调用 EndAccept 并访问 RemoteEndPoint 后套接字未连接异常

Question

When accessing the RemoteEndPoint property of an accepted socket on the server, a SocketException: 'The socket is not connected' is sometimes thrown.

This happens occasionally (like once in 100 connection attempts) and only when there is a significant latency over the network (eg 10ms+, wifi network). Seems to be some kind of race condition but I'm not sure what can be done to prevent this.

Server code:

this.Socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
this.Socket.Bind(new IPEndPoint(IPAddress.Parse("0.0.0.0"), port));
this.Socket.Listen(0);
this.Socket.BeginAccept(OnSocketAccepted, null);

private void OnSocketAccepted(IAsyncResult ar)
{
    var socket = Socket.EndAccept(ar);
    var endPoint = socket.RemoteEndPoint; // SocketException: The socket is not connected
}

Client code:

var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
socket.Connect(ip, port);
// times out after a couple of seconds if the exception happened on the server

Server is running on Mono 6.4.0.198 on ARMv7 rev4 Raspberry Pi 3b+

Client is running on Windows 10 .NET framework 4.6.1 console app

I thought EndAccept basically guaranteed that the socket would be in a connected state and the RemoteEndPoint would be set. I've gone through the whole msdn EndAccept and RemoteEndPoint documentation and can't find any obvious mistakes or misunderstandings.

Answer 1

The Socket can close after you have called EndAccept() and before you have queried RemoteEndPoint. It can also close before your callback calls EndAccept(), this will also throw a Socket Exception.

There aa number of race conditions that can happen in here, particularly if your system is slow. You can check Socket.Connected but even that isn't guaranteed.

The correct approach is a Try/Catch and gracefully exit or throw an unrecoverable socket.

eg,

        catch (SocketException ex)
        #region SocketException
        {
            /// Some SocketExceptions we manage quietly.  Others we surface as critical errors.
            if ((SocketError)ex.ErrorCode == SocketError.AddressAlreadyInUse)
            {
                Log.Error("Endpoint failed to start. IPEndPoint {ipEndPoint} already in use.", this.localIPEndPoint.ToString());
                // Execution will continue after finally{}
            }
            else
            {
                Log.Error(ex, "Endpoint has ended with an unrecoverable socket error:", Name);
                throw;
            }
        }