mailsrv/services/sender.go

package services

import (
	"fmt"
	cfg "mailsrv/config"
	"mailsrv/mail"
	"mailsrv/runtime"
	"os"
	"os/signal"
	"path"
	"strings"
	"syscall"
	"time"

	"net/smtp"

	"github.com/rs/zerolog/log"
)

const (
	TickerInterval time.Duration = 10 * time.Second
	JSONSuffix     string        = ".json"
	ErrorSuffix    string        = ".err"
)

type Sender struct {
	smtpConfig cfg.SMTPConfig
	// fetch this directory to collect `.json` e-mail format
	outboxPath string
	queue      *runtime.Queue
}

func NewSender(config cfg.SMTPConfig, outboxPath string) Sender {
	return Sender{
		smtpConfig: config,
		outboxPath: outboxPath,
		queue:      runtime.NewQueue(),
	}
}

func (s Sender) SendMail(email mail.Email) error {
	auth := smtp.PlainAuth("", s.smtpConfig.User, s.smtpConfig.Password, s.smtpConfig.Url)
	log.Debug().Msg("SMTP authentication succeed")

	if err := smtp.SendMail(s.smtpConfig.GetFullUrl(), auth, email.Sender, email.GetReceivers(), email.Generate()); err != nil {
		log.Err(err).Msg("error while sending email")
		return err
	}

	log.Debug().Msg("mail send successfully")
	return nil
}

// watchOutbox reads the `outbox` directory every `TickInterval` and put JSON format e-mail in the queue.
func (s Sender) watchOutbox() {
	log.Info().Str("outbox", s.outboxPath).Msg("start watching outbox directory")

	ticker := time.NewTicker(TickerInterval)

	go func() {
		for range ticker.C {
			log.Debug().Str("action", "retrieving json e-mail format...").Str("path", s.outboxPath)

			files, err := os.ReadDir(s.outboxPath)
			if err != nil && !os.IsExist(err) {
				log.Err(err).Msg("outbox directory does not exist")
				s.queue.Shutdown()
			}

			for _, file := range files {
				filename := file.Name()
				if !strings.HasSuffix(filename, JSONSuffix) {
					log.Debug().Str("filename", filename).Msg("incorrect suffix")
					continue
				}

				path := path.Join(s.outboxPath, filename)
				email, err := mail.FromJSON(path)

				if err != nil {
					log.Err(err).Str("path", path).Msg("unable to parse JSON email")

					// if JSON parsing failed the `path` is renamed with an error suffix to not watch it again
					newPath := fmt.Sprintf("%s%s", path, ErrorSuffix)
					if err := os.Rename(path, newPath); err != nil {
						log.Err(err).Str("path", path).Str("new path", newPath).Msg("unable to rename bad JSON email path")
					}

					continue
				}

				email.Path = path
				s.queue.Add(email)
			}
		}
	}()
}

// processNextEmail iterates over the queue and send email.
func (s Sender) processNextEmail() bool {
	item, quit := s.queue.Get()
	if quit {
		return false
	}
	defer s.queue.Done(item)

	email, ok := item.(mail.Email)
	if !ok {
		log.Error().Any("item", item).Msg("unable to cast queue item into mail.Email")
		return true
	}

	if err := s.SendMail(email); err != nil {
		log.Err(err).Msg("unable to send the email")
	}

	if path := email.Path; path != "" {
		if err := os.Remove(path); err != nil {
			// this is a fatal error, can't send same e-mail indefinitely
			if !os.IsExist(err) {
				log.Err(err).Str("path", path).Msg("unable to remove the JSON email")
				s.queue.Shutdown()
			}
		}
	}

	return true
}

// run starts processing the queue.
func (s Sender) run() <-chan struct{} {
	chQueue := make(chan struct{})
	go func() {
		for s.processNextEmail() {
		}
		chQueue <- struct{}{}
	}()
	return chQueue
}

// Run launches the queue processing and the outbox watcher.
// It catches `SIGINT` and `SIGTERM` to properly stopped the queue.
func (s Sender) Run() {
	log.Info().Msg("sender service is running")

	chSignal := make(chan os.Signal, 1)
	signal.Notify(chSignal, os.Interrupt, syscall.SIGTERM)

	s.watchOutbox()
	chQueue := s.run()

	select {
	case <-chSignal:
		log.Warn().Msg("stop signal received, stopping e-mail queue...")
		s.queue.Shutdown()
	case <-chQueue:
		log.Info().Msg("e-mail queue stopped successfully")
	}

	log.Info().Msg("sender service stopped successfully")
}