what-is-my-ip/server.go

package main

import (
	"bytes"
	"html/template"
	"io"
	"net"
	"net/http"
	"strings"

	"github.com/pkg/errors"
)

// Server that handles http responses
type Server struct {
	headerNames []string
	logger      LoggerHandler
	tmpl        *template.Template
}

type responseFormat int

const (
	unknownResponse responseFormat = iota
	jsonResponse
	textResponse
	htmlResponse
)

func (s *Server) ServeHTTP(resp http.ResponseWriter, req *http.Request) {
	s.handleHTTP(resp, req, s.getResponseType(req))
}

func (s *Server) handleHTTP(resp http.ResponseWriter, req *http.Request, responseType responseFormat) {
	for _, headerName := range s.headerNames {
		possibleIP := req.Header.Get(headerName)
		if possibleIP == "" {
			continue
		}

		ip, err := NewIP(possibleIP)
		if err != nil {
			continue
		}

		s.sendResponse(resp, ip, responseType)
		return
	}

	addr, _, err := net.SplitHostPort(req.RemoteAddr)
	if err != nil {
		s.handleError(resp, http.StatusBadRequest, err, "Could not get IP address from request")
		return
	}

	ip, err := NewIP(addr)
	if err != nil {
		s.handleError(resp, http.StatusBadRequest, err, "Could not parse IP address"+addr)
		return
	}

	s.sendResponse(resp, ip, responseType)
}

func hasType(accepts, valid []string) bool {
	for _, accept := range accepts {
		value := strings.Trim(strings.Split(accept, ";")[0], " ")
		for _, check := range valid {
			if check == value {
				return true
			}
		}
	}
	return false
}

func (s *Server) getResponseType(req *http.Request) responseFormat {
	accepts := strings.Split(req.Header.Get("Accept"), ",")
	if hasType(accepts, []string{"text/html", "application/xhtml+xml"}) {
		return htmlResponse
	}
	if hasType(accepts, []string{"application/json", "text/json", "text/javascript"}) {
		return jsonResponse
	}
	if hasType(accepts, []string{"text/plain"}) {
		return textResponse
	}
	return unknownResponse
}

func (s *Server) handleHTTPAs(responseType responseFormat) func(http.ResponseWriter, *http.Request) {
	return func(resp http.ResponseWriter, req *http.Request) {
		s.handleHTTP(resp, req, responseType)
	}
}

func (s *Server) sendResponse(resp http.ResponseWriter, ip *IP, responseType responseFormat) {
	s.logger.Printf("Request from %s %s\n", ip.version, ip)

	var body []byte
	var contentType = "text/plain; charset=utf-8"
	var err error

	switch responseType {

	case jsonResponse:
		jsonBody, marshalErr := ip.MarshalJSON()
		if marshalErr != nil {
			err = errors.Wrap(marshalErr, "could not marshal json")
		}
		contentType = "application/json; charset=utf-8"
		body = jsonBody

	case htmlResponse:
		buffer := bytes.NewBuffer([]byte{})
		exeErr := s.tmpl.Execute(buffer, map[string]string{"IP": ip.String()})
		if exeErr != nil {
			err = errors.Wrap(exeErr, "could not get html")
		}
		contentType = "text/html; charset=utf-8"
		body = buffer.Bytes()

	default:
		body = []byte(ip.String())
	}

	if err != nil {
		const errorMessage = "could not send response"
		s.handleError(resp, http.StatusInternalServerError, errors.Wrap(err, errorMessage), errorMessage)
		return
	}

	resp.Header().Set("Content-Type", contentType)
	resp.Write(body)
}

func (s *Server) handleError(resp http.ResponseWriter, status int, err error, message string) {
	resp.WriteHeader(status)
	io.WriteString(resp, message)
	s.logger.Printf("Error handling request: %s (%s)", message, err)
}