Defer more field detachments to passive phase

This allows us to use those fields during passive unmount traversal.

Author: acdlite

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

@@ -1069,30 +1069,42 @@ function commitNestedUnmounts(
 }
 
 function detachFiberMutation(fiber: Fiber) {
-  // Cut off the return pointers to disconnect it from the tree. Ideally, we
-  // should clear the child pointer of the parent alternate to let this
+  // Cut off the return pointer to disconnect it from the tree.
+  // This enables us to detect and warn against state updates on an unmounted component.
+  // It also prevents events from bubbling from within disconnected components.
+  //
+  // Ideally, we should also clear the child pointer of the parent alternate to let this
   // get GC:ed but we don't know which for sure which parent is the current
-  // one so we'll settle for GC:ing the subtree of this child. This child
-  // itself will be GC:ed when the parent updates the next time.
-  // Note: we cannot null out sibling here, otherwise it can cause issues
-  // with findDOMNode and how it requires the sibling field to carry out
-  // traversal in a later effect. See PR #16820. We now clear the sibling
-  // field after effects, see: detachFiberAfterEffects.
+  // one so we'll settle for GC:ing the subtree of this child.
+  // This child itself will be GC:ed when the parent updates the next time.
   //
-  // Don't disconnect stateNode now; it will be detached in detachFiberAfterEffects.
-  // It may be required if the current component is an error boundary,
-  // and one of its descendants throws while unmounting a passive effect.
-  fiber.alternate = null;
+  // Note that we can't clear child or sibling pointers yet.
+  // They're needed for passive effects and for findDOMNode.
+  // We defer those fields, and all other cleanup, to the passive phase (see detachFiberAfterEffects).
+  const alternate = fiber.alternate;
+  if (alternate !== null) {
+    alternate.return = null;
+    fiber.alternate = null;
+  }
+  fiber.return = null;
+}
+
+export function detachFiberAfterEffects(fiber: Fiber): void {
+  // Null out fields to improve GC for references that may be lingering (e.g. DevTools).
+  // Note that we already cleared the return pointer in detachFiberMutation().
   fiber.child = null;
   fiber.deletions = null;
   fiber.dependencies = null;
-  fiber.firstEffect = null;
-  fiber.lastEffect = null;
   fiber.memoizedProps = null;
   fiber.memoizedState = null;
   fiber.pendingProps = null;
-  fiber.return = null;
+  fiber.sibling = null;
+  fiber.stateNode = null;
   fiber.updateQueue = null;
+  fiber.nextEffect = null;
+  fiber.firstEffect = null;
+  fiber.lastEffect = null;
+
   if (__DEV__) {
     fiber._debugOwner = null;
   }

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

@@ -121,6 +121,7 @@ import {
   Update,
   PlacementAndUpdate,
   Deletion,
+  ChildDeletion,
   Ref,
   ContentReset,
   Snapshot,
@@ -193,6 +194,7 @@ import {
   commitResetTextContent,
   isSuspenseBoundaryBeingHidden,
   commitPassiveMountEffects,
+  detachFiberAfterEffects,
 } from './ReactFiberCommitWork.new';
 import {enqueueUpdate} from './ReactUpdateQueue.new';
 import {resetContextDependencies} from './ReactFiberNewContext.new';
@@ -2116,13 +2118,21 @@ function commitRootImpl(root, renderPriorityLevel) {
   } else {
     // We are done with the effect chain at this point so let's clear the
     // nextEffect pointers to assist with GC. If we have passive effects, we'll
-    // clear this in flushPassiveEffects.
+    // clear this in flushPassiveEffects
+    // TODO: We should always do this in the passive phase, by scheduling
+    // a passive callback for every deletion.
     nextEffect = firstEffect;
     while (nextEffect !== null) {
       const nextNextEffect = nextEffect.nextEffect;
       nextEffect.nextEffect = null;
-      if (nextEffect.flags & Deletion) {
-        detachFiberAfterEffects(nextEffect);
+      if (nextEffect.flags & ChildDeletion) {
+        const deletions = nextEffect.deletions;
+        if (deletions !== null) {
+          for (let i = 0; i < deletions.length; i++) {
+            const deletion = deletions[i];
+            detachFiberAfterEffects(deletion);
+          }
+        }
       }
       nextEffect = nextNextEffect;
     }
@@ -3687,8 +3697,3 @@ export function act(callback: () => Thenable<mixed>): Thenable<void> {
     };
   }
 }
-
-function detachFiberAfterEffects(fiber: Fiber): void {
-  fiber.sibling = null;
-  fiber.stateNode = null;
-}