same as io.CopyBuffer from stdlib. except I add a cancellation channel and prioritize the allocated buffer over the io.ReadFrom and io.WriteTo interfaces.

CopyBuffer.go

package customio

import (
	"errors"
	"io"
)

var ErrCanceled = errors.New("operation canceled")

func CopyBuffer(dst io.Writer, src io.Reader, buf []byte, cancel chan struct{}) (written int64, err error) {
	if buf == nil {
		//allocated buffer has higer priority over io.ReadFrom and io.WriteTo interface.
		if wt, ok := src.(io.WriterTo); ok {
			return wt.WriteTo(dst)
		}
		if rt, ok := dst.(io.ReaderFrom); ok {
			return rt.ReadFrom(src)
		}
		size := 32 * 1024
		if l, ok := src.(*io.LimitedReader); ok && int64(size) > l.N {
			if l.N < 1 {
				size = 1
			} else {
				size = int(l.N)
			}
		}
		buf = make([]byte, size)
	}
loop:
	for {
		select {
		case <-cancel:
			err = ErrCanceled
			break loop
		default:
		}
		nr, er := src.Read(buf)
		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 {
			if er != io.EOF {
				err = er
			}
			break
		}
	}
	return written, err
}