Fix: Move destroy field to shared instance object

This fixes the "double free" bug illustrated by the regression test
added in the previous commit.

The underlying issue is that `effect.destroy` field is a mutable field
but we read it during render. This is a concurrency bug — if we had a
borrow checker, it would not allow this.

It's rare in practice today because the field is updated during the
commit phase, which takes a lock on the fiber tree until all the effects
have fired. But it's still theoretically wrong because you can have
multiple Fiber copies each with their own reference to a single
destroy function, and indeed we discovered in production a scenario
where this happens via our current APIs.

In the future these types of scenarios will be much more common because
we will introduce features where effects may run concurrently with the
render phase — i.e. an imperative `hide` method that synchronously hides
a React tree and unmounts all its effects without entering the render
phase, and without interrupting a render phase that's already
in progress.

A future version of React may also be able to run the entire commit
phase concurrently with a subsequent render phase. We can't do this now
because our data structures are not fully thread safe (see: the Fiber
alternate model) but we should be able to do this in the future.

The fix I've introduced in this commit is to move the `destroy` field to
a separate object. The effect "instance" is a shared object that remains
the same for the entire lifetime of an effect. In Rust terms, a RefCell.
The field is `undefined` if the effect is unmounted, or if the effect
ran but is not stateful. We don't explicitly track whether the effect is
mounted or unmounted because that can be inferred by the hiddenness of
the fiber in the tree, i.e. whether there is a hidden Offscreen fiber
above it.

It's unfortunate that this is stored on a separate object, because it
adds more memory per effect instance, but it's conceptually sound. I
think there's likely a better data structure we could use for effects;
perhaps just one array of effect instances per fiber. But I think this
is OK for now despite the additional memory and we can follow up with
performance optimizations later.

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -592,9 +592,10 @@ function commitHookEffectListUnmount(
     do {
       if ((effect.tag & flags) === flags) {
         // Unmount
-        const destroy = effect.destroy;
-        effect.destroy = undefined;
+        const inst = effect.inst;
+        const destroy = inst.destroy;
         if (destroy !== undefined) {
+          inst.destroy = undefined;
           if (enableSchedulingProfiler) {
             if ((flags & HookPassive) !== NoHookEffect) {
               markComponentPassiveEffectUnmountStarted(finishedWork);
@@ -653,7 +654,9 @@ function commitHookEffectListMount(flags: HookFlags, finishedWork: Fiber) {
             setIsRunningInsertionEffect(true);
           }
         }
-        effect.destroy = create();
+        const inst = effect.inst;
+        const destroy = create();
+        inst.destroy = destroy;
         if (__DEV__) {
           if ((flags & HookInsertion) !== NoHookEffect) {
             setIsRunningInsertionEffect(false);
@@ -669,7 +672,6 @@ function commitHookEffectListMount(flags: HookFlags, finishedWork: Fiber) {
         }
 
         if (__DEV__) {
-          const destroy = effect.destroy;
           if (destroy !== undefined && typeof destroy !== 'function') {
             let hookName;
             if ((effect.tag & HookLayout) !== NoFlags) {
@@ -2190,9 +2192,12 @@ function commitDeletionEffectsOnFiber(
 
             let effect = firstEffect;
             do {
-              const {destroy, tag} = effect;
+              const tag = effect.tag;
+              const inst = effect.inst;
+              const destroy = inst.destroy;
               if (destroy !== undefined) {
                 if ((tag & HookInsertion) !== NoHookEffect) {
+                  inst.destroy = undefined;
                   safelyCallDestroy(
                     deletedFiber,
                     nearestMountedAncestor,
@@ -2205,13 +2210,15 @@ function commitDeletionEffectsOnFiber(
 
                   if (shouldProfile(deletedFiber)) {
                     startLayoutEffectTimer();
+                    inst.destroy = undefined;
                     safelyCallDestroy(
                       deletedFiber,
                       nearestMountedAncestor,
                       destroy,
                     );
                     recordLayoutEffectDuration(deletedFiber);
                   } else {
+                    inst.destroy = undefined;
                     safelyCallDestroy(
                       deletedFiber,
                       nearestMountedAncestor,

packages/react-reconciler/src/ReactFiberHooks.js

@@ -180,11 +180,29 @@ export type Hook = {
   next: Hook | null,
 };
 
+// The effect "instance" is a shared object that remains the same for the entire
+// lifetime of an effect. In Rust terms, a RefCell. We use it to store the
+// "destroy" function that is returned from an effect, because that is stateful.
+// The field is `undefined` if the effect is unmounted, or if the effect ran
+// but is not stateful. We don't explicitly track whether the effect is mounted
+// or unmounted because that can be inferred by the hiddenness of the fiber in
+// the tree, i.e. whether there is a hidden Offscreen fiber above it.
+//
+// It's unfortunate that this is stored on a separate object, because it adds
+// more memory per effect instance, but it's conceptually sound. I think there's
+// likely a better data structure we could use for effects; perhaps just one
+// array of effect instances per fiber. But I think this is OK for now despite
+// the additional memory and we can follow up with performance
+// optimizations later.
+type EffectInstance = {
+  destroy: void | (() => void),
+};
+
 export type Effect = {
   tag: HookFlags,
   create: () => (() => void) | void,
-  destroy: (() => void) | void,
-  deps: Array<mixed> | void | null,
+  inst: EffectInstance,
+  deps: Array<mixed> | null,
   next: Effect,
 };
 
@@ -1662,7 +1680,7 @@ function mountSyncExternalStore<T>(
   pushEffect(
     HookHasEffect | HookPassive,
     updateStoreInstance.bind(null, fiber, inst, nextSnapshot, getSnapshot),
-    undefined,
+    createEffectInstance(),
     null,
   );
 
@@ -1719,7 +1737,7 @@ function updateSyncExternalStore<T>(
     pushEffect(
       HookHasEffect | HookPassive,
       updateStoreInstance.bind(null, fiber, inst, nextSnapshot, getSnapshot),
-      undefined,
+      createEffectInstance(),
       null,
     );
 
@@ -1860,13 +1878,13 @@ function rerenderState<S>(
 function pushEffect(
   tag: HookFlags,
   create: () => (() => void) | void,
-  destroy: (() => void) | void,
-  deps: Array<mixed> | void | null,
+  inst: EffectInstance,
+  deps: Array<mixed> | null,
 ): Effect {
   const effect: Effect = {
     tag,
     create,
-    destroy,
+    inst,
     deps,
     // Circular
     next: (null: any),
@@ -1891,6 +1909,10 @@ function pushEffect(
   return effect;
 }
 
+function createEffectInstance(): EffectInstance {
+  return {destroy: undefined};
+}
+
 let stackContainsErrorMessage: boolean | null = null;
 
 function getCallerStackFrame(): string {
@@ -1994,7 +2016,7 @@ function mountEffectImpl(
   hook.memoizedState = pushEffect(
     HookHasEffect | hookFlags,
     create,
-    undefined,
+    createEffectInstance(),
     nextDeps,
   );
 }
@@ -2007,16 +2029,16 @@ function updateEffectImpl(
 ): void {
   const hook = updateWorkInProgressHook();
   const nextDeps = deps === undefined ? null : deps;
-  let destroy = undefined;
+  const effect: Effect = hook.memoizedState;
+  const inst = effect.inst;
 
   // currentHook is null when rerendering after a render phase state update.
   if (currentHook !== null) {
-    const prevEffect = currentHook.memoizedState;
-    destroy = prevEffect.destroy;
     if (nextDeps !== null) {
+      const prevEffect: Effect = currentHook.memoizedState;
       const prevDeps = prevEffect.deps;
       if (areHookInputsEqual(nextDeps, prevDeps)) {
-        hook.memoizedState = pushEffect(hookFlags, create, destroy, nextDeps);
+        hook.memoizedState = pushEffect(hookFlags, create, inst, nextDeps);
         return;
       }
     }
@@ -2027,7 +2049,7 @@ function updateEffectImpl(
   hook.memoizedState = pushEffect(
     HookHasEffect | hookFlags,
     create,
-    destroy,
+    inst,
     nextDeps,
   );
 }