Harden channel select concurrency

[attahn]

Concurrent channel selectors shared result and round-robin state, allowing tasks to race and read another selector's value.

• Initialize the shared select wakeup exactly once.
• Keep integer, string, and pointer-backed select state thread-local.
• Add replayable channel/select stress coverage under TSan.

Verified with cargo test --bin mako and the fixed TSan stress seeds.

Summary by CodeRabbit

• Bug Fixes

  • Improved concurrent channel selection to isolate selector state across threads and prevent data races.
  • Improved select wakeup initialization for reliable multithreaded operation.

• Tests

  • Added deterministic, seed-controlled stress coverage for channel selection, timeouts, closures, and typed selectors.
  • Added ThreadSanitizer replay scenarios using configurable seeds and rounds.

• Documentation

  • Updated concurrency, soundness, roadmap, status, and CLI documentation with stress-test coverage and replay instructions.

[attahn]

@coderabbitai[bot] review

[coderabbitai]

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📝 Walkthrough

Walkthrough

The runtime now stores channel-select values and round-robin cursors per thread, initializes shared wakeup synchronization once, and adds deterministic integer, string, and struct channel-select stress coverage with fixed-seed TSan replay documentation.

Changes

Channel select stress

Layer / File(s)	Summary
Thread-local select state and wakeup initialization runtime/mako_rt.h	Select values and round-robin cursors use TLS, while shared wakeup synchronization uses platform-specific one-time initialization.
Seeded integer channel stress scenarios examples/testing/chan_select_stress_test.mko	Deterministic producers and seeded rounds cover fan-in, close behavior, concurrent selectors, uniqueness, and runtime statistics.
Concurrent typed selectors and timeout cleanup examples/testing/chan_select_stress_test.mko	String and struct selectors run concurrently, and the timeout/default test closes its channel after validation.
TSan replay and soundness records .github/workflows/ci.yml, docs/BUILTINS.md, docs/ROADMAP.md, docs/SOUNDNESS.md, docs/STATUS.md, CHANGELOG.md	CI and documentation record fixed-seed replay commands, seeded stress coverage, and updated RT-005 status text.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant StressTest
  participant Producers
  participant Selectors
  participant Channels
  StressTest->>Producers: start deterministic seeded producers
  Producers->>Channels: send values and close selected channels
  Selectors->>Channels: perform timed select receives
  Channels-->>StressTest: return received values for uniqueness and statistics checks

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Suggested reviewers: loreste

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 14.29% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: hardening concurrent channel select behavior and stress coverage.

✨ Finishing Touches

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• Create PR with unit tests

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[coderabbitai]

@attahn I’ll review the changes in #9, with particular attention to select-state isolation, one-time wakeup initialization, and concurrency-test coverage.

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[attahn]

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