[cleanup] remove deletedTreeCleanUpLevel feature flag (#25529)

I noticed this was an experiment concluded 16 months ago (#21679) that
this extra work is beneficial
to break up cycles leaking memory in product code.

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -46,7 +46,6 @@ import {
   enableSchedulingProfiler,
   enableSuspenseCallback,
   enableScopeAPI,
-  deletedTreeCleanUpLevel,
   enableUpdaterTracking,
   enableCache,
   enableTransitionTracing,
@@ -1658,74 +1657,43 @@ function detachFiberAfterEffects(fiber: Fiber) {
     detachFiberAfterEffects(alternate);
   }
 
-  // Note: Defensively using negation instead of < in case
-  // `deletedTreeCleanUpLevel` is undefined.
-  if (!(deletedTreeCleanUpLevel >= 2)) {
-    // This is the default branch (level 0).
-    fiber.child = null;
-    fiber.deletions = null;
-    fiber.dependencies = null;
-    fiber.memoizedProps = null;
-    fiber.memoizedState = null;
-    fiber.pendingProps = null;
-    fiber.sibling = null;
-    fiber.stateNode = null;
-    fiber.updateQueue = null;
+  // Clear cyclical Fiber fields. This level alone is designed to roughly
+  // approximate the planned Fiber refactor. In that world, `setState` will be
+  // bound to a special "instance" object instead of a Fiber. The Instance
+  // object will not have any of these fields. It will only be connected to
+  // the fiber tree via a single link at the root. So if this level alone is
+  // sufficient to fix memory issues, that bodes well for our plans.
+  fiber.child = null;
+  fiber.deletions = null;
+  fiber.sibling = null;
 
-    if (__DEV__) {
-      fiber._debugOwner = null;
-    }
-  } else {
-    // Clear cyclical Fiber fields. This level alone is designed to roughly
-    // approximate the planned Fiber refactor. In that world, `setState` will be
-    // bound to a special "instance" object instead of a Fiber. The Instance
-    // object will not have any of these fields. It will only be connected to
-    // the fiber tree via a single link at the root. So if this level alone is
-    // sufficient to fix memory issues, that bodes well for our plans.
-    fiber.child = null;
-    fiber.deletions = null;
-    fiber.sibling = null;
-
-    // The `stateNode` is cyclical because on host nodes it points to the host
-    // tree, which has its own pointers to children, parents, and siblings.
-    // The other host nodes also point back to fibers, so we should detach that
-    // one, too.
-    if (fiber.tag === HostComponent) {
-      const hostInstance: Instance = fiber.stateNode;
-      if (hostInstance !== null) {
-        detachDeletedInstance(hostInstance);
-      }
-    }
-    fiber.stateNode = null;
-
-    // I'm intentionally not clearing the `return` field in this level. We
-    // already disconnect the `return` pointer at the root of the deleted
-    // subtree (in `detachFiberMutation`). Besides, `return` by itself is not
-    // cyclical — it's only cyclical when combined with `child`, `sibling`, and
-    // `alternate`. But we'll clear it in the next level anyway, just in case.
-
-    if (__DEV__) {
-      fiber._debugOwner = null;
-    }
-
-    if (deletedTreeCleanUpLevel >= 3) {
-      // Theoretically, nothing in here should be necessary, because we already
-      // disconnected the fiber from the tree. So even if something leaks this
-      // particular fiber, it won't leak anything else
-      //
-      // The purpose of this branch is to be super aggressive so we can measure
-      // if there's any difference in memory impact. If there is, that could
-      // indicate a React leak we don't know about.
-      fiber.return = null;
-      fiber.dependencies = null;
-      fiber.memoizedProps = null;
-      fiber.memoizedState = null;
-      fiber.pendingProps = null;
-      fiber.stateNode = null;
-      // TODO: Move to `commitPassiveUnmountInsideDeletedTreeOnFiber` instead.
-      fiber.updateQueue = null;
+  // The `stateNode` is cyclical because on host nodes it points to the host
+  // tree, which has its own pointers to children, parents, and siblings.
+  // The other host nodes also point back to fibers, so we should detach that
+  // one, too.
+  if (fiber.tag === HostComponent) {
+    const hostInstance: Instance = fiber.stateNode;
+    if (hostInstance !== null) {
+      detachDeletedInstance(hostInstance);
     }
   }
+  fiber.stateNode = null;
+
+  if (__DEV__) {
+    fiber._debugOwner = null;
+  }
+
+  // Theoretically, nothing in here should be necessary, because we already
+  // disconnected the fiber from the tree. So even if something leaks this
+  // particular fiber, it won't leak anything else.
+  fiber.return = null;
+  fiber.dependencies = null;
+  fiber.memoizedProps = null;
+  fiber.memoizedState = null;
+  fiber.pendingProps = null;
+  fiber.stateNode = null;
+  // TODO: Move to `commitPassiveUnmountInsideDeletedTreeOnFiber` instead.
+  fiber.updateQueue = null;
 }
 
 function emptyPortalContainer(current: Fiber) {
@@ -3986,31 +3954,29 @@ export function commitPassiveUnmountEffects(finishedWork: Fiber): void {
 }
 
 function detachAlternateSiblings(parentFiber: Fiber) {
-  if (deletedTreeCleanUpLevel >= 1) {
-    // 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.
+  // 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 = parentFiber.alternate;
-    if (previousFiber !== null) {
-      let detachedChild = previousFiber.child;
-      if (detachedChild !== null) {
-        previousFiber.child = null;
-        do {
-          // $FlowFixMe[incompatible-use] found when upgrading Flow
-          const detachedSibling = detachedChild.sibling;
-          // $FlowFixMe[incompatible-use] found when upgrading Flow
-          detachedChild.sibling = null;
-          detachedChild = detachedSibling;
-        } while (detachedChild !== null);
-      }
+  const previousFiber = parentFiber.alternate;
+  if (previousFiber !== null) {
+    let detachedChild = previousFiber.child;
+    if (detachedChild !== null) {
+      previousFiber.child = null;
+      do {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
+        const detachedSibling = detachedChild.sibling;
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
+        detachedChild.sibling = null;
+        detachedChild = detachedSibling;
+      } while (detachedChild !== null);
     }
   }
 }
@@ -4196,8 +4162,7 @@ function commitPassiveUnmountEffectsInsideOfDeletedTree_begin(
     resetCurrentDebugFiberInDEV();
 
     const child = fiber.child;
-    // TODO: Only traverse subtree if it has a PassiveStatic flag. (But, if we
-    // do this, still need to handle `deletedTreeCleanUpLevel` correctly.)
+    // TODO: Only traverse subtree if it has a PassiveStatic flag.
     if (child !== null) {
       child.return = fiber;
       nextEffect = child;
@@ -4217,23 +4182,13 @@ function commitPassiveUnmountEffectsInsideOfDeletedTree_complete(
     const sibling = fiber.sibling;
     const returnFiber = fiber.return;
 
-    if (deletedTreeCleanUpLevel >= 2) {
-      // Recursively traverse the entire deleted tree and clean up fiber fields.
-      // This is more aggressive than ideal, and the long term goal is to only
-      // have to detach the deleted tree at the root.
-      detachFiberAfterEffects(fiber);
-      if (fiber === deletedSubtreeRoot) {
-        nextEffect = null;
-        return;
-      }
-    } else {
-      // This is the default branch (level 0). We do not recursively clear all
-      // the fiber fields. Only the root of the deleted subtree.
-      if (fiber === deletedSubtreeRoot) {
-        detachFiberAfterEffects(fiber);
-        nextEffect = null;
-        return;
-      }
+    // Recursively traverse the entire deleted tree and clean up fiber fields.
+    // This is more aggressive than ideal, and the long term goal is to only
+    // have to detach the deleted tree at the root.
+    detachFiberAfterEffects(fiber);
+    if (fiber === deletedSubtreeRoot) {
+      nextEffect = null;
+      return;
     }
 
     if (sibling !== null) {

packages/shared/ReactFeatureFlags.js

@@ -101,19 +101,6 @@ export const enableHostSingletons = true;
 
 export const enableFloat = true;
 
-// When a node is unmounted, recurse into the Fiber subtree and clean out
-// references. Each level cleans up more fiber fields than the previous level.
-// As far as we know, React itself doesn't leak, but because the Fiber contains
-// cycles, even a single leak in product code can cause us to retain large
-// amounts of memory.
-//
-// The long term plan is to remove the cycles, but in the meantime, we clear
-// additional fields to mitigate.
-//
-// It's an enum so that we can experiment with different levels of
-// aggressiveness.
-export const deletedTreeCleanUpLevel = 3;
-
 export const enableUseHook = true;
 
 // Enables unstable_useMemoCache hook, intended as a compilation target for

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

@@ -59,7 +59,6 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
-export const deletedTreeCleanUpLevel = 3;
 export const enableGetInspectorDataForInstanceInProduction = true;
 export const deferRenderPhaseUpdateToNextBatch = false;
 

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

@@ -49,7 +49,6 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
-export const deletedTreeCleanUpLevel = 3;
 export const enableGetInspectorDataForInstanceInProduction = false;
 export const deferRenderPhaseUpdateToNextBatch = false;
 

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

@@ -49,7 +49,6 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
-export const deletedTreeCleanUpLevel = 3;
 export const enableGetInspectorDataForInstanceInProduction = false;
 export const deferRenderPhaseUpdateToNextBatch = false;
 

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

@@ -41,7 +41,6 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
-export const deletedTreeCleanUpLevel = 3;
 export const enableGetInspectorDataForInstanceInProduction = false;
 export const deferRenderPhaseUpdateToNextBatch = false;
 export const enableSuspenseAvoidThisFallback = false;

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

@@ -49,7 +49,6 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
-export const deletedTreeCleanUpLevel = 3;
 export const enableGetInspectorDataForInstanceInProduction = false;
 export const deferRenderPhaseUpdateToNextBatch = false;
 

packages/shared/forks/ReactFeatureFlags.testing.js

@@ -49,7 +49,6 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = false;
-export const deletedTreeCleanUpLevel = 3;
 export const enableGetInspectorDataForInstanceInProduction = false;
 export const deferRenderPhaseUpdateToNextBatch = false;
 

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

@@ -49,7 +49,6 @@ export const enableLegacyFBSupport = !__EXPERIMENTAL__;
 export const enableFilterEmptyStringAttributesDOM = false;
 export const disableNativeComponentFrames = false;
 export const skipUnmountedBoundaries = true;
-export const deletedTreeCleanUpLevel = 3;
 export const enableGetInspectorDataForInstanceInProduction = false;
 export const deferRenderPhaseUpdateToNextBatch = false;
 

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

@@ -19,7 +19,6 @@ export const enableFilterEmptyStringAttributesDOM = __VARIANT__;
 export const enableLegacyFBSupport = __VARIANT__;
 export const skipUnmountedBoundaries = __VARIANT__;
 export const enableUseRefAccessWarning = __VARIANT__;
-export const deletedTreeCleanUpLevel: number = __VARIANT__ ? 3 : 1;
 export const enableProfilerNestedUpdateScheduledHook = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
 export const enableLazyContextPropagation = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -95,8 +95,6 @@ export const disableTextareaChildren = __EXPERIMENTAL__;
 
 export const allowConcurrentByDefault = true;
 
-export const deletedTreeCleanUpLevel = 3;
-
 export const consoleManagedByDevToolsDuringStrictMode = true;
 export const enableServerContext = true;