Flaky CI: GC-dependent lifecycle tests fail intermittently on .NET Framework

[mattleibow]

Problem

Several tests that verify object lifecycle and GC behavior fail intermittently on Windows .NET Framework. They all share the same pattern: call CollectGarbage(), then assert that objects have been finalized or reference counts have changed.

Failing tests (from 10 recent main builds)

Test	Failures	Error
SKColorSpaceTest.ColorSpaceIsNotDisposedPrematurely	2x	Expected refcount 4, got 3
SKImageTest.DataOutLivesImageUntilFinalizersRun	1x	Expected refcount 3, got 2
SKCodecTest.StreamLosesOwnershipAndCanBeGarbageCollected	1x	Assert.True — object not collected
SKTypefaceTest.StreamLosesOwnershipAndCanBeGarbageCollected	1x	Assert.True — object not collected
SKManagedWStreamTest.StreamIsNotDisposedWhenReferencedIsDisposed	1x	Assert.True — stream unexpectedly disposed
SKManagedWStreamTest.StreamIsReferencedAndNotDisposedPrematurely	1x	NullReferenceException (GC race)

Already skipped on Mono

Several of these tests call SkipOnMono(), acknowledging that GC behavior varies by runtime. The same non-determinism applies to .NET Framework under CI load.

Root Cause

All these tests assert specific GC/finalizer behavior:

• CollectGarbage() + assert object is collected — GC is non-deterministic, especially on .NET Framework under heavy CI load with concurrent test execution. Finalizers may not run within the expected window.
• Reference count assertions — Tests check native refcounts at specific points, but concurrent finalizer threads can alter refcounts between the CollectGarbage() call and the assertion.
• The NullReferenceException in StreamIsReferencedAndNotDisposedPrematurely suggests a race where the finalizer disposes the stream between the GC call and the assertion.

Impact

Low individually (each fails ~1-2x per 10 builds), but collectively they contribute to CI noise. Combined with the x86 memory exhaustion tests (#3608), they make the Tests stage fail on nearly every build.

Recommendations

1. Add retry/tolerance to GC assertions — Instead of a single CollectGarbage() + assert, retry the GC+assert loop a few times:

   // Instead of:
   CollectGarbage();
   Assert.True(collected);
   
   // Use:
   for (int i = 0; i < 5; i++)
   {
       CollectGarbage();
       if (collected) break;
       Thread.Sleep(100);
   }
   Assert.True(collected);

2. Make reference count checks tolerant — For ColorSpaceIsNotDisposedPrematurely, the exact refcount may vary by ±1 depending on internal caching. Consider Assert.InRange instead of Assert.Equal.

3. Gate on runtime — If .NET Framework's GC is too unpredictable, consider SkipOnNetFramework() for the most fragile tests, similar to the existing SkipOnMono().

4. Improve CollectGarbage() helper — If it doesn't already, ensure it calls GC.Collect(), GC.WaitForPendingFinalizers(), GC.Collect() (double-collect pattern) and possibly with a small delay.