CODEWITHSUNDEEP
go
Eric Broda
Eric Broda

Reputation: 7231

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

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?

Upvotes: 0

Views: 1869

Answers (1)

fl0cke
fl0cke

Reputation: 2884

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

Upvotes: 2

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