gorilla handlers write to file

import (
	"github.com/gorilla/handlers"
)

func main(){
	...
	
	err := http.ListenAndServe(":9000", access_log(r))
	if err != nil {
		log.Fatal("HTTP server: ", err)
	}
}

func access_log(r http.Handler) http.Handler {
	f, err := os.OpenFile("log/access.log", os.O_CREATE | os.O_WRONLY | os.O_APPEND, 0666)
	if err != nil {
		log.Panic("Access log: ", err)
	}
	
	return handlers.LoggingHandler(io.Writer(f), r)
}

The file is created but nothing is written to the file on each request

#update

This works when the output is written to os.Stdout

func access_log(r http.Handler) http.Handler {
	/*f, err := os.OpenFile("log/access.log", os.O_CREATE | os.O_WRONLY | os.O_APPEND, 0666)
	if err != nil {
		log.Panic("Access log: ", err)
	}
	
	return handlers.LoggingHandler(io.Writer(f), r)*/
	
	return handlers.LoggingHandler(os.Stdout, r)
}

#update II

func main(){
	runtime.GOMAXPROCS(runtime.NumCPU())
	
	r := mux.NewRouter()
	
	www := r.Host("secure.domain.com").Subrouter()
	www.HandleFunc("/", Handler_www)
	
	api := r.Host("api.domain.com").Subrouter()
	api.HandleFunc("/", Handler_api)
	
	err := http.ListenAndServe(":9000", access_log(r))
	if err != nil {
		log.Fatal("HTTP server: ", err)
	}
}

func access_log(r http.Handler) http.Handler {
	f, err := os.OpenFile("log/access.log", os.O_CREATE | os.O_WRONLY | os.O_APPEND, 0666)
	if err != nil {
		log.Panic("Access log: ", err)
	}
	
	return handlers.LoggingHandler(f, r)
}

Try starting from a working example, as described in "Making and Using HTTP Middleware":

package main

import (
  "github.com/gorilla/handlers"
  "net/http"
  "os"
)

func main() {
  finalHandler := http.HandlerFunc(final)

  logFile, err := os.OpenFile("server.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666)
  if err != nil {
    panic(err)
  }

  http.Handle("/", handlers.LoggingHandler(logFile, finalHandler))
  http.ListenAndServe(":3000", nil)
}

func final(w http.ResponseWriter, r *http.Request) {
  w.Write([]byte("OK"))
}

The handlers.LoggingHandler() uses the file directly, not io.Writer(f).

In your case:

return handlers.LoggingHandler(f, r)