IssueTracker

Component used: Paging Version used: 3.0.0-alpha10 Devices/Android versions reproduced on: Pixel 1, Android 9.x

I'm seeing the following exception when using the latest alpha when I set a maxSize. Without it, there's no crash - our objects are fairly heavy memory wise so I do want to set it.

pageSize/prefetchDistance are both 20 FWIW.

To reproduce, I do the following:

• set maxSize to 200
• open paged list in app
• scroll down a lot - beyond 200 items
• scroll up to a section where there are placeholders again, which means the page has been dropped and needs to be read from db again
• observe the below crash

    java.lang.IllegalArgumentException: Additional prepend event after prepend state is done
        at androidx.paging.SeparatorState.onInsert(Separators.kt:223)
        at androidx.paging.SeparatorState.onEvent(Separators.kt:173)
        at androidx.paging.SeparatorsKt$insertEventSeparators$$inlined$map$1$2.emit(Collect.kt:135)
        at androidx.paging.PagingDataKt$map$$inlined$transform$1$2.emit(Collect.kt:136)
        at androidx.paging.PagingDataKt$map$$inlined$transform$1$2.emit(Collect.kt:136)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt.emitAllImpl$FlowKt__ChannelsKt(Channels.kt:61)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt$emitAllImpl$1.invokeSuspend(Unknown Source:11)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:69)
        at kotlinx.coroutines.DispatchedTaskKt.resumeUnconfined(DispatchedTask.kt:236)
        at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:161)
        at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:362)
        at kotlinx.coroutines.CancellableContinuationImpl.completeResume(CancellableContinuationImpl.kt:479)
        at kotlinx.coroutines.channels.AbstractChannel$ReceiveElement.completeResumeReceive(AbstractChannel.kt:899)
        at kotlinx.coroutines.channels.ArrayChannel.offerInternal(ArrayChannel.kt:84)
        at kotlinx.coroutines.channels.AbstractSendChannel.send(AbstractChannel.kt:135)
        at androidx.paging.PageFetcherSnapshot.doLoad(PageFetcherSnapshot.kt:492)
        at androidx.paging.PageFetcherSnapshot$doLoad$1.invokeSuspend(Unknown Source:12)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6718)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

I have tried the same reproduction steps on alpha09 and can confirm that I see the crash there too.

Seems potentially related to https://android-review.googlesource.com/c/platform/frameworks/support/+/1482701 which has a similar stacktrace when it crashes?