Swift中传递CVPixelBuffer给CoreImage/UIKit后如何释放？

我用CVPixelBufferCreateWithBytes(_:_:_:_:_:_:_:_:_:_:)从磁盘读取24位BMP，代码如下：

var data = try! Data(contentsOf: url.appendingPathExtension(for: .bmp))
data.withUnsafeMutableBytes { (unsafeMutableRawBufferPointer: UnsafeMutableRawBufferPointer) in
  var pixelBuffer: CVPixelBuffer?
  CVPixelBufferCreateWithBytes(
    kCFAllocatorDefault,
    imageWidth,
    imageHeight,
    kCVPixelFormatType_24RGB,
    unsafeMutableRawBufferPointer.baseAddress!,
    bytesPerRow,
    nil, nil, nil,
    &pixelBuffer
  )
}

尝试将其转换为JPEG时，所有相关方法都存在内存泄漏，例如：

try! UIImage(ciImage: CIImage(cvPixelBuffer: pixelBuffer!))
    .jpegData(compressionQuality: 0.0)!
    .write(to: someUrl)

以及Core Image的写入方法：

try! CIContext().writeJPEGRepresentation(
    of: CIImage(cvPixelBuffer: pixelBuffer!),
    to: someUrl,
    colorSpace: CGColorSpaceCreateDeviceRGB()
)

使用autoreleasepool包裹代码也无法解决泄漏问题：

autoreleasepool {
  var pixelBuffer: CVPixelBuffer?
  CVPixelBufferCreateWithBytes(
    kCFAllocatorDefault,
    imageWidth,
    imageHeight,
    kCVPixelFormatType_24RGB,
    unsafeMutableRawBufferPointer.baseAddress!,
    bytesPerRow,
    nil, nil, nil,
    &pixelBuffer
  )
  try! UIImage(ciImage: CIImage(cvPixelBuffer: pixelBuffer!))
    .jpegData(compressionQuality: 0.0)!
    .write(to: someUrl)
}

同时，所有*Release方法均不可用，编译器报错：'CFRelease' is unavailable: Core Foundation objects are automatically memory managed。

进一步测试发现，泄漏仅发生在调用JPEG相关方法时：

• 存在泄漏的代码：

let uiImage = UIImage(ciImage: CIImage(cvPixelBuffer: pixelBuffer!))
    .jpegData(compressionQuality: 0.0)!

• 无泄漏的代码：

let uiImage = UIImage(ciImage: CIImage(cvPixelBuffer: pixelBuffer!))

Core Image也遵循同样的规律：

• 无泄漏的代码：

CIContext().createCGImage(CIImage(cvPixelBuffer: pixelBuffer!), from: CGRect(x: 0, y: 0, width: imageWidth, height: imageHeight))

• 存在泄漏的代码：

CIContext().writeJPEGRepresentation(
    of: CIImage(cvPixelBuffer: pixelBuffer!),
    to: someUrl,
    colorSpace: CGColorSpaceCreateDeviceRGB()
)

我猜测需要通过向CVPixelBufferCreateWithBytes传递CVPixelBufferReleaseBytesCallback来管理内存，但不清楚正确的用法。多数示例给出的回调写法如下：

let releaseCallBack: CVPixelBufferReleaseBytesCallback = { _, pointer in
  if let pointer = pointer {
     free(UnsafeMutableRawPointer(mutating: pointer))
  }
}

但使用该回调会导致autoreleasepool执行时抛出异常，原因是回调试图释放已不存在的内存；如果移除该回调，又会导致对象持续泄漏。手动调用指针的deallocate方法也无法解决问题。

内容的提问来源于stack exchange，提问作者Tae