golang - Exit http handler from anywhere

Question

I'm using the net/http package and wondering how I can exit the handler from anywhere in the code. Say I have this code:

func main() {
    http.HandleFunc("/", handler)
    http.ListenAndServe(":8080", nil)
}

func handler(w http.ResponseWriter, r *http.Request){
    err := checkSomeThing(w, r)

    if err != nil {
        return
    }

    fmt.Println("End of Handler.")
    return
}

func checkSomeThing(w http.ResponseWriter, r *http.Request) error{
    http.Error(w, "Bad Request!", http.StatusBadRequest) 
    
    return errors.New("bad request")
}

Ideally I'd like to exit the handler from within the checkSomeThing function without having to return and then return again up a level, which will get worse as the application grows. This is purely for code readability.

Answer 1

The idiomatic approach is to check error returns up the call chain.

To exit the the handler from anywhere, use panic and recover following the pattern in the encoding/json package.

Define a unique type for panic:

type httpError struct {
    status  int
    message string
}

Write a function to be used in a defer statement. The function checks for the type and handles the error as appropriate. Otherwise, the function continues the panic.

func handleExit(w http.ResponseWriter) {
    if r := recover(); r != nil {
        if he, ok := r.(httpError); ok {
            http.Error(w, he.message, he.status)
        } else {
            panic(r)
        }
    }
}

Write a helper function for the call to panic:

func exit(status int, message string) {
    panic(httpError{status: status, message: message})
}

Use the functions like this:

func example() {
   exit(http.StatusBadRequest, "Bad!")
}

func someHandler(w http.ResponseWriter, r *http.Request) {
   defer handleExit(w)
   example()
}