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golang实现可中断的流式下载

最近有一个需要实现下载功能：

1. 从服务器上读取文件，返回一个ReadCloser
2. 在用户磁盘上创建文件，通过io.Copy实现文件下载（io.Copy是流式的操作，不会出现因文件过大而内存暴涨的问题）
3. 通过context实现暂停

1 流式下载：io.Copy

这里拷贝文件我们选择的是io.Copy而非是通过ioutil.ReadAll()将body中返回的数据一次性读取到内存

通过io.Copy可以保证内存占用一直处于一个比较稳定的水平

2 可中断：context

通过封装io.Copy实现

• 将io.Copy封装为一个方法，方法里传入context,外部通过context.WithCancel()控制流式拷贝的暂停

3 全部代码

这里演示我通过读取S3的一个对象下载到本地

/*通过io.Copy实现可中断的流复制
*/
var (ak       = "99999999999999999999"sk       = "9999999999999999999999999999999999999999"endpoint = "http://xx.xx.xx.xx:8060"bucket   = "test-bucket"key      = "d_xp/2G/2G.txt"
)func main() {s3Client := osg.Client.GetS3Client(ak, sk, endpoint)ctx, cancelFunc := context.WithCancel(context.Background())object, err := s3Client.GetObject(ctx, &s3.GetObjectInput{Bucket: aws.String(bucket),Key:    aws.String(key),})go func() {time.Sleep(time.Second * 10)cancelFunc()log.Infof("canceled...")}()if err != nil {log.Errorf("%v", err)return}body := object.Bodydefer body.Close()file, err := os.Create("/Users/ziyi/GolandProjects/MyTest/demo_home/io_demo/target.txt")if err != nil {log.Errorf("%v", err)return}defer file.Close()_, err = FileService.Copy(ctx, file, body)if err != nil {log.Errorf("%v", err)return}}type fileService struct {sem *semaphore.Weighted
}var FileService = &fileService{sem: semaphore.NewWeighted(1),
}type IoCopyCancelledErr struct {errMsg string
}func (e *IoCopyCancelledErr) Error() string {return fmt.Sprintf("io copy error, %s", e.errMsg)
}func NewIoCopyCancelledErr(msg string) *IoCopyCancelledErr {return &IoCopyCancelledErr{errMsg: msg,}
}type readerFunc func(p []byte) (n int, err error)func (rf readerFunc) Read(p []byte) (n int, err error) { return rf(p) }//通过ctx实现可中断的流拷贝
// Copy closable copy
func (s *fileService) Copy(ctx context.Context, dst io.Writer, src io.Reader) (int64, error) {// Copy will call the Reader and Writer interface multiple time, in order// to copy by chunk (avoiding loading the whole file in memory).// I insert the ability to cancel before read time as it is the earliest// possible in the call process.size, err := io.Copy(dst, readerFunc(func(p []byte) (int, error) {select {// if context has been canceledcase <-ctx.Done():// stop process and propagate "context canceled" errorreturn 0, NewIoCopyCancelledErr(ctx.Err().Error())default:// otherwise just run default io.Reader implementationreturn src.Read(p)}}))return size, err
}