Prerender during same pass if blocked anyway

If something suspends in the shell — i.e. we won't replace the
suspended content with a fallback — we might as well prerender the
siblings during the current render pass, instead of spawning a
separate prerender pass.

This is implemented by setting the "is prerendering" flag to true
whenever we suspend in the shell. But only if we haven't already skipped
over some siblings, because if so, then we need to schedule a separate
prerender pass regardless.

packages/react-cache/src/__tests__/ReactCacheOld-test.internal.js

@@ -238,12 +238,14 @@ describe('ReactCache', () => {
     await act(() => jest.advanceTimersByTime(100));
     assertLog([
       'Promise resolved [4]',
+
       1,
       4,
       'Suspend! [5]',
       1,
       4,
       'Suspend! [5]',
+
       'Promise resolved [5]',
       1,
       4,
@@ -274,12 +276,14 @@ describe('ReactCache', () => {
     await act(() => jest.advanceTimersByTime(100));
     assertLog([
       'Promise resolved [2]',
+
       1,
       2,
       'Suspend! [3]',
       1,
       2,
       'Suspend! [3]',
+
       'Promise resolved [3]',
       1,
       2,

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

@@ -744,7 +744,7 @@ describe('ReactDOMFiberAsync', () => {
       // Because it suspended, it remains on the current path
       expect(div.textContent).toBe('/path/a');
     });
-    assertLog(gate('enableSiblingPrerendering') ? ['Suspend! [/path/b]'] : []);
+    assertLog([]);
 
     await act(async () => {
       resolvePromise();

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

@@ -699,15 +699,7 @@ describe('ReactDOMForm', () => {
     // This should suspend because form actions are implicitly wrapped
     // in startTransition.
     await submit(formRef.current);
-    assertLog([
-      'Pending...',
-      'Suspend! [Updated]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [Updated]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Pending...', 'Suspend! [Updated]', 'Loading...']);
     expect(container.textContent).toBe('Pending...Initial');
 
     await act(() => resolveText('Updated'));
@@ -744,15 +736,7 @@ describe('ReactDOMForm', () => {
 
     // Update
     await submit(formRef.current);
-    assertLog([
-      'Pending...',
-      'Suspend! [Count: 1]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [Count: 1]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Pending...', 'Suspend! [Count: 1]', 'Loading...']);
     expect(container.textContent).toBe('Pending...Count: 0');
 
     await act(() => resolveText('Count: 1'));
@@ -761,15 +745,7 @@ describe('ReactDOMForm', () => {
 
     // Update again
     await submit(formRef.current);
-    assertLog([
-      'Pending...',
-      'Suspend! [Count: 2]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [Count: 2]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Pending...', 'Suspend! [Count: 2]', 'Loading...']);
     expect(container.textContent).toBe('Pending...Count: 1');
 
     await act(() => resolveText('Count: 2'));
@@ -813,14 +789,7 @@ describe('ReactDOMForm', () => {
     assertLog(['Async action started', 'Pending...']);
 
     await act(() => resolveText('Wait'));
-    assertLog([
-      'Suspend! [Updated]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [Updated]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Suspend! [Updated]', 'Loading...']);
     expect(container.textContent).toBe('Pending...Initial');
 
     await act(() => resolveText('Updated'));
@@ -1506,15 +1475,7 @@ describe('ReactDOMForm', () => {
     // Now dispatch inside of a transition. This one does not trigger a
     // loading state.
     await act(() => startTransition(() => dispatch()));
-    assertLog([
-      'Count: 1',
-      'Suspend! [Count: 2]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [Count: 2]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Count: 1', 'Suspend! [Count: 2]', 'Loading...']);
     expect(container.textContent).toBe('Count: 1');
 
     await act(() => resolveText('Count: 2'));

packages/react-reconciler/src/ReactFiberHooks.js

@@ -1666,7 +1666,7 @@ function mountSyncExternalStore<T>(
     }
 
     const rootRenderLanes = getWorkInProgressRootRenderLanes();
-    if (!includesBlockingLane(root, rootRenderLanes)) {
+    if (!includesBlockingLane(rootRenderLanes)) {
       pushStoreConsistencyCheck(fiber, getSnapshot, nextSnapshot);
     }
   }
@@ -1778,7 +1778,7 @@ function updateSyncExternalStore<T>(
       );
     }
 
-    if (!isHydrating && !includesBlockingLane(root, renderLanes)) {
+    if (!isHydrating && !includesBlockingLane(renderLanes)) {
       pushStoreConsistencyCheck(fiber, getSnapshot, nextSnapshot);
     }
   }

packages/react-reconciler/src/ReactFiberLane.js

@@ -579,7 +579,7 @@ export function includesOnlyTransitions(lanes: Lanes): boolean {
   return (lanes & TransitionLanes) === lanes;
 }
 
-export function includesBlockingLane(root: FiberRoot, lanes: Lanes): boolean {
+export function includesBlockingLane(lanes: Lanes): boolean {
   const SyncDefaultLanes =
     InputContinuousHydrationLane |
     InputContinuousLane |

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -907,7 +907,7 @@ export function performConcurrentWorkOnRoot(
   // bug we're still investigating. Once the bug in Scheduler is fixed,
   // we can remove this, since we track expiration ourselves.
   const shouldTimeSlice =
-    !includesBlockingLane(root, lanes) &&
+    !includesBlockingLane(lanes) &&
     !includesExpiredLane(root, lanes) &&
     (disableSchedulerTimeoutInWorkLoop || !didTimeout);
   let exitStatus = shouldTimeSlice
@@ -1968,6 +1968,18 @@ export function renderDidSuspend(): void {
 export function renderDidSuspendDelayIfPossible(): void {
   workInProgressRootExitStatus = RootSuspendedWithDelay;
 
+  if (
+    !workInProgressRootDidSkipSuspendedSiblings &&
+    !includesBlockingLane(workInProgressRootRenderLanes)
+  ) {
+    // This render may not have originally been scheduled as a prerender, but
+    // something suspended inside the visible part of the tree, which means we
+    // won't be able to commit a fallback anyway. Let's proceed as if this were
+    // a prerender so that we can warm up the siblings without scheduling a
+    // separate pass.
+    workInProgressRootIsPrerendering = true;
+  }
+
   // Check if there are updates that we skipped tree that might have unblocked
   // this render.
   if (

packages/react-reconciler/src/__tests__/ActivitySuspense-test.js

@@ -215,15 +215,7 @@ describe('Activity Suspense', () => {
         );
       });
     });
-    assertLog([
-      'Open',
-      'Suspend! [Async]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Open', 'Suspend! [Async]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Open', 'Suspend! [Async]', 'Loading...']);
     // It should suspend with delay to prevent the already-visible Suspense
     // boundary from switching to a fallback
     expect(root).toMatchRenderedOutput(<span>Closed</span>);
@@ -284,15 +276,7 @@ describe('Activity Suspense', () => {
         );
       });
     });
-    assertLog([
-      'Open',
-      'Suspend! [Async]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Open', 'Suspend! [Async]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Open', 'Suspend! [Async]', 'Loading...']);
     // It should suspend with delay to prevent the already-visible Suspense
     // boundary from switching to a fallback
     expect(root).toMatchRenderedOutput(

packages/react-reconciler/src/__tests__/ReactActWarnings-test.js

@@ -349,14 +349,7 @@ describe('act warnings', () => {
           root.render(<App showMore={true} />);
         });
       });
-      assertLog([
-        'Suspend! [Async]',
-        'Loading...',
-
-        ...(gate('enableSiblingPrerendering')
-          ? ['Suspend! [Async]', 'Loading...']
-          : []),
-      ]);
+      assertLog(['Suspend! [Async]', 'Loading...']);
       expect(root).toMatchRenderedOutput('(empty)');
 
       // This is a ping, not a retry, because no fallback is showing.

packages/react-reconciler/src/__tests__/ReactAsyncActions-test.js

@@ -303,7 +303,7 @@ describe('ReactAsyncActions', () => {
       'Suspend! [A1]',
 
       ...(gate('enableSiblingPrerendering')
-        ? ['Pending: false', 'Suspend! [A1]', 'Suspend! [B1]', 'Suspend! [C1]']
+        ? ['Suspend! [B1]', 'Suspend! [C1]']
         : []),
     ]);
     expect(root).toMatchRenderedOutput(
@@ -322,9 +322,7 @@ describe('ReactAsyncActions', () => {
       'A1',
       'Suspend! [B1]',
 
-      ...(gate('enableSiblingPrerendering')
-        ? ['Pending: false', 'A1', 'Suspend! [B1]', 'Suspend! [C1]']
-        : []),
+      ...(gate('enableSiblingPrerendering') ? ['Suspend! [C1]'] : []),
     ]);
     expect(root).toMatchRenderedOutput(
       <>
@@ -333,16 +331,7 @@ describe('ReactAsyncActions', () => {
       </>,
     );
     await act(() => resolveText('B1'));
-    assertLog([
-      'Pending: false',
-      'A1',
-      'B1',
-      'Suspend! [C1]',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Pending: false', 'A1', 'B1', 'Suspend! [C1]']
-        : []),
-    ]);
+    assertLog(['Pending: false', 'A1', 'B1', 'Suspend! [C1]']);
     expect(root).toMatchRenderedOutput(
       <>
         <span>Pending: true</span>
@@ -715,10 +704,6 @@ describe('ReactAsyncActions', () => {
       // automatically reverted.
       'Pending: false',
       'Suspend! [B]',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Pending: false', 'Suspend! [B]']
-        : []),
     ]);
 
     // Resolve the transition

packages/react-reconciler/src/__tests__/ReactConcurrentErrorRecovery-test.js

@@ -209,16 +209,7 @@ describe('ReactConcurrentErrorRecovery', () => {
         root.render(<App step={2} />);
       });
     });
-    assertLog([
-      'Suspend! [A2]',
-      'Loading...',
-      'Suspend! [B2]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [A2]', 'Loading...', 'Suspend! [B2]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Suspend! [A2]', 'Loading...', 'Suspend! [B2]', 'Loading...']);
     // Because this is a refresh, we don't switch to a fallback
     expect(root).toMatchRenderedOutput('A1B1');
 
@@ -229,16 +220,7 @@ describe('ReactConcurrentErrorRecovery', () => {
 
     // Because we're still suspended on A, we can't show an error boundary. We
     // should wait for A to resolve.
-    assertLog([
-      'Suspend! [A2]',
-      'Loading...',
-      'Error! [B2]',
-      'Oops!',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [A2]', 'Loading...', 'Error! [B2]', 'Oops!']
-        : []),
-    ]);
+    assertLog(['Suspend! [A2]', 'Loading...', 'Error! [B2]', 'Oops!']);
     // Remain on previous screen.
     expect(root).toMatchRenderedOutput('A1B1');
 
@@ -299,16 +281,7 @@ describe('ReactConcurrentErrorRecovery', () => {
         root.render(<App step={2} />);
       });
     });
-    assertLog([
-      'Suspend! [A2]',
-      'Loading...',
-      'Suspend! [B2]',
-      'Loading...',
-
-      ...(gate('enableSiblingPrerendering')
-        ? ['Suspend! [A2]', 'Loading...', 'Suspend! [B2]', 'Loading...']
-        : []),
-    ]);
+    assertLog(['Suspend! [A2]', 'Loading...', 'Suspend! [B2]', 'Loading...']);
     // Because this is a refresh, we don't switch to a fallback
     expect(root).toMatchRenderedOutput('A1B1');
 
@@ -364,11 +337,7 @@ describe('ReactConcurrentErrorRecovery', () => {
         root.render(<AsyncText text="Async" />);
       });
     });
-    assertLog([
-      'Suspend! [Async]',
-
-      ...(gate('enableSiblingPrerendering') ? ['Suspend! [Async]'] : []),
-    ]);
+    assertLog(['Suspend! [Async]']);
     expect(root).toMatchRenderedOutput(null);
 
     // This also works if the suspended component is wrapped with an error
@@ -384,11 +353,7 @@ describe('ReactConcurrentErrorRecovery', () => {
         );
       });
     });
-    assertLog([
-      'Suspend! [Async]',
-
-      ...(gate('enableSiblingPrerendering') ? ['Suspend! [Async]'] : []),
-    ]);
+    assertLog(['Suspend! [Async]']);
     expect(root).toMatchRenderedOutput(null);
 
     // Continues rendering once data resolves
@@ -445,7 +410,7 @@ describe('ReactConcurrentErrorRecovery', () => {
         'Suspend! [Async]',
 
         ...(gate('enableSiblingPrerendering')
-          ? ['Suspend! [Async]', 'Caught an error: Oops!']
+          ? ['Caught an error: Oops!']
           : []),
       ]);
       // The render suspended without committing the error.
@@ -468,7 +433,7 @@ describe('ReactConcurrentErrorRecovery', () => {
         'Suspend! [Async]',
 
         ...(gate('enableSiblingPrerendering')
-          ? ['Suspend! [Async]', 'Caught an error: Oops!']
+          ? ['Caught an error: Oops!']
           : []),
       ]);
       expect(root).toMatchRenderedOutput(null);