Updating an async atom twice from a writer atom never resolves

[martinhath]

Bug Description

I've found what I think is a bug and have written two test cases that are seemingly equivalent, but where one succeeds and one fails.

The succeding one creates an atom<Promise<number>> and a writer atom that increments the promised value twice. All is well.

it('write to async atom twice', async () => {
  const asyncAtom = atom(Promise.resolve(2))

  const writer = atom(null, async (get, set) => {
    let val = await get(asyncAtom)
    set(asyncAtom, Promise.resolve(val + 1))
    val = await get(asyncAtom)
    set(asyncAtom, Promise.resolve(val + 1))
    return get(asyncAtom)
  })

  const Component = () => {
    const load = useAtomValue(loadable(asyncAtom))
    const write = useSetAtom(writer)
    const val = load.state === 'hasData' ? load.data : 'loading'
    return (
      <>
        <div>atom count: {val}</div>
        <button onClick={() => write()}>dispatch</button>
      </>
    )
  }

  const { findByText, getByText } = render(
    <StrictMode>
      <Component />
    </StrictMode>,
  )

  await findByText('atom count: 2')

  await userEvent.click(getByText('dispatch'))
  await findByText('atom count: 4', undefined, { timeout: 1000 })
})

However, if we change the writer to use an update function instead:

  const writer = atom(null, (get, set) => {
    set(asyncAtom, async (c) => (await c) + 1)
    set(asyncAtom, async (c) => (await c) + 1) // This await gets stuck
    return get(asyncAtom)
  })

it does not work. Some console.log debugging shows that the returned promise doesn't resolve, and that the second await c doesn't resolve.

Reproduction Link

https://codesandbox.io/p/sandbox/serene-paper-v9n5qy?file=%2Fsrc%2FApp.tsx

[dai-shi]

Thanks for reporting!
Can you try if previous versions have worked before? Especially, v2.8.4.

[martinhath]

I quickly ran through all 2.x.0 versions (and also 2.8.4) on codesandbox.io, and it seems that the only version that works is 2.0; 2.1 is broken. All patch versions 2.0.x work.

[dai-shi]

Thanks. That's very old. I think it worked in v2.0.x by chance.

[dai-shi]

Reproduction with StackBlitz: https://stackblitz.com/edit/vitejs-vite-indg9c?file=src%2FApp.tsx

If we add a setTimeout wait, it works. This is a bug, but I'm not sure how easy a fix would be. Would anyone like to try?

[dai-shi]

a failing test: #2695

[dai-shi]

okay, it's like a deep issue. I'll work on it.

[dai-shi]

I'm still investigating it, but it may require a big architectural change. If that's the case, it will be a minor bump and may take time.

[dai-shi]

#2695 is done. Can you try it please?

https://stackblitz.com/edit/vitejs-vite-rhna75?file=package.json seems to work.

[martinhath]

That's great! It seems like promises are no longer stuck, but I did hit this error immediately:

Uncaught (in promise) Error: [Bug] p is not updated even after cancelation

I can see if I can get a small reproduction; this is from a 300kloc app 😅