iOS/macOS crash: os_unfair_lock_corruption_abort in Vision — sample-buffer delegate runs on a concurrent queue

[JoacimWall]

Summary

On iOS, the app crashes with SIGTRAP / _os_unfair_lock_corruption_abort inside Apple's Vision framework, called from BarcodeAnalyzer.DidOutputSampleBuffer. The root cause is that the AVCaptureVideoDataOutput sample-buffer delegate is set on a concurrent dispatch queue, while BarcodeAnalyzer uses a single shared, non-thread-safe VNSequenceRequestHandler.

Environment

• BarcodeScanning.Native.Maui 3.0.3 (also present on master)
• .NET MAUI on net10.0-ios
• Device: iPhone 15 (iPhone15,4), iOS 17.6.1 — reproducible on fast devices that deliver overlapping frames

Symbolicated crash (crashing thread)

0  libsystem_platform   _os_unfair_lock_corruption_abort
1  libsystem_platform   _os_unfair_lock_lock_slow
2  Vision               -[VNRequestPerformer _performOrderedRequests:inContext:error:]
3  Vision               -[VNRequestPerformer _performRequests:onBehalfOfRequest:inContext:error:]
4  Vision               -[VNSequenceRequestHandler _performRequests:onImageBuffer:gatheredForensics:error:]
5  Vision               -[VNSequenceRequestHandler performRequests:onCMSampleBuffer:orientation:...]
6  (managed -> native)  VNSequenceRequestHandler.Perform
...
12 App                  -[BarcodeScanning_BarcodeAnalyzer captureOutput:didOutputSampleBuffer:fromConnection:]
13 AVFCapture           -[AVCaptureVideoDataOutput _processSampleBuffer:]

A second thread was simultaneously blocked in CMCapture on _pthread_mutex_firstfit_lock_slow — i.e. a second frame was being delivered at the same instant the crashing frame was running Vision.

Root cause

In Platform/MaciOS/CameraManager.cs (Start()), the delegate is attached to the concurrent global queue:

_videoDataOutput?.SetSampleBufferDelegate(_barcodeAnalyzer, DispatchQueue.DefaultGlobalQueue);

Apple's contract for setSampleBufferDelegate:queue: requires a serial queue:

The queue you specify must be a serial dispatch queue to ensure that frames are delivered in order.

Because the queue is concurrent, DidOutputSampleBuffer can run on multiple threads simultaneously. BarcodeAnalyzer holds one shared VNSequenceRequestHandler:

private readonly VNSequenceRequestHandler _sequenceRequestHandler;
...
_sequenceRequestHandler.Perform([_detectBarcodesRequest], sampleBuffer, out _);

VNSequenceRequestHandler is not thread-safe, so concurrent Perform calls corrupt Vision's internal os_unfair_lock, aborting the process. AlwaysDiscardsLateVideoFrames = true does not prevent this, since the "drop frame while busy" behavior depends on the delegate queue being serial.

Note: a serial queue (com.barcodescanning.maui.sessionQueue) is already created in the class but is only used for session configuration, not for the delegate.

Proposed fix

Attach the sample-buffer delegate to a dedicated serial queue instead of DispatchQueue.DefaultGlobalQueue, e.g.:

// field
private readonly DispatchQueue? _cameraQueue = new("com.barcodescanning.maui.cameraQueue");

// in Start()
_videoDataOutput?.SetSampleBufferDelegate(null, null);
_videoDataOutput?.SetSampleBufferDelegate(_barcodeAnalyzer, _cameraQueue);

(Don't reuse _dispatchQueue for the delegate, since it's used with DispatchBarrierAsync for start/stop and would let frame processing block session reconfiguration.)