streamedQuery reducer being called twice

[iSplasher]

Describe the bug

It seems the streamedQuery's reducer is being called twice, resulting in duplicated values.
Please see the relevant code:

https://github.com/TanStack/query/blob/ecdf96296b1dfcee617a5a5b9afdf2a42ad436ba/packages/query-core/src/streamedQuery.ts#L83C1-L89C38

      // don't append to the cache directly when replace-refetching
      if (!isRefetch || refetchMode !== 'replace') {
        context.client.setQueryData<TData>(context.queryKey, (prev) =>
          reducer(prev === undefined ? initialValue : prev, chunk),
        )
      }
      result = reducer(result, chunk)

As you can see, reducer gets called once in the setQueryData under special conditions, and then again at the end of the loop. This can mess up a stateful object in a custom reducer.

Your minimal, reproducible example

It's very simple

Steps to reproduce

const arr = [];

queryOptions({
    queryKey: ["somekey"],
    queryFn: streamedQuery({
      streamFn: async function* (ctx) {
        const v = [1, 2, 3];
        yield* v;
      },
      reducer: (oldStream, newChunk) => {
        arr.push(newChunk);
        return [...oldStream, newChunk];
      },
      initialValue: [],
      refetchMode: "replace",
    }),

arr.length == 6

Expected behavior

Reducer gets called once per chunk.

How often does this bug happen?

None

Screenshots or Videos

No response

Platform

Windows

Tanstack Query adapter

None

TanStack Query version

v5.90.5

TypeScript version

No response

Additional context

No response

[TkDodo]

reducer is supposed to be a pure function, not something that writes to a global value. In that sense, I don’t think it matters much, but we can also capture the result of the reducer in the if statement and re-use for the result assignment.

If you want to improve this, please add a failing test-case & a fix and open a PR 🙏