func LoggingHandler(out io.Writer, h http.Handler) http.Handler {
	return loggingHandler{out, h, writeLog}
}
LoggingHandler return a http.Handler that wraps h and logs requests to out in
Apache Common Log Format (CLF).

See http://httpd.apache.org/docs/2.2/logs.html#common for a description of this format.

LoggingHandler always sets the ident field of the log to -

Example:

r := mux.NewRouter()
r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("This is a catch-all route"))
})
loggedRouter := handlers.LoggingHandler(os.Stdout, r)
http.ListenAndServe(":1123", loggedRouter)
package handlers
import (
	"io"
	"net"
	"net/http"
	"net/url"
	"strconv"
	"time"
	"unicode/utf8"
)
type LogFormatterParams struct {
	Request    *http.Request
	URL        url.URL
	TimeStamp  time.Time
	StatusCode int
	Size       int
}
type LogFormatter func(writer io.Writer, params LogFormatterParams)
type loggingHandler struct {
	writer    io.Writer
	handler   http.Handler
	formatter LogFormatter
}
func (h loggingHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	t := time.Now()
	logger := makeLogger(w)
	url := *req.URL
	h.handler.ServeHTTP(logger, req)
	if req.MultipartForm != nil {
		req.MultipartForm.RemoveAll()
	}
	params := LogFormatterParams{
		Request:    req,
		URL:        url,
		TimeStamp:  t,
		StatusCode: logger.Status(),
		Size:       logger.Size(),
	}
	h.formatter(h.writer, params)
}
func makeLogger(w http.ResponseWriter) loggingResponseWriter {
	var logger loggingResponseWriter = &responseLogger{w: w, status: http.StatusOK}
	if _, ok := w.(http.Hijacker); ok {
		logger = &hijackLogger{responseLogger{w: w, status: http.StatusOK}}
	}
	h, ok1 := logger.(http.Hijacker)
	c, ok2 := w.(http.CloseNotifier)
	if ok1 && ok2 {
		return hijackCloseNotifier{logger, h, c}
	}
	if ok2 {
		return &closeNotifyWriter{logger, c}
	}
	return logger
}
type commonLoggingResponseWriter interface {
	http.ResponseWriter
	http.Flusher
	Status() int
	Size() int
}
const lowerhex = "0123456789abcdef"
func appendQuoted(buf []byte, s string) []byte {
	var runeTmp [utf8.UTFMax]byte
	for width := 0; len(s) > 0; s = s[width:] {
		r := rune(s[0])
		width = 1
		if r >= utf8.RuneSelf {
			r, width = utf8.DecodeRuneInString(s)
		}
		if width == 1 && r == utf8.RuneError {
			buf = append(buf, `\x`...)
			buf = append(buf, lowerhex[s[0]>>4])
			buf = append(buf, lowerhex[s[0]&0xF])
			continue
		}
		if r == rune('"') || r == '\\' {  
			buf = append(buf, '\\')
			buf = append(buf, byte(r))
			continue
		}
		if strconv.IsPrint(r) {
			n := utf8.EncodeRune(runeTmp[:], r)
			buf = append(buf, runeTmp[:n]...)
			continue
		}
		switch r {
		case '\a':
			buf = append(buf, `\a`...)
		case '\b':
			buf = append(buf, `\b`...)
		case '\f':
			buf = append(buf, `\f`...)
		case '\n':
			buf = append(buf, `\n`...)
		case '\r':
			buf = append(buf, `\r`...)
		case '\t':
			buf = append(buf, `\t`...)
		case '\v':
			buf = append(buf, `\v`...)
		default:
			switch {
			case r < ' ':
				buf = append(buf, `\x`...)
				buf = append(buf, lowerhex[s[0]>>4])
				buf = append(buf, lowerhex[s[0]&0xF])
			case r > utf8.MaxRune:
				r = 0xFFFD
				fallthrough
			case r < 0x10000:
				buf = append(buf, `\u`...)
				for s := 12; s >= 0; s -= 4 {
					buf = append(buf, lowerhex[r>>uint(s)&0xF])
				}
			default:
				buf = append(buf, `\U`...)
				for s := 28; s >= 0; s -= 4 {
					buf = append(buf, lowerhex[r>>uint(s)&0xF])
				}
			}
		}
	}
	return buf
}
func buildCommonLogLine(req *http.Request, url url.URL, ts time.Time, status int, size int) []byte {
	username := "-"
	if url.User != nil {
		if name := url.User.Username(); name != "" {
			username = name
		}
	}
	host, _, err := net.SplitHostPort(req.RemoteAddr)
	if err != nil {
		host = req.RemoteAddr
	}
	uri := req.RequestURI
	if req.ProtoMajor == 2 && req.Method == "CONNECT" {
		uri = req.Host
	}
	if uri == "" {
		uri = url.RequestURI()
	}
	buf := make([]byte, 0, 3*(len(host)+len(username)+len(req.Method)+len(uri)+len(req.Proto)+50)/2)
	buf = append(buf, host...)
	buf = append(buf, " - "...)
	buf = append(buf, username...)
	buf = append(buf, " ["...)
	buf = append(buf, ts.Format("02/Jan/2006:15:04:05 -0700")...)
	buf = append(buf, `] "`...)
	buf = append(buf, req.Method...)
	buf = append(buf, " "...)
	buf = appendQuoted(buf, uri)
	buf = append(buf, " "...)
	buf = append(buf, req.Proto...)
	buf = append(buf, `" `...)
	buf = append(buf, strconv.Itoa(status)...)
	buf = append(buf, " "...)
	buf = append(buf, strconv.Itoa(size)...)
	return buf
}
func writeLog(writer io.Writer, params LogFormatterParams) {
	buf := buildCommonLogLine(params.Request, params.URL, params.TimeStamp, params.StatusCode, params.Size)
	buf = append(buf, '\n')
	writer.Write(buf)
}
func writeCombinedLog(writer io.Writer, params LogFormatterParams) {
	buf := buildCommonLogLine(params.Request, params.URL, params.TimeStamp, params.StatusCode, params.Size)
	buf = append(buf, ` "`...)
	buf = appendQuoted(buf, params.Request.Referer())
	buf = append(buf, `" "`...)
	buf = appendQuoted(buf, params.Request.UserAgent())
	buf = append(buf, '"', '\n')
	writer.Write(buf)
}
func CombinedLoggingHandler(out io.Writer, h http.Handler) http.Handler {
	return loggingHandler{out, h, writeCombinedLog}
}