Detach sibling pointers in old child list

When a fiber is deleted, it's still part of the previous (alternate)
parent fiber's list of children. Because children are a linked list, an
earlier sibling that's still alive will be connected to the deleted
fiber via its alternate:


  live fiber
  --alternate--> previous live fiber
  --sibling--> deleted fiber

We can't disconnect `alternate` on nodes that haven't been deleted
yet, but we can disconnect the `sibling` and `child` pointers.

Will use this feature flag to test the memory impact.

packages/react-reconciler/src/ReactFiberCommitWork.new.js

@@ -35,7 +35,11 @@ import {
   enableSuspenseCallback,
   enableScopeAPI,
   enableStrictEffects,
+<<<<<<< current
   enableStrongMemoryCleanup,
+=======
+  enableDetachOldChildList,
+>>>>>>> patched
 } from 'shared/ReactFeatureFlags';
 import {
   FunctionComponent,
@@ -2323,6 +2327,33 @@ function commitPassiveUnmountEffects_begin() {
             detachFiberAfterEffects(alternate);
           }
         }
+
+        if (enableDetachOldChildList) {
+          // A fiber was deleted from this parent fiber, but it's still part of
+          // the previous (alternate) parent fiber's list of children. Because
+          // children are a linked list, an earlier sibling that's still alive
+          // will be connected to the deleted fiber via its `alternate`:
+          //
+          //   live fiber
+          //   --alternate--> previous live fiber
+          //   --sibling--> deleted fiber
+          //
+          // We can't disconnect `alternate` on nodes that haven't been deleted
+          // yet, but we can disconnect the `sibling` and `child` pointers.
+          const previousFiber = fiber.alternate;
+          if (previousFiber !== null) {
+            let detachedChild = previousFiber.child;
+            if (detachedChild !== null) {
+              previousFiber.child = null;
+              do {
+                const detachedSibling = detachedChild.sibling;
+                detachedChild.sibling = null;
+                detachedChild = detachedSibling;
+              } while (detachedChild !== null);
+            }
+          }
+        }
+
         nextEffect = fiber;
       }
     }

packages/react-reconciler/src/ReactFiberCommitWork.old.js

@@ -17,10 +17,10 @@ import type {
 } from './ReactFiberHostConfig';
 import type {Fiber} from './ReactInternalTypes';
 import type {FiberRoot} from './ReactInternalTypes';
-import type {LanePriority, Lanes} from './ReactFiberLane.old';
-import type {SuspenseState} from './ReactFiberSuspenseComponent.old';
-import type {UpdateQueue} from './ReactUpdateQueue.old';
-import type {FunctionComponentUpdateQueue} from './ReactFiberHooks.old';
+import type {LanePriority, Lanes} from './ReactFiberLane.new';
+import type {SuspenseState} from './ReactFiberSuspenseComponent.new';
+import type {UpdateQueue} from './ReactUpdateQueue.new';
+import type {FunctionComponentUpdateQueue} from './ReactFiberHooks.new';
 import type {Wakeable} from 'shared/ReactTypes';
 import type {OffscreenState} from './ReactFiberOffscreenComponent';
 import type {HookFlags} from './ReactHookEffectTags';
@@ -35,7 +35,11 @@ import {
   enableSuspenseCallback,
   enableScopeAPI,
   enableStrictEffects,
+<<<<<<< current
   enableStrongMemoryCleanup,
+=======
+  enableDetachOldChildList,
+>>>>>>> patched
 } from 'shared/ReactFeatureFlags';
 import {
   FunctionComponent,
@@ -87,18 +91,18 @@ import {
   setCurrentFiber as setCurrentDebugFiberInDEV,
 } from './ReactCurrentFiber';
 
-import {onCommitUnmount} from './ReactFiberDevToolsHook.old';
-import {resolveDefaultProps} from './ReactFiberLazyComponent.old';
+import {onCommitUnmount} from './ReactFiberDevToolsHook.new';
+import {resolveDefaultProps} from './ReactFiberLazyComponent.new';
 import {
   isCurrentUpdateNested,
   getCommitTime,
   recordLayoutEffectDuration,
   startLayoutEffectTimer,
   recordPassiveEffectDuration,
   startPassiveEffectTimer,
-} from './ReactProfilerTimer.old';
+} from './ReactProfilerTimer.new';
 import {ProfileMode} from './ReactTypeOfMode';
-import {commitUpdateQueue} from './ReactUpdateQueue.old';
+import {commitUpdateQueue} from './ReactUpdateQueue.new';
 import {
   getPublicInstance,
   supportsMutation,
@@ -134,14 +138,14 @@ import {
   resolveRetryWakeable,
   markCommitTimeOfFallback,
   enqueuePendingPassiveProfilerEffect,
-} from './ReactFiberWorkLoop.old';
+} from './ReactFiberWorkLoop.new';
 import {
   NoFlags as NoHookEffect,
   HasEffect as HookHasEffect,
   Layout as HookLayout,
   Passive as HookPassive,
 } from './ReactHookEffectTags';
-import {didWarnAboutReassigningProps} from './ReactFiberBeginWork.old';
+import {didWarnAboutReassigningProps} from './ReactFiberBeginWork.new';
 import {doesFiberContain} from './ReactFiberTreeReflection';
 
 let didWarnAboutUndefinedSnapshotBeforeUpdate: Set<mixed> | null = null;
@@ -2323,6 +2327,33 @@ function commitPassiveUnmountEffects_begin() {
             detachFiberAfterEffects(alternate);
           }
         }
+
+        if (enableDetachOldChildList) {
+          // A fiber was deleted from this parent fiber, but it's still part of
+          // the previous (alternate) parent fiber's list of children. Because
+          // children are a linked list, an earlier sibling that's still alive
+          // will be connected to the deleted fiber via its `alternate`:
+          //
+          //   live fiber
+          //   --alternate--> previous live fiber
+          //   --sibling--> deleted fiber
+          //
+          // We can't disconnect `alternate` on nodes that haven't been deleted
+          // yet, but we can disconnect the `sibling` and `child` pointers.
+          const previousFiber = fiber.alternate;
+          if (previousFiber !== null) {
+            let detachedChild = previousFiber.child;
+            if (detachedChild !== null) {
+              previousFiber.child = null;
+              do {
+                const detachedSibling = detachedChild.sibling;
+                detachedChild.sibling = null;
+                detachedChild = detachedSibling;
+              } while (detachedChild !== null);
+            }
+          }
+        }
+
         nextEffect = fiber;
       }
     }

packages/shared/ReactFeatureFlags.js

@@ -112,8 +112,12 @@ export const disableNativeComponentFrames = false;
 // If there are no still-mounted boundaries, the errors should be rethrown.
 export const skipUnmountedBoundaries = false;
 
+<<<<<<< current
 // When a node is unmounted, recurse into the Fiber subtree and clean out references.
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 // --------------------------
 // Future APIs to be deprecated

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

@@ -46,7 +46,11 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
+<<<<<<< current
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = true;

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

@@ -45,7 +45,11 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
+<<<<<<< current
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = true;

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

@@ -45,7 +45,11 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
+<<<<<<< current
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = true;

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

@@ -45,7 +45,11 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
+<<<<<<< current
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = true;

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

@@ -45,7 +45,11 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
+<<<<<<< current
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = true;

packages/shared/forks/ReactFeatureFlags.testing.js

@@ -45,7 +45,11 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
+<<<<<<< current
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = true;

packages/shared/forks/ReactFeatureFlags.testing.www.js

@@ -45,7 +45,11 @@ export const enableLegacyFBSupport = !__EXPERIMENTAL__;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = true;
+<<<<<<< current
 export const enableStrongMemoryCleanup = false;
+=======
+export const enableDetachOldChildList = false;
+>>>>>>> patched
 
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = true;

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -52,7 +52,11 @@ export const disableNativeComponentFrames = false;
 export const createRootStrictEffectsByDefault = false;
 export const enableStrictEffects = false;
 export const enableUseRefAccessWarning = __VARIANT__;
+<<<<<<< current
 export const enableStrongMemoryCleanup = __VARIANT__;
+=======
+export const enableDetachOldChildList = __VARIANT__;
+>>>>>>> patched
 
 export const enableProfilerNestedUpdateScheduledHook = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -34,6 +34,7 @@ export const {
   enableSyncMicroTasks,
   enableLazyContextPropagation,
   enableStrongMemoryCleanup,
+  enableDetachOldChildList,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.