Fix exception

[samoylenkodmitry]

Another rare crash that our monkey found, 2 crashes/day

Non-fatal Exception: java.util.ConcurrentModificationException
       at java.util.ArrayList$Itr.next(ArrayList.java:860)
       at leakcanary.internal.activity.db.HeapAnalysisTable$notifyUpdateOnMainThread$1.run(HeapAnalysisTable.kt:207)
       at android.os.Handler.handleCallback(Handler.java:790)
       at android.os.Handler.dispatchMessage(Handler.java:99)
       at android.os.Looper.loop(Looper.java:164)
       at android.app.ActivityThread.main(ActivityThread.java:6494)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

[pyricau]

Thanks for the exception. Can you provide a repro scenario?

HeapAnalysisTable.updateListeners should only be accessed from the main thread, so this should never happens. If it's accessed from a background thread somewhere, we need to identify where and fix that. I don't think we should use thread safe data structure here unless we come up with a good reason for accessing updateListeners from a background thread.

[pyricau]

I looked into this further. This isn't a concurrency issue, HeapAnalysisTable.updateListeners is only accessed from the main thread. It's the good old "Removed an entry from a list while iterating on it". Right now the two screens that have listeners will refresh themselves when triggered, ie remove and redisplay themselves, which triggers a removal of the listener.

However that works fine because there's usually only one listener at a time, so we foreach on the first entry, remove the first entry in that foreach, but then there's not a second entry to loop again so no crash.

The Monkey here probably found a way to have two screens in flight and cause this. It's a good find :)

[pyricau]

I just reproduced by adding right after HeapAnalysisTable.onUpdate in HeapDumpsScreen.createView:

      HeapAnalysisTable.onUpdate {
      }

and then going to the heap dump screen and clicking "Dump Now". Confirmed this PR fixes the issue, merging.