echo server distorting image response body - go

Question

Trying to write a simple echo server for images, but it distorts the file. What's going wrong?
package main
import (
"fmt"
"io"
"net/http"
)
type FlushWriter struct {
w io.Writer
}
func (fw *FlushWriter) Write(bytes []byte) (int, error) {
count, e := fw.w.Write(bytes)
fw.w.(http.Flusher).Flush()
return count, e
}
func main() {
http.HandleFunc("/", index)
fmt.Println("listening on 8000")
http.ListenAndServe(":8000", nil)
}
func index(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "image/jpeg")
fw := &FlushWriter{ w: w }
io.Copy(fw, r.Body)
}
And to test it.
$ curl --data-binary #image.jpg -o test.jpg localhost:8000

Answer 1

You're ignoring errors in your code, and missing a ErrBodyReadAfterClose.
You can't read from the http.Request.Body once you start writing to the http.ResponseWriter
http://golang.org/pkg/net/http/#pkg-variables
ErrBodyReadAfterClose is returned when reading a Request or Response
Body after the body has been closed. This typically happens when the
body is read after an HTTP Handler calls WriteHeader or Write on its
ResponseWriter
You'll need to buffer the image on the server before writing it back.
Aside from the fact that Go won't let you do this, even if you were to make a handler that worked, most clients would cause this to deadlock with images larger than the sum of all the buffers involved. This requires a client that can simultaniously send and receive, which very few, if any would do.