fix(query-core): Keep observer notifications stable

.changeset/stable-observers-notify.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Notify every query observer when another observer unsubscribes synchronously during the same query update.

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

@@ -824,6 +824,27 @@ describe('query', () => {
  notifySpy.mockRestore()
  })
 
+ it('should notify remaining observers when one unsubscribes during an update', () => {
+ const key = queryKey()
+ const options = { queryKey: key, enabled: false }
+ const firstObserver = new QueryObserver(queryClient, options)
+ const secondObserver = new QueryObserver(queryClient, options)
+ const secondListener = vi.fn()
+
+ let unsubscribeFirst: () => void = () => undefined
+ unsubscribeFirst = firstObserver.subscribe(() => {
+ unsubscribeFirst()
+ })
+ const unsubscribeSecond = secondObserver.subscribe(secondListener)
+
+ queryClient.setQueryData(key, 'data')
+
+ expect(secondListener).toHaveBeenCalledTimes(1)
+ expect(secondObserver.getCurrentResult().data).toBe('data')
+
+ unsubscribeSecond()
+ })
+
  it('should not change state on invalidate() if already invalidated', async () => {
  const key = queryKey()
 

packages/query-core/src/query.ts

@@ -704,7 +704,9 @@ export class Query<
  this.state = reducer(this.state)
 
  notifyManager.batch(() => {
- this.observers.forEach((observer) => {
+ // Keep the current iteration stable if an observer unsubscribes
+ // synchronously while it is being notified.
+ this.observers.slice().forEach((observer) => {
  observer.onQueryUpdate()
  })