Initial structure

internal/alerts/smtp.go

@@ -0,0 +1,78 @@
+package alerts
+
+import (
+	"crypto/tls"
+	"errors"
+	"fmt"
+	"net/smtp"
+	"strings"
+
+	"github.com/jasper/quptime/internal/config"
+)
+
+// sendSMTP delivers msg through the alert's SMTP relay. STARTTLS is
+// negotiated whenever the alert has SMTPStartTLS true; the smtp
+// server is responsible for advertising the extension.
+func sendSMTP(a *config.Alert, msg Message) error {
+	if a.SMTPHost == "" || a.SMTPPort == 0 {
+		return errors.New("smtp host/port not set")
+	}
+	if a.SMTPFrom == "" || len(a.SMTPTo) == 0 {
+		return errors.New("smtp from/to not set")
+	}
+
+	addr := fmt.Sprintf("%s:%d", a.SMTPHost, a.SMTPPort)
+	client, err := smtp.Dial(addr)
+	if err != nil {
+		return fmt.Errorf("dial smtp: %w", err)
+	}
+	defer client.Close()
+
+	if a.SMTPStartTLS {
+		if ok, _ := client.Extension("STARTTLS"); !ok {
+			return errors.New("server does not support STARTTLS")
+		}
+		if err := client.StartTLS(&tls.Config{ServerName: a.SMTPHost, MinVersion: tls.VersionTLS12}); err != nil {
+			return fmt.Errorf("starttls: %w", err)
+		}
+	}
+
+	if a.SMTPUser != "" {
+		auth := smtp.PlainAuth("", a.SMTPUser, a.SMTPPassword, a.SMTPHost)
+		if err := client.Auth(auth); err != nil {
+			return fmt.Errorf("smtp auth: %w", err)
+		}
+	}
+
+	if err := client.Mail(a.SMTPFrom); err != nil {
+		return fmt.Errorf("mail from: %w", err)
+	}
+	for _, rcpt := range a.SMTPTo {
+		if err := client.Rcpt(rcpt); err != nil {
+			return fmt.Errorf("rcpt %s: %w", rcpt, err)
+		}
+	}
+	w, err := client.Data()
+	if err != nil {
+		return fmt.Errorf("data: %w", err)
+	}
+	if _, err := w.Write(buildRFC822(a.SMTPFrom, a.SMTPTo, msg)); err != nil {
+		return fmt.Errorf("write body: %w", err)
+	}
+	if err := w.Close(); err != nil {
+		return fmt.Errorf("close body: %w", err)
+	}
+	return client.Quit()
+}
+
+func buildRFC822(from string, to []string, msg Message) []byte {
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "From: %s\r\n", from)
+	fmt.Fprintf(&sb, "To: %s\r\n", strings.Join(to, ", "))
+	fmt.Fprintf(&sb, "Subject: %s\r\n", msg.Subject)
+	fmt.Fprintf(&sb, "MIME-Version: 1.0\r\n")
+	fmt.Fprintf(&sb, "Content-Type: text/plain; charset=UTF-8\r\n")
+	fmt.Fprintf(&sb, "\r\n")
+	sb.WriteString(msg.Body)
+	return []byte(sb.String())
+}