defer func() {
		if err != nil {
			// block and abort remote upload upon failure.
			attempts := 1
			for attempts <= 3 {
				actx, acancel := context.WithTimeout(ctx, time.Minute)
				aerr := c.AbortMultipartUpload(actx, bucket, object, uploadID)
				acancel()
				if aerr == nil {
					return
				}
				attempts++
				time.Sleep(time.Duration(rand.Int63n(int64(time.Second))))
			}
		}
	}()