Investigating #8825: prefetchInfiniteQuery SSR Hydration Issue

[chosule]

Hi @TkDodo,
I've been looking into #8825 and think I found the root cause. Want to run my approach by you before submitting a PR.

Setup I tested

Server prefetch: fails with 500
Client fetch: succeeds
Result: Works! Data shows up after client retry

What's happening

1. Type info gets lost during hydration
   DehydratedQuery doesn't track if it's an infinite query
2. infiniteQueryBehavior not attached
   Regular queries work, but infinite queries lose their behavior
3. Failed promise blocks retry
   The rejected promise prevents client-side refetch

Proposed fix

1. Add a queryType field to track infinite queries:

interface DehydratedQuery {
  queryType?: 'infiniteQuery' | 'normal'
}

2. Detect during dehydration:

const isInfinite = 'initialPageParam' in query.options
const queryType = isInfinite ? 'infiniteQuery' : 'normal'

3. Attach behavior during hydration:

if (queryType === 'infiniteQuery') {
  queryOptions.behavior = infiniteQueryBehavior(undefined)
}

4. Handle failed promises:

const promiseHandler = Promise.resolve(promise)
  .then(data => ({ success: true, data }))
  .catch(error => ({ success: false, error }))

promiseHandler.then(result => {
  if (result.success) {
    // Pass successful promise as initialPromise
    query.fetch(undefined, { initialPromise: promise })
  } else {
    // Don't pass failed promise - allows client retry
    console.log('Server failed, client will retry')
  }
})

Question

I saw #9633 tried something similar but was closed.
You mentioned concerns about it "failing in the normal case of a successfully streamed infinite query."

Does my approach handle both cases correctly?
The key difference is I only skip initialPromise on failures, so successful prefetches should still work.

Should I:

1. Keep testing and open a draft PR
2. Discuss more here first
3. Try a different approach

I've reviewed:

• Issue Next.js: exception encountered in prefetchInfiniteQuery causes useSuspenseInfiniteQuery to fail #8825 and reproduction sandbox
• PR fix(query-core,react-query): preserve infinite query type through failed SSR hydration #9633 and your feedback
• dehydrate.ts and hydrate.ts in query-core
• infiniteQueryBehavior implementation

Let me know what you think!