function variable in struct resets to nil - go

Question

I'm trying to make HTTP proxy using go net/http. I made struct type named HTTPProxy to store http.Server object and some hooks to modify Request and Response.
There're two hooks in my proxy(Request/Response hook). The hooks can be set with interface named SetRequestHook and SetResultHook. I checked value of HTTPProxy struct using fmt.Printf("%#v\n", httpProxy) before/after SetRequestHook to check the set of request hook was successful. This is the value of httpProxy before/after SetRequestHook.
webproxy.HTTPProxy{server:(*http.Server)(0xc000154000), IsStarted:false, requestHook:(webproxy.RequestHookFunc)(nil), responseHook:(webproxy.ResponseHookFunc)(nil)}
webproxy.HTTPProxy{server:(*http.Server)(0xc000154000), IsStarted:false, requestHook:(webproxy.RequestHookFunc)(0x75ce60), responseHook:(webproxy.ResponseHookFunc)(nil)}
As you can see address of request hook function stored well in httpProxy struct. But if I call another interface names Start requestHook in struct resets to nil for no reason. This is the result of fmt.Printf("%#v\n", httpProxy) in Start interface.
webproxy.HTTPProxy{server:(*http.Server)(0xc000154000), IsStarted:false, requestHook:(webproxy.RequestHookFunc)(nil), responseHook:(webproxy.ResponseHookFunc)(nil)}
I haven't reset my struct since initialize so I'm not even sure what is happening to my struct. How can I make my request hook to not reset to nil? This is my current code having this problem.
cmd/runserver/main.go
package main
import (
"log"
"net/http"
"test/pkg/webproxy"
)
var a int
func test(request *http.Request) {
log.Printf("New Request: %d\n", a)
a++
}
func main() {
a = 1
httpProxy := webproxy.NewHTTPProxy()
httpProxy.SetRequestHook(test)
httpProxy.Start("", 3000)
}
pkg/webproxy/webproxy.go
// Package webproxy provides http and https proxy with request and response hooks
package webproxy
import (
"context"
"fmt"
"log"
"net"
"net/http"
)
// RequestHookFunc typed functions provided to proxy will modify request before sent to the server.
type RequestHookFunc func(request *http.Request)
// ResponseHookFunc typed functions provided to proxy will modify response before sent to the client.
type ResponseHookFunc func(response *http.Response)
// HTTPProxy will store some items that are used in HTTP proxy and state.
type HTTPProxy struct {
server *http.Server
IsStarted bool
requestHook RequestHookFunc
responseHook ResponseHookFunc
}
// ProxyManager provide methods to manage HTTPProxy and HTTPSProxy.
type ProxyManager interface {
Start(host string, port uint16)
Stop()
GetAddr()
SetRequestHook(hook RequestHookFunc)
SetResponseHook(hook ResponseHookFunc)
}
// NewHTTPProxy creates default HTTPProxy struct.
func NewHTTPProxy() *HTTPProxy {
return &HTTPProxy{
server: new(http.Server),
IsStarted: false,
requestHook: nil,
responseHook: nil,
}
}
// Start serves HTTPProxy on host:port.
func (httpProxy HTTPProxy) Start(host string, port uint16) {
if httpProxy.IsStarted {
log.Println("[HTTP] The proxy is already running.")
return
} else if len(host) != 0 && net.ParseIP(host) == nil {
log.Fatal("[HTTP] Inserted host is not valid.")
}
addr := fmt.Sprintf("%s:%d", host, port)
fmt.Printf("%#v\n", httpProxy) // requestHook = nil. Why?? I called SetRequestHook before run Start
httpProxy.server.Handler = getHandler(httpProxy.requestHook, httpProxy.responseHook)
httpProxy.server.Addr = addr
log.Printf("[HTTP] The proxy will be served on %s:%d\n", host, port)
if err := httpProxy.server.ListenAndServe(); err != http.ErrServerClosed {
log.Fatalf("[HTTP] The proxy ListenAndServer: %v\n", err)
}
}
// Stop will stop HTTPProxy immediately.
func (httpProxy HTTPProxy) Stop() {
if !httpProxy.IsStarted {
log.Println("[HTTP] The proxy is not running.")
return
}
httpProxy.server.Shutdown(context.Background())
}
// GetAddr will return HTTPProxy's host:port.
func (httpProxy HTTPProxy) GetAddr() string {
return httpProxy.server.Addr
}
// SetRequestHook setup request hook into the proxy
func (httpProxy HTTPProxy) SetRequestHook(hook RequestHookFunc) {
if httpProxy.IsStarted {
log.Println("[HTTP] You can't set request hook while the proxy is working.")
return
}
fmt.Printf("%#v\n", httpProxy) // requestHook = nil
httpProxy.requestHook = hook
fmt.Printf("%#v\n", httpProxy) // requestHook = [hook's address]
}
// SetResponseHook setup response hook into the proxy.
func (httpProxy HTTPProxy) SetResponseHook(hook ResponseHookFunc) {
if httpProxy.IsStarted {
log.Println("[HTTP] You can't set response hook while the proxy is working.")
return
}
httpProxy.responseHook = hook
}
pkg/webproxy/proxyhandler.go
package webproxy
import (
"fmt"
"io"
"net/http"
)
type proxyHandler struct {
requestHook RequestHookFunc
responseHook ResponseHookFunc
}
func getHandler(requestHookIn RequestHookFunc, responseHookIn ResponseHookFunc) http.Handler {
fmt.Printf("%#v\n", requestHookIn)
return &proxyHandler{requestHook: requestHookIn, responseHook: responseHookIn}
}
// ServeHttp
func (handler *proxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
fmt.Printf("%#v\n", handler.requestHook)
if handler.requestHook != nil {
fmt.Println("Yaha")
handler.requestHook(r)
}
w.Header().Set("Content-Type", "text/plain")
w.WriteHeader(http.StatusOK)
io.WriteString(w, "This HTTP response has both headers before this text and trailers at the end.\n")
}