Not buffered http.ResponseWritter in Golang - http

Question

I'm writing a simple web app in Go and I want my responses to be streamed to the client (i.e. not buffered and sent in blocks once the request is fully processed) :
func handle(res http.ResponseWriter, req *http.Request) {
fmt.Fprintf(res, "sending first line of data")
sleep(10) //not real code
fmt.Fprintf(res, "sending second line of data")
}
From the client point of view, the two lines will be sent at the same time. Any suggestions are appreciated :)
Edit after #dystroy answer
It's possible to flush after each write I personally make, but in my use case it's not enough:
cmd := exec.Command("a long command that outputs lots of lines")
cmd.Stdout = res //where res is a http.ResponseWritter
cmd.Stderr = res
err := cmd.Run()
I want the output of my cmd to be flushed as well. Anyway to "autoflush" the ResponseWritter ?
Solution
I found help on golang's mailing list. There is 2 way to achieve this: using hijacker that allow to take over the underlying TCP connection of HTTP, or piping the stdout and stderr of the command in a go routine that will write and flush :
pipeReader, pipeWriter := io.Pipe()
cmd.Stdout = pipeWriter
cmd.Stderr = pipeWriter
go writeCmdOutput(res, pipeReader)
err := cmd.Run()
pipeWriter.Close()
//---------------------
func writeCmdOutput(res http.ResponseWriter, pipeReader *io.PipeReader) {
buffer := make([]byte, BUF_LEN)
for {
n, err := pipeReader.Read(buffer)
if err != nil {
pipeReader.Close()
break
}
data := buffer[0:n]
res.Write(data)
if f, ok := res.(http.Flusher); ok {
f.Flush()
}
//reset buffer
for i := 0; i < n; i++ {
buffer[i] = 0
}
}
}
Last update
Even nicer: http://play.golang.org/p/PpbPyXbtEs

Answer 1

As implied in the documentation, some ResponseWriter may implement the Flusher interface.
This means you can do something like this :
func handle(res http.ResponseWriter, req *http.Request) {
fmt.Fprintf(res, "sending first line of data")
if f, ok := res.(http.Flusher); ok {
f.Flush()
} else {
log.Println("Damn, no flush");
}
sleep(10) //not real code
fmt.Fprintf(res, "sending second line of data")
}
Be careful that buffering can occur in many other places in the network or client side.

Answer 2

Sorry if I've misunderstood your question, but would something like the below do the trick?
package main
import (
"bytes"
"fmt"
"net/http"
)
func handler(w http.ResponseWriter, r *http.Request) {
body := make([]byte, int(r.ContentLength))
b := bytes.NewBuffer(body)
if _, err := b.ReadFrom(r.Body); err != nil {
fmt.Fprintf(w, "%s", err)
}
if _, err := b.WriteTo(w); err != nil {
fmt.Fprintf(w, "%s", err)
}
}
func main() {
http.HandleFunc("/", handler)
if err := http.ListenAndServe(":8080", nil); err != nil {
panic(err)
}
}
$ curl --data "param1=value1&param2=value2" http://localhost:8080
returns:
param1=value1&param2=value2
You could always append whatever data you wanted to body, or read more bytes into the buffer from elsewhere before writing it all out again.