Frequent failure: TryFindClass_Utf8_DoesNotLeakGlobalRefs

[simonrozsival]

The TryFindClass_Utf8_DoesNotLeakGlobalRefs device test is failing frequently and needs investigation.

Example failing build:
https://dev.azure.com/dnceng-public/public/_build/results?buildId=1498907&view=results

Failure:

failed TryFindClass_Utf8_DoesNotLeakGlobalRefs
    TryFindClass for non-existent classes should not leak global references
  Expected: 492
  But was:  546

   at Java.InteropTests.JniTypeUtf8Test.TryFindClass_Utf8_DoesNotLeakGlobalRefs()

  from /Users/runner/work/1/s/bin/TestDebug/net11.0-android/Mono.Android.NET-Tests.dll (net11.0|x64)
    TryFindClass for non-existent classes should not leak global references
    Expected: 492
    But was:  546
  
     at Java.InteropTests.JniTypeUtf8Test.TryFindClass_Utf8_DoesNotLeakGlobalRefs()

Test source:
external/Java.Interop/tests/Java.Interop-Tests/Java.Interop/JniTypeUtf8Test.cs

[simonrozsival]

PR #12032 has been updated with a root-cause test fix instead of a temporary ignore. The likely issue is that the test captured its gref baseline before the first failed TryFindClass fallback path had initialized one-time JNI/runtime caches, so it counted setup refs as a leak.

[simonrozsival]

Update after looking at the failure:

The earlier first-use-cache hypothesis does not fit the CI log. TryFindClass_Utf8 ran immediately before TryFindClass_Utf8_DoesNotLeakGlobalRefs and already performed a missing UTF-8 lookup, so the failing test was not the first failed UTF-8 path.

More likely, this assertion is flaky because Android's gref count is process-wide. The on-device NUnit runner is configured with NumberOfTestWorkers=0, so tests themselves are serialized, but unrelated runtime/test-infrastructure activity can still happen on other threads while the test snapshots GlobalReferenceCount. In the failing log, runtime work on another thread appears immediately before the failing test. A +54 delta is consistent with process-wide ambient gref allocation/noise, not necessarily a per-call leak in TryFindClass.

PR #12032 is being kept as a temporary disable for this specific exact-count assertion. We should investigate a better leak-test strategy that isolates the operation under test or amplifies per-call leaks above process-wide background noise.