Bug: Extra render pass when reverting to client render

I noticed while working on a PR that when an error happens during
hydration, and we revert to client rendering, React actually does _two_
additional render passes instead of just one. We didn't notice it
earlier because none of our tests happened to assert on how many renders
it took to recover, only on the final output.

It's possible this extra render pass had other consequences that I'm
not aware of, like messing with some assumption in the recoverable
errors logic.

This adds a test to demonstrate the issue. (One problem is that we
don't have much test coverage of this scenario in the first place, which
likely would have caught this earlier.)

Author: acdlite

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -19,6 +19,7 @@ let ReactFeatureFlags;
 let Suspense;
 let SuspenseList;
 let Offscreen;
+let useSyncExternalStore;
 let act;
 let IdleEventPriority;
 let waitForAll;
@@ -113,6 +114,7 @@ describe('ReactDOMServerPartialHydration', () => {
     Scheduler = require('scheduler');
     Suspense = React.Suspense;
     Offscreen = React.unstable_Offscreen;
+    useSyncExternalStore = React.useSyncExternalStore;
     if (gate(flags => flags.enableSuspenseList)) {
       SuspenseList = React.SuspenseList;
     }
@@ -480,20 +482,42 @@ describe('ReactDOMServerPartialHydration', () => {
   });
 
   it('recovers with client render when server rendered additional nodes at suspense root', async () => {
+    function CheckIfHydrating({children}) {
+      // This is a trick to check whether we're hydrating or not, since React
+      // doesn't expose that information currently except
+      // via useSyncExternalStore.
+      let serverOrClient = '(unknown)';
+      useSyncExternalStore(
+        () => {},
+        () => {
+          serverOrClient = 'Client rendered';
+          return null;
+        },
+        () => {
+          serverOrClient = 'Server rendered';
+          return null;
+        },
+      );
+      Scheduler.log(serverOrClient);
+      return null;
+    }
+
     const ref = React.createRef();
     function App({hasB}) {
       return (
         <div>
           <Suspense fallback="Loading...">
             <span ref={ref}>A</span>
             {hasB ? <span>B</span> : null}
+            <CheckIfHydrating />
           </Suspense>
           <div>Sibling</div>
         </div>
       );
     }
 
     const finalHTML = ReactDOMServer.renderToString(<App hasB={true} />);
+    assertLog(['Server rendered']);
 
     const container = document.createElement('div');
     container.innerHTML = finalHTML;
@@ -514,12 +538,12 @@ describe('ReactDOMServerPartialHydration', () => {
       });
     }).toErrorDev('Did not expect server HTML to contain a <span> in <div>');
 
-    jest.runAllTimers();
-
     expect(container.innerHTML).toContain('<span>A</span>');
     expect(container.innerHTML).not.toContain('<span>B</span>');
 
     assertLog([
+      'Server rendered',
+      'Client rendered',
       'There was an error while hydrating this Suspense boundary. ' +
         'Switched to client rendering.',
     ]);