sgmail-skill/internal/smtp/smtp.go

package smtp

import (
	"bytes"
	"context"
	"crypto/rand"
	"crypto/tls"
	"encoding/hex"
	"fmt"
	"io"
	"mime"
	"net"
	"net/mail"
	stdsmtp "net/smtp"
	"os"
	"strings"
	"time"

	"internal-mail-skill/internal/config"
)

type SendRequest struct {
	FromAddress     string
	FromName        string
	To              []string
	Cc              []string
	Bcc             []string
	Subject         string
	Body            string
	InReplyTo       string
	References      string
	MessageIDHeader string
}

type SendResult struct {
	MessageIDHeader string
	Raw             []byte
	Recipients      []string
	SentAt          time.Time
}

type Sender struct {
	cfg config.SMTPConfig
}

func NewSender(cfg config.SMTPConfig) *Sender {
	return &Sender{cfg: cfg}
}

func (s *Sender) Send(ctx context.Context, req SendRequest) (SendResult, error) {
	if req.FromAddress == "" {
		req.FromAddress = s.cfg.FromAddress
	}
	if req.FromName == "" {
		req.FromName = s.cfg.FromName
	}
	raw, recipients, err := BuildMessage(req)
	if err != nil {
		return SendResult{}, err
	}
	if len(recipients) == 0 {
		return SendResult{}, fmt.Errorf("no recipients")
	}
	if s.cfg.Host == "" {
		return SendResult{}, fmt.Errorf("SMTP_HOST is required")
	}
	auth, err := authForConfig(s.cfg)
	if err != nil {
		return SendResult{}, err
	}
	if err := s.sendRaw(ctx, req.FromAddress, recipients, raw, auth); err != nil {
		return SendResult{}, err
	}
	return SendResult{
		MessageIDHeader: extractHeader(raw, "Message-ID"),
		Raw:             raw,
		Recipients:      recipients,
		SentAt:          time.Now().UTC(),
	}, nil
}

func BuildMessage(req SendRequest) ([]byte, []string, error) {
	if req.FromAddress == "" {
		return nil, nil, fmt.Errorf("from address is required")
	}
	recipients := append([]string{}, req.To...)
	recipients = append(recipients, req.Cc...)
	recipients = append(recipients, req.Bcc...)
	if len(recipients) == 0 {
		return nil, nil, fmt.Errorf("at least one recipient is required")
	}
	messageID := req.MessageIDHeader
	if messageID == "" {
		messageID = newMessageID()
	}
	var b bytes.Buffer
	writeHeader(&b, "From", (&mail.Address{Name: req.FromName, Address: req.FromAddress}).String())
	writeHeader(&b, "To", strings.Join(req.To, ", "))
	if len(req.Cc) > 0 {
		writeHeader(&b, "Cc", strings.Join(req.Cc, ", "))
	}
	writeHeader(&b, "Subject", mime.QEncoding.Encode("utf-8", req.Subject))
	writeHeader(&b, "Date", time.Now().Format(time.RFC1123Z))
	writeHeader(&b, "Message-ID", messageID)
	if req.InReplyTo != "" {
		writeHeader(&b, "In-Reply-To", req.InReplyTo)
	}
	if req.References != "" {
		writeHeader(&b, "References", req.References)
	}
	writeHeader(&b, "MIME-Version", "1.0")
	writeHeader(&b, "Content-Type", `text/plain; charset="utf-8"`)
	writeHeader(&b, "Content-Transfer-Encoding", "8bit")
	b.WriteString("\r\n")
	body := strings.ReplaceAll(req.Body, "\n", "\r\n")
	body = strings.ReplaceAll(body, "\r\r\n", "\r\n")
	b.WriteString(body)
	if !strings.HasSuffix(body, "\r\n") {
		b.WriteString("\r\n")
	}
	return b.Bytes(), recipients, nil
}

func (s *Sender) sendRaw(ctx context.Context, from string, recipients []string, raw []byte, auth stdsmtp.Auth) error {
	timeout := s.cfg.Timeout()
	if timeout <= 0 {
		timeout = 30 * time.Second
	}
	address := net.JoinHostPort(s.cfg.Host, fmt.Sprintf("%d", s.cfg.Port))
	dialer := &net.Dialer{Timeout: timeout}
	var conn net.Conn
	var err error
	if s.cfg.UseTLS {
		conn, err = tls.DialWithDialer(dialer, "tcp", address, &tls.Config{ServerName: s.cfg.Host, MinVersion: tls.VersionTLS12})
	} else {
		conn, err = dialer.DialContext(ctx, "tcp", address)
	}
	if err != nil {
		return fmt.Errorf("connect SMTP: %w", err)
	}
	defer conn.Close()

	client, err := stdsmtp.NewClient(conn, s.cfg.Host)
	if err != nil {
		return fmt.Errorf("create SMTP client: %w", err)
	}
	defer client.Close()

	if s.cfg.UseSTARTTLS {
		if err := client.StartTLS(&tls.Config{ServerName: s.cfg.Host, MinVersion: tls.VersionTLS12}); err != nil {
			return fmt.Errorf("starttls: %w", err)
		}
	}
	if auth != nil {
		if err := client.Auth(auth); err != nil {
			return fmt.Errorf("smtp auth: %w", err)
		}
	}
	if err := client.Mail(from); err != nil {
		return fmt.Errorf("smtp MAIL FROM: %w", err)
	}
	for _, recipient := range recipients {
		if strings.TrimSpace(recipient) == "" {
			continue
		}
		if err := client.Rcpt(recipient); err != nil {
			return fmt.Errorf("smtp RCPT TO %s: %w", recipient, err)
		}
	}
	writer, err := client.Data()
	if err != nil {
		return fmt.Errorf("smtp DATA: %w", err)
	}
	if _, err := io.Copy(writer, bytes.NewReader(raw)); err != nil {
		_ = writer.Close()
		return fmt.Errorf("write SMTP body: %w", err)
	}
	if err := writer.Close(); err != nil {
		return fmt.Errorf("close SMTP body: %w", err)
	}
	if err := client.Quit(); err != nil {
		return fmt.Errorf("smtp quit: %w", err)
	}
	return nil
}

func authForConfig(cfg config.SMTPConfig) (stdsmtp.Auth, error) {
	if !cfg.AuthRequired {
		return nil, nil
	}
	if !cfg.UseTLS && !cfg.UseSTARTTLS {
		if cfg.AllowInsecureAuth {
			return plainAuthNoTLSCheck{username: cfg.Username, password: cfg.Password}, nil
		}
		if cfg.Host != "localhost" && cfg.Host != "127.0.0.1" && cfg.Host != "::1" {
			return nil, fmt.Errorf("SMTP auth over cleartext requires SMTP_USE_TLS=true, SMTP_USE_STARTTLS=true, or SMTP_ALLOW_INSECURE_AUTH=true")
		}
	}
	if cfg.UseTLS {
		return plainAuthNoTLSCheck{username: cfg.Username, password: cfg.Password}, nil
	}
	return stdsmtp.PlainAuth("", cfg.Username, cfg.Password, cfg.Host), nil
}

type plainAuthNoTLSCheck struct {
	username string
	password string
}

func (a plainAuthNoTLSCheck) Start(server *stdsmtp.ServerInfo) (string, []byte, error) {
	resp := []byte("\x00" + a.username + "\x00" + a.password)
	return "PLAIN", resp, nil
}

func (a plainAuthNoTLSCheck) Next(fromServer []byte, more bool) ([]byte, error) {
	if more {
		return nil, fmt.Errorf("unexpected server challenge for PLAIN auth")
	}
	return nil, nil
}

func writeHeader(b *bytes.Buffer, key, value string) {
	if strings.TrimSpace(value) == "" {
		return
	}
	b.WriteString(key)
	b.WriteString(": ")
	b.WriteString(value)
	b.WriteString("\r\n")
}

func newMessageID() string {
	host, err := os.Hostname()
	if err != nil || host == "" {
		host = "localhost"
	}
	var random [8]byte
	if _, err := rand.Read(random[:]); err != nil {
		return fmt.Sprintf("<%d@%s>", time.Now().UnixNano(), host)
	}
	return fmt.Sprintf("<%d.%s@%s>", time.Now().UnixNano(), hex.EncodeToString(random[:]), host)
}

func extractHeader(raw []byte, name string) string {
	prefix := strings.ToLower(name) + ":"
	for _, line := range strings.Split(string(raw), "\r\n") {
		if strings.HasPrefix(strings.ToLower(line), prefix) {
			return strings.TrimSpace(line[len(name)+1:])
		}
	}
	return ""
}