Transfer mounted effects on suspend in legacy mode

In legacy mode, a component that suspends bails out and commit in
its previous state. If the component previously had mounted effects,
we must transfer those to the work-in-progress so they don't
get dropped.

Author: acdlite

packages/react-reconciler/src/ReactFiberThrow.js

@@ -199,9 +199,11 @@ function throwException(
       // to render it.
       let currentSource = sourceFiber.alternate;
       if (currentSource) {
+        sourceFiber.updateQueue = currentSource.updateQueue;
         sourceFiber.memoizedState = currentSource.memoizedState;
         sourceFiber.expirationTime = currentSource.expirationTime;
       } else {
+        sourceFiber.updateQueue = null;
         sourceFiber.memoizedState = null;
       }
     }

packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.internal.js

@@ -1448,6 +1448,53 @@ function loadModules({
             'Caught an error: Error in host config.',
           );
         });
+
+        it('does not drop mounted effects', async () => {
+          let never = {then() {}};
+
+          let setShouldSuspend;
+          function App() {
+            const [shouldSuspend, _setShouldSuspend] = React.useState(0);
+            setShouldSuspend = _setShouldSuspend;
+            return (
+              <Suspense fallback="Loading...">
+                <Child shouldSuspend={shouldSuspend} />
+              </Suspense>
+            );
+          }
+
+          function Child({shouldSuspend}) {
+            if (shouldSuspend === 1) {
+              throw never;
+            }
+
+            React.useEffect(() => {
+              Scheduler.unstable_yieldValue('Mount');
+              return () => {
+                Scheduler.unstable_yieldValue('Unmount');
+              };
+            }, []);
+
+            return 'Child';
+          }
+
+          const root = ReactNoop.createLegacyRoot(null);
+          await ReactNoop.act(async () => {
+            root.render(<App />);
+          });
+          expect(Scheduler).toHaveYielded(['Mount']);
+
+          // Suspend the child. This puts it into an inconsistent state.
+          await ReactNoop.act(async () => {
+            setShouldSuspend(true);
+          });
+
+          // Unmount everying
+          await ReactNoop.act(async () => {
+            root.render(null);
+          });
+          expect(Scheduler).toHaveYielded(['Unmount']);
+        });
       });
 
       it('does not call lifecycles of a suspended component', async () => {