Potential inconsistent MemoryCache stats

[verdie-g]

Description

The stats provided by MemoryCache might randomly decrease for a short time when a thread dies.

That problem did not occur to me in production but I would like to use a similar pattern in my library, so I would be interested to double check that the pattern is valid.

Reproduction Steps

1. A thread dies
2. The thread and thread locals get collected
3. The WeakReference in MemoryCache._allStats should now point to null
4. During that time, the thread local stats is not referenced by the _allStats list anymore and is not in the dead threads accumulator either
5. The Stats finalizer is called and the value of the thread counter is now added to the accumulators.

Expected behavior

The MemoryCache stats should always increase and never decrease.

Actual behavior

The MemoryCache stats might decrease in some rare case.

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[verdie-g]

@maryamariyan @tarekgh could you have a look at this issue 🙏

Again, I'm not sure this is a real problem but I'm really interested by this pattern to have a low-cost Interlocked.Increment.

[tarekgh]

@adamsitnik @jozkee could you please help looking at this issue? Thanks!

[jeffhandley]

Thank you for a good explanation, @verdie-g. I do not expect our team will be able to investigate this during .NET 10, so I'm marking it as help wanted [up-for-grabs] Good issue for external contributors and moving it to the Future milestone. I think the trickiest part will be setting up a minimal and reliable repro that can be used to validate a change on this.