如何停止io.CopyN

Question

I have some code that copies from a file to a tcp socket (like an ftp server) and want to be able to abort this copy if needed.

Im just using io.CopyN(socket, file, size) and cant see a way to signal an abort. Any ideas?

Answer 1

CopyN tries hard to copy N bytes. If you want to optionally copy less than N bytes then don't use CopyN in the first place. I would probably adapt the original code to something like (untested code):

func copyUpToN(dst Writer, src Reader, n int64, signal chan int) (written int64, err error) {
    buf := make([]byte, 32*1024)
    for written < n {
        select {
        default:
        case <-signal:
            return 0, fmt.Errorf("Aborted") // or whatever
        }

        l := len(buf)
        if d := n - written; d < int64(l) {
            l = int(d)
        }
        nr, er := src.Read(buf[0:l])
        if nr > 0 {
            nw, ew := dst.Write(buf[0:nr])
            if nw > 0 {
                written += int64(nw)
            }
            if ew != nil {
                err = ew
                break
            }
            if nr != nw {
                err = io.ErrShortWrite
                break
            }
        }
        if er != nil {
            err = er
            break
        }
    }
    return written, err
}