CommonExporterVerifyTests randomly failed when called from multiple thread. #2927
Description
CommonExporterVerifyTests failed on latest CI.
https://github.com/dotnet/BenchmarkDotNet/actions/runs/20255647506/job/58156934897
Failed Log
2025-12-16T03:44:34.1544745Z [xUnit.net 00:00:01.16] Exporters(cultureInfoName: "") [FAIL]
2025-12-16T03:44:34.1546551Z [xUnit.net 00:00:01.16] VerifyException : Directory: /home/runner/work/BenchmarkDotNet/BenchmarkDotNet/tests/BenchmarkDotNet.Tests/Exporters/VerifiedFiles
2025-12-16T03:44:34.1548056Z [xUnit.net 00:00:01.16] NotEqual:
2025-12-16T03:44:34.1549309Z [xUnit.net 00:00:01.16] - Received: CommonExporterVerifyTests.Exporters_Invariant.DotNet8_0.received.txt
2025-12-16T03:44:34.1551023Z [xUnit.net 00:00:01.16] Verified: CommonExporterVerifyTests.Exporters_Invariant.verified.txt
The verified content contains JSON that is incorrectly indented.
It looks like SimpleJsonSerializer doesn't support concurrent calls from multiple threads.
(indentationLevel is stored on static variable)
Currently SimpleJson is not expected to be called from multiple threads except for UnitTests.
Though, I thought it is desirable to migrate to System.Text.Json based implementation in the future.