fix(server): stop a busy terminal from starving other terminals' persistence

[derektrimm]

When a key in KeyedCoalescingWorker became dirty again while its batch was processing, processKey recursed on that same key. A terminal running a redraw-heavy loop keeps its key permanently dirty, so the single worker fiber never returned to the queue: every other key's persistence stalled, drainKey never resolved, and terminal.close hung holding the thread lock, blocking new terminals in that thread until a reconnect interrupted it.

Instead of recursing, a batch that completes with newer data pending now atomically moves its key from active back to queued and offers it at the queue tail, in the same transaction. Other keys interleave fairly, and updates that land while the key waits at the tail still coalesce into one merged batch.

The two new tests fail against the previous implementation (starved quiet key processed out of order) and pass with the change; the existing coalescing and failure-requeue tests are unchanged and stay green, as do the terminal manager's tests.

Fixes #8872

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Note

Medium Risk
Changes core scheduling in a shared worker used for terminal persistence; behavior is well-tested but affects concurrency and shutdown/drain timing across terminals.

Overview
Fixes worker starvation when one key stays dirty during processing (e.g. a terminal in a redraw loop blocking other terminals’ persistence and drainKey / terminal.close).

In KeyedCoalescingWorker, when a batch finishes but latestByKey still has that key, the worker no longer recurses into processKey on the same fiber. It clears the key from activeKeys, adds it to queuedKeys, and TxQueue.offers it at the queue tail in the same transaction so other keys can run in between.

Adds two live tests: one asserts quiet runs before a requeued busy key (busy:busy-1, quiet:quiet-1, busy:busy-2); one asserts merges while waiting at the tail still coalesce (busy:a+b+c).

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Note

Stop busy terminal starvation by requeuing keys in KeyedCoalescingWorker

• Changes processKey in makeKeyedCoalescingWorker to requeue dirty keys to the queue tail instead of recursively processing them immediately.
• Prevents persistence starvation by allowing other terminal keys to run between batches when one key is continuously dirty.
• Adds tests for cross-key fairness and tail-queue coalescing in KeyedCoalescingWorker.test.ts.
• Behavioral Change: processKey no longer extracts pending values and recurses; it removes the key from activeKeys and offers it to the queue if latestByKey has pending work.

^{Macroscope summarized 0427b5f.}

Summary by CodeRabbit

• Bug Fixes
  • Improved background processing so continuously updated items no longer monopolize the queue.
  • Ensured work for other items can continue while one item is busy.
  • Combined updates that arrive while an item is waiting into a single processing batch.

[coderabbitai]

📝 Walkthrough

Walkthrough

The keyed worker now requeues keys with newer work at the queue tail instead of processing them recursively. Two live tests verify fair processing across keys and coalescing of updates received while a key waits.

Changes

Keyed worker fairness

Layer / File(s)	Summary
Worker requeue behavior and live validation packages/shared/src/KeyedCoalescingWorker.ts, packages/shared/src/KeyedCoalescingWorker.test.ts	processKey clears the active status and requeues newer work at the queue tail. Live tests verify that other keys can process and that waiting updates merge into one batch.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 0427b

The PR changes worker scheduling to fairly requeue busy terminal keys, with tests covering fairness and coalescing. The package-placement concern is a bounded code-organization follow-up, and no actionable merge-blocking risk remains.

Suggested reviewers: juliusmarminge

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary fix: preventing busy terminals from starving persistence. It is concise and directly related to the changes.
Description check	✅ Passed	The description clearly explains the problem, the implementation, the impact, the tests, and the linked issue. It does not use the template headings or include the checklist, but it is otherwise compl…
Linked Issues check	✅ Passed	The changes address issue [#8872] by requeueing continuously dirty keys at the queue tail instead of recursively processing them. This allows other terminal keys to run, prevents persistence starvatio…
Out of Scope Changes check	✅ Passed	The changes are limited to KeyedCoalescingWorker scheduling logic and focused tests. They directly support the linked issue and stated objectives, with no unrelated code changes identified.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…

Full details: Description check

Explanation

The description clearly explains the problem, the implementation, the impact, the tests, and the linked issue. It does not use the template headings or include the checklist, but it is otherwise complete and focused.

Full details: Linked Issues check

Explanation

The changes address issue [#8872] by requeueing continuously dirty keys at the queue tail instead of recursively processing them. This allows other terminal keys to run, prevents persistence starvation, and preserves coalescing behavior. The added tests cover fairness and tail-queue coalescing.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2 files.

✨ Finishing Touches

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• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@packages/shared/src/KeyedCoalescingWorker.ts`:
- Around line 42-55: Move the KeyedCoalescingWorker implementation, including
its scheduling logic around activeKeys, latestByKey, and queuedKeys, from
packages/shared to packages/client-runtime. Update all consumers and imports to
use the client-runtime location, removing the shared implementation without
changing its behavior.

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• Create a new PR with the fixes

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 6d15c5b and 0427b5f.

📒 Files selected for processing (2)

• packages/shared/src/KeyedCoalescingWorker.test.ts
• packages/shared/src/KeyedCoalescingWorker.ts

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[macroscopeapp]

Approvability

Verdict: Approved at 0427b5f

Macroscope's review found this PR approvable — This is a narrowly scoped, well-tested fix to shared terminal-persistence scheduling: continuously dirty terminals are requeued fairly instead of monopolizing the worker. It introduces no schema, deployment, security, billing, default, or static-analysis changes.

Notes:

• Diff unchanged. Approvability was decided on eligibility alone.

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[lnieuwenhuis]

Maintainer verification: drain semantics hold. The requeue path sets queuedKeys and re-offers at the tail in the same transaction, so drainKey keeps retrying rather than dropping the write — consistent with the existing cleanupFailedKey pattern. The two live tests pin both fairness ordering and merge-while-waiting coalescing. One note: this is fairness, not termination — a permanently dirty key still loops by design. Looks merge-ready to me.

[derektrimm]

Thanks for reading it closely. The fairness-not-termination point is intended: a key that never stops changing keeps getting its turn at the tail, and the goal is only that it stops taking everyone else's.

[derektrimm]

Still current against main and merge-clean. It only touches the keyed coalescing worker and its test, so it has stayed conflict-free through the CLI rework. Ready whenever someone has a minute.

Dismissing prior approval to re-evaluate 0427b5f

[Yash-Singh1]

Closing in favor of newer #17181