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# Description `FairRequestQueueTests.WeightedServiceHoldsUnderConcurrentWorkersAsync` failed once on GitHub Actions macOS net10.0 (#4753, run 38349863236): light=124 heavy=276, ratio 2.23 against an expected 2.5-3.5. It passed on every other CI run and 30/30 locally. **Root cause.** `TrySelect` dequeues an owner's turn token from the lock-free ready queue. On the last token of a turn, `TakeTurnToken` CASes the turn and calls `PushTurn` for the next one. A worker preempted between those two steps leaves the owner with no queued token while it is stalled. The work-conserving dispatch serves the other owner in the meantime, and that share is never made up. Delay injection confirms this: sleeping 1 ms on every 7th turn publish collapses 1:3 to roughly 1:1 (light=199 heavy=201). Publishing a turn ahead of time would only cover stalls shorter than one round (microseconds), while real preemption lasts milliseconds. Making up the lost share (deficit or virtual time) would mean redesigning the scheduler. So this PR documents the bounded skew and makes the test robust instead. **Changes** - `FairRequestQueue` remarks: the weighting holds over time, not in every window, and a preempted worker can cost an owner its share for the length of the stall. - `WeightedServiceHoldsUnderConcurrentWorkersAsync` now measures 3 independent windows through `MeasureWeightedServiceAsync` and asserts that the **median** heavy/light ratio is in 2.5-3.5. The failure message lists all three windows. - Each window uses perOwner = 480 and measured = 560, with queue limits of `2 * perOwner`. That stays under the TestProvider's 1000-request cap and keeps both owners backlogged. - Each window still checks that both owners stay backlogged, then drains the queue and checks that everything is back to zero. The median tolerates one skewed window and still fails when the weighting is lost systematically. With stalls injected into one window only, the median is 3.0 and the test passes (e.g. `light=238 heavy=322; light=140 heavy=420; light=140 heavy=420`). With stalls injected into every window, it still fails. **Validation:** the `FairRequestQueueTests` fixture passes 93/93 on net10.0 Release. The weighted test passed 30/30 runs pinned to 2 cores (`start /affinity 3`). `dotnet format whitespace --verify-no-changes` is clean, and the build has 0 warnings. ## Related Issues - Flaky test seen on #4753 after #4750 (FairRequestQueue without a queue-wide lock) ## Checklist - [x] I have signed the [CLA](https://opcfoundation.org/license/cla/ContributorLicenseAgreementv1.0.pdf) and read the [CONTRIBUTING](https://github.com/OPCFoundation/UA-.NETStandard/blob/master/CONTRIBUTING.md) doc. - [x] I have added tests that prove my fix is effective or that my feature works and increased code coverage. - [x] I have added all necessary documentation. - [x] I have verified that my changes do not introduce (new) build or analyzer warnings. - [ ] I ran **all** tests locally using the **UA.slnx** solution against at least .net **framework** and .net **10**, and all passed. - [ ] I fixed **all** failing and flaky tests in the CI pipelines and **all** CodeQL warnings. - [ ] I have addressed **all** PR feedback received. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>