How to stream an io.ReadCloser into a http.ResponseWriter

Question

I have a client which makes a request to download a file, the web server forwards this request on to the resource server that actually holds the file. The *http.Response back from the resource server has the Body io.ReaderCloser streaming the file contents from the resource server. But I'm then at the point where I want to start writing that into the original http.ResponseWriter that came from the client. Looking at the http.ResponseWriter interface it only contains a Write method which takes a slice of bytes, which makes me think that the only way to get the file contents back to the client is to read the Body io.ReaderCloser into a buffer and then put that into the http.ResponseWriter's Write method. which I dont want to do since that is hugely inefficient and it would be much better to stream it through my web server. Is this possible?

here is some code to illustrate:

getFile() *http.Response {
    //make a request to resource server and return the response object
}

// handle request from client
http.HandleFunc("/getFile", func(w http.ResponseWriter, r *http.Request){
    res := getFile()
    //how can I stream res.Body into w without buffering ?
})

Answer 1

You can use io.Copy() which does exactly this.

Copy copies from src to dst until either EOF is reached on src or an error occurs. It returns the number of bytes copied and the first error encountered while copying, if any.

n, err := io.Copy(w, res.Body)
// check err

Also note that Copy() will not return io.EOF but nil because if it can "copy" everything until src reports io.EOF, that is not considered an error.