Bugfix: useFormState queues actions in wrong order (facebook#27570)

I neglected to update the "last" pointer of the action queue. Since the
queue is circular, rather than dropping the update, the effect was to
add the update to the front of the queue instead of the back. I didn't
notice earlier because in my demos/tests, the actions would either
resolve really quickly or the actions weren't order dependent (like
incrementing a counter).

packages/react-dom/src/__tests__/ReactDOMForm-test.js

@@ -90,7 +90,7 @@ describe('ReactDOMForm', () => {
       const thenable = record.value;
       record.status = 'resolved';
       record.value = text;
-      thenable.pings.forEach(t => t());
+      thenable.pings.forEach(t => t(text));
     }
   }
 
@@ -1082,6 +1082,35 @@ describe('ReactDOMForm', () => {
     });
   });
 
+  // @gate enableFormActions
+  // @gate enableAsyncActions
+  test('queues multiple actions and runs them in order', async () => {
+    let action;
+    function App() {
+      const [state, dispatch] = useFormState(
+        async (s, a) => await getText(a),
+        'A',
+      );
+      action = dispatch;
+      return <Text text={state} />;
+    }
+
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => root.render(<App />));
+    assertLog(['A']);
+
+    await act(() => action('B'));
+    await act(() => action('C'));
+    await act(() => action('D'));
+
+    await act(() => resolveText('B'));
+    await act(() => resolveText('C'));
+    await act(() => resolveText('D'));
+
+    assertLog(['D']);
+    expect(container.textContent).toBe('D');
+  });
+
   // @gate enableFormActions
   // @gate enableAsyncActions
   test('useFormState: warns if action is not async', async () => {

packages/react-reconciler/src/ReactFiberHooks.js

@@ -1915,7 +1915,7 @@ function dispatchFormState<S, P>(
       payload,
       next: first,
     };
-    last.next = newLast;
+    actionQueue.pending = last.next = newLast;
   }
 }