From aa75e94831df7ec94ee4cbc4df0de453c486022e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rub=C3=A9n=20Norte?= <rubennorte@meta.com>
Date: Tue, 18 Mar 2025 05:59:50 -0700
Subject: [PATCH] Avoid errors when dispatching mount operations within mount
 hooks (#50091)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50091

Changelog: [internal]

If a library uses mount hooks to perform mount operations, it's possible to get concurrent modifications of the list of pending surface IDs to report.

This fixes that potential error by making a copy of the list before dispatching the mount notifications.

Fixes https://github.com/facebook/react-native/issues/49783.

Reviewed By: javache

Differential Revision: D71387739

fbshipit-source-id: 96c723ef2d6bcc659c4452434b7a4d5af26117ef
---
 .../react/fabric/FabricUIManager.java         | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
index a7389a8b48f..2e8cab11087 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
@@ -173,7 +173,7 @@ public class FabricUIManager
   private final CopyOnWriteArrayList<UIManagerListener> mListeners = new CopyOnWriteArrayList<>();
 
   private boolean mMountNotificationScheduled = false;
-  private final List<Integer> mMountedSurfaceIds = new ArrayList<>();
+  private List<Integer> mSurfaceIdsWithPendingMountNotification = new ArrayList<>();
 
   @ThreadConfined(UI)
   @NonNull
@@ -1250,12 +1250,13 @@ public class FabricUIManager
 
       // Collect surface IDs for all the mount items
       for (MountItem mountItem : mountItems) {
-        if (mountItem != null && !mMountedSurfaceIds.contains(mountItem.getSurfaceId())) {
-          mMountedSurfaceIds.add(mountItem.getSurfaceId());
+        if (mountItem != null
+            && !mSurfaceIdsWithPendingMountNotification.contains(mountItem.getSurfaceId())) {
+          mSurfaceIdsWithPendingMountNotification.add(mountItem.getSurfaceId());
         }
       }
 
-      if (!mMountNotificationScheduled && !mMountedSurfaceIds.isEmpty()) {
+      if (!mMountNotificationScheduled && !mSurfaceIdsWithPendingMountNotification.isEmpty()) {
         mMountNotificationScheduled = true;
 
         // Notify mount when the effects are visible and prevent mount hooks to
@@ -1267,17 +1268,19 @@ public class FabricUIManager
                   public void run() {
                     mMountNotificationScheduled = false;
 
+                    // Create a copy in case mount hooks trigger more mutations
+                    final List<Integer> surfaceIdsToReportMount =
+                        mSurfaceIdsWithPendingMountNotification;
+                    mSurfaceIdsWithPendingMountNotification = new ArrayList<>();
+
                     final @Nullable FabricUIManagerBinding binding = mBinding;
                     if (binding == null || mDestroyed) {
-                      mMountedSurfaceIds.clear();
                       return;
                     }
 
-                    for (int surfaceId : mMountedSurfaceIds) {
+                    for (int surfaceId : surfaceIdsToReportMount) {
                       binding.reportMount(surfaceId);
                     }
-
-                    mMountedSurfaceIds.clear();
                   }
                 });
       }