Repository: facebook/react
Pull request: Bugfix: Remove extra render pass when reverting to client render

(This was reviewed and approved as part of #26380; I'm extracting it into its own PR so that it can bisected later if it causes an issue.)

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.)

Changed files:
filename: packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
patch: @@ -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.',
     ]);

filename: packages/react-reconciler/src/ReactFiberCompleteWork.js
patch: @@ -85,8 +85,6 @@ import {
   StaticMask,
   MutationMask,
   Passive,
-  Incomplete,
-  ShouldCapture,
   ForceClientRender,
   SuspenseyCommit,
   ScheduleRetry,
@@ -839,7 +837,7 @@ function completeDehydratedSuspenseBoundary(
   ) {
     warnIfUnhydratedTailNodes(workInProgress);
     resetHydrationState();
-    workInProgress.flags |= ForceClientRender | Incomplete | ShouldCapture;
+    workInProgress.flags |= ForceClientRender | DidCapture;
 
     return false;
   }
@@ -1284,7 +1282,7 @@ function completeWork(
             nextState,
           );
         if (!fallthroughToNormalSuspensePath) {
-          if (workInProgress.flags & ShouldCapture) {
+          if (workInProgress.flags & ForceClientRender) {
             // Special case. There were remaining unhydrated nodes. We treat
             // this as a mismatch. Revert to client rendering.
             return workInProgress;