Download a stream of unknown size from an Amazon S3 object using an AWS SDK

import ArgumentParser
import AWSClientRuntime
import AWSS3
import Foundation
import Smithy
import SmithyHTTPAPI
import SmithyStreams


    /// Download a file from the specified bucket.
    ///
    /// - Parameters:
    ///   - bucket: The Amazon S3 bucket name to get the file from.
    ///   - key: The name (or path) of the file to download from the bucket.
    ///   - destPath: The pathname on the local filesystem at which to store
    ///     the downloaded file.
    func downloadFile(bucket: String, key: String, destPath: String?) async throws {
        let fileURL: URL

        // If no destination path was provided, use the key as the name to use
        // for the file in the downloads folder.
        
        if destPath == nil {
            do {
                try fileURL = FileManager.default.url(
                    for: .downloadsDirectory,
                    in: .userDomainMask,
                    appropriateFor: URL(string: key),
                    create: true
                ).appendingPathComponent(key)
            } catch {
                throw TransferError.directoryError
            }
        } else {
            fileURL = URL(fileURLWithPath: destPath!)
        }
                
        let config = try await S3Client.S3ClientConfiguration(region: region)
        let s3Client = S3Client(config: config)

        // Create a `FileHandle` referencing the local destination. Then
        // create a `ByteStream` from that.

        FileManager.default.createFile(atPath: fileURL.path, contents: nil, attributes: nil)
        let fileHandle = try FileHandle(forWritingTo: fileURL)

        // Download the file using `GetObject`.
        
        let getInput = GetObjectInput(
            bucket: bucket,
            key: key
        )

        do {
            let getOutput = try await s3Client.getObject(input: getInput)

            guard let body = getOutput.body else {
                throw TransferError.downloadError("Error: No data returned for download")
            }

            // If the body is returned as a `Data` object, write that to the
            // file. If it's a stream, read the stream chunk by chunk,
            // appending each chunk to the destination file.

            switch body {
            case .data:
                guard let data = try await body.readData() else {
                    throw TransferError.downloadError("Download error")
                }

                // Write the `Data` to the file.

                do {
                    try data.write(to: fileURL)
                } catch {
                    throw TransferError.writeError
                }
                break

            case .stream(let stream as ReadableStream):
                while (true) {
                    let chunk = try await stream.readAsync(upToCount: 5 * 1024 * 1024)
                    guard let chunk = chunk else {
                        break
                    }

                    // Write the chunk to the destination file.

                    do {
                        try fileHandle.write(contentsOf: chunk)
                    } catch {
                        throw TransferError.writeError
                    }
                }

                break
            default:
                throw TransferError.downloadError("Received data is unknown object type")
            }
        } catch {
            throw TransferError.downloadError("Error downloading the file: \(error)")
        }

        print("File downloaded to \(fileURL.path).")
    }