[Android] ConcurrentModificationException in FabricUIManager

[kagod-dev]

Description

In this commit you have changed the flag mMountNotificationScheduled from AtomicBoolean to boolean

-  @NonNull private final AtomicBoolean mMountNotificationScheduled = new AtomicBoolean(false);
+  private boolean mMountNotificationScheduled = false;

but the line changing its value to true was removed

-      boolean mountNotificationScheduled = mMountNotificationScheduled.getAndSet(true);

so now the flag is always false.
This is probably the reason of the ConcurrentModificationException.

Steps to reproduce

Couldn't provide minimum steps. It happens when animating the image to slide in. Reanimated version: 3.16.1

React Native Version

0.76.7

Affected Platforms

Runtime - Android

Areas

Fabric - The New Renderer

Output of npx @react-native-community/cli info

System:
  OS: macOS 15.3.1
  CPU: (10) arm64 Apple M1 Pro
  Memory: 158.77 MB / 32.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.12.0
    path: ~/.volta/tools/image/node/22.12.0/bin/node
  Yarn:
    version: 1.22.22
  npm:
    version: 10.9.0
    path: ~/.volta/tools/image/node/22.12.0/bin/npm
  Watchman:
    version: 2025.02.17.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.0
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 24.1
      - iOS 18.1
      - macOS 15.1
      - tvOS 18.1
      - visionOS 2.1
      - watchOS 11.1
  Android SDK:
    API Levels:
      - "33"
      - "34"
      - "35"
    Build Tools:
      - 33.0.1
      - 34.0.0
      - 35.0.0
      - 35.0.1
      - 36.0.0
    System Images:
      - android-31 | Google Play ARM 64 v8a
      - android-34 | Google Play ARM 64 v8a
      - android-35 | Google APIs ARM 64 v8a
      - android-35 | Google Play ARM 64 v8a
    Android NDK: 26.3.11579264
IDEs:
  Android Studio: 2024.2 AI-242.23726.103.2422.12816248
  Xcode:
    version: 16.1/16B40
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.13
    path: /usr/bin/javac
  Ruby:
    version: 3.1.0
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 18.3.1
    wanted: 18.3.1
  react-native:
    installed: 0.76.7
    wanted: 0.76.7
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: false

Stacktrace or Logs

E  FATAL EXCEPTION: main
java.util.ConcurrentModificationException                                                                                                    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1111)
        at java.util.ArrayList$Itr.next(ArrayList.java:1064)
        at com.facebook.react.fabric.FabricUIManager$MountItemDispatchListener$1.run(FabricUIManager.java:1281)
        at android.os.Handler.handleCallback(Handler.java:958)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loopOnce(Looper.java:230)
        at android.os.Looper.loop(Looper.java:319)
        at android.app.ActivityThread.main(ActivityThread.java:8919)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:578)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1103)

Reproducer

https://github.com

Screenshots and Videos

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[react-native-bot]

Warning

Missing reproducer: We could not detect a reproducible example in your issue report. Please provide either:

• If your bug is UI related: a Snack
• If your bug is build/upgrade related: a project using our Reproducer Template
• Otherwise send us a Pull Request with the RNTesterPlayground.js edited to reproduce your bug.

[cortinico]

cc @rubennorte

[kagod-dev]

hey @rubennorte, unfortunately I didn't manage to create a reproducible example.

[rubennorte]

Hi @kagod-dev! Thanks for reporting this. I created #50090 to add the flag back.

The only case I was thinking could cause this is if you're making mutations in mount hooks, which has nothing to do with that PR (my intuition tells me the PR only avoids unnecessary tasks, but it's functionally the same).

I'll put up another PR to avoid it even in that case (edit: it's #50091 ).