middleware.go

package main

import (
        "net/http"
        "net/http/httptest"
)

type FooRequest struct {
        Payload string
}

type ModifierMiddleware struct {
        handler *FooRequest
}

func (m *ModifierMiddleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
        rec := httptest.NewRecorder()
        // passing a ResponseRecorder instead of the original RW
        foo := m.handler
        foo.Payload = "payload from middleware"
        m.handler.ServeHTTP(rec, r)
        // after this finishes, we have the response recorded
        // and can modify it before copying it to the original RW

        // we copy the original headers first
        for k, v := range rec.Header() {
                w.Header()[k] = v
        }
        // and set an additional one
        w.Header().Set("X-We-Modified-This", "Yup")
        w.Header().Set("X-Payload", foo.Payload)
        // only then the status code, as this call writes the headers as well
        w.WriteHeader(418)
        // the body hasn't been written yet, so we can prepend some data.
        w.Write([]byte("Middleware says hello again. "))
        // then write out the original body
        w.Write(rec.Body.Bytes())
}

func (self *FooRequest) ServeHTTP(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte(self.Payload))
        self.Payload = "bla bla bla"
}

func barHandler(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("bar!"))
}

func main() {
        foo := &FooRequest{}
        mid := &ModifierMiddleware{foo}

        println("Listening on port 7080")
        http.ListenAndServe(":7080", mid)
}