Sync metadata reads skip an earlier held transaction's writes while an optimistic transaction persists #2220

• I've validated the bug against the latest version of DB packages (@tanstack/db 0.15.2, @tanstack/react-db 0.5.12)

Describe the bug

While an optimistic transaction is persisting, core holds committed sync transactions until it settles. During that window, metadata.collection.get(key) in a sync function only reads the newest pending sync transaction. If that one didn't write key, the read falls through to the applied metadata. A value an earlier held transaction wrote is skipped, and the read returns the old value (or undefined). metadata.collection.list() has the same gap.

This bites a sync source that keeps a resume cursor in collection metadata. It writes the cursor in one transaction and rows in the next, then reads the cursor back to fetch the next page. The read returns the old cursor, so the source fetches pages it already has. Once the optimistic transaction settles and the held transactions apply, the read is right again.

Cause

CollectionSyncManager.createSyncMetadataApi (packages/db/src/collection/sync.ts) reads this.state.pendingSyncedTransactions.at(-1) and then syncedCollectionMetadata. It doesn't walk the earlier entries of pendingSyncedTransactions. collection.list merges only .at(-1) too, and row.get uses the same pattern. I've reproduced collection.get and collection.list; row.get is from reading the source.

The persisted wrapper already reads its own staged writes newest first (readStagedMetadata in db-sqlite-persistence-core createWrappedSyncConfig). Once it has handed a transaction to core, though, the read lands back in core's .at(-1) lookup.

To Reproduce

Fails on 0.15.2:

import { createCollection, createTransaction } from '@tanstack/db'
import type { SyncConfig } from '@tanstack/db'
import { expect, it } from 'vitest'

type Todo = { id: string; title: string }

it(`reads collection metadata written by an earlier held sync transaction`, async () => {
  let sync!: Parameters<SyncConfig<Todo, string>[`sync`]>[0]
  const todos = createCollection<Todo, string>({
    id: `todos`,
    getKey: (todo) => todo.id,
    startSync: true,
    sync: {
      sync: (params) => {
        sync = params
        params.begin()
        params.commit()
        params.markReady()
      },
    },
  })
  await todos.preload()

  // A persisting optimistic transaction makes core hold committed sync transactions.
  let release!: () => void
  const gate = new Promise<void>((resolve) => {
    release = resolve
  })
  const tx = createTransaction({ autoCommit: false, mutationFn: () => gate })
  tx.mutate(() => todos.insert({ id: `1`, title: `optimistic` }))
  const persisted = tx.commit()

  sync.begin()
  sync.metadata?.collection.set(`cursor`, 5)
  sync.commit()

  sync.begin()
  sync.write({ type: `insert`, value: { id: `2`, title: `from server` } })
  sync.commit()

  expect(sync.metadata?.collection.get(`cursor`)).toBe(5) // received undefined
  expect(sync.metadata?.collection.list()).toEqual([{ key: `cursor`, value: 5 }]) // received []

  release()
  await persisted
  expect(sync.metadata?.collection.get(`cursor`)).toBe(5) // passes
})

Expected behavior

A metadata read inside a sync function sees every committed sync transaction, newest write first, before the applied metadata, whether or not core is holding them. list() merges the same way.

Workaround

We keep a map of the metadata our own held transactions wrote and read it before metadata.collection.get, dropping each entry once that transaction's commit receipt settles.

Additional context

main still has the .at(-1) reads. A fix could walk pendingSyncedTransactions from the newest entry back, stopping at the first write for the key, or at a truncate for row metadata as the current code does. list() would apply them oldest first over syncedCollectionMetadata.