System.Threading.Thread tests crashing on Mono

[stephentoub]

https://dev.azure.com/dnceng/public/_build/results?buildId=1037924&view=ms.vss-test-web.build-test-results-tab&runId=32176598&paneView=attachments&resultId=180347
https://helixre8s23ayyeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-49582-merge-946cc614d5d64401bf/System.Threading.Thread.Tests/console.348ec2f2.log?sv=2019-07-07&se=2021-04-02T17%3A22%3A03Z&sr=c&sp=rl&sig=y5h54yQ5kT6ij4I3eLYK4J2iA8kFcMStqR5%2Bglu45Vk%3D

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
   at System.Threading.WaitSubsystem.OnThreadExiting(Thread thread) in /_/src/libraries/System.Private.CoreLib/src/System/Threading/WaitSubsystem.Unix.cs:line 430
   at System.Threading.Thread.Finalize() in /_/src/mono/System.Private.CoreLib/src/System/Threading/Thread.Mono.cs:line 85
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
   at System.Threading.WaitSubsystem.OnThreadExiting(Thread thread) in /_/src/libraries/System.Private.CoreLib/src/System/Threading/WaitSubsystem.Unix.cs:line 430
   at System.Threading.Thread.Finalize() in /_/src/mono/System.Private.CoreLib/src/System/Threading/Thread.Mono.cs:line 85

[jkotas]

This looks like a regression from #47333. cc @CoffeeFlux

[CoffeeFlux]

Yep, will take a look. I have a couple other fixes in the pipeline as well, so I'll try to have a PR out in a bit.

[CoffeeFlux]

I can reproduce this locally on MacOS by running the suite in a loop, though it's somewhat rare.

[benaadams]

Seen in #49608 runtime (Libraries Test Run release mono OSX x64 Debug)

https://helixre107v0xdeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-49608-merge-452b8f6c8f6b417182/System.Threading.Thread.Tests/console.11032fa9.log?sv=2019-07-07&se=2021-04-03T23%3A52%3A11Z&sr=c&sp=rl&sig=aRKn6vyA2vMqWsnV9sa22QB22zjiuAlILDvd6fJ0npU%3D

=== TEST EXECUTION SUMMARY ===
   System.Threading.Thread.Tests  Total: 53, Errors: 0, Failed: 0, Skipped: 2, Time: 2.894s

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
   at System.Threading.WaitSubsystem.OnThreadExiting(Thread thread) in /_/src/libraries/System.Private.CoreLib/src/System/Threading/WaitSubsystem.Unix.cs:line 430
   at System.Threading.Thread.Finalize() in /_/src/mono/System.Private.CoreLib/src/System/Threading/Thread.Mono.cs:line 85
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
   at System.Threading.WaitSubsystem.OnThreadExiting(Thread thread) in /_/src/libraries/System.Private.CoreLib/src/System/Threading/WaitSubsystem.Unix.cs:line 430
   at System.Threading.Thread.Finalize() in /_/src/mono/System.Private.CoreLib/src/System/Threading/Thread.Mono.cs:line 85
/private/tmp/helix/working/A8F30958/w/B84F09F9/e