I’ve been struggling with a very frustrating issue using the new iOS 26 Swift Concurrency APIs for video processing. My pipeline reads frames using AVAssetReader, processes them via CIContext (Lanczos upscale), and then appends the result to an AVAssetWriter using the new PixelBufferReceiver.
The Problem: The execution randomly stops at the ]await append(...)] call. The task suspends and never resumes.
It is completely unpredictable: It might hang on the very first run, or it might work fine for 4-5 runs and then hang on the next one.
It is independent of video duration: It happens with 5-second clips just as often as with long videos.
No feedback from the system: There is no crash, no error thrown, and CPU usage drops to zero. The thread just stays in the suspended state indefinitely.
If I manually cancel the operation and restart the VideoEngine, it usually starts working again for a few more attempts, which makes me suspect some internal resource exhaustion or a deadlock between the GPU context and the writer's input.
The Code: Here is a simplified version of my processing loop:
private func proccessVideoPipeline(
readerOutputProvider: AVAssetReaderOutput.Provider<CMReadySampleBuffer<CMSampleBuffer.DynamicContent>>,
pixelBufferReceiver: AVAssetWriterInput.PixelBufferReceiver,
nominalFrameRate: Float,
targetSize: CGSize
) async throws {
while !Task.isCancelled, let payload = try await readerOutputProvider.next() {
let sampleBufferInfo: (imageBuffer: CVPixelBuffer?, presentationTimeStamp: CMTime) = payload.withUnsafeSampleBuffer { sampleBuffer in
return (sampleBuffer.imageBuffer, sampleBuffer.presentationTimeStamp)
}
guard let currentPixelBuffer = sampleBufferInfo.imageBuffer else {
throw AsyncFrameProcessorError.missingImageBuffer
}
guard let pixelBufferPool = pixelBufferReceiver.pixelBufferPool else {
throw NSError(domain: "PixelBufferPool", code: -1, userInfo: [NSLocalizedDescriptionKey: "No pixel buffer pool available"])
}
let newPixelBuffer = try pixelBufferPool.makeMutablePixelBuffer()
let newCVPixelBuffer = newPixelBuffer.withUnsafeBuffer({ $0 })
try upscale(currentPixelBuffer, outputPixelBuffer: newCVPixelBuffer, targetSize: targetSize )
let presentationTime = sampleBufferInfo.presentationTimeStamp
try await pixelBufferReceiver.append(.init(unsafeBuffer: newCVPixelBuffer), with: presentationTime)
}
}
Does anyone know how to fix it?
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