IssueTracker

Component used: WorkManager
Version used: 1.0.0-alpha06
Devices/Android versions reproduced on: Many

We can see a large number of ConcurrentModificationExceptions in WorkManager:

java.lang.RuntimeException: Error receiving broadcast Intent { act=android.net.conn.CONNECTIVITY_CHANGE flg=0x4000010 bqHint=4 (has extras) } in vu@2406837b
        at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:988)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:145)
        at android.app.ActivityThread.main(ActivityThread.java:7007)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
Caused by: java.util.ConcurrentModificationException
        at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:346)
        at java.util.LinkedHashMap$KeyIterator.next(LinkedHashMap.java:366)
        at androidx.work.impl.constraints.trackers.ConstraintTracker.setState(OperaSrc:89)
        at androidx.work.impl.constraints.trackers.NetworkStateTracker$NetworkStateBroadcastReceiver.onReceive(OperaSrc:157)
        at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:978)

It looks like the mListeners set is being modified during the loop in ConstraintTracker.setState(). I see two potential issues with this method:
1. Maybe a listener is added or removed in onConstraintChanged(). I haven't seen this happen, but it might be good to make a copy of mListeners and iterate over the copy.
2. The NetworkStateTracker.mNetworkCallback is called on a different thread than the main thread. In consequence setState() is called on a different thread so there is a possibility that the main thread adds or removes a listener.  This cannot be the (only) reason because the error happens in response to the CONNECTIVITY_CHANGE.
This may be fixed with synchronization or by registering mNetworkCallback to run on the main thread.