protocol DataAcquirer {
    func acquireData(url: URL) -> Future<Data>
}

final class NetworkFacade: DataAcquirer {
    private let queue: DispatchQueue
    private let session: URLSession

    func acquireData(url: URL) -> Future<Data> {
        let promise = Promise<Data>()
        let task = session.downloadTask(with: url) { [queue] (resultUrl, response, error) in
            queue.async {
                let data = try! Data(contentsOf: resultUrl!)
                DispatchQueue.main.async {
                    promise.resolve(data)
                }
            }
        }
        task.resume()
        return promise.future
    }
}

let facade: NetworkFacade
// No race condition: future is accessible only from DispatchQueue.main
let imageAquired = facade.acquireData(url: imageUrl).then(decodeImage(from:))