[Beta6] NoSuchElementException (Nexus 4 only)

[ghost]

Device: Nexus 4 running 5.1.1

    java.util.NoSuchElementException
        at java.util.LinkedList.removeFirstImpl(LinkedList.java:689)
        at java.util.LinkedList.removeFirst(LinkedList.java:676)
        at java.util.LinkedList.poll(LinkedList.java:895)
        at no.nordicsemi.android.ble.BleManager$BleManagerGattCallback.nextRequest(BleManager.java:2864)
        at no.nordicsemi.android.ble.BleManager$BleManagerGattCallback.onCharacteristicWriteSafe(BleManager.java:2482)
        at no.nordicsemi.android.ble.MainThreadBluetoothGattCallback.lambda$onCharacteristicWrite$3(MainThreadBluetoothGattCallback.java:128)
        at no.nordicsemi.android.ble.-$$Lambda$MainThreadBluetoothGattCallback$dAnXisth7CAlJFFhbDJ5X_192I0.run(lambda)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)```

[ghost]

It fails consistently on Nexus4, but the same operations on other devices never trigger this issue

[philips77]

This indeed is very interesting. The poll() should return null if the list is empty:
https://android.googlesource.com/platform/libcore/+/lollipop-mr1-release/luni/src/main/java/java/util/LinkedList.java#895

But if size != null, then it calls removeFirst() which actually may throw an exception in that case:
https://android.googlesource.com/platform/libcore/+/lollipop-mr1-release/luni/src/main/java/java/util/LinkedList.java#713
The question is, why the list is empty and has size > 0, or where does it get messed up. Looks like race condition.

I tried running nRF Blinky on Nexus 4 with Android 5.1.1, but didn't have problem like this. Do you add or remove tasks in some weird way in your app?
The only solution I can find is to enclose it all in try-catch to avoid crashes like this.

[ghost]

I'm enqueuing multiple write requests at the same time. I don't see any failure before this NPE.

I'm thinking this is a bug on my side, they should be queued one after the other, but it's working on most devices.

Once next version is released, I'll test again and close the issue if I can't reproduce it. Thanks for the effort!

[philips77]

I've released 2.0-beta7 with all the recent changes (except Reliable Write).