Move suspended render logic to ensureRootIsScheduled (#26328)

When the work loop is suspended, we shouldn't schedule a new render task
until the promise has resolved. When I originally implemented this, I
wasn't sure where to put this logic — `ensureRootIsScheduled` is the
more natural place for it, but that's also a really hot path, so I chose
to do it elsewhere, and left a TODO to reconsider later.

Now it's later. I'm working on a refactor to move the
`ensureRootIsScheduled` call to always happen in a microtask, so that if
there are multiple updates/pings in a single event, they get batched
into a single operation. Which means I can put the logic in that
function where it belongs.

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -322,7 +322,7 @@ const SuspendedOnError: SuspendedReason = 1;
 const SuspendedOnData: SuspendedReason = 2;
 const SuspendedOnImmediate: SuspendedReason = 3;
 const SuspendedOnDeprecatedThrowPromise: SuspendedReason = 4;
-const SuspendedAndReadyToUnwind: SuspendedReason = 5;
+const SuspendedAndReadyToContinue: SuspendedReason = 5;
 const SuspendedOnHydration: SuspendedReason = 6;
 
 // When this is true, the work-in-progress fiber just suspended (or errored) and
@@ -892,6 +892,18 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     return;
   }
 
+  // If this root is currently suspended and waiting for data to resolve, don't
+  // schedule a task to render it. We'll either wait for a ping, or wait to
+  // receive an update.
+  if (
+    workInProgressSuspendedReason === SuspendedOnData &&
+    workInProgressRoot === root
+  ) {
+    root.callbackPriority = NoLane;
+    root.callbackNode = null;
+    return;
+  }
+
   // We use the highest priority lane to represent the priority of the callback.
   const newCallbackPriority = getHighestPriorityLane(nextLanes);
 
@@ -1153,20 +1165,6 @@ function performConcurrentWorkOnRoot(
   if (root.callbackNode === originalCallbackNode) {
     // The task node scheduled for this root is the same one that's
     // currently executed. Need to return a continuation.
-    if (
-      workInProgressSuspendedReason === SuspendedOnData &&
-      workInProgressRoot === root
-    ) {
-      // Special case: The work loop is currently suspended and waiting for
-      // data to resolve. Unschedule the current task.
-      //
-      // TODO: The factoring is a little weird. Arguably this should be checked
-      // in ensureRootIsScheduled instead. I went back and forth, not totally
-      // sure yet.
-      root.callbackPriority = NoLane;
-      root.callbackNode = null;
-      return null;
-    }
     return performConcurrentWorkOnRoot.bind(null, root);
   }
   return null;
@@ -1858,7 +1856,7 @@ function handleThrow(root: FiberRoot, thrownValue: any): void {
       case SuspendedOnData:
       case SuspendedOnImmediate:
       case SuspendedOnDeprecatedThrowPromise:
-      case SuspendedAndReadyToUnwind: {
+      case SuspendedAndReadyToContinue: {
         const wakeable: Wakeable = (thrownValue: any);
         markComponentSuspended(
           erroredWork,
@@ -2216,6 +2214,17 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
             // `status` field, but if the promise already has a status, we won't
             // have added a listener until right here.
             const onResolution = () => {
+              // Check if the root is still suspended on this promise.
+              if (
+                workInProgressSuspendedReason === SuspendedOnData &&
+                workInProgressRoot === root
+              ) {
+                // Mark the root as ready to continue rendering.
+                workInProgressSuspendedReason = SuspendedAndReadyToContinue;
+              }
+              // Ensure the root is scheduled. We should do this even if we're
+              // currently working on a different root, so that we resume
+              // rendering later.
               ensureRootIsScheduled(root, now());
             };
             thenable.then(onResolution, onResolution);
@@ -2225,10 +2234,10 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
             // If this fiber just suspended, it's possible the data is already
             // cached. Yield to the main thread to give it a chance to ping. If
             // it does, we can retry immediately without unwinding the stack.
-            workInProgressSuspendedReason = SuspendedAndReadyToUnwind;
+            workInProgressSuspendedReason = SuspendedAndReadyToContinue;
             break outer;
           }
-          case SuspendedAndReadyToUnwind: {
+          case SuspendedAndReadyToContinue: {
             const thenable: Thenable<mixed> = (thrownValue: any);
             if (isThenableResolved(thenable)) {
               // The data resolved. Try rendering the component again.

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

@@ -650,6 +650,47 @@ describe('ReactThenable', () => {
     assertLog(['Something different']);
   });
 
+  // @gate enableUseHook
+  test('when waiting for data to resolve, an update on a different root does not cause work to be dropped', async () => {
+    const getCachedAsyncText = cache(getAsyncText);
+
+    function App() {
+      return <Text text={use(getCachedAsyncText('Hi'))} />;
+    }
+
+    const root1 = ReactNoop.createRoot();
+    await act(async () => {
+      root1.render(<Suspense fallback={<Text text="Loading..." />} />);
+    });
+
+    // Start a transition on one root. It will suspend.
+    await act(async () => {
+      startTransition(() => {
+        root1.render(
+          <Suspense fallback={<Text text="Loading..." />}>
+            <App />
+          </Suspense>,
+        );
+      });
+    });
+    assertLog(['Async text requested [Hi]']);
+
+    // While we're waiting for the first root's data to resolve, a second
+    // root renders.
+    const root2 = ReactNoop.createRoot();
+    await act(async () => {
+      root2.render('Do re mi');
+    });
+    expect(root2).toMatchRenderedOutput('Do re mi');
+
+    // Once the first root's data is ready, we should finish its transition.
+    await act(async () => {
+      await resolveTextRequests('Hi');
+    });
+    assertLog(['Hi']);
+    expect(root1).toMatchRenderedOutput('Hi');
+  });
+
   // @gate enableUseHook
   test('while suspended, hooks cannot be called (i.e. current dispatcher is unset correctly)', async () => {
     function App() {