http_json_body_decode.go

package main

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

const (
	maxHttpJsonBodySize = 256 * 1024
)

const (
	EmptyRequestBody = "Request body must not be empty"
)

type MalformedRequest struct {
	Code int         `json:"code"`
	Msg  string      `json:"msg"`
	Data interface{} `json:"data,omitempty"`
}

func (mr *MalformedRequest) Error() string {
	return mr.Msg
}

func DecodeJSONBody(body io.ReadCloser, dst interface{}) error {
	dec := json.NewDecoder(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{Code: http.StatusBadRequest, Msg: msg}

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

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

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

	return nil
}

func HttpDecodeJSONBody(w http.ResponseWriter, r *http.Request, dst interface{}) error {
	// Use http.MaxBytesReader to enforce a maximum read of 1MB from the
	// response body. A request body larger than that will now result in
	// Decode() returning a "http: request body too large" error.
	r.Body = http.MaxBytesReader(w, r.Body, maxHttpJsonBodySize)
	return DecodeJSONBody(r.Body, dst)
}