CleanupQueue schedules GC deadlines on wall-clock time but services them with an elapsed-time timer

[benjosity]

CleanupQueue schedules GC deadlines on wall-clock time but services them with an elapsed-time timer

Package: @tanstack/db 0.8.7 — packages/db/src/collection/cleanup-queue.ts (unchanged on main at the time of writing).

What happens

CleanupQueue.schedule records executeAt = Date.now() + gcTime. updateTimeout arms setTimeout(process, executeAt - Date.now()), and process runs a task only when Date.now() >= task.executeAt, otherwise re-arming for the remainder.

setTimeout measures elapsed time; Date.now() is the realtime clock. When realtime moves backwards between schedule and process — an NTP step, a VM or container guest resynchronising with its host, a manual clock change — process finds now < executeAt and re-arms for executeAt - now, which is roughly the size of the step. Every pending cleanup is delayed by that amount regardless of its gcTime.

For a live query collection this delays the release of its subscriptions to its source collections: a component unmounts, the live query's gcTime (1 ms under @tanstack/react-db's useLiveQuery) elapses, and the source still counts it as a subscriber seconds later.

How it was observed

In a Vitest lane that waits for source collections to reach zero subscribers before disposing them, the wait timed out intermittently with the queue holding one task whose executeAt was 1.7–2.7 s ahead of Date.now() and its timer armed for that difference. A sampler on the same machine (a WSL2 guest with an in-guest time-sync service) recorded the realtime clock stepping back ~3.7 s periodically while performance.now() advanced steadily. The environment is only how it was noticed; any realtime step reproduces it.

Deterministic reproduction

import { createCollection, createLiveQueryCollection } from '@tanstack/db'

const source = createCollection<{ id: string }>({
  id: 'source',
  getKey: (row) => row.id,
  startSync: true,
  sync: { sync: ({ markReady }) => { markReady(); return () => {} } },
})
const live = createLiveQueryCollection({
  query: (q) => q.from({ s: source }),
  gcTime: 1,
  startSync: true,
})
const subscription = live.subscribeChanges(() => {})
console.log(source.subscriberCount) // 1

subscription.unsubscribe() // live query's GC is scheduled: executeAt = Date.now() + 1
const realNow = Date.now
Date.now = () => realNow() - 3000 // realtime steps back 3 s before the timer fires

setTimeout(() => console.log(source.subscriberCount), 100) // still 1
setTimeout(() => console.log(source.subscriberCount), 3500) // 0 — released ~3 s late

With a monotonic deadline the second log would already read 0 at 100 ms.

Suggested property

Deadlines and the timer that services them should share a clock: compute executeAt from a monotonic elapsed-time source (for example performance.now(), with Date.now() only as a fallback where it is unavailable) so that a realtime step cannot move a scheduled cleanup. The specific implementation is up to you; the property is that process runs a task once the elapsed time since schedule reaches gcTime.