根据Swift.org的建议，重叠访问修复不起作用。有什么想法吗？

Question

我有一个以前在Swift 3中可用的方法。它是BZCompression算法的端口。该方法分配内存以对其进行压缩，并使其在运行时可用。使用Xcode 10.1时，代码无法编译。该方法会重新评估内存以运行算法，这在swift中是不允许的，但在swift 4中是更严格的。

这是我在Xcode中收到的错误消息： 对“ buff”的访问重叠，但是修改需要排他访问；考虑复制到本地变量

var bzCompressedData: Data? {
        get {
            if self.count == 0 {
                return self
            }
            var compressed = [UInt8]()
            var stream = bz_stream()
            var mself = self
            var success = true
            mself.withUnsafeMutableBytes { (selfBuff: UnsafeMutablePointer<Int8>) -> Void in
                stream.next_in = selfBuff
                stream.avail_in = UInt32(self.count)
                var buff = Data(capacity: Int(BZCompressionBufferSize))
                var copyBuff = buff
                copyBuff.withUnsafeMutableBytes({ (outBuff: UnsafeMutablePointer<Int8>) -> Void in
                    stream.next_out = outBuff
                    stream.avail_out = BZCompressionBufferSize
                    var bzret = BZ2_bzCompressInit(&stream, BZDefaultBlockSize, 0, BZDefaultWorkFactor)
                    if bzret != BZ_OK {
                        print("failed compression init")
                        success = false
                        return
                    }
                    repeat {
                        bzret = BZ2_bzCompress(&stream, stream.avail_in > 0 ? BZ_RUN : BZ_FINISH)
                        if bzret < BZ_OK {
                            print("failed compress")
                            success = false
                            return
                        }
                        buff.withUnsafeBytes({ (bp: UnsafePointer<UInt8>) -> Void in
                            let bpp = UnsafeBufferPointer(
                                start: bp, count: (Int(BZCompressionBufferSize) - Int(stream.avail_out)))
                            compressed.append(contentsOf: bpp)
                        })
                        stream.next_out = outBuff
                        stream.avail_out = BZCompressionBufferSize
                    } while bzret != BZ_STREAM_END
                })
            }
            BZ2_bzCompressEnd(&stream)
            if !success {
                return nil
            }
            return Data(bytes: compressed)
        }
    }

这设法允许进行编译，但是我可以看到原始缓冲区已过度分配并在运行时崩溃。