[epoxy-paging] IndexOutOfBoundsException

[denis-bezrukov]

We encountered the following crash in our app:

java.lang.IndexOutOfBoundsException: Index: 81, Size: 81
    at androidx.paging.PagedStorage.get(PagedStorage.java:138)
    at androidx.paging.PagedList.get(PagedList.java:410)
    at com.airbnb.epoxy.paging.PagedListModelCache.getModels(PagedListModelCache.kt:200)
    at com.airbnb.epoxy.paging.PagedListEpoxyController.buildModels(PagedListEpoxyController.kt:74)
    at com.airbnb.epoxy.EpoxyController$1.run(EpoxyController.java:276)
    at android.os.Handler.handleCallback(Handler.java:815)
    at android.os.Handler.dispatchMessage(Handler.java:104)
    at android.os.Looper.loop(Looper.java:207)
    at android.os.HandlerThread.run(HandlerThread.java:61)

After a quick look at the lines from the stacktrace I found that access to modelCache in PagedListModelCache may be non thread safe.

(0 until modelCache.size).forEach { position ->
    if (modelCache[position] == null) { // <- crash here, so it's concurrent modification issue
        modelCache[position] = modelBuilder(position, currentList[position])
    }
}

functions that accesses to modelCache are marked by @Synchronized but synchronization performed on different objects:
getModels is synchronized on PagedListModelCache instance while onRemoved is synchronized on updateCallback object. So there is no synchronization between these two functions.

Fix will be to use the same monitor object for synchronization these functions. e.g. by making

override fun onRemoved(position: Int, count: Int) = synchronized(this@PagedListModelCache) {
    assertUpdateCallbacksAllowed()
    (0 until count).forEach {
        modelCache.removeAt(position)
     }
     rebuildCallback()
}

[elihart]

thanks for the report! are you interested in putting up the fix?