copyZeroAlloc allocates when interface uses genericWriteTo

[lunixbochs]

it looks like go's io.CopyBuffer always uses the WriterTo interface if it's available, much like copyZeroAlloc

however, File and TCPConn unfortunately have a fallback in their WriterTo that calls io.Copy if sendfile isn't possible, which allocates an implicit buffer

https://github.com/golang/go/blob/15c558016088d6aaf103b4f0fd2b716a4573e5a2/src/net/net.go#L789
https://github.com/golang/go/blob/15c558016088d6aaf103b4f0fd2b716a4573e5a2/src/os/file.go#L274

this is with an os.File reaching writeBodyFixedSize

[lunixbochs]

it doesn't look like the golang zero copy backend works with bufio.Writer at all, despite the comment here

fasthttp/http.go

Lines 2205 to 2211 in 40bdc4a

	if earlyFlush {
	// w buffer must be empty for triggering
	// sendfile path in bufio.Writer.ReadFrom.
	if err := w.Flush(); err != nil {
	return err
	}
	}

the os.File writeTo implementation will see that the destination is not a network socket and immediately fall back to io.Copy, which allocates an implicit buffer:
https://github.com/golang/go/blob/15c558016088d6aaf103b4f0fd2b716a4573e5a2/src/os/zero_copy_linux.go#L19

func (f *File) writeTo(w io.Writer) (written int64, handled bool, err error) {
	pfd, network := getPollFDAndNetwork(w)
	// TODO(panjf2000): same as File.spliceToFile.
	if pfd == nil || !pfd.IsStream || !isUnixOrTCP(string(network)) {
		return
	}

https://github.com/golang/go/blob/15c558016088d6aaf103b4f0fd2b716a4573e5a2/src/os/zero_copy_linux.go#L163

func getPollFDAndNetwork(i any) (*poll.FD, poll.String) {
	sc, ok := i.(syscall.Conn)
	if !ok {
		return nil, ""
	}