How to do error http error handling in Go language - go

Question

I am little confused on how error handling should be done in go. I have read so many posts about it but still am not able to apply them on my structure of code. I am new to go so please help.
There is a main function which handles two apis: api1 and api2
func main() {
http.HandleFunc("/offers", api1)
http.HandleFunc("/getOffersList", api2)
err := http.ListenAndServe(":8080", nil)
if err != nil {
log.Fatal("ListenAndServe: ", err)
}
}
func api1(w http.ResponseWriter, req *http.Request) {
validateRequestHeader(w, req, "GET")
// code here....
}
func api2(w http.ResponseWriter, req *http.Request) {
validateRequestHeader(w, req, "POST")
//code here....
}
func validateRequestHeader(w http.ResponseWriter, req *http.Request, allowedMethod string) {
// allow cross domain AJAX requests
w.Header().Set("Content-Type", "application/json")
if origin := req.Header.Get("Origin"); origin != "" {
w.Header().Set("Access-Control-Allow-Origin", origin)
w.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS, PUT, DELETE")
w.Header().Set("Access-Control-Allow-Headers",
"Accept, Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization")
}
// Stop here if its Preflighted OPTIONS request
if req.Method == "OPTIONS" {
return
}
if req.Method != allowedMethod {
response := "Only " + allowedMethod + " requests are allowed"
http.Error(w, response, http.StatusMethodNotAllowed)
return
}
}
In both api1 and api2 , function validateRequestHeader is getting called. If this gets true, then in the api1/api2 further code is getting executed which I do not want. How it should be handled?
if req.Method != allowedMethod {
response := "Only " + allowedMethod + " requests are allowed"
http.Error(w, response, http.StatusMethodNotAllowed)
return
}

Answer 1

This blog post goes into some details on how to chain multiple handler functions, which is what you should be doing. That way, you can have a validation handler, a logging handler, an authorization handler, etc etc. and just chain them together.
Essentially
func validator(next http.Handler) http.Handler {
fn := func(w http.ResponseWriter, req *http.Request) {
if isRequestValid(req) {
// a valid request is passed on to next handler
next.ServeHTTP(w, req)
} else {
// otherwise, respond with an error
http.Error(w, "Bad request - Go away!", 400)
}
}
return http.HandlerFunc(fn)
}
func api1() http.Handler {
fn := func(w http.ResponseWriter, req *http.Request) {
// api 1 code
}
return http.HandlerFunc(fn)
}
func api2() http.Handler {
fn := func(w http.ResponseWriter, req *http.Request) {
// api 2 code
}
return http.HandlerFunc(fn)
}
And then chain them up in your main function.
func main() {
http.Handler("/offers", validate(api1()))
http.Handler("/getOffersList", validate(api2()))
err := http.ListenAndServe(":8080", nil)
if err != nil {
log.Fatal("ListenAndServe: ", err)
}
}