fix(node): do not reclaim READY SAB slots in beginFrame

[RtlZeroMemory]

Summary

• follow up on PR feat: add backend beginFrame zero-copy SAB frame path #213 review comment: feat: add backend beginFrame zero-copy SAB frame path #213 (comment)
• make SAB slot acquisition configurable and keep latest-wins READY reclamation enabled by default
• call SAB slot acquisition from beginFrame() with READY reclamation disabled, so beginFrame only uses FREE slots
• add a worker integration regression test proving beginFrame() returns null under single-slot READY pressure instead of reclaiming a published slot

Why

Reclaiming a READY slot in beginFrame() creates a window where the worker can still observe old mailbox metadata but find slot state changed to WRITING with the same token, which can escalate to a backend fatal.

Testing

• npm run build -- --pretty false
• node --test --test-concurrency=1 packages/node/dist/__tests__/worker_integration.test.js

Summary by CodeRabbit

• Bug Fixes

  • Prevents unintended slot reclamation during high-pressure frame submission, reducing contention under load.
  • Ensures oversized bracketed-paste is only treated as a paste event when an explicit oversized marker is present.

• Tests

  • Adds integration tests verifying SAB-based frame submission behavior under backpressure.
  • Adds tests validating marker-based oversized-paste detection.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between c97234e and 6f293bf.

📒 Files selected for processing (1)

• packages/node/src/__e2e__/terminal_io_contract.e2e.test.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/node/src/e2e/terminal_io_contract.e2e.test.ts

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📝 Walkthrough

Walkthrough

Adds an option to SAB slot acquisition to prevent reclaiming READY slots under pressure and updates Node backend calls to use it; also adds a unit test exercising the new behavior and adjusts an e2e test's oversized-paste detection.

Changes

Cohort / File(s)	Summary
Unit Test packages/node/src/__tests__/worker_integration.test.ts	New test "backend: SAB beginFrame does not reclaim READY slot under pressure" exercising SAB-based frame submission with backpressure and single-slot configuration.
Backend SAB Acquisition Logic packages/node/src/backend/nodeBackend.ts	acquireSabSlot gains an optional opts: { allowReadyReclaim?: boolean } (default true). When allowReadyReclaim is false and no free slot exists, the function returns -1 instead of reclaiming READY slots. Calls in beginFrame and initial SAB acquisition now pass { allowReadyReclaim: false }.
E2E Test (paste handling) packages/node/src/__e2e__/terminal_io_contract.e2e.test.ts	Changed oversized paste detection to use a marker-prefixed payload and a pre-collection pass; final assertion now checks for paste events containing the marker rather than asserting no paste events at all.

Sequence Diagram(s)

(omitted)

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• feat: add backend beginFrame zero-copy SAB frame path #213: Related changes touching Node backend beginFrame and SAB writer logic that intersect with SAB slot acquisition and reclaim behavior.

Poem

🐇 I hopped through queues and counted each slot,
Kept READY untouched when the pressure was hot.
A gentle refrain, no snatch in the night,
I waited my turn till the frame came to light.
🥕

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title 'fix(node): do not reclaim READY SAB slots in beginFrame' accurately and specifically describes the main change: disabling READY slot reclamation in the beginFrame function.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/pr213-beginframe-ready-slot

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@packages/node/src/__e2e__/terminal_io_contract.e2e.test.ts`:
- Around line 896-904: The test block around
oversizedPaste/findIndex/assert.equal is failing formatting; run the project's
formatter (Prettier) or lint autofix to reformat this snippet so it matches
project style (e.g., run npm run lint -- --fix or the repo's formatter) and
commit the formatted changes; focus on the lines referencing oversizedEvents,
findIndex, textDecoder.decode(ev.bytes).includes(oversizedMarker),
oversizedPaste, and the assert.equal call to ensure spacing/line breaks match
project formatting rules.

---

ℹ️ Review info

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 6e0d34a and c97234e.

📒 Files selected for processing (1)

• packages/node/src/__e2e__/terminal_io_contract.e2e.test.ts