fix(query-core): notify sibling observers when one unsubscribes during dispatch

[contactjawad]

Summary

Query.#dispatch can skip a still-subscribed observer when another observer on the same query unsubscribes during notification, leaving it with a stale pending result after the query has already resolved.

Root cause

removeObserver mutates the observer array in place (this.observers.splice(index, 1)). #dispatch iterates that same live array with this.observers.forEach(...) and calls onQueryUpdate() synchronously inside the open notifyManager batch. If a listener unsubscribes a same-query observer during that loop, an element is spliced out mid-iteration and forEach skips the next observer.

Fix

Iterate over a snapshot ([...this.observers]) so an observer unsubscribing during notification can't cause a sibling to be skipped.

Test

Added a regression test in queryObserver.test.tsx: two observers subscribe to the same query; observer A unsubscribes itself when it first sees success, and the test asserts observer B still receives the resolved success data. It fails on main (expected 'pending' to be 'success') and passes with this change. A changeset is included.

Summary by CodeRabbit

• Bug Fixes

  • Fixed query updates being skipped for some observers when another observer unsubscribes during notification.
  • Ensured all still-subscribed observers receive successful query results reliably, including when subscriptions change during an update.

• Tests

  • Added regression coverage for observer notifications during subscription changes and asynchronous query completion.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: 2342084c-e7a6-4158-b876-40f872a43816

📥 Commits

Reviewing files that changed from the base of the PR and between 76bf8ef and 4f220aa.

📒 Files selected for processing (1)

• packages/query-core/src/__tests__/queryObserver.test.tsx

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/query-core/src/tests/queryObserver.test.tsx

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📝 Walkthrough

Walkthrough

The query dispatch loop now iterates over an observer snapshot. A regression test covers observer unsubscription during dispatch, and a patch changeset documents the fix.

Changes

Query observer dispatch

Layer / File(s)	Summary
Snapshot-based observer notification packages/query-core/src/query.ts, packages/query-core/src/__tests__/queryObserver.test.tsx, .changeset/fix-query-dispatch-observer-skip.md	Query.#dispatch notifies observers from a snapshot. The regression test verifies that a sibling observer receives successful data after another observer unsubscribes. The changeset records the patch release.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 4f220

This change snapshots observers during query notifications so unsubscribing one observer cannot prevent another from receiving the resolved result; no actionable merge-blocking risk remains beyond normal checks and review.

Possibly related PRs

• TanStack/query#11214: Both changes modify observer-array handling in packages/query-core/src/query.ts.

Suggested reviewers: tkdodo

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the observer notification bug and its fix.
Description check	✅ Passed	The description clearly explains the problem, root cause, fix, regression test, and changeset, but omits the template checklist and release impact sections.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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☁️ Nx Cloud last updated this comment at 2026-08-19 19:20:44 UTC

[TkDodo]

merging this one:

• fix(query-core): Keep observer notifications stable #11234