ConcurrentModificationException in MediaLibrarySessionImpl.getSubscribedControllers() #3056

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1. jjohnstz

   added

   bug

   needs triage

   on Feb 4, 2026
2. marcbaechinger

   self-assigned this

   on Feb 5, 2026
3. marcbaechinger commented on Feb 5, 2026

   

   marcbaechinger

   on Feb 5, 2026

   Contributor

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   Thanks for reporting!

   If you find some time to tell us a bit about the threading model of your app in which that call is executed and producing the exception that would be great. I have some assumptions, but I think it's more efficient to get some information from the pros. It would allow me to try to reproduce exact your case and make a unit test to test the regression.

   I'm aware you can't reproduce yourself. Let us know about the app architecture specifically the threading. This may help us to a) give you some ideas how you could fix that on your end without needing to wait for a fix from our end, and b) to fix this on our end to either make it thread safe or add some checks from what threads a given method is allowed to be accessed.
4. marcbaechinger

   removed

   needs triage

   on Feb 5, 2026
5. jjohnstz commented on Feb 10, 2026

   

   jjohnstz

   on Feb 10, 2026

   Author

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   getSubscribedControllers() is being called from a Kotlin coroutine running on a custom IO thread pool (unbounded Executors.newCachedThreadPool()), not from the main thread.

   We have a coroutine flow that monitors database changes and notifies subscribed controllers:

   mediaRepo.getFolderUpdateFlow(folder)
       .onEach { count ->
           val controllers = mediaLibrarySession?.getSubscribedControllers(mediaId) // ← CRASH
           controllers?.forEach { controller ->
               mediaLibrarySession?.notifyChildrenChanged(controller, mediaId, count, null)
           }
       }
       .launchIn(ioScope) // Custom IO thread pool
   

   The issue

   Thread A (our IO pool thread): Calls getSubscribedControllers() → iterates collection
   Thread B (Media3's thread): Controller subscribes/unsubscribes → modifies collection
   Result: ConcurrentModificationException in ImmutableList.copyOf()

   Question

   Should getSubscribedControllers() only be called from the main thread? If yes, we can wrap it with:

   withContext(Dispatchers.Main) {
       mediaLibrarySession?.getSubscribedControllers(mediaId)
   }
   

   Or should the method be made thread-safe for calls from any thread?
6. marcbaechinger commented on Feb 10, 2026

   

   marcbaechinger

   on Feb 10, 2026

   Contributor

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   Thanks for confirming!

   Or should the method be made thread-safe for calls from any thread?

   All methods on 'MediaSession' must be thread safe as per documentation. We have a fix for this and will update this issue when it landed in main.

   Thank you for reporting and adding details, very much appreciated!
7. copybara-service

   added a commit that references this issue on Feb 11, 2026

   Make subscriptions for onChilldrenChanged thread safe

   ...

   e086991
8. marcbaechinger commented on Feb 11, 2026

   

   marcbaechinger

   on Feb 11, 2026

   Contributor

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   The commit above should fix this.
9. marcbaechinger

   closed this as completedon Feb 12, 2026
10. androidx

    locked and limited conversation to collaborators on Apr 13, 2026