func fetchThumbnails(for itemIdentifiers: [NSFileProviderItemIdentifier], requestedSize size: CGSize, perThumbnailCompletionHandler: @escaping (NSFileProviderItemIdentifier, Data?, Error?) -> Void, completionHandler: @escaping (Error?) -> Void) -> Progress {
    
    os_log("fetchThumbnails: Called with itemIdentifiers = %{public}@, requestedSize = (width: %ld, height: x%ld)",
            log: OSLog.fileprovider, FileProviderUtils.toString(itemIdentifiers), size.width, size.height)
    
    let progress = Progress(totalUnitCount: Int64(itemIdentifiers.count))
    let group = DispatchGroup()
    
    for itemIdentifier in itemIdentifiers {
        let identifierComponents = itemIdentifier.rawValue.components(separatedBy: "/")
        let photoIdentifier = identifierComponents[0]
        let filename = identifierComponents[1]
        os_log("fetchThumbnails: Fetching thumbnail for %{public}@", log: OSLog.fileprovider, photoIdentifier)
        
        let request = ThumbnailerClient.shared.thumbnailRequest(identifier: photoIdentifier)
        let urlSession = URLSession(configuration: ThumbnailerClient.shared.getConfiguration())
        
        // Download the thumbnail to disk into a temporary location.
        group.enter()
        let urlSessionDownloadTask = urlSession.downloadTask(with: request) { temporaryURL, response, error in
            if let error = error {
                os_log("fetchThumbnails: Error: %{public}@", log: OSLog.fileprovider, error.localizedDescription)
                completionHandler(error)
                return
            }
            guard let httpStatusCode = HTTPUtils.getStatusCode(from: response) else {
                os_log("fetchThumbnails: This is not supposed to happen. httpStatusCode was nil.")
                return
            }
            if (httpStatusCode != 200) {
                os_log("fetchThumbnails: Download task for item identifier %{public}@ failed with http status code %ld",
                        log: OSLog.fileprovider, itemIdentifier.rawValue, httpStatusCode)
                completionHandler(NSError.fileProviderErrorForNonExistentItem(withIdentifier: itemIdentifier))
                return
            }
            os_log("fetchThumbnaisl: Download task for item identifier %{public}@ succeeded with http status code %ld",
                    log: OSLog.fileprovider, itemIdentifier.rawValue, httpStatusCode)
            
            // If the download succeeds, map a data object to the file.
            assert(temporaryURL != nil, "temporaryURL is nil")
            var thumbnailData: Data? = nil
            do {
                if let temporaryURL = temporaryURL {
                    thumbnailData = try Data(contentsOf: temporaryURL, options: Data.ReadingOptions.alwaysMapped)
                }
            }
            catch let mappingError {
                perThumbnailCompletionHandler(itemIdentifier, nil, mappingError)
                group.leave()
                return
            }
            
            // The thumbnail download was successful, so we can call the perThumbnailCompletionHandler
            // and leave the dispatch group
            perThumbnailCompletionHandler(itemIdentifier, thumbnailData, nil)
            group.leave()
        }
        
        // Add the download task's progress as a child to the overall progress.
        progress.addChild(urlSessionDownloadTask.progress, withPendingUnitCount: 1)
        
        // Start the download task.
        urlSessionDownloadTask.resume()
    }
    
    // Run the completionHandler as soon as all thumbnails have been downloaded.
    group.notify(queue: DispatchQueue.global()) {
        completionHandler(nil)
    }
    
    return progress
}