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

.changeset/fix-query-dispatch-observer-skip.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Fix an observer being skipped during query notification when another observer on the same query unsubscribes mid-dispatch. `Query.#dispatch` iterated the live `observers` array while `onQueryUpdate()` could splice it, so a still-subscribed sibling could be left with a stale `pending` result after the query resolved. The notification now iterates over a snapshot of the observers.

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

@@ -31,6 +31,39 @@ describe('queryObserver', () => {
     vi.useRealTimers()
   })
 
+  it('should notify a sibling observer when another observer on the same query unsubscribes during dispatch', async () => {
+    const key = queryKey()
+    const queryFn = vi
+      .fn()
+      .mockImplementation(() => sleep(10).then(() => 'data'))
+
+    const observerA = new QueryObserver(queryClient, {
+      queryKey: key,
+      queryFn,
+      staleTime: Infinity,
+    })
+    const observerB = new QueryObserver(queryClient, {
+      queryKey: key,
+      queryFn,
+      staleTime: Infinity,
+    })
+
+    let unsubscribeA = () => {}
+    unsubscribeA = observerA.subscribe((result) => {
+      if (result.status === 'success') {
+        unsubscribeA()
+      }
+    })
+    const unsubscribeB = observerB.subscribe(() => undefined)
+
+    await vi.advanceTimersByTimeAsync(15)
+
+    expect(observerB.getCurrentResult().status).toBe('success')
+    expect(observerB.getCurrentResult().data).toBe('data')
+
+    unsubscribeB()
+  })
+
   it('should trigger a fetch when subscribed', () => {
     const key = queryKey()
     const queryFn = vi

packages/query-core/src/query.ts

@@ -704,7 +704,9 @@ export class Query<
     this.state = reducer(this.state)
 
     notifyManager.batch(() => {
-      this.observers.forEach((observer) => {
+      // Iterate over a snapshot so that an observer unsubscribing during
+      // notification cannot cause a sibling observer to be skipped.
+      ;[...this.observers].forEach((observer) => {
         observer.onQueryUpdate()
       })