Golang custom logger that provides the request headers to log result.

custom-header-logger.go

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"os"
	"runtime"
	"time"
)

func main() {
	http.Handle("/", errorHandler(index))
	http.ListenAndServe(":8000", nil)
}

func index(w http.ResponseWriter, r *http.Request) {
	panic("Something went wrong :/")
}

// Error Handler
type errorHandler func(http.ResponseWriter, *http.Request)

func (fn errorHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	ctx := addHeadersForLogging(r) // Required to read headers from request context.
	defer func() {
		if rec := recover(); rec != nil {
			logError(ctx, "Panic: %+v", rec)
			w.WriteHeader(http.StatusInternalServerError)
		}
	}()
	fn(w, r.WithContext(ctx))
}

// Error log
func logError(ctx context.Context, format string, v ...interface{}) {
	// message
	message := fmt.Sprintf(format, v...)
	// headers
	h := ctx.Value("logkey").(*Headers)
	// stack trace
	buf := make([]byte, 1024)
	stackTrace := runtime.Stack(buf, true)
	// log (use sync.Once for getting instance of new logger).
	log.New(os.Stdout, "", 0).Printf("error: %s %s \"%s\"\n%s", time.Now().UTC().Format(time.RFC3339Nano), h.RequestID, message, buf[0:stackTrace])
}

// Headers contains header values to logging.
type Headers struct {
	RequestID string
}

// AddHeaders will add any common HTTP headers
func addHeadersForLogging(r *http.Request) context.Context {
	headers := &Headers{
		RequestID: r.Header.Get("x-request-id"),
	}
	return context.WithValue(r.Context(), "logkey", headers)
}