Passing context between golang's http handlers using unsafe pointer

passing_context.go

package main

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


type ExtRequest struct {
  *http.Request

  message string
}

func main() {
  handlerReader := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request){
    extReq := (*ExtRequest)(unsafe.Pointer(req))
    fmt.Println(extReq.message)
    w.WriteHeader(http.StatusOK)
  })

  handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
    extReq := &ExtRequest{req, "this is message passed between handlers"}
    handlerReader(w, (*http.Request)(unsafe.Pointer(extReq)))
  })

  mux := http.NewServeMux()
  mux.Handle("/", handler)

  srv := httptest.NewServer(mux)
  defer srv.Close()

  req, _ := http.NewRequest("GET", srv.URL, nil)
  http.DefaultClient.Do(req)
}