Terminating or aborting an HTTP request

What's the way to abort my API serving with some error message?

Link to call my service:

http://creative.test.spoti.io/api/getVastPlayer?add=
    {"Json":Json}&host=api0.spoti.io&domain=domain&userAgent=userAgent&mobile=true

To call my service the client need to send a Json and some params.

I want to test if the params that I get are correct, if not I want send a error message.

The response should be a Json Code {"Result":"Result","Error":"error message"}

I tried log.fatal and os.Exit(1) they stop the service, not just the call request. panic aborts the call but it prevents me to send a http.ResponseWriter which is my error message.

I read something about panic, defer, recover but I don't really know how can I use them to solve this problem.

return works:

mobile :=query.Get("mobile")
if mobile=="mobile" {
			str:=`{"Resultt":"","Error":"No valide Var"}`
			fmt.Fprint(w, str)		
			fmt.Println("No successfull Operation!!")
	        return}  

But I can use it just in the main function, because in the other functions it exits just the func not the caller function (request).

Terminating the serving of an HTTP request is nothing more than to return from the ServeHTTP() method, e.g.:

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
	// examine incoming params
	if !ok {
		str := `{"Result":"","Error":"No valide Var"}`
		fmt.Fprint(w, str)
		return
	}

	// Do normal API serving
})

panic(http.ListenAndServe(":8080", nil))

Notes:

If the input params of your API service are invalid, you should consider returning an HTTP error code instead of the implied default 200 OK. For this you can use the http.Error() function, for example:

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
	// examine incoming params
	if !ok {
		http.Error(w, `Invalid input params!`, http.StatusBadRequest) 
		return
	}

	// Do normal API serving
})

For a more sophisticated example where you send back JSON data along with the error code:

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
	// examine incoming params
	if !ok {
		w.Header().Set("Content-Type", "application/json")
		w.WriteHeader(http.StatusBadRequest)
		str := `{"Result":"","Error":"No valide Var"}`
		fmt.Fprint(w, str)
		return
	}

	// Do normal API serving
})

Example showing how to propagate "returning"

If the error is detected outside of ServeHTTP(), e.g. in a function that is called from ServeHTTP(), you have to return this error state so that ServeHTTP() can return.

Let's assume you have the following custom type for your required parameters and a function which is responsible to decode them from a request:

type params struct {
	// fields for your params 
}

func decodeParams(r *http.Request) (*params, error) {
	p := new(params)
	// decode params, if they are invalid, return an error:
	
	if !ok {
		return nil, errors.New("Invalid params")
	}
	
	// If everything goes well:
	return p, nil
}

Using these:

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
	p, err := decodeParams(r)
	if err != nil {
		http.Error(w, `Invalid input params!`, http.StatusBadRequest)
		return
	}

	// Do normal API serving
})

Also see this related question: https://stackoverflow.com/questions/30858823/golang-how-to-return-in-func-from-another-func