main.go

package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log"
	"net/http"
	"strings"

	"github.com/golang/gddo/httputil/header"
)

type Person struct {
	Name string
	Age  int
}

func personCreate(w http.ResponseWriter, r *http.Request) {
	var p Person

	err := decodeJSONBody(w, r, &p)
	if err != nil {
		var mr *malformedRequest
		if errors.As(err, &mr) {
			http.Error(w, mr.msg, mr.status)
		} else {
			log.Print(err.Error())
			http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
		}
		return
	}

	fmt.Fprintf(w, "Person: %+v", p)
}

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("/person/create", personCreate)

	log.Print("Starting server on :4000...")
	err := http.ListenAndServe(":4000", mux)
	log.Fatal(err)
}

type malformedRequest struct {
	status int
	msg    string
}

func (mr *malformedRequest) Error() string {
	return mr.msg
}

func decodeJSONBody(w http.ResponseWriter, r *http.Request, dst interface{}) error {
	if r.Header.Get("Content-Type") != "" {
		value, _ := header.ParseValueAndParams(r.Header, "Content-Type")
		if value != "application/json" {
			msg := "Content-Type header is not application/json"
			return &malformedRequest{status: http.StatusUnsupportedMediaType, msg: msg}
		}
	}

	r.Body = http.MaxBytesReader(w, r.Body, 1048576)

	dec := json.NewDecoder(r.Body)
	dec.DisallowUnknownFields()

	err := dec.Decode(&dst)
	if err != nil {
		var syntaxError *json.SyntaxError
		var unmarshalTypeError *json.UnmarshalTypeError

		switch {
		case errors.As(err, &syntaxError):
			msg := fmt.Sprintf("Request body contains badly-formed JSON (at position %d)", syntaxError.Offset)
			return &malformedRequest{status: http.StatusBadRequest, msg: msg}

		case errors.Is(err, io.ErrUnexpectedEOF):
			msg := fmt.Sprintf("Request body contains badly-formed JSON")
			return &malformedRequest{status: http.StatusBadRequest, msg: msg}

		case errors.As(err, &unmarshalTypeError):
			msg := fmt.Sprintf("Request body contains an invalid value for the %q field (at position %d)", unmarshalTypeError.Field, unmarshalTypeError.Offset)
			return &malformedRequest{status: http.StatusBadRequest, msg: msg}

		case strings.HasPrefix(err.Error(), "json: unknown field "):
			fieldName := strings.TrimPrefix(err.Error(), "json: unknown field ")
			msg := fmt.Sprintf("Request body contains unknown field %s", fieldName)
			return &malformedRequest{status: http.StatusBadRequest, msg: msg}

		case errors.Is(err, io.EOF):
			msg := "Request body must not be empty"
			return &malformedRequest{status: http.StatusBadRequest, msg: msg}

		case err.Error() == "http: request body too large":
			msg := "Request body must not be larger than 1MB"
			return &malformedRequest{status: http.StatusRequestEntityTooLarge, msg: msg}

		default:
			return err
		}
	}

	err = dec.Decode(&struct{}{})
	if err != io.EOF {
		msg := "Request body must only contain a single JSON object"
		return &malformedRequest{status: http.StatusBadRequest, msg: msg}
	}

	return nil
}