feat: email adapter with SMTP TLS support

internal/adapter/adapter.go

@@ -22,7 +22,7 @@ func NewSender(channelType string, smtpCfg *config.SMTPConfig) (ChannelSender, e
 	case "bark":
 		return &BarkSender{}, nil
 	case "email":
-		return nil, fmt.Errorf("email sender not yet implemented (see Task 10)")
+		return NewEmailSender(*smtpCfg), nil
 	default:
 		return nil, fmt.Errorf("unknown channel type: %s", channelType)
 	}

internal/adapter/email.go

@@ -0,0 +1,123 @@
+package adapter
+
+import (
+	"crypto/tls"
+	"encoding/json"
+	"fmt"
+	"net"
+	"net/smtp"
+	"strings"
+
+	"aiaa-notification-service/internal/config"
+)
+
+type emailConfig struct {
+	To []string `json:"to"`
+}
+
+type EmailSender struct {
+	cfg config.SMTPConfig
+}
+
+func NewEmailSender(cfg config.SMTPConfig) *EmailSender {
+	return &EmailSender{cfg: cfg}
+}
+
+func (s *EmailSender) Type() string { return "email" }
+
+func (s *EmailSender) Send(title, content string, config json.RawMessage) error {
+	var ecfg emailConfig
+	if err := json.Unmarshal(config, &ecfg); err != nil {
+		return fmt.Errorf("parse email config: %w", err)
+	}
+	if len(ecfg.To) == 0 {
+		return fmt.Errorf("email: 'to' list is empty")
+	}
+
+	msg := buildEmail(s.cfg.From, ecfg.To, title, content)
+	addr := fmt.Sprintf("%s:%d", s.cfg.Host, s.cfg.Port)
+
+	if s.cfg.Port == 465 {
+		return s.sendTLS(addr, ecfg.To, msg)
+	}
+	return s.sendSTARTTLS(addr, ecfg.To, msg)
+}
+
+func (s *EmailSender) sendTLS(addr string, to []string, msg []byte) error {
+	tlsCfg := &tls.Config{ServerName: s.cfg.Host}
+	conn, err := tls.Dial("tcp", addr, tlsCfg)
+	if err != nil {
+		return fmt.Errorf("tls dial: %w", err)
+	}
+	defer conn.Close()
+
+	client, err := smtp.NewClient(conn, s.cfg.Host)
+	if err != nil {
+		return fmt.Errorf("smtp client: %w", err)
+	}
+	defer client.Quit()
+
+	return s.authAndSend(client, to, msg)
+}
+
+func (s *EmailSender) sendSTARTTLS(addr string, to []string, msg []byte) error {
+	conn, err := net.Dial("tcp", addr)
+	if err != nil {
+		return fmt.Errorf("dial: %w", err)
+	}
+	defer conn.Close()
+
+	client, err := smtp.NewClient(conn, s.cfg.Host)
+	if err != nil {
+		return fmt.Errorf("smtp client: %w", err)
+	}
+	defer client.Quit()
+
+	if ok, _ := client.Extension("STARTTLS"); ok {
+		tlsCfg := &tls.Config{ServerName: s.cfg.Host}
+		if err := client.StartTLS(tlsCfg); err != nil {
+			return fmt.Errorf("starttls: %w", err)
+		}
+	}
+
+	return s.authAndSend(client, to, msg)
+}
+
+func (s *EmailSender) authAndSend(client *smtp.Client, to []string, msg []byte) error {
+	if s.cfg.User != "" {
+		auth := smtp.PlainAuth("", s.cfg.User, s.cfg.Password, s.cfg.Host)
+		if err := client.Auth(auth); err != nil {
+			return fmt.Errorf("auth: %w", err)
+		}
+	}
+
+	if err := client.Mail(s.cfg.From); err != nil {
+		return fmt.Errorf("mail: %w", err)
+	}
+	for _, recipient := range to {
+		if err := client.Rcpt(recipient); err != nil {
+			return fmt.Errorf("rcpt %s: %w", recipient, err)
+		}
+	}
+	w, err := client.Data()
+	if err != nil {
+		return fmt.Errorf("data: %w", err)
+	}
+	_, err = w.Write(msg)
+	if err != nil {
+		return fmt.Errorf("write: %w", err)
+	}
+	return w.Close()
+}
+
+func buildEmail(from string, to []string, subject, body string) []byte {
+	var sb strings.Builder
+	sb.WriteString(fmt.Sprintf("From: %s\r\n", from))
+	sb.WriteString(fmt.Sprintf("To: %s\r\n", strings.Join(to, ", ")))
+	sb.WriteString(fmt.Sprintf("Subject: %s\r\n", subject))
+	sb.WriteString("MIME-Version: 1.0\r\n")
+	sb.WriteString("Content-Type: text/plain; charset=utf-8\r\n")
+	sb.WriteString("\r\n")
+	sb.WriteString(body)
+	return []byte(sb.String())
+}