IOOB exception when using DiffUtil on separate thread

About this issue

When using FastAdapterDiffUtil.calculateDiff() on any thread different from Main, there is a chance of IndexOutOfBoundsException being thrown. It is reproducible on modified DiffUtilActivity from FastAdapter's sample: https://github.com/boguszpawlowski/FastAdapter/blob/diffutil_bug/app/src/main/java/com/mikepenz/fastadapter/app/DiffUtilActivity.kt Steps to reproduce:

• Open DiffUtil Screen
• Start scrolling
• After hitting a specific time window, a crash will happen:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.mikepenz.fastadapter.app, PID: 2075
    java.lang.IndexOutOfBoundsException: Index: 26, Size: 19
        at java.util.ArrayList.get(ArrayList.java:437)
        at com.mikepenz.fastadapter.utils.DefaultItemListImpl.get(DefaultItemListImpl.kt:23)
        at com.mikepenz.fastadapter.adapters.ModelAdapter.getAdapterItem(ModelAdapter.kt:168)
        at com.mikepenz.fastadapter.FastAdapter.getItem(FastAdapter.kt:507)
        at com.mikepenz.fastadapter.FastAdapter.getItemViewType(FastAdapter.kt:581)
        at androidx.recyclerview.widget.RecyclerView$Recycler.validateViewHolderForOffsetPosition(RecyclerView.java:5978)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6158)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6118)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6114)
        at androidx.recyclerview.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2303)
        at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1627)
        at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1587)
        at androidx.recyclerview.widget.LinearLayoutManager.scrollBy(LinearLayoutManager.java:1391)
        at androidx.recyclerview.widget.LinearLayoutManager.scrollVerticallyBy(LinearLayoutManager.java:1128)
        at androidx.recyclerview.widget.RecyclerView.scrollStep(RecyclerView.java:1841)
        at androidx.recyclerview.widget.RecyclerView$ViewFlinger.run(RecyclerView.java:5302)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1031)
        at android.view.Choreographer.doCallbacks(Choreographer.java:854)
        at android.view.Choreographer.doFrame(Choreographer.java:785)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1016)
        at android.os.Handler.handleCallback(Handler.java:914)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:224)
        at android.app.ActivityThread.main(ActivityThread.java:7560)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

Observations:

• Error only occurs when diff calculation is made on thread different from Main
• Error only occurs when you are interacting with a screen in the same time (e.g. scrolling) and large number of items is removed.

My conclusion: If diff calculation is happening at the same time as some changes on the screen (user interaction, change in view hierarchy), and is done concurrently, due to item list modified and read on different threads, crash can happen.

Details

• Used library version - 5.3.5
• Used support library version - N/A
• Used gradle build tools version - 4.1.2
• Used tooling / Android Studio version - Arctic Fox 2021 Canary 4
• Other used libraries, potential conflicting libraries - N/A

Checklist

• Searched for similar issues
• Checked out the sample application
• Read the README

Similar/Connected issues: https://github.com/mikepenz/FastAdapter/issues/772 - potential solution https://github.com/mikepenz/FastAdapter/issues/959 - key difference is that I'm NOT using Objects.hash for creating id, they are 100% stable and unique.