Decide NativeAnimated backend per-instance instead of live flag

Summary:
## Changelog:

[Internal][Fixed] - Fix EXC_BAD_ACCESS / KERN_INVALID_ADDRESS in C++ Animated when `useSharedAnimatedBackend()` flips across a reused runtime

C++ Animated chose between the legacy and shared-`AnimationBackend` code paths by reading `ReactNativeFeatureFlags::useSharedAnimatedBackend()` live, in many places. That flag is a process-global singleton (`ReactNativeFeatureFlags::accessor_`). On some app the global is reset and re-applied on every user switch (`FBReactModule setUpReactNativeFeatureFlags` -> `dangerouslyReset()`/`override()`), and the previous user's runtime is kept alive and reused. The shared `AnimationBackend` is attached only once, when an instance's `Scheduler` is constructed, gated on the flag at that moment. On a multi-account device the global flag could therefore read true on a reused instance whose backend was never attached, so `getOrCreate` took the shared path and dereferenced a null backend. It also let JS and C++ disagree, since JS caches the flag per runtime while C++ followed the mutated global.

Fix: make the per-instance decision once and use it everywhere instead of the live flag.

- `NativeAnimatedNodesManagerProvider::getOrCreate` selects the path by whether the shared `AnimationBackend` actually exists for this instance (`unstable_getAnimationBackend().lock() != nullptr`).
- `NativeAnimatedNodesManager` stores a `const bool useSharedAnimatedBackend_`, latched in its constructor (true for the shared-backend ctor, false for the legacy ctor), and exposes it via `useSharedAnimatedBackend()`. All internal reads now use the member.
- `PropsAnimatedNode` reads the decision through `manager_->useSharedAnimatedBackend()`.

The attach side (`Scheduler`) is intentionally unchanged: it remains the single construction-time read that latches the per-instance decision the rest of the code now follows. This keeps JS and C++ consistent across global flag flips and removes the null dereference.

Reviewed By: sbuggay

Differential Revision: D108428720

Author: zeyap

packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManager.cpp

@@ -73,7 +73,8 @@ NativeAnimatedNodesManager::NativeAnimatedNodesManager(
     StartOnRenderCallback&& startOnRenderCallback,
     StopOnRenderCallback&& stopOnRenderCallback,
     FrameRateListenerCallback&& frameRateListenerCallback) noexcept
-    : directManipulationCallback_(std::move(directManipulationCallback)),
+    : useSharedAnimatedBackend_(false),
+      directManipulationCallback_(std::move(directManipulationCallback)),
       fabricCommitCallback_(std::move(fabricCommitCallback)),
       resolvePlatformColor_(std::move(resolvePlatformColor)),
       startOnRenderCallback_(std::move(startOnRenderCallback)),
@@ -97,7 +98,7 @@ NativeAnimatedNodesManager::NativeAnimatedNodesManager(
 
 NativeAnimatedNodesManager::NativeAnimatedNodesManager(
     std::shared_ptr<UIManagerAnimationBackend> animationBackend) noexcept
-    : animationBackend_(animationBackend) {}
+    : animationBackend_(animationBackend), useSharedAnimatedBackend_(true) {}
 
 NativeAnimatedNodesManager::~NativeAnimatedNodesManager() noexcept {
   stopRenderCallbackIfNeeded(true);
@@ -256,7 +257,7 @@ void NativeAnimatedNodesManager::disconnectAnimatedNodeFromView(
   auto node = getAnimatedNode<PropsAnimatedNode>(propsNodeTag);
   if (node != nullptr) {
     node->disconnectFromView(viewTag);
-    if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+    if (useSharedAnimatedBackend_) {
       node->disconnectFromShadowNodeFamily();
     }
     {
@@ -521,7 +522,7 @@ void NativeAnimatedNodesManager::handleAnimatedEvent(
     // That's why, in case this is called from the UI thread, we need to
     // proactivelly trigger the animation loop to avoid showing stale
     // frames.
-    if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+    if (useSharedAnimatedBackend_) {
       if (auto animationBackend = animationBackend_.lock()) {
         animationBackend->pushAnimationMutations(
             [this](AnimationTimestamp timestamp) -> AnimationMutations {
@@ -561,7 +562,7 @@ void NativeAnimatedNodesManager::startRenderCallbackIfNeeded(bool isAsync) {
     return;
   }
 
-  if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+  if (useSharedAnimatedBackend_) {
     if (auto animationBackend = animationBackend_.lock()) {
       auto weak = weak_from_this();
       animationBackendCallbackId_ = animationBackend->start(
@@ -589,7 +590,7 @@ void NativeAnimatedNodesManager::stopRenderCallbackIfNeeded(
   // stopRenderCallbackIfNeeded is always called from the UI thread.
   auto isRenderCallbackStarted = isRenderCallbackStarted_.exchange(false);
 
-  if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+  if (useSharedAnimatedBackend_) {
     if (isRenderCallbackStarted) {
       if (auto animationBackend = animationBackend_.lock()) {
         animationBackend->stop(animationBackendCallbackId_);
@@ -922,7 +923,7 @@ void NativeAnimatedNodesManager::schedulePropsCommit(
     bool layoutStyleUpdated,
     bool forceFabricCommit,
     ShadowNodeFamily::Weak shadowNodeFamily) noexcept {
-  if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+  if (useSharedAnimatedBackend_) {
     if (forceFabricCommit) {
       shouldRequestAsyncFlush_.insert(viewTag);
     }
@@ -1029,7 +1030,7 @@ AnimationMutations NativeAnimatedNodesManager::onAnimationFrameForBackend(
 
 AnimationMutations NativeAnimatedNodesManager::pullAnimationMutations(
     AnimationTimestamp timestamp) {
-  if (!ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+  if (!useSharedAnimatedBackend_) {
     return {};
   }
   TraceSection s(
@@ -1119,7 +1120,7 @@ void NativeAnimatedNodesManager::flushAnimatedNodesCreatedAsync() noexcept {
 }
 
 void NativeAnimatedNodesManager::onRender() {
-  if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+  if (useSharedAnimatedBackend_) {
     return;
   }
   TraceSection s(

packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManager.h

@@ -78,6 +78,14 @@ class NativeAnimatedNodesManager : public std::enable_shared_from_this<NativeAni
   NativeAnimatedNodesManager(NativeAnimatedNodesManager &&) = delete;
   NativeAnimatedNodesManager &operator=(NativeAnimatedNodesManager &&) = delete;
 
+  // Whether this instance was constructed to use the shared AnimationBackend.
+  // Latched at construction, so it is immune to later flips of the
+  // process-global useSharedAnimatedBackend() flag.
+  bool useSharedAnimatedBackend() const noexcept
+  {
+    return useSharedAnimatedBackend_;
+  }
+
   template <typename T, typename = std::enable_if_t<std::is_base_of_v<AnimatedNode, T>>>
   T *getAnimatedNode(Tag tag) const
     requires(std::is_base_of_v<AnimatedNode, T>)
@@ -219,6 +227,12 @@ class NativeAnimatedNodesManager : public std::enable_shared_from_this<NativeAni
 
   std::weak_ptr<UIManagerAnimationBackend> animationBackend_;
 
+  // Latched per-instance copy of which backend this manager uses, set from the
+  // constructor that ran (true for the shared-AnimationBackend ctor). Reads stay
+  // stable even when the global useSharedAnimatedBackend() flag is re-overridden
+  // on another RN runtime.
+  const bool useSharedAnimatedBackend_;
+
   std::unique_ptr<AnimatedNode> animatedNode(Tag tag, const folly::dynamic &config) noexcept;
 
   static thread_local bool isOnRenderThread_;

packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManagerProvider.cpp

@@ -8,7 +8,6 @@
 #include "NativeAnimatedNodesManagerProvider.h"
 
 #include <glog/logging.h>
-#include <react/featureflags/ReactNativeFeatureFlags.h>
 #include <react/renderer/animated/MergedValueDispatcher.h>
 #include <react/renderer/animated/internal/AnimatedMountingOverrideDelegate.h>
 #include <react/renderer/animated/internal/primitives.h>
@@ -51,8 +50,9 @@ NativeAnimatedNodesManagerProvider::getOrCreate(
 
   auto* uiManager = &UIManagerBinding::getBinding(runtime)->getUIManager();
 
-  if (!ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
-    // === PATH 1: Legacy Backend (useSharedAnimatedBackend = false) ===
+  auto animationBackend = uiManager->unstable_getAnimationBackend().lock();
+  if (animationBackend == nullptr) {
+    // === PATH 1: Legacy Backend (no shared AnimationBackend attached) ===
     // Uses the architecture with MergedValueDispatcher and
     // AnimatedMountingOverrideDelegate
 
@@ -130,13 +130,10 @@ NativeAnimatedNodesManagerProvider::getOrCreate(
 
     uiManager->setNativeAnimatedDelegate(nativeAnimatedDelegate_);
   } else {
-    // === PATH 2: Shared AnimationBackend (useSharedAnimatedBackend = true) ===
+    // === PATH 2: Shared AnimationBackend ===
     // Uses the shared AnimationBackend from UIManager. The backend handles all
-    // animation commits and platform integration internally.
-
-    auto animationBackend = uiManager->unstable_getAnimationBackend().lock();
-    react_native_assert(
-        animationBackend != nullptr && "animationBackend is nullptr");
+    // animation commits and platform integration internally. It is guaranteed
+    // non-null here because it was successfully locked above.
     animationBackend->registerJSInvoker(jsInvoker);
 
     nativeAnimatedNodesManager_ =

packages/react-native/ReactCommon/react/renderer/animated/nodes/PropsAnimatedNode.cpp

@@ -12,7 +12,6 @@
 #include "PropsAnimatedNode.h"
 
 #include <react/debug/react_native_assert.h>
-#include <react/featureflags/ReactNativeFeatureFlags.h>
 #include <react/renderer/animated/NativeAnimatedNodesManager.h>
 #include <react/renderer/animated/nodes/ColorAnimatedNode.h>
 #include <react/renderer/animated/nodes/ObjectAnimatedNode.h>
@@ -84,7 +83,7 @@ void PropsAnimatedNode::disconnectFromView(Tag viewTag) {
 void PropsAnimatedNode::restoreDefaultValues() {
   // If node is already disconnected from View, we cannot restore default values
   if (connectedViewTag_ != animated::undefinedAnimatedNodeIdentifier) {
-    if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+    if (manager_->useSharedAnimatedBackend()) {
       manager_->schedulePropsCommit(
           connectedViewTag_,
           folly::dynamic::object(),
@@ -166,7 +165,7 @@ void PropsAnimatedNode::update(bool forceFabricCommit) {
 
   layoutStyleUpdated_ = isLayoutStyleUpdated(getConfig()["props"], *manager_);
 
-  if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+  if (manager_->useSharedAnimatedBackend()) {
     manager_->schedulePropsCommit(
         connectedViewTag_,
         props_,