ci: flag condition polls bounded by a Task.yield() count

[teamleaderleo]

Summary

check-test-determinism.py reported 0 findings in cmuxTests, yet 27 waits there polled a condition for a fixed number of Task.yield() calls (#13903). A yield waits for nothing. N yields take however long N reschedules happen to take, so the bound shrinks exactly when the runner is busy, and a slow pass becomes a failure. That is the shape test-determinism.md bans, and it produced the SimulatorPanelThemeTests flake that #13907 fixed. The checker could not see it because it only matched sleep call sites.

The checker now has a yield-count-poll rule for Swift test files. It flags a for _ in 0..<N / for _ in A...B loop that awaits Task.yield(), exits on a condition (break or return), and reads no clock. It does not flag:

• loops that already read a deadline or clock (the count only caps spinning)
• bare for _ in 0..<N { await Task.yield() } drains with no condition. Test determinism: 27 waits bound a poll by Task.yield() count, not a deadline — the checker sees none of them #13903's follow-up comment separates these out: they wait for a non-event and need a positive signal, not a deadline.
• loops whose only exit is Task.isCancelled
• loops with a named index, which do per-iteration work

On current main the rule finds 126 sites in 58 files. That includes the 17 remaining cmuxTests sites #13903 lists after #13907, and a random sample of the Packages/**/Tests hits were all real condition polls. They are allowlisted per file in .github/test-determinism-allowlist.txt, with a reason pointing at #13903, so the --strict gate stays green. New test files with the shape fail it. The allowlist is keyed by (file, rule), as for the existing rules, so a new site added to an already-listed file is not caught. The rules doc now names the shape beside the sleep ban.

Refs #13903. This is the checker half. Converting the 126 sites is separate work, and each file comes off the allowlist once it is converted.

Testing

• Added 4 positive and 6 negative self-test fixtures: the SimulatorPanelThemeTests shape, return and guard exits, a one-line loop, a deadline-bounded loop, a bare drain, a cancellation-only exit, a named index, a Python analogue, and a string literal. Commit 1 adds only the fixtures, and --self-test fails with 4 missing positives. Commit 2 passes (142 positive + 92 negative).
• python3 scripts/check-test-determinism.py --strict on the branch rebased onto current main: 0 active, 126 allowlisted.
• I ran every guard command in ci-guards.yml locally, and all passed except three that fail for environment reasons: test_ghostty_zig_version_sync.sh and lint-stored-dispatch-work-items.py need submodules this checkout lacks, and the app-host catalog-diff line needs $RUNNER_TEMP and a base SHA. The ci-cache-receipts.yml invocation (--roots tests/test_ci_cache_restore_receipt.py --strict) passes.

Demo Video

Not applicable (CI tooling change).

Checklist

• I tested the change locally
• I added or updated tests for behavior changes
• For iOS connectivity, auth, lifecycle, workspace or terminal changes, I updated the deterministic soak coverage (not applicable: static checker only)
• I updated docs/changelog if needed (.github/review-bot-rules/test-determinism.md)
• I requested bot reviews after my latest commit
• All code review bot comments are resolved
• All human review comments are resolved

— Ophelia g1 🍄
Run: run_cmux_main_red_triage_app_host_census_and_pr_review_20260923_07d8d17b

🤖 Generated with Claude Code

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Summary by cubic

Adds a yield-count-poll rule to check-test-determinism.py that flags Swift loops which poll a condition for a fixed number of Task.yield() calls. A yield waits for nothing, so the bound shrinks exactly when the runner is busy and a slow pass becomes a failure; the checker previously missed this shape because it only matched sleep call sites (refs #13903).

• Skips loops that read a clock or deadline, drain yields with no condition, exit only on cancellation, or use a named index.
• Allowlists the 126 existing sites in 58 files by (file, rule) so the --strict gate stays green; new files with the shape now fail it.
• Adds 4 positive and 6 negative self-test fixtures and names the shape in the rules doc.

Migration

• Existing test files must come off the allowlist one at a time as they convert to deadline-bounded polls.

^{Written for commit aa3b9ab. Summary will update on new commits.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 4b82298 and aa3b9ab.

📒 Files selected for processing (3)

• .github/review-bot-rules/test-determinism.md
• .github/test-determinism-allowlist.txt
• scripts/check-test-determinism.py

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[teamleaderleo]

Self-review. What I verified:

• Commit 1 (fixtures only) fails --self-test with 4 missing positives, and commit 2 passes all 234 fixtures.
• --strict on the branch rebased onto current main reports 0 active findings and 126 allowlisted. The ci-cache-receipts invocation (--roots tests/test_ci_cache_restore_receipt.py --strict) is unaffected.
• The 17 cmuxTests hits are exactly the remaining sites Test determinism: 27 waits bound a poll by Task.yield() count, not a deadline — the checker sees none of them #13903 lists after test(simulator): bound the panel waits by a deadline, not a yield count #13907. I read a random sample of 12 Packages hits, and each is a condition poll bounded only by a yield count.
• The detector only runs on .swift files and only on a for _ in <literal range> { header, so it adds no cost or findings for the other languages the checker scans.
• The full ci-guards.yml guard sweep passed locally, apart from three environment-only failures listed in the PR body.

CI tooling and docs only, so I'm enabling squash auto-merge.

— Ophelia g1 🍄

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