[Android] Fix ConcurrentModificationException in InteropUIBlockListener

[jkoutavas]

Summary:

This React Native Android fix hardens the Fabric InteropUiBlockListener against ConcurrentModificationException crashes in willMountItems and didMountItems. By iterating over a shallow copy of the UI‐block lists and catching any mid‐iteration mutations, we ensure the listener never throws during a UI frame and always clears its pending blocks.

The issue was first reported in React Native Issue #49783, and although React Native PR#50091 was closed and not merged, the work in that PR did make it to React Native in commit 17da3cb. However, the fix didn't go far enough. We saw intermittent examples of this exception being thrown when swiping through a carousel from react-native-reanimated-carousel. This fix goes right to the exception site itself.

Changelog:

[ANDROID] [FIXED] - Hardened the Fabric InteropUiBlockListener against ConcurrentModificationException crashes in willMountItems and didMountItems

Test Plan:

The only way to test this is to develop a standalone app that emulates what we've been seeing in our commercially available RN app. We have done extensive testing of before (intermittent crashes) and after the fix (no crashes) and things have been standing up very well for us.

Here is a the Red Box we see right at the time of the crash, before this fix:

And here is the beginning of the logcat crash log:

2025-05-07 16:01:49.212 unknown:BridgelessReact com.aura.suite                       W  ReactHost{0}.handleHostException(message = "null")
2025-05-07 16:01:49.212 unknown:ReactNative     com.aura.suite                       E  Exception in native call
                                                                                        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.internal.interop.InteropUIBlockListener.willMountItems(InteropUiBlockListener.kt:72)
                                                                                        	at com.facebook.react.fabric.FabricUIManager$MountItemDispatchListener.willMountItems(FabricUIManager.java:1235)
                                                                                        	at com.facebook.react.fabric.mounting.MountItemDispatcher.dispatchMountItems(MountItemDispatcher.java:184)
                                                                                        	at com.facebook.react.fabric.mounting.MountItemDispatcher.tryDispatchMountItems(MountItemDispatcher.java:122)
                                                                                        	at com.facebook.react.fabric.FabricUIManager$3.runGuarded(FabricUIManager.java:820)
                                                                                        	at com.facebook.react.bridge.GuardedRunnable.run(GuardedRunnable.java:29)
                                                                                        	at com.facebook.react.fabric.FabricUIManager.scheduleMountItem(FabricUIManager.java:824)
                                                                                        	at com.facebook.react.fabric.FabricUIManagerBinding.reportMount(Native Method)
                                                                                        	at com.facebook.react.fabric.FabricUIManager$MountItemDispatchListener$1.run(FabricUIManager.java:1282)
                                                                                        	at android.os.Handler.handleCallback(Handler.java:959)
                                                                                        	at android.os.Handler.dispatchMessage(Handler.java:100)
                                                                                        	at android.os.Looper.loopOnce(Looper.java:232)
                                                                                        	at android.os.Looper.loop(Looper.java:317)
                                                                                        	at android.app.ActivityThread.main(ActivityThread.java:8592)
                                                                                        	at java.lang.reflect.Method.invoke(Native Method)
                                                                                        	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:580)
                                                                                        	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:878)

All of this goes away with this fix. We're using React Native 0.79.2 and this is the first time we've open a PR for react-native. I hope this is enough info as far as testing goes.

Can we see a 0.79.x release with this fix, please?

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[jkoutavas]

I did a cleanup of the changes and retested on our app. All checked-out okay.

[javache]

Sorry, my previous comment was wrong. Synchronized is a recursive lock in JVM, so you can still have concurrent modification here. The right fix is to do the copy in a synchronized block though.

[jkoutavas]

Sorry, my previous comment was wrong. Synchronized is a recursive lock in JVM, so you can still have concurrent modification here. The right fix is to do the copy in a synchronized block though.

No problem, @javache, I reverted the latest commit.

[javache]

The copying of beforeUiblocks and clearing of it still needs to happen in a synchronized block.

[jkoutavas]

The copying of beforeUiblocks and clearing of it still needs to happen in a synchronized block.

✔️

[facebook-github-bot]

@javache has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[jkoutavas]

@javache thank you so much for helping me get through this PR and being so responsive. It was really unexpectedly wonderful. Having never tried to open a PR against such a big repo before, I didn't know how long this was going to take, so I had expected "awhile."

Now that you've imported it into your own system, can you have this be a fix for not only the main line but also for v0.79.x?

[jkoutavas]

Now that you've imported it into your own system, can you have this be a fix for not only the main line but also for v0.79.x?

Ah, react-native-releases may become my eventual stop for a "Pick Request"?

[facebook-github-bot]

@javache merged this pull request in 48b2dd3.

[react-native-bot]

This pull request was successfully merged by @jkoutavas in 48b2dd3

^{When will my fix make it into a release? | How to file a pick request?}