dotnet test hangs when using `"maxParallelThreads": "unlimited"` in xunit.runner.json

[meziantou]

Here's the repro steps:

• demo.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>net9.0</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
    <PackageReference Include="xunit.v3" Version="1.0.0" />
    <PackageReference Include="xunit.runner.visualstudio" Version="3.0.0" />
  </ItemGroup>

  <ItemGroup>
    <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
  </ItemGroup>
</Project>

• demo.cs

using Xunit;

public sealed class Repro
{
    [Fact]
    public void Demo()
    {
    }
}

• xunit.runner.json

{
  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
  "maxParallelThreads": "unlimited",
  "parallelizeAssembly": true
}

• run dotnet test

Actual behavior: dotnet test never completes

Notes: This configuration seems ok with xunit 2

dotnet info

.NET SDK:
 Version:           9.0.200-preview.0.24575.35
 Commit:            81b5f040be
 Workload version:  9.0.200-manifests.054dcb45
 MSBuild version:   17.13.0-preview-24569-04+8f6b8ad0a

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.26100
 OS Platform: Windows
 RID:         win-arm64
 Base Path:   C:\Program Files\dotnet\sdk\9.0.200-preview.0.24575.35\

.NET workloads installed:
 [aspire]
   Installation Source: VS 17.13.35617.110
   Manifest Version:    8.2.2/8.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.aspire\8.2.2\WorkloadManifest.json
   Install Type:              Msi

 [wasm-tools]
   Installation Source: VS 17.13.35617.110
   Manifest Version:    9.0.0/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.workload.mono.toolchain.current\9.0.0\WorkloadManifest.json
   Install Type:              Msi

Configured to use loose manifests when installing new manifests.

Host:
  Version:      9.0.0
  Architecture: arm64
  Commit:       9d5a6a9aa4

.NET SDKs installed:
  8.0.404 [C:\Program Files\dotnet\sdk]
  9.0.101 [C:\Program Files\dotnet\sdk]
  9.0.200-preview.0.24575.35 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x64   [C:\Program Files\dotnet\x64]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x64\InstallLocation]
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

[bradwilson]

Very odd. It doesn't hang with dotnet run, so this seems like a problem with xunit.runner.visualstudio. I'm going to transfer the issue over there.

[bradwilson]

Turns out I was wrong, the problem actually is in the core framework. Oops. 😄 But the fix will need to end up showing up in xunit.runner.visualstudio since the code is only being exercised in multi-assembly runners and not with dotnet run.

[bradwilson]

Fix available for VSTest runner in xunit.runner.visualstudio version 3.0.1-pre.4
Fix available for console and MSBuild runners in xunit.v3.* version 1.0.1-pre.6

https://xunit.net/docs/using-ci-builds