.NET 9 Applications can randomly freeze when being debugged

[auott]

Description

Debugging a .NET 9 application with Visual Studio can randomly end up freezing the debuggee.
See this issue

Reproduction Steps

Debug an application with many threads that exit while trying to step through code.

Expected behavior

Debuggee does not freeze.

Actual behavior

Debuggee gets stuck with the runtime waiting for all threads to synchronize, but one of the threads being waited has exited so it will never complete.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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[Ollhax]

Hi, I'm the one who reported this issue initially. I'm still seeing the problem a few times a day, let me know if I can try any settings or send you any information that could help you track this down.

[tommcdon]

Hi @Ollhax! We have a speculative fix for the issue. Would you mind giving it a try to determine if it resolves the issue? I suggest testing on a non-production machine as these are test-only binaries I built locally. To test the changes:

1. Make a backup copy of the .NET 9 runtime files located in %ProgramFiles%\dotnet\shared\Microsoft.NETCore.App\9.0.2
2. Download gh112747.zip from https://github.com/tommcdon/runtime/releases/tag/9.0_x64_fix-net9-app-freeze_112747
3. Copy clrjit.dll, coreclr.dll, mscordaccore.dll, mscordbi.dll, System.Private.CoreLib.dll to %ProgramFiles%\dotnet\shared\Microsoft.NETCore.App\9.0.2\
4. Follow the instructions on allowing unsigned debugger libraries per https://aka.ms/vs/unsigned-dotnet-debugger-lib
5. Try to reproduce the debugger bug in Visual Studio

After testing is complete, please restore the original clrjit.dll, coreclr.dll, mscordaccore.dll, mscordbi.dll, System.Private.CoreLib.dll back to %ProgramFiles%\dotnet\shared\Microsoft.NETCore.App\9.0.2, and re-enable debugger library signing checks.

[Ollhax]

Hi @Ollhax! We have a speculative fix for the issue. Would you mind giving it a try to determine if it resolves the issue? I suggest testing on a non-production machine as these are test-only binaries I built locally. To test the changes:

1. Make a backup copy of the .NET 9 runtime files located in %ProgramFiles%\dotnet\shared\Microsoft.NETCore.App\9.0.2

2. Download gh112747.zip from https://github.com/tommcdon/runtime/releases/tag/9.0_x64_fix-net9-app-freeze_112747

3. Copy clrjit.dll, coreclr.dll, mscordaccore.dll, mscordbi.dll, System.Private.CoreLib.dll to  %ProgramFiles%\dotnet\shared\Microsoft.NETCore.App\9.0.2\

4. Follow the instructions on allowing unsigned debugger libraries per https://aka.ms/vs/unsigned-dotnet-debugger-lib

5. Try to reproduce the debugger bug in Visual Studio

After testing is complete, please restore the original clrjit.dll, coreclr.dll, mscordaccore.dll, mscordbi.dll, System.Private.CoreLib.dll back to %ProgramFiles%\dotnet\shared\Microsoft.NETCore.App\9.0.2, and re-enable debugger library signing checks.

Alright, I'll give this a try tomorrow 👍