dance-lessons-coach/pkg/email/smtp_sender.go

package email

import (
	"context"
	"errors"
	"fmt"
	"net/smtp"
	"net/textproto"
	"strings"
	"time"
)

// SMTPConfig configures an SMTPSender. Defaults match local Mailpit
// (host=localhost, port=1025, no TLS, no auth).
type SMTPConfig struct {
	Host     string
	Port     int
	Username string // empty means no AUTH
	Password string // empty means no AUTH
	UseTLS   bool   // STARTTLS — false for Mailpit local
	// Timeout bounds Send to avoid hanging forever on a stuck server.
	// Defaults to 10s when zero.
	Timeout time.Duration
}

// SMTPSender sends mail through an SMTP server. Configured by SMTPConfig
// with sensible Mailpit-friendly defaults.
type SMTPSender struct {
	cfg SMTPConfig
}

// NewSMTPSender returns a Sender backed by SMTP. The SMTPConfig is copied —
// mutating the caller's struct after this call has no effect.
func NewSMTPSender(cfg SMTPConfig) *SMTPSender {
	if cfg.Host == "" {
		cfg.Host = "localhost"
	}
	if cfg.Port == 0 {
		cfg.Port = 1025
	}
	if cfg.Timeout == 0 {
		cfg.Timeout = 10 * time.Second
	}
	return &SMTPSender{cfg: cfg}
}

// Send delivers the message via SMTP. Returns an error if the message is
// invalid (missing required fields), if connecting / authenticating fails,
// or if the SMTP server rejects the envelope or data.
func (s *SMTPSender) Send(ctx context.Context, msg Message) error {
	if err := validateMessage(msg); err != nil {
		return err
	}

	addr := fmt.Sprintf("%s:%d", s.cfg.Host, s.cfg.Port)
	body := buildRFC5322(msg)

	var auth smtp.Auth
	if s.cfg.Username != "" {
		auth = smtp.PlainAuth("", s.cfg.Username, s.cfg.Password, s.cfg.Host)
	}

	// Run the SMTP exchange in a goroutine so we can honour the
	// context's cancellation independently of net/smtp's own timeouts.
	done := make(chan error, 1)
	go func() {
		done <- smtp.SendMail(addr, auth, msg.From, []string{msg.To}, body)
	}()

	select {
	case err := <-done:
		if err != nil {
			return fmt.Errorf("smtp send to %s: %w", msg.To, err)
		}
		return nil
	case <-ctx.Done():
		return ctx.Err()
	case <-time.After(s.cfg.Timeout):
		return errors.New("smtp send: timeout")
	}
}

// validateMessage checks the minimum required fields for an outgoing email.
func validateMessage(msg Message) error {
	if msg.To == "" {
		return errors.New("email: To is required")
	}
	if msg.From == "" {
		return errors.New("email: From is required")
	}
	if msg.BodyText == "" && msg.BodyHTML == "" {
		return errors.New("email: at least one of BodyText or BodyHTML is required")
	}
	return nil
}

// buildRFC5322 builds an RFC 5322 message body from Message. Plain-text
// only when BodyHTML is empty ; multipart/alternative when both are set.
//
// Keep in mind: this is the body sent to net/smtp.SendMail, which adds
// no headers itself. We add the canonical From, To, Subject, MIME-Version,
// Content-Type, and any caller-provided custom headers.
func buildRFC5322(msg Message) []byte {
	var b strings.Builder

	// Custom headers first so they show up early in the message.
	for k, v := range msg.Headers {
		// Normalise header name case (Foo-Bar, not foo-BAR)
		k = textproto.CanonicalMIMEHeaderKey(k)
		fmt.Fprintf(&b, "%s: %s\r\n", k, v)
	}

	fmt.Fprintf(&b, "From: %s\r\n", msg.From)
	fmt.Fprintf(&b, "To: %s\r\n", msg.To)
	fmt.Fprintf(&b, "Subject: %s\r\n", msg.Subject)
	fmt.Fprintf(&b, "MIME-Version: 1.0\r\n")

	if msg.BodyHTML == "" {
		// Plain text only
		fmt.Fprintf(&b, "Content-Type: text/plain; charset=utf-8\r\n")
		fmt.Fprintf(&b, "\r\n")
		b.WriteString(msg.BodyText)
		return []byte(b.String())
	}

	// multipart/alternative — boundary is deterministic for tests but unique
	// enough not to collide with body content. We don't put random bytes in
	// the boundary because the alphabetical "alt-" prefix is recognisably
	// ours and the rest is timestamped.
	boundary := fmt.Sprintf("alt-%d", time.Now().UnixNano())
	fmt.Fprintf(&b, "Content-Type: multipart/alternative; boundary=%q\r\n", boundary)
	fmt.Fprintf(&b, "\r\n")

	// Plain part (always first — clients that only render text/plain see
	// it ; clients preferring HTML go to the HTML part)
	fmt.Fprintf(&b, "--%s\r\n", boundary)
	fmt.Fprintf(&b, "Content-Type: text/plain; charset=utf-8\r\n\r\n")
	if msg.BodyText != "" {
		b.WriteString(msg.BodyText)
	} else {
		b.WriteString("(see HTML version)")
	}
	fmt.Fprintf(&b, "\r\n")

	// HTML part
	fmt.Fprintf(&b, "--%s\r\n", boundary)
	fmt.Fprintf(&b, "Content-Type: text/html; charset=utf-8\r\n\r\n")
	b.WriteString(msg.BodyHTML)
	fmt.Fprintf(&b, "\r\n")

	// Close boundary
	fmt.Fprintf(&b, "--%s--\r\n", boundary)

	return []byte(b.String())
}