C#•3mo ago

Canceling TcpClient after TcpListener has been stopped

So, I may have done this completely right, but I'm a bit unsure on the behavior. I'm creating TcpClients with AcceptTcpClientAsync and passing it a CancellationToken which does seem to end the TcpClients, but I guess I'm expecting to see a more graceful end in my TcpCleints which I am not. To wit, I'm expecting to end up hitting client.Close and then get the "Client disconnected" message, which isn't happening.

28 Replies

maxmahem•3mo ago

public sealed class EchoService(IPAddress address, ushort port) : IService, IDisposable
{
    const uint BUFFER_SIZE = 1024;

    readonly TcpListener listener = new(address, port);
    EndPoint? server;

    readonly CancellationTokenSource cancellationTokenSource = new();

    public async Task StartAsync()
    {
        if (cancellationTokenSource.IsCancellationRequested)
            throw new OperationCanceledException("This object cannot be restarted after being canceled", cancellationTokenSource.Token);

        this.listener.Start();
        this.server = this.listener.Server.LocalEndPoint;
        OnMessage(this.server, "Echo Service started... ");

        while (!cancellationTokenSource.IsCancellationRequested) {
            using TcpClient client = await listener.AcceptTcpClientAsync(cancellationTokenSource.Token);
            OnMessage(client.Client.RemoteEndPoint, "Client connected.");
            await HandleClientAsync(client, cancellationTokenSource.Token);
        }
    }

    public async Task StopAsync()
    {
        await cancellationTokenSource.CancelAsync();
        this.listener.Stop();
        OnMessage(this.server, "Echo Service stopped.");
    }

    private async Task HandleClientAsync(TcpClient client, CancellationToken cancellationToken)
    {
        using NetworkStream stream = client.GetStream();
        var endpoint = client.Client.RemoteEndPoint;
        var buffer = new byte[BUFFER_SIZE];

        while (!cancellationToken.IsCancellationRequested) { 
            int bytesRead = await stream.ReadAsync(buffer, cancellationToken);
            if (bytesRead <= 0) break;

            string message = TranslateMessage(buffer.AsSpan()[..bytesRead]);
            OnMessage(endpoint, $"Received: {message}");

            await stream.WriteAsync(buffer.AsMemory(0, bytesRead), cancellationToken);
        }

        client.Close();
        OnMessage(endpoint, "Client disconnected.");
    }
}

public sealed class EchoService(IPAddress address, ushort port) : IService, IDisposable
{
    const uint BUFFER_SIZE = 1024;

    readonly TcpListener listener = new(address, port);
    EndPoint? server;

    readonly CancellationTokenSource cancellationTokenSource = new();

    public async Task StartAsync()
    {
        if (cancellationTokenSource.IsCancellationRequested)
            throw new OperationCanceledException("This object cannot be restarted after being canceled", cancellationTokenSource.Token);

        this.listener.Start();
        this.server = this.listener.Server.LocalEndPoint;
        OnMessage(this.server, "Echo Service started... ");

        while (!cancellationTokenSource.IsCancellationRequested) {
            using TcpClient client = await listener.AcceptTcpClientAsync(cancellationTokenSource.Token);
            OnMessage(client.Client.RemoteEndPoint, "Client connected.");
            await HandleClientAsync(client, cancellationTokenSource.Token);
        }
    }

    public async Task StopAsync()
    {
        await cancellationTokenSource.CancelAsync();
        this.listener.Stop();
        OnMessage(this.server, "Echo Service stopped.");
    }

    private async Task HandleClientAsync(TcpClient client, CancellationToken cancellationToken)
    {
        using NetworkStream stream = client.GetStream();
        var endpoint = client.Client.RemoteEndPoint;
        var buffer = new byte[BUFFER_SIZE];

        while (!cancellationToken.IsCancellationRequested) { 
            int bytesRead = await stream.ReadAsync(buffer, cancellationToken);
            if (bytesRead <= 0) break;

            string message = TranslateMessage(buffer.AsSpan()[..bytesRead]);
            OnMessage(endpoint, $"Received: {message}");

            await stream.WriteAsync(buffer.AsMemory(0, bytesRead), cancellationToken);
        }

        client.Close();
        OnMessage(endpoint, "Client disconnected.");
    }
}

SineѶeҀҬOӶ⒉⓸⎤ᚙ▟ ▞╸•3mo ago

If you're looking for a graceful end. You need to call .disconnect() first before closing.

Unknown User•3mo ago

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SineѶeҀҬOӶ⒉⓸⎤ᚙ▟ ▞╸•3mo ago

If you cancel first. It's gonna throw a System.OperationCancelled exception.

Unknown User•3mo ago

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maxmahem•3mo ago

graceful end isn't the right word yeah.

Unknown User•3mo ago

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maxmahem•3mo ago

but yeah I guess I need to wrap it. I'll try that.

Unknown User•3mo ago

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SineѶeҀҬOӶ⒉⓸⎤ᚙ▟ ▞╸•3mo ago

That's a server setup.

Unknown User•3mo ago

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SineѶeҀҬOӶ⒉⓸⎤ᚙ▟ ▞╸•3mo ago

Oh yeah I only just noticed that yeh. That'll definitely be an issue

Unknown User•3mo ago

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maxmahem•3mo ago

hmm, no point in catching the exception really, I have nothing to do with it. I'll just put in a finally.

Unknown User•3mo ago

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maxmahem•3mo ago

it's fine for this.

Unknown User•3mo ago

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maxmahem•3mo ago

I mean, this is just an echo service. There isn't any way I can "gracefully" tell the client "I'm not going to handle your requests anymore"

SineѶeҀҬOӶ⒉⓸⎤ᚙ▟ ▞╸•3mo ago

It depends on what exceptions are too sometimes. Usually something with a SocketException is an ungraceful disconnect e.g. timeouts and connection resets. With OperationCancelledException. You might want to gracefully disconnect before closing.

Unknown User•3mo ago

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maxmahem•3mo ago

I can call client.Stop in the finally

private async Task HandleClientAsync(TcpClient client, CancellationToken cancellationToken)
{
    using NetworkStream stream = client.GetStream();
    var endpoint = client.Client.RemoteEndPoint;
    var buffer = new byte[BUFFER_SIZE];

    try {
        while (!cancellationToken.IsCancellationRequested) {
            int bytesRead = await stream.ReadAsync(buffer, cancellationToken);
            if (bytesRead <= 0)
                break;

            string message = TranslateMessage(buffer.AsSpan()[..bytesRead]);
            OnMessage(endpoint, $"Received: {message}");

            await stream.WriteAsync(buffer.AsMemory(0, bytesRead), cancellationToken);
        }
    } finally { 
        client.Close();
        OnMessage(endpoint, "Client disconnected.");
    }
}

private async Task HandleClientAsync(TcpClient client, CancellationToken cancellationToken)
{
    using NetworkStream stream = client.GetStream();
    var endpoint = client.Client.RemoteEndPoint;
    var buffer = new byte[BUFFER_SIZE];

    try {
        while (!cancellationToken.IsCancellationRequested) {
            int bytesRead = await stream.ReadAsync(buffer, cancellationToken);
            if (bytesRead <= 0)
                break;

            string message = TranslateMessage(buffer.AsSpan()[..bytesRead]);
            OnMessage(endpoint, $"Received: {message}");

            await stream.WriteAsync(buffer.AsMemory(0, bytesRead), cancellationToken);
        }
    } finally { 
        client.Close();
        OnMessage(endpoint, "Client disconnected.");
    }
}

SineѶeҀҬOӶ⒉⓸⎤ᚙ▟ ▞╸•3mo ago

That'll just completely throw a SocketException on the client side.

Unknown User•3mo ago

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maxmahem•3mo ago

no particular reason, I just don't really have anything to do with the exception here.

Unknown User•3mo ago

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maxmahem•3mo ago

the exception isn't bubling out of here.

Unknown User•3mo ago

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maxmahem•3mo ago

ahhhh I know why. But that's okay as well. hmm... finally does let me avoid some code duplication (catch path isn't taken on a normal client exit). But hmm... maybe I'll look into just registering some cleanup methods in the token instead.

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Canceling TcpClient after TcpListener has been stopped