Peer list concurrent access crash (fix #71)

[rom1v]

I had the same error as you had in bug #71.

Here is my analysis.

When we need to access and modify an ArrayList from two different threads, there are typically two solutions:

1. synchronize access (mutex);
2. serialize the calls in a FIFO queue for making them happen in one thread.

As peers variable is used as PeerListAdapter backend, we must choose the second solution, and the thread must be the UI thread.

Currently, wrong access to peers occur at two places:

• in listener, peerChanged(...) modifies peers from outside the UI thread;
• in refresh(), the AbstractJniResults.putBlob(...) method does the same.

The comments on the sort() method suggests the problem already happened and has been workarounded:

    // list sorter that preserves new items added to the end in a different
    // thread, unlike Collections.sort which just crashes.
    private void sort() {
        IPeer array[] = peers.toArray(new IPeer[peers.size()]);
        Arrays.sort(array, new PeerComparator());
        for (int i = 0; i < peers.size() && i < array.length; i++)
            peers.set(i, array[i]);
    }

This kind of workaround avoids to crash, but can result in an incorrect display (a missing item for example).

The fix I propose consists in always accessing/modifying peers in the UI thread.
It removes the need of the handler, the refreshing variable and the custom sort() method.
It also allows to sort and notify only when needed (when an item is added).

[rom1v]

By the way, I don't see any piece of code which removes an item when a peer is not reachable anymore. Did I miss something?

[lakeman]

Looks like this issue is cropping up again in the latest build on some phones;

java.lang.IllegalStateException: The content of the adapter has changed but ListView did not receive a notification. Make sure the content of your adapter is not modified from a background thread, but only from the UI thread. [in ListView(16908298, class android.widget.ListView) with Adapter(class org.servalproject.PeerListAdapter)]
at android.widget.ListView.layoutChildren(ListView.java:1492)
at android.widget.AbsListView.onTouchModeChanged(AbsListView.java:1960)
at android.view.ViewTreeObserver.dispatchOnTouchModeChanged(ViewTreeObserver.java:591)
at android.view.ViewRoot.ensureTouchModeLocally(ViewRoot.java:2021)
at android.view.ViewRoot.ensureTouchMode(ViewRoot.java:2005)
at android.view.ViewRoot.handleMessage(ViewRoot.java:1774)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:123)
at android.app.ActivityThread.main(ActivityThread.java:4627)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:521)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
at dalvik.system.NativeStart.main(Native Method)

[rom1v]

@lakeman In PeerList, a notification was missing after peers change.
I added a commit on top of bug71 that you can merge in, but github don't add it to this pull request automatically.

While searching for the problem, I also read CallDirector (because it uses PeerListAdapter too), and this piece of code seems strange:

@Override
protected void onProgressUpdate(DnaResult... values) {
    if (adapter.getPosition(values[0]) < 0)
        adapter.add(values[0]);
    }
}

I think this method should iterate over each DnaResult in values, no?