Extending GoLang's http.ResponseWriter functionality to pre/post process responses - go

Question

I am trying to write a simple http MiddleWare handler that will process an http response. Unfortunately, it does not work and I cannot figure out what mistake I am making. Any/all help is appreciated!
I am using Go Gorilla mux router
Here are illustrative parts of the code:
import (
"fmt"
"log"
"github.com/gorilla/mux"
)
:
func Start() {
router := mux.NewRouter()
router.HandleFunc("/", myHandler)
:
log.Fatal(http.ListenAndServe(":8088", Middleware(router)))
}
func myHandler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "myHandler called")
}
func Middleware(h http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
neww := NewProcessor(w)
h.ServeHTTP(neww, r)
})
}
type Processor struct {
http.ResponseWriter
}
func (r *Processor) Write(b []byte) (int, error) {
fmt.Printf("******* Processor writing...")
log.Print(string(b)) // log it out
return r.Write(b) // pass it to the original ResponseWriter
}
func NewProcessor(w http.ResponseWriter) http.ResponseWriter {
fmt.Printf("******* Creating new Processor...")
return &Processor{ResponseWriter: w}
}
The output I get is listed below (extra logging text omitted for clarity):
******* Creating new Processor
myHandler called
However, notice the message "******* Processor writing..." was not displayed, suggesting that the "Write" function did not get called.
What changes need to be made to allow the "Write" function to be called?

Answer 1

return r.Write(b) caused an infinite loop of calls to the Processor's Write() method. Replacing it with return r.ResponseWriter.Write(b) fixed the bug.
Here is the corrected code:
package main
import (
"fmt"
"log"
"net/http"
)
func main() {
mux := http.NewServeMux()
mux.HandleFunc("/", myHandler)
log.Fatal(http.ListenAndServe(":8088", Middleware(mux)))
}
func myHandler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "myHandler called")
}
func Middleware(h http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
new := NewProcessor(w)
h.ServeHTTP(new, r)
})
}
type Processor struct {
http.ResponseWriter
}
func (r *Processor) Write(b []byte) (int, error) {
log.Print("******* Processor writing...")
log.Print(string(b)) // log it out
return r.ResponseWriter.Write(b) // pass it to the original ResponseWriter
}
func NewProcessor(w http.ResponseWriter) http.ResponseWriter {
log.Print("******* Creating new Processor...")
return &Processor{ResponseWriter: w}
}
Output:
2016/07/21 22:59:08 ******* Creating new Processor...
2016/07/21 22:59:08 ******* Processor writing...
2016/07/21 22:59:08 myHandler called