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 https://github.com/valyala/fasthttp/blob/40bdc4abc74e2760e4c3b3153b2d4c2357eb7f4e/http.go#L2205-L2211

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, ""
    }