useDeferredValue should skip initialValue if it suspends (#27509)

### Based on #27505

If a parent render spawns a deferred task with useDeferredValue, but the
parent render suspends, we should not wait for the parent render to
complete before attempting to render the final value.

The reason is that the initialValue argument to useDeferredValue is
meant to represent an immediate preview of the final UI. If we can't
render it "immediately", we might as well skip it and go straight to the
"real" value.

This is an improvement over how a userspace implementation of
useDeferredValue would work, because a userspace implementation would
have to wait for the parent task to commit (useEffect) before spawning
the deferred task, creating a waterfall.

Author: acdlite

packages/react-devtools-shared/src/__tests__/TimelineProfiler-test.js

@@ -2001,15 +2001,15 @@ describe('Timeline profiler', () => {
             524288 => "Transition",
             1048576 => "Transition",
             2097152 => "Transition",
-            4194304 => "Transition",
+            4194304 => "Retry",
             8388608 => "Retry",
             16777216 => "Retry",
             33554432 => "Retry",
-            67108864 => "Retry",
-            134217728 => "SelectiveHydration",
-            268435456 => "IdleHydration",
-            536870912 => "Idle",
-            1073741824 => "Offscreen",
+            67108864 => "SelectiveHydration",
+            134217728 => "IdleHydration",
+            268435456 => "Idle",
+            536870912 => "Offscreen",
+            1073741824 => "Deferred",
           },
           "laneToReactMeasureMap": Map {
             1 => [],
@@ -2269,15 +2269,15 @@ describe('Timeline profiler', () => {
             524288 => "Transition",
             1048576 => "Transition",
             2097152 => "Transition",
-            4194304 => "Transition",
+            4194304 => "Retry",
             8388608 => "Retry",
             16777216 => "Retry",
             33554432 => "Retry",
-            67108864 => "Retry",
-            134217728 => "SelectiveHydration",
-            268435456 => "IdleHydration",
-            536870912 => "Idle",
-            1073741824 => "Offscreen",
+            67108864 => "SelectiveHydration",
+            134217728 => "IdleHydration",
+            268435456 => "Idle",
+            536870912 => "Offscreen",
+            1073741824 => "Deferred",
           },
           "laneToReactMeasureMap": Map {
             1 => [],

packages/react-devtools-shared/src/__tests__/preprocessData-test.js

@@ -61,16 +61,17 @@ import {
   NoLane,
   SyncLane,
   OffscreenLane,
+  DeferredLane,
   NoLanes,
   isSubsetOfLanes,
   includesBlockingLane,
   includesOnlyNonUrgentLanes,
-  claimNextTransitionLane,
   mergeLanes,
   removeLanes,
   intersectLanes,
   isTransitionLane,
   markRootEntangled,
+  includesSomeLane,
 } from './ReactFiberLane';
 import {
   ContinuousEventPriority,
@@ -101,6 +102,7 @@ import {
   getWorkInProgressRootRenderLanes,
   scheduleUpdateOnFiber,
   requestUpdateLane,
+  requestDeferredLane,
   markSkippedUpdateLanes,
   isInvalidExecutionContextForEventFunction,
 } from './ReactFiberWorkLoop';
@@ -2665,16 +2667,21 @@ function rerenderDeferredValue<T>(value: T, initialValue?: T): T {
 }
 
 function mountDeferredValueImpl<T>(hook: Hook, value: T, initialValue?: T): T {
-  if (enableUseDeferredValueInitialArg && initialValue !== undefined) {
+  if (
+    enableUseDeferredValueInitialArg &&
     // When `initialValue` is provided, we defer the initial render even if the
     // current render is not synchronous.
-    // TODO: However, to avoid waterfalls, we should not defer if this render
-    // was itself spawned by an earlier useDeferredValue. Plan is to add a
-    // Deferred lane to track this.
+    initialValue !== undefined &&
+    // However, to avoid waterfalls, we do not defer if this render
+    // was itself spawned by an earlier useDeferredValue. Check if DeferredLane
+    // is part of the render lanes.
+    !includesSomeLane(renderLanes, DeferredLane)
+  ) {
+    // Render with the initial value
     hook.memoizedState = initialValue;
 
-    // Schedule a deferred render
-    const deferredLane = claimNextTransitionLane();
+    // Schedule a deferred render to switch to the final value.
+    const deferredLane = requestDeferredLane();
     currentlyRenderingFiber.lanes = mergeLanes(
       currentlyRenderingFiber.lanes,
       deferredLane,
@@ -2710,7 +2717,7 @@ function updateDeferredValueImpl<T>(
 
     if (!is(value, prevValue)) {
       // Schedule a deferred render
-      const deferredLane = claimNextTransitionLane();
+      const deferredLane = requestDeferredLane();
       currentlyRenderingFiber.lanes = mergeLanes(
         currentlyRenderingFiber.lanes,
         deferredLane,

packages/react-reconciler/src/ReactFiberHooks.js

@@ -52,7 +52,7 @@ export const SyncUpdateLanes: Lane = enableUnifiedSyncLane
   : SyncLane;
 
 const TransitionHydrationLane: Lane = /*                */ 0b0000000000000000000000001000000;
-const TransitionLanes: Lanes = /*                       */ 0b0000000011111111111111110000000;
+const TransitionLanes: Lanes = /*                       */ 0b0000000001111111111111110000000;
 const TransitionLane1: Lane = /*                        */ 0b0000000000000000000000010000000;
 const TransitionLane2: Lane = /*                        */ 0b0000000000000000000000100000000;
 const TransitionLane3: Lane = /*                        */ 0b0000000000000000000001000000000;
@@ -68,24 +68,24 @@ const TransitionLane12: Lane = /*                       */ 0b0000000000001000000
 const TransitionLane13: Lane = /*                       */ 0b0000000000010000000000000000000;
 const TransitionLane14: Lane = /*                       */ 0b0000000000100000000000000000000;
 const TransitionLane15: Lane = /*                       */ 0b0000000001000000000000000000000;
-const TransitionLane16: Lane = /*                       */ 0b0000000010000000000000000000000;
 
-const RetryLanes: Lanes = /*                            */ 0b0000111100000000000000000000000;
-const RetryLane1: Lane = /*                             */ 0b0000000100000000000000000000000;
-const RetryLane2: Lane = /*                             */ 0b0000001000000000000000000000000;
-const RetryLane3: Lane = /*                             */ 0b0000010000000000000000000000000;
-const RetryLane4: Lane = /*                             */ 0b0000100000000000000000000000000;
+const RetryLanes: Lanes = /*                            */ 0b0000011110000000000000000000000;
+const RetryLane1: Lane = /*                             */ 0b0000000010000000000000000000000;
+const RetryLane2: Lane = /*                             */ 0b0000000100000000000000000000000;
+const RetryLane3: Lane = /*                             */ 0b0000001000000000000000000000000;
+const RetryLane4: Lane = /*                             */ 0b0000010000000000000000000000000;
 
 export const SomeRetryLane: Lane = RetryLane1;
 
-export const SelectiveHydrationLane: Lane = /*          */ 0b0001000000000000000000000000000;
+export const SelectiveHydrationLane: Lane = /*          */ 0b0000100000000000000000000000000;
 
-const NonIdleLanes: Lanes = /*                          */ 0b0001111111111111111111111111111;
+const NonIdleLanes: Lanes = /*                          */ 0b0000111111111111111111111111111;
 
-export const IdleHydrationLane: Lane = /*               */ 0b0010000000000000000000000000000;
-export const IdleLane: Lane = /*                        */ 0b0100000000000000000000000000000;
+export const IdleHydrationLane: Lane = /*               */ 0b0001000000000000000000000000000;
+export const IdleLane: Lane = /*                        */ 0b0010000000000000000000000000000;
 
-export const OffscreenLane: Lane = /*                   */ 0b1000000000000000000000000000000;
+export const OffscreenLane: Lane = /*                   */ 0b0100000000000000000000000000000;
+export const DeferredLane: Lane = /*                    */ 0b1000000000000000000000000000000;
 
 // Any lane that might schedule an update. This is used to detect infinite
 // update loops, so it doesn't include hydration lanes or retries.
@@ -135,6 +135,9 @@ export function getLabelForLane(lane: Lane): string | void {
     if (lane & OffscreenLane) {
       return 'Offscreen';
     }
+    if (lane & DeferredLane) {
+      return 'Deferred';
+    }
   }
 }
 
@@ -180,7 +183,6 @@ function getHighestPriorityLanes(lanes: Lanes | Lane): Lanes {
     case TransitionLane13:
     case TransitionLane14:
     case TransitionLane15:
-    case TransitionLane16:
       return lanes & TransitionLanes;
     case RetryLane1:
     case RetryLane2:
@@ -195,6 +197,10 @@ function getHighestPriorityLanes(lanes: Lanes | Lane): Lanes {
       return IdleLane;
     case OffscreenLane:
       return OffscreenLane;
+    case DeferredLane:
+      // This shouldn't be reachable because deferred work is always entangled
+      // with something else.
+      return NoLanes;
     default:
       if (__DEV__) {
         console.error(
@@ -367,7 +373,6 @@ function computeExpirationTime(lane: Lane, currentTime: number) {
     case TransitionLane13:
     case TransitionLane14:
     case TransitionLane15:
-    case TransitionLane16:
       return currentTime + 5000;
     case RetryLane1:
     case RetryLane2:
@@ -383,6 +388,7 @@ function computeExpirationTime(lane: Lane, currentTime: number) {
     case IdleHydrationLane:
     case IdleLane:
     case OffscreenLane:
+    case DeferredLane:
       // Anything idle priority or lower should never expire.
       return NoTimestamp;
     default:
@@ -616,7 +622,11 @@ export function markRootUpdated(root: FiberRoot, updateLane: Lane) {
   }
 }
 
-export function markRootSuspended(root: FiberRoot, suspendedLanes: Lanes) {
+export function markRootSuspended(
+  root: FiberRoot,
+  suspendedLanes: Lanes,
+  spawnedLane: Lane,
+) {
   root.suspendedLanes |= suspendedLanes;
   root.pingedLanes &= ~suspendedLanes;
 
@@ -631,13 +641,21 @@ export function markRootSuspended(root: FiberRoot, suspendedLanes: Lanes) {
 
     lanes &= ~lane;
   }
+
+  if (spawnedLane !== NoLane) {
+    markSpawnedDeferredLane(root, spawnedLane, suspendedLanes);
+  }
 }
 
 export function markRootPinged(root: FiberRoot, pingedLanes: Lanes) {
   root.pingedLanes |= root.suspendedLanes & pingedLanes;
 }
 
-export function markRootFinished(root: FiberRoot, remainingLanes: Lanes) {
+export function markRootFinished(
+  root: FiberRoot,
+  remainingLanes: Lanes,
+  spawnedLane: Lane,
+) {
   const noLongerPendingLanes = root.pendingLanes & ~remainingLanes;
 
   root.pendingLanes = remainingLanes;
@@ -683,6 +701,37 @@ export function markRootFinished(root: FiberRoot, remainingLanes: Lanes) {
 
     lanes &= ~lane;
   }
+
+  if (spawnedLane !== NoLane) {
+    markSpawnedDeferredLane(
+      root,
+      spawnedLane,
+      // This render finished successfully without suspending, so we don't need
+      // to entangle the spawned task with the parent task.
+      NoLanes,
+    );
+  }
+}
+
+function markSpawnedDeferredLane(
+  root: FiberRoot,
+  spawnedLane: Lane,
+  entangledLanes: Lanes,
+) {
+  // This render spawned a deferred task. Mark it as pending.
+  root.pendingLanes |= spawnedLane;
+  root.suspendedLanes &= ~spawnedLane;
+
+  // Entangle the spawned lane with the DeferredLane bit so that we know it
+  // was the result of another render. This lets us avoid a useDeferredValue
+  // waterfall — only the first level will defer.
+  const spawnedLaneIndex = laneToIndex(spawnedLane);
+  root.entangledLanes |= spawnedLane;
+  root.entanglements[spawnedLaneIndex] |=
+    DeferredLane |
+    // If the parent render task suspended, we must also entangle those lanes
+    // with the spawned task.
+    entangledLanes;
 }
 
 export function markRootEntangled(root: FiberRoot, entangledLanes: Lanes) {
@@ -795,7 +844,6 @@ export function getBumpedLaneForHydration(
       case TransitionLane13:
       case TransitionLane14:
       case TransitionLane15:
-      case TransitionLane16:
       case RetryLane1:
       case RetryLane2:
       case RetryLane3: