main.go template for cli tool or server

main.go

package main

import (
	"context"
	"fmt"
	"os"
	"os/signal"
	"path/filepath"
	"syscall"

	"go.uber.org/zap"
	"go.uber.org/zap/zapcore"
	"gopkg.in/alecthomas/kingpin.v2"
)

var Version = "unknown"

type Configuration struct {
	logLevel string
}

func main() {
	finalizers := new(Finalizers)

	cfg, err := configure(os.Args)
	FatalIfError(err, "configuration failed", finalizers)

	log, err := newLogger(cfg.logLevel)
	FatalIfError(err, "logger configuration failed", finalizers)

	log.Info("starting", zap.String("app", os.Args[0]), zap.String("version", Version))

	_, cancel := context.WithCancel(context.Background())
	finalizers.Add(cancel)

	WaitShutdownRequest(func(sig os.Signal) {
		log.Info("initiate shutdown", zap.Stringer("signal", sig))
		// cancel global context
		cancel()
		// stop all exposed services
		// stop all the rest
		finalizers.Execute()
	})

	log.Info("done")
}

func newLogger(level string) (*zap.Logger, error) {
	cfg := zap.NewProductionEncoderConfig()
	cfg.EncodeLevel = zapcore.CapitalColorLevelEncoder
	cfg.TimeKey = ""
	enc := zapcore.NewConsoleEncoder(cfg)
	syncer := zapcore.Lock(os.Stdout)
	var priority zapcore.Level
	if err := priority.Set(level); err != nil {
		return nil, err
	}
	return zap.New(zapcore.NewCore(enc, syncer, priority)), nil
}

func configure(args []string) (Configuration, error) {
	cfg := Configuration{}

	app := kingpin.New(filepath.Base(args[0]), "IPForensics: Internet Protocols security toolkit")
	app.Version(Version)

	// application level parameters
	app.Flag("log-priority", "Priority of logging").Short('l').
		Envar("LOG_PRIORITY").Default("info").EnumVar(&cfg.logLevel, "debug", "info", "error")

	kingpin.MustParse(app.Parse(args[1:]))

	return cfg, nil
}

func WaitShutdownRequest(f func(os.Signal)) {
	signals := make(chan os.Signal)
	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM)
	f(<-signals)
}

type Finalizers []func()

func (c *Finalizers) Add(f func()) { *c = append(*c, f) }

func (c *Finalizers) Execute() {
	for _, f := range *c {
		f()
	}
}

func FatalIfError(err error, lastWords string, finalizers *Finalizers) {
	if err == nil {
		return
	}
	finalizers.Execute()
	fmt.Printf("FATAL %s: %s", lastWords, err)
	os.Exit(1)
}