reusableBuffer, 0, 0);
                if (newMessageTask == null)
                    newMessageTask = state.OutgoingMessageQueue.ReceiveAsync();
                var completed = await Task.WhenAny(nsReadTask, newMessageTask).ConfigureAwait(false);
                if (completed == newMessageTask) {
                    var result = await newMessageTask;
                    // do stuff
                    newMessageTask = null;
                }
                else {
                    var bytesRead = await nsReadTask;
                    nsReadTask = null;
                }
            }
        }
    }
}

OutgoingMessageTcs作为奖励，这个版本（我认为）是线程安全的，而您当前的版本不是，因为您正在使用潜在的多个线程（调用者的线程RunAsync和线程）执行非线程安全的操作EnqueueMessages。

如果由于某种原因您不喜欢BufferBlock- 您可以以完全相同的方式使用 nuget 包AsyncCollection。Nito.AsyncEx初始化变为：

internal readonly AsyncCollection<Message> OutgoingMessageQueue = new AsyncCollection<Message>(new ConcurrentQueue<Message>());

并获取：

if (newMessageTask == null)
   newMessageTask = state.OutgoingMessageQueue.TakeAsync();