io-transform.go

package main

import (
	"compress/gzip"
	"io"
	"log"
	"net"
)

type hello struct {
	msg     []byte
	r       io.Reader
	written int
}

func newHello(r io.Reader, name string) *hello {
	return &hello{
		msg:     []byte("Hello, " + name + "!\n"),
		r:       r,
		written: 0,
	}
}

func (h *hello) Read(p []byte) (int, error) {
	if h.written < len(h.msg) {
		n := copy(p, h.msg[h.written:])
		h.written += n
		return n, nil
	}
	return h.r.Read(p)
}

type transformer func(r io.Reader) (io.Reader, error)

func transform(r io.Reader, t transformer) io.Reader {
	pr, pw := io.Pipe()
	go func() {
		tr, err := t(r)
		if err != nil {
			pw.CloseWithError(err)
			return
		}
		_, err = io.Copy(pw, tr)
		if err != nil {
			pw.CloseWithError(err)
			return
		}
		pw.Close()
	}()
	return pr
}

func onConnected(c net.Conn) {
	defer c.Close()
	io.Copy(c, transform(
		transform(
			c,
			func(r io.Reader) (io.Reader, error) {
				return gzip.NewReader(r)
			},
		),
		func(r io.Reader) (io.Reader, error) {
			return newHello(r, "world"), nil
		},
	))
}

func main() {
	l, err := net.Listen("tcp", ":5000")
	if err != nil {
		log.Fatal(err)
	}
	for {
		conn, err := l.Accept()
		if err != nil {
			log.Fatal(err)
		}
		go onConnected(conn)
	}
}