Gelf with log15 UPD: use go-gelf.v2 by Graylog2

log15_gelf_handler.go

package gelflog15

import (
	"os"
	"strconv"
	"strings"
	"gopkg.in/inconshreveable/log15.v2"
	"gopkg.in/Graylog2/go-gelf.v2/gelf"
)

// Handler sends logs to Graylog in GELF.
type Handler struct {
	writer     gelf.Writer
	host       string
	facility   string
}

// New creates a log15 handler that emits GELF to addr. It is already wrapped
// in log15's LazyHandler and SyncHandler helpers. Its error is non-nil if there
// is a problem creating the GELF writer or determining our hostname.
func New(addr string, facility string) (log15.Handler, error) {
	w, err := gelf.NewUDPWriter(addr)
	if err != nil {
		return nil, err
	}

	host, err := os.Hostname()
	if err != nil {
		return nil, err
	}

	return log15.CallerFileHandler(log15.LazyHandler(log15.SyncHandler(Handler{
		writer:   w,
		host:     host,
		facility: facility,
	}))), nil
}

// Log forwards a log message to the specified receiever.
func (h Handler) Log(r *log15.Record) error {
	short, full := shortAndFull(r.Msg)

	ctx := ctxToMap(r.Ctx)
	callerFile, callerLine := caller(ctx)
	delete(ctx, "_caller")
       	ctx["_file"] = callerFile
  	ctx["_line"] = callerLine

	m := &gelf.Message{
		Version:  "1.1",
		Host:     h.host,
		Short:    short,
		Full:     full,
		TimeUnix: float64(r.Time.Unix()),
		Level:    log15LevelsToSyslog[r.Lvl],
		Facility: h.facility,
		Extra:    ctx,
	}

	return h.writer.WriteMessage(m)
}

func ctxToMap(ctx []interface{}) map[string]interface{} {
	m := make(map[string]interface{}, len(ctx)/2)
	for i := 0; i < len(ctx); i += 2 {
		s := ctx[i].(string)
		m["_" + s] = ctx[i+1]
	}

	return m
}

func shortAndFull(msg string) (short, full string) {
	lines := strings.SplitN(msg, "\n", 2)
	short = lines[0]
	if len(lines) > 1 {
		full = msg
	}

	return
}

// caller searches a context list for an entry called "caller" and splits it into
// filename and line number.
func caller(ctx map[string]interface{}) (string, int) {
	info, present := ctx["_caller"]
	if !present {
		return "", 0
	}

	parts := strings.Split(info.(string), ":")
	line, _ := strconv.Atoi(parts[1])
	return parts[0], line
}

// source: http://www.cisco.com/c/en/us/td/docs/security/asa/syslog-guide/syslogs/logsevp.html
var log15LevelsToSyslog = map[log15.Lvl]int32{
	log15.LvlCrit:  2,
	log15.LvlError: 3,
	log15.LvlWarn:  4,
	log15.LvlInfo:  6,
	log15.LvlDebug: 7,
}