Function copyZeroAlloc

http.go:2565–2605

copyZeroAlloc optimizes io.Copy by calling ReadFrom or WriteTo only when copying between os.File and net.TCPConn. If the reader has a WriteTo method, it uses WriteTo for copying; if the writer has a ReadFrom method, it uses ReadFrom for copying. If neither method is available, it gets a buffer from

(w io.Writer, r io.Reader)

//
//nolint:dupword
func copyZeroAlloc(w io.Writer, r io.Reader) (int64, error) {
	var readerIsFile, readerIsConn bool

	switch r := r.(type) {
	case *os.File:
		readerIsFile = true
	case *net.TCPConn:
		readerIsConn = true
	case io.WriterTo:
		return r.WriteTo(w)
	}

	switch w := w.(type) {
	case *os.File:
		if readerIsConn {
			return w.ReadFrom(r)
		}
	case *net.TCPConn:
		if readerIsFile {
			// net.WriteTo requires go1.22 or later
			// Benchmark tests show that on Windows, WriteTo performs
			// significantly better than ReadFrom. On Linux, however,
			// ReadFrom slightly outperforms WriteTo. When possible,
			// copyZeroAlloc aims to perform  better than or as well
			// as io.Copy, so we use WriteTo whenever possible for
			// optimal performance.
			if rt, ok := r.(io.WriterTo); ok {
				return rt.WriteTo(w)
			}
			return w.ReadFrom(r)
		}
	case io.ReaderFrom:
		return w.ReadFrom(r)
	}

	vbuf := copyBufPool.Get()
	buf := vbuf.([]byte)
	n, err := copyBuffer(w, r, buf)
	copyBufPool.Put(vbuf)
	return n, err
}

// copyBuffer is rewritten from io.copyBuffer. We do not check if src has a
// WriteTo method, if dst has a ReadFrom method, or if buf is empty.