__copy_helper_block_ crash in AVFoundation

Question

I have a strange crash in my video processing app. It uses AVFoundation to work with video & audio and GPUImage for filtering. I have never faced this issue myself but after releasing it to the App Store it appeared in Crashlytics quite often. Here is the log of the crash:

Thread : Crashed: AVPlayerItemOutput queue
0  libobjc.A.dylib                0x00000001986f80b4 objc_retain + 20
1  libsystem_blocks.dylib         0x0000000198d79bf8 _Block_object_assign + 320
2  AVFoundation                   0x0000000186895a34 __copy_helper_block_171 + 36
3  libsystem_blocks.dylib         0x0000000198d79738 _Block_copy_internal + 384
4  libdispatch.dylib              0x0000000198d252fc _dispatch_Block_copy + 36
5  libdispatch.dylib              0x0000000198d2685c dispatch_async + 68
6  AVFoundation                   0x00000001868959ac -[AVPlayerItemVideoOutput _dispatchOutputSequenceWasFlushed] + 112
7  libdispatch.dylib              0x0000000198d2536c _dispatch_client_callout + 16
8  libdispatch.dylib              0x0000000198d2e6e8 _dispatch_barrier_sync_f_invoke + 76
9  AVFoundation                   0x00000001868940a8 AVPlayerItemVideoOutput_figVCSequentialAvailable + 196
10 MediaToolbox                   0x000000018a3c16f8 FigVisualContextImageAvailableSequential + 108
11 MediaToolbox                   0x000000018a348ce8 itemremote_postNotificationWithPayload + 3996
12 MediaToolbox                   0x000000018a342d60 FigPlayerRemoteCallbacksServer_SendNotifyPing + 924
13 MediaToolbox                   0x000000018a342998 _XSendNotifyPing + 60
14 MediaToolbox                   0x000000018a33f0d4 figmoviecallbacks_server + 112
15 MediaToolbox                   0x000000018a33f018 fpr_ClientPortCallBack + 208
16 CoreFoundation                 0x0000000187f44ce0 __CFMachPortPerform + 180
17 CoreFoundation                 0x0000000187f598fc __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 56
18 CoreFoundation                 0x0000000187f5985c __CFRunLoopDoSource1 + 436
19 CoreFoundation                 0x0000000187f577dc __CFRunLoopRun + 1640
20 CoreFoundation                 0x0000000187e851f4 CFRunLoopRunSpecific + 396
21 GraphicsServices               0x00000001910135a4 GSEventRunModal + 168
22 UIKit                          0x000000018c7b6784 UIApplicationMain + 1488
23 MerryVideoEditor               0x000000010024b804 main (main.m:16)
24 libdyld.dylib                  0x0000000198d4ea08 start + 4

This is how I connect AVFoundation to GPUImage:

class ProjectEditorViewController: UIViewController {
   private var videoPlayerView = VideoPlayerView()
   private var movieFile: GPUImageMovie!   
   private var currentComposition: AVComposition!
   //...and other properties
}

// MARK: - Filtering & Playback
extension ProjectEditorViewController{
   func updatePlayer() {

      currentFilter.removeAllTargets()
      movieFile?.removeAllTargets()
      movieFile?.endProcessing()

      let time = self.videoPlayerView.player?.currentItem.currentTime() ?? kCMTimeZero

      let (composition, audioMix) = compositionBuilder.buildCompositionFromTimeLine(timeLine)
      videoPlayerView.setAsset(composition)
      videoPlayerView.playerItem.audioMix = audioMix

      movieFile = GPUImageMovie(playerItem: videoPlayerView.player.currentItem)
      currentFilter.applyFromOutput(movieFile, toInput: gpuPlayerView)
      movieFile.startProcessing()

      addSyncLayerIfNeededForComposition(composition)

      videoPlayerView.player.seekToTime(time, toleranceBefore: kPlayerToleranceSeekTime, toleranceAfter: kPlayerToleranceSeekTime)
      currentComposition = composition

   }

   func updatePlayerFilter(){          
      if movieFile != nil{
         movieFile.removeAllTargets()
         currentFilter.applyFromOutput(movieFile, toInput: gpuPlayerView)
         if(!videoPlayerView.isPlaying) { movieFile.startProcessing() }
         addSyncLayerIfNeededForComposition(currentComposition)
      }else{
         updatePlayer()
      }
   }       
}

Any ideas what is wrong with my code? Any questions, comments, hints and answers are highly appreciated.

Answer 1

This is an issue in GPUImageMovie - when instance adds itself as AVPlayerItemVideoOutput delegate: [playerItemOutput setDelegate:self queue:videoProcessingQueue], playerItemOutput doesn't release its delegate when GPUImageMovie instance is deallocating. Later, this results in a method call from the deallocated object (outputSequenceWasFlushed:) and you get the crash. This was found with help of NSZombie detector and I fixed it by adding this in GPUImageMovie dealloc method: [playerItemOutput setDelegate:nil queue:nil];

Good luck, Nikita ;)