FileHandle causing memory leak in iOS

Question

The background

I am working on a project which uses a third party SDK to process files in a particular format. The SDK takes Data as parameter to process. In case of large files, since iOS Data object doesn't support buffered reading, the SDK also allows processing a file chunk by chunk.

Issue

I am trying to use FileHandle to read huge files chunk by chunk and pass it to the SDK to process. following is the code which I am using:

let chunkSize = 10 * 1024 * 1024
let outputFileHandle = try! FileHandle(forReadingFrom: urls[0])
var datas = outputFileHandle.readData(ofLength: chunkSize)
while !(datas.isEmpty) {
    try! $0.process(datas)
    datas = outputFileHandle.readData(ofLength: chunkSize)
}

The above piece of code lies in a subscript of the SDK's FileProcessor object. I tried passing a 5GB file to the above code, but as each chunk it read, it is gradually eating up memory, till it reaches a point where it crashes due to memory issue. I was expecting the FileHandler to release already read Data.

I tried using autoreleasepool like below:

autoreleasepool {
    while let data = self.nextData() {
        print("")
    }
}

func nextData(fileHandle: FileHandle, chunkSize: Int) -> Data? {
    return fileHandle.readData(ofLength: chunkSize)
}

The above code works like a charm and doesn't clog memory.

However, when I use it in the subscript, it shows two compile time errors. For one, as you can see above, there is a mandatory throwable code in the subscript. This cannot be avoided. The two errors I am seeing are shown below:

Cannot convert value of type '() -> ()' to expected argument type '((FileEncryptProcess) throws -> Void)?'

The subscript expects a throw, which doesn't get detected due to autoreleasepool(That is my assumption reading from the error message)

autoreleasepool {
    while let data = self.nextData() {
        try! $0.process(datas)
    }
}

Secondly, since the autoreleasepool doesn't expect a throw from within its body, the following error is also thrown(Again my assumption reading from the error message):

Contextual closure type '() throws -> ()' expects 0 arguments, but 1 was used in closure body

Any approach for reading a file data part by part in iOS is sufficient for me. This is the closest I could reach.