Disposing socket on Linux does not interrupt poll/select

[drieseng]

On Linux, disposing a socket on a given thread does not interrupt poll/select on another thread.

To reproduce, compile & run the following code and press enter when prompted:

// Comment this define to test using Socket.Select(...)
#define POLL

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        var endPoint = new IPEndPoint(IPAddress.Loopback, 8080);
        var listener = new TcpListener(endPoint);
        listener.Start();

        var client = new Socket(endPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp) {NoDelay = true};
        client.Connect(endPoint);

        var listenerTask = Task.Factory.StartNew(s =>
            {
                var socket = (Socket) s;

#if POLL
                if (socket.Poll(-1, SelectMode.SelectRead))
                {
                    if (!socket.Connected)
                    {
                        Console.WriteLine("Connection closed.");
                    }
                }
#else
                var readSockets = new System.Collections.Generic.List<Socket> {socket};
                Socket.Select(readSockets, null, null, -1);
                if (!socket.Connected)
                {
                    Console.WriteLine("Connection closed.");
                }
#endif
            }, client, TaskCreationOptions.LongRunning);

        Console.WriteLine("Press enter to dispose the socket.");
        Console.ReadLine();

        Console.WriteLine("Disposing socket...");
        client.Dispose();
        Console.WriteLine("Socket disposed.");

        if (!listenerTask.Wait(5000))
            Console.WriteLine("Socket did not break out of Select call within 5 seconds.");

        Console.WriteLine("Done.");
    }
}

Expected result:

The following output is written to stdout:

Disposing socket...
Connection closed.
Socket disposed.
Done.

This is the result you get on the desktop CLR, and .NET Core on Windows.

Actual result:

Disposing socket...
(hang)

Info

.NET Core version: 2.1.302
OS: Ubuntu 14.04

[stephentoub]

Have you tried the equivalent in C? Generally closing a socket while it's in use by synchronous operations is considered undefined behavior by the Berkeley sockets specification. On Windows it may work most of the time, but it's also possible to lead to crashes and other corruption. On Linux, it often results in hangs.

@wfurt has been looking into ways to make this better when used from managed code, but it would only be working around the inherent limitations of the underlying APIs / OSes, rather than it being a bug in managed code.

[wfurt]

dotnet/corefx#26898 should resolve this and you can call shutdown() as workaround. As @stephentoub mentioned, we are trying to bend OS behavior where blocking IO operation behaves different on Linux/MacOS.

[wfurt]

I verified that dotnet/corefx#32087 fixes this issue.
In mean time you can workaround it by adding Shutdown() before the Dispose()

        Console.WriteLine("Disposing socket...");
+       client.Shutdown(SocketShutdown.Both);
        client.Dispose();
        Console.WriteLine("Socket disposed.");

[wfurt]

fixed by dotnet/corefx#38804