fix(query-core): reattach 'MutationObserver' to its mutation in 'onSubscribe'

.changeset/khaki-hoops-shake.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Reattach `MutationObserver` to its current mutation when a listener subscribes again, so a `useMutation` result no longer stays `pending` after React tears down and re-establishes the subscription mid-mutation.

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

@@ -62,6 +62,48 @@ describe('mutationObserver', () => {
     expect(queryClient.getMutationCache().findAll()).toHaveLength(0)
   })
 
+  it('resubscribing should reattach the observer to the in-flight mutation', async () => {
+    const mutation = new MutationObserver(queryClient, {
+      mutationFn: (text: string) => sleep(20).then(() => text),
+    })
+
+    const unsubscribe = mutation.subscribe(vi.fn())
+
+    mutation.mutate('input')
+
+    unsubscribe()
+
+    const subscriptionHandler = vi.fn()
+    mutation.subscribe(subscriptionHandler)
+
+    await vi.advanceTimersByTimeAsync(20)
+    expect(mutation.getCurrentResult()).toMatchObject({
+      status: 'success',
+      data: 'input',
+    })
+    expect(subscriptionHandler).toHaveBeenCalledTimes(1)
+  })
+
+  it('resubscribing should pick up a mutation that settled while unsubscribed', async () => {
+    const mutation = new MutationObserver(queryClient, {
+      mutationFn: (text: string) => sleep(20).then(() => text),
+    })
+
+    const unsubscribe = mutation.subscribe(vi.fn())
+
+    mutation.mutate('input')
+
+    unsubscribe()
+
+    await vi.advanceTimersByTimeAsync(20)
+    mutation.subscribe(vi.fn())
+
+    expect(mutation.getCurrentResult()).toMatchObject({
+      status: 'success',
+      data: 'input',
+    })
+  })
+
   it('reset should remove observer to trigger GC', async () => {
     const mutation = new MutationObserver(queryClient, {
       mutationFn: (text: string) => sleep(5).then(() => text),

packages/query-core/src/mutationObserver.ts

@@ -93,6 +93,14 @@ export class MutationObserver<
     }
   }
 
+  protected onSubscribe(): void {
+    if (this.listeners.size === 1 && this.#currentMutation) {
+      this.#currentMutation.addObserver(this)
+
+      this.#updateResult()
+    }
+  }
+
   protected onUnsubscribe(): void {
     if (!this.hasListeners()) {
       this.#currentMutation?.removeObserver(this)