Constrained tests tool implementation + skill (#5873)

* constrained tests tool implementation + skill * pr suggestions
2026-11-07 12:58:39 +00:00 · 2026-07-15 12:17:51 +02:00
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+---
+name: constrained-tests
+description: Slow-CI simulation — run unit tests on saturated CPU to reproduce CI-style flakiness locally. Suggest when new/reworked suite lean heavy on awaits (deferFulfillment, waitForConfirmation, TestClock, timeouts) or test fail only on CI. Not for every change.
+---
+
+# Constrained test runs
+
+CI runners shared + slow. Background tasks starve, awaits stretch, scheduler races flake there, pass locally. `--constrained` saturate CPU with hog tasks — same conditions, local.
+
+## When
+
+- PR add/rework suite heavy on timing-sensitive async: `deferFulfillment`, `waitForConfirmation`, `TestClock`, `transitionValues`, timeouts.
+- Test fail on CI, pass locally.
+- Not routine changes, not sync suites — slow, no gain.
+
+## How
+
+Full unit test workflow (what CI run):
+
+```bash
+swift run tools ci unit-tests --constrained
+```
+
+One suite, iterate on new tests (prefer while developing):
+
+```bash
+swift run tools ci run-tests --scheme UnitTests --test-name MyNewTests --constrained
+```
+
+Hog count automatic (2× cores). Suite survive ~10 constrained iterations → good shape.
+
+## Failures mean
+
+Failure under constraint = real bug in test (sometimes code under test): assertion race async work, transient state observation missed, timeout too tight. Fix synchronisation, no blind timeout bump. Helpers in `UnitTests/Sources/TestUtilities/` (`deferFulfillment`, `deferFailure`, `waitForConfirmation`). Wait on durable state, not transient.