test: eliminate flaky time.sleep() tests with deterministic coordination

[27Bslash6]

Summary

• Adds ThreadGate helper to tests/utils/timing_helper.py — named synchronization points for deterministic thread coordination
• Replaces 26 flaky time.sleep() calls across 5 test files with deterministic primitives
• Fixes test_version_mismatch_aborts_refresh_concurrent flake blocking release-please PR chore(main): release 0.6.1 #112

Changes by file

File	Before	After
tests/unit/test_l1_swr.py	patch("time.time") + sleep-based thread ordering	time-machine + threading.Event
tests/unit/test_load_control.py	time.sleep(0.05) "let thread acquire"	ThreadGate + wait_for_condition
tests/critical/test_backpressure_load_control.py	time.sleep(0.05) "let it acquire"	ThreadGate + wait_for_condition
tests/unit/test_metrics_collection.py	time.sleep(0.1) "allow time for processing"	collector.flush() (existing API)
tests/critical/conftest.py	backpressure tests required Redis setup	skipped (they don't use Redis)

What was NOT changed

• time.sleep inside thread callbacks simulating real work (valid usage)
• Integration tests waiting for real Redis/memcached TTL expiry
• Performance tests measuring real elapsed time

Test plan

• All 1400 unit tests pass
• All 11 critical backpressure tests pass
• test_version_mismatch_aborts_refresh_concurrent passes 50/50 in tight loop
• Ruff lint + format clean
• basedpyright passes
• CI passes on this PR
• Re-run release-please PR chore(main): release 0.6.1 #112 checks after merge

Summary by CodeRabbit

• Tests
  • Improved test reliability by replacing timing-dependent sleeps with deterministic thread coordination across backpressure control, cache revalidation, and metrics collection tests.
  • Reduced flakiness in critical load control and race condition testing scenarios.
  • Enhanced test infrastructure with new utilities for more robust concurrent testing.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 95480ddb-8120-4054-89c2-81b90bcebabd

📥 Commits

Reviewing files that changed from the base of the PR and between f51d3b1 and 5c5ee92.

📒 Files selected for processing (6)

• tests/critical/conftest.py
• tests/critical/test_backpressure_load_control.py
• tests/unit/test_l1_swr.py
• tests/unit/test_load_control.py
• tests/unit/test_metrics_collection.py
• tests/utils/timing_helper.py

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📝 Walkthrough

Walkthrough

This PR reduces test timing flakiness by replacing sleep-based delays with deterministic synchronization. A new ThreadGate utility enables explicit thread coordination in load control tests; time_machine controls time in stale-while-revalidate tests; and collector.flush() drains background queues in metrics tests.

Changes

Test Timing Determinism

Layer / File(s)	Summary
Thread coordination infrastructure tests/utils/timing_helper.py	New ThreadGate class manages named synchronization gates via signal(name) and wait(name, timeout). TimingHelper.wait_for_condition adds configurable polling interval parameter.
Backpressure and load control test determinism tests/critical/conftest.py, tests/critical/test_backpressure_load_control.py, tests/unit/test_load_control.py	Backpressure controller tests in both critical and unit suites replace sleep-based blocking with ThreadGate signaling and TimingHelper.wait_for_condition polling. Redis fixtures are skipped for backpressure tests. Tests coordinate permit acquisition and queue formation deterministically.
L1 Cache stale-while-revalidate test determinism tests/unit/test_l1_swr.py	SWR tests use time_machine.travel() for deterministic time control instead of mocking time.time. Critical race condition test adds threading.Event coordination between refresher and invalidator threads to order operations deterministically.
Metrics collection test determinism tests/unit/test_metrics_collection.py	Metrics tests replace sleep delays with collector.flush() calls to ensure background queue draining before assertions, removing timing dependencies across counter, histogram, gauge, and Prometheus integration tests.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 Sleeps and timers, gone away,
Gates and signals lead the way,
Time machines spin backward, forward bright,
Tests now dance without the fight!
Determinism blooms, no more delay—
Flakiness has had its day! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: eliminating flaky time.sleep() tests by implementing deterministic thread coordination.
Description check	✅ Passed	The description covers motivation, detailed changes by file, scope clarification, and test results. All major required sections from the template are addressed with substantive content.
Docstring Coverage	✅ Passed	Docstring coverage is 82.98% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/eliminate-flaky-sleep-tests

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