Socket.Select() method doesn't work as expected on macOS

[earlbread]

On macOS, Socket.Select method doesn't work as expected. In .NET Framework, it works as expected.

using System;
using System.Net;
using System.Net.Sockets;
using System.Collections.Generic;

namespace SelectTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Socket receiver = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Unspecified);
            Socket sender = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Unspecified);

            receiver.Bind(new IPEndPoint(IPAddress.Loopback, 0));
            receiver.Listen(1);
            sender.Connect(receiver.LocalEndPoint);
            receiver = receiver.Accept();

            sender.Send(new byte[] { 1 });

            var readList = new List<Socket> { receiver };
            var errorList = new List<Socket> { receiver };

            Socket.Select(readList, null, errorList, -1);
            Console.WriteLine($"readList Count: {readList.Count}");
            Console.WriteLine($"errorList Count: {errorList.Count}");
        }
    }
}

Expected result:

The following output is written to stdout:

readList Count: 1
errorList Count: 0

Actual result:

The following output is written to stdout:

readList Count: 0
errorList Count: 0

Dotnet Info

.NET Core SDK (reflecting any global.json):
 Version:   2.2.105
 Commit:    7cecb35b92

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  10.14
 OS Platform: Darwin
 RID:         osx.10.14-x64
 Base Path:   /usr/local/share/dotnet/sdk/2.2.105/

Host (useful for support):
  Version: 2.2.3
  Commit:  6b8ad509b6

.NET Core SDKs installed:
  2.2.105 [/usr/local/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.2.3 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.2.3 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.2.3 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

[davidsh]

@wfurt @tmds

[wfurt]

It seems like we use poll() under the cover to avoid select() limitations. I did some tracing and I see broken behavior described here: https://daniel.haxx.se/blog/2016/10/11/poll-on-mac-10-12-is-broken/

poll() system calls returns immediately with 0 and no events even if infinite (or long) time is specified.
It seems like curl project ditched Apple's poll() altogether curl/curl#1057

While this looks like system bug we can either use select() on OSX or use kqueue under the cover.

from OSX man page:
[EINVAL] ndfs is greater than FD_SETSIZE and _DARWIN_UNLIMITED_SELECT is not defined.

It seems like there is option to avoid limits we were trying to workaround in first place.

https://github.com/dotnet/corefx/blob/master/src/System.Net.Sockets/src/System/Net/Sockets/SocketPal.Unix.cs#L1386-L1398

[karelz]

Looks like most people are using async for these scenarios. Only 1 complaint so far.
When we fix that, we should add a test.

Compat angle: It works on other platforms.

[karelz]

This might be fairly straightforward.

[mattjbird]

Is there any update on this issue?

When I run the above snippet in dotnet 6.0.102 on macOS the Select call hangs. If I compile the same code using mono it works as expected. Happy to provide more info if that'd help.