Add eager alternate.stateNode cleanup (#33161)

This is a fix for a problem where React retains shadow nodes longer than
it needs to. The behaviour is shown in React Native test:
https://github.com/facebook/react-native/blob/main/packages/react-native/src/private/__tests__/utilities/__tests__/ShadowNodeReferenceCounter-itest.js#L169

When React commits a new shadow tree, old shadow nodes are stored inside
`fiber.alternate.stateNode`. This is not cleared up until React clones
the node again. This may be problematic if mutation deletes a subtree,
in that case `fiber.alternate.stateNode` will retain entire subtree
until next update. In case of image nodes, this means retaining entire
images.

So when React goes from revision A: `<View><View /></View>` to revision
B: `<View />`, `fiber.alternate.stateNode` will be pointing to Shadow
Node that represents revision A..

![image](https://github.com/user-attachments/assets/076b677e-d152-4763-8c9d-4f923212b424)

To fix this, this PR adds a new feature flag
`enableEagerAlternateStateNodeCleanup`. When enabled,
`alternate.stateNode` is proactively pointed towards finishedWork's
stateNode, releasing resources sooner.

I have verified this fixes the issue [demonstrated by React Native
tests](https://github.com/facebook/react-native/blob/main/packages/react-native/src/private/__tests__/utilities/__tests__/ShadowNodeReferenceCounter-itest.js#L169).
All existing React tests pass when the flag is enabled.

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -57,6 +57,7 @@ import {
   enableComponentPerformanceTrack,
   enableViewTransition,
   enableFragmentRefs,
+  enableEagerAlternateStateNodeCleanup,
 } from 'shared/ReactFeatureFlags';
 import {
   FunctionComponent,
@@ -1947,6 +1948,20 @@ function commitMutationEffectsOnFiber(
             }
           }
         }
+      } else {
+        if (enableEagerAlternateStateNodeCleanup) {
+          if (supportsPersistence) {
+            if (finishedWork.alternate !== null) {
+              // `finishedWork.alternate.stateNode` is pointing to a stale shadow
+              // node at this point, retaining it and its subtree. To reclaim
+              // memory, point `alternate.stateNode` to new shadow node. This
+              // prevents shadow node from staying in memory longer than it
+              // needs to. The correct behaviour of this is checked by test in
+              // React Native: ShadowNodeReferenceCounter-itest.js#L150
+              finishedWork.alternate.stateNode = finishedWork.stateNode;
+            }
+          }
+        }
       }
       break;
     }

packages/shared/ReactFeatureFlags.js

@@ -135,6 +135,8 @@ export const enableShallowPropDiffing = false;
 
 export const enableSiblingPrerendering = true;
 
+export const enableEagerAlternateStateNodeCleanup = true;
+
 /**
  * Enables an expiration time for retry lanes to avoid starvation.
  */

packages/shared/forks/ReactFeatureFlags.native-fb-dynamic.js

@@ -22,6 +22,7 @@ export const enableObjectFiber = __VARIANT__;
 export const enableHiddenSubtreeInsertionEffectCleanup = __VARIANT__;
 export const enablePersistedModeClonedFlag = __VARIANT__;
 export const enableShallowPropDiffing = __VARIANT__;
+export const enableEagerAlternateStateNodeCleanup = __VARIANT__;
 export const passChildrenWhenCloningPersistedNodes = __VARIANT__;
 export const enableSiblingPrerendering = __VARIANT__;
 export const enableUseEffectCRUDOverload = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -25,6 +25,7 @@ export const {
   enablePersistedModeClonedFlag,
   enableShallowPropDiffing,
   enableUseEffectCRUDOverload,
+  enableEagerAlternateStateNodeCleanup,
   passChildrenWhenCloningPersistedNodes,
   enableSiblingPrerendering,
   enableFastAddPropertiesInDiffing,

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -49,6 +49,7 @@ export const enableSchedulingProfiler = __PROFILE__;
 export const enableComponentPerformanceTrack = false;
 export const enableScopeAPI = false;
 export const enableShallowPropDiffing = false;
+export const enableEagerAlternateStateNodeCleanup = false;
 export const enableSuspenseAvoidThisFallback = false;
 export const enableSuspenseCallback = false;
 export const enableTaint = true;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -65,6 +65,7 @@ export const enableShallowPropDiffing = false;
 export const enableSiblingPrerendering = true;
 
 export const enableUseEffectCRUDOverload = false;
+export const enableEagerAlternateStateNodeCleanup = false;
 
 export const enableYieldingBeforePassive = true;
 

packages/shared/forks/ReactFeatureFlags.test-renderer.native-fb.js

@@ -47,6 +47,7 @@ export const enableSchedulingProfiler = __PROFILE__;
 export const enableComponentPerformanceTrack = false;
 export const enableScopeAPI = false;
 export const enableShallowPropDiffing = false;
+export const enableEagerAlternateStateNodeCleanup = false;
 export const enableSuspenseAvoidThisFallback = false;
 export const enableSuspenseCallback = false;
 export const enableTaint = true;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -74,6 +74,7 @@ export const enableShallowPropDiffing = false;
 export const enableSiblingPrerendering = true;
 
 export const enableUseEffectCRUDOverload = false;
+export const enableEagerAlternateStateNodeCleanup = false;
 
 export const enableHydrationLaneScheduling = true;
 

packages/shared/forks/ReactFeatureFlags.www.js

@@ -110,6 +110,8 @@ export const disableLegacyMode = true;
 
 export const enableShallowPropDiffing = false;
 
+export const enableEagerAlternateStateNodeCleanup = false;
+
 export const enableLazyPublicInstanceInFabric = false;
 
 export const enableSwipeTransition = false;