Control HTTP headers from outer Go middleware

Question

Let's say I have a middleware in Go where I want to override any existing Server headers with my own value.

// Server attaches a Server header to the response.
func Server(h http.Handler, serverName string) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Server", serverName)
        h.ServeHTTP(w, r)
    })
}

Then I add it to a response chain like this

http.Handle("/", authHandler)
http.ListenAndServe(":8000", Server(http.DefaultServeMux, "myapp"))

Unfortunately, if the authHandler or anything handled by DefaultServeMux calls w.Header().Add("Server", "foo") (like, say, httputil.ReverseProxy.ServeHTTP), I end up with two Server headers in the response.

$ http localhost:5962/v1/hello_world
HTTP/1.1 200 OK
Content-Length: 11
Content-Type: text/plain
Date: Tue, 12 Jul 2016 04:54:04 GMT
Server: inner-middleware
Server: myapp

What I really want is something like this:

// Server attaches a Server header to the response.
func Server(h http.Handler, serverName string) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        h.ServeHTTP(w, r)
        w.Header().Set("Server", serverName)
    })
}

However, this is disallowed by the semantics of ServeHTTP:

ServeHTTP should write reply headers and data to the ResponseWriter and then return. Returning signals that the request is finished; it is not valid to use the ResponseWriter or read from the Request.Body after or concurrently with the completion of the ServeHTTP call.

I have seen some fairly ugly hacks around this, for example the code in httputil.ReverseProxy.ServeHTTP, which copies headers and rewrites the response code, or using httptest.ResponseRecorder, which reads the entire body into a byte buffer.

I could also reverse the traditional middleware order and put my Server middleware last, somehow, or make it the inner-most middleware.

Is there any simple method I'm missing?

Kevin Burke · Accepted Answer · 2018-03-28 09:36:36Z

4

You can define a custom type which wraps a ResponseWriter and inserts the Server header just before all headers are written, at the cost of an additional layer of indirection. Here is an example:

type serverWriter struct {
    w           http.ResponseWriter
    name        string
    wroteHeader bool
}

func (s serverWriter) WriteHeader(code int) {
    if s.wroteHeader == false {
        s.w.Header().Set("Server", s.name)
        s.wroteHeader = true
    }
    s.w.WriteHeader(code)
}

func (s serverWriter) Write(b []byte) (int, error) {
    return s.w.Write(b)
}

func (s serverWriter) Header() http.Header {
    return s.w.Header()
}

// Server attaches a Server header to the response.
func Server(h http.Handler, serverName string) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        sw := serverWriter{
            w:           w,
            name:        serverName,
            wroteHeader: false,
        }
        h.ServeHTTP(sw, r)
    })
}

I wrote more about this here. https://kev.inburke.com/kevin/how-to-write-go-middleware/

edited Mar 28, 2018 at 9:36

answered Jul 12, 2016 at 17:15

Kevin Burke

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2 Comments

dlsniper Over a year ago

This is a perfect example of overengineering it. Really all that you need to do is change the place of where the header gets written and write it before calling WriteHeader().

Kevin Burke Over a year ago

I'm really confused by your comment. It's impossible to override an internal middleware from an outer middleware. In this case I'm calling httputil.ReverseProxy.ServeHTTP to proxy to code I don't control. ReverseProxy.ServeHTTP calls WriteHeader on whatever the proxy returns; I can't control that.

dlsniper · Accepted Answer · 2016-07-12 16:16:06Z

0

It's not possible to change headers (add/remove/modify) after you use WriteHeader(Status).

For this to work, change to:

// Server attaches a Server header to the response.
func Server(h http.Handler, serverName string) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Server", serverName)
    h.ServeHTTP(w, r)
  })
}

answered Jul 12, 2016 at 16:16

dlsniper

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4 Comments

Kevin Burke Over a year ago

sorry - I don't see how that differs from my first code snippet?

dlsniper Over a year ago

The position of write header vs h.ServeHTTP, diff the code.

Kevin Burke Over a year ago

Ah - I have two code snippets in my question, the first matches what you have written, the second is an example of what I want to do, and I note that it's incorrect.

Kugel Over a year ago

How does this answer the question? You just repeated the code in the Question.

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