Fix race conditions in queuePerItem promise lifecycle

[Copilot]

Two race conditions in queuePerItem cause promise coalescing to silently fail under concurrent access, producing duplicate task executions and stale results.

Race Condition 1 — Registration gap before sentinel

state.promises.set(key, promise) was called after task() inside runTask(). Any same-key call occurring synchronously within task() would find no entry and spawn a duplicate task.

Fix: Create a deferred sentinel promise and register it in state.promises before invoking task(), then forward the task result to the sentinel:

// Before: task() runs first, then state.promises.set()
promise = task().then(...);
state.promises.set(key, promise);

// After: sentinel registered before task() runs
const sentinel = new Promise<unknown>((res, rej) => { resolveSentinel = res; rejectSentinel = rej; });
state.promises.set(key, sentinel);          // registered before task()
task().then(result => { onComplete(true); resolveSentinel(result); }, ...);
return sentinel;

Race Condition 2 — forceRefresh chained promise not stored

When forceRefresh: true and a promise is in-flight, a chained promise was created and returned but never written back to state.promises. The next caller with forceRefresh: false would coalesce onto the stale original instead of the refresh chain.

Fix: One line — store the chained promise before returning:

state.promises.set(key, chained);
return chained;

Tests

Two new tests directly exercise each race condition, verifying no duplicate task execution and correct promise identity.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation

Checklist

• npm run lint passes
• npx tsc --noEmit passes
• npm run test:run passes
• npm run build succeeds

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Critical/Bug] Race conditions in queuePerItem promise lifecycle</issue_title>
<issue_description>## Description

Two related race conditions in the eval queue's queuePerItem function cause promise coalescing to fail under concurrent access.

Race Condition 1: Promise registration gap

lib/eval-queue.ts:341-438: Between the runTask() call (line ~415) and state.promises.set(key, promise) (line ~410 inside runTask), there's a window where the key isn't in state.promises. A concurrent call for the same key during this window bypasses coalescing entirely, causing duplicate work.

Race Condition 2: forceRefresh chained promise not stored

lib/eval-queue.ts:334-338: When forceRefresh is true, a new chained promise is created and returned to the caller, but it is NOT stored back into state.promises. A third caller with forceRefresh: false sees the stale original promise and gets outdated results.

Proposed Fix

1. Register a sentinel promise in state.promises before execution begins
2. When creating a forceRefresh chain, store the new promise back in state.promises so all subsequent callers see the refreshed version</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Critical/Bug] Race conditions in queuePerItem promise lifecycle #50

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