smtp/new.go

package smtp

import (
	"bytes"
	"context"
	"encoding/base64"
	"fmt"
	"io"
	"mime/multipart"
	"mime/quotedprintable"
	"net"
	netsmtp "net/smtp"
	"net/textproto"
	"strings"

	"code.nochebuena.dev/einherjar/contracts/logging"
	"code.nochebuena.dev/einherjar/contracts/observability"
	"code.nochebuena.dev/einherjar/core/xerrors"
)

// Compile-time interface verification (I-8 / CT-5).
var _ Component = (*smtpClient)(nil)
var _ Component = (*noopClient)(nil)
var _ observability.Identifiable = (*smtpClient)(nil)
var _ observability.Identifiable = (*noopClient)(nil)

// New returns a Component backed by the given configuration.
// When cfg.Host is empty, New returns a no-op client that logs a warning
// and silently discards every message — SMTP absence must never block a transaction.
func New(logger logging.Logger, cfg Config) Component {
	if cfg.Host == "" {
		logger.Warn("smtp: host not configured, using no-op sender")
		return &noopClient{logger: logger}
	}
	return &smtpClient{logger: logger, cfg: cfg}
}

// --- smtpClient ---

type smtpClient struct {
	logger logging.Logger
	cfg    Config
}

func (c *smtpClient) OnInit() error  { return nil }
func (c *smtpClient) OnStart() error { return nil }
func (c *smtpClient) OnStop() error  { return nil }

func (c *smtpClient) Name() string                  { return "smtp" }
func (c *smtpClient) Priority() observability.Level { return observability.LevelDegraded }

func (c *smtpClient) HealthCheck(ctx context.Context) error {
	addr := fmt.Sprintf("%s:%d", c.cfg.Host, c.cfg.Port)
	conn, err := (&net.Dialer{}).DialContext(ctx, "tcp", addr)
	if err != nil {
		return xerrors.New(xerrors.ErrUnavailable, "smtp: unreachable").WithError(err)
	}
	_ = conn.Close()
	return nil
}

func (c *smtpClient) Send(_ context.Context, msg Message) error {
	raw, err := buildRawMessage(c.cfg.From, msg)
	if err != nil {
		return xerrors.New(xerrors.ErrInternal, "smtp: build message").WithError(err)
	}

	all := make([]string, 0, len(msg.To)+len(msg.CC)+len(msg.BCC))
	all = append(all, msg.To...)
	all = append(all, msg.CC...)
	all = append(all, msg.BCC...)

	addr := fmt.Sprintf("%s:%d", c.cfg.Host, c.cfg.Port)
	var auth netsmtp.Auth
	if c.cfg.User != "" {
		auth = netsmtp.PlainAuth("", c.cfg.User, c.cfg.Password, c.cfg.Host)
	}
	if err := netsmtp.SendMail(addr, auth, c.cfg.From, all, raw); err != nil {
		return xerrors.New(xerrors.ErrInternal, "smtp: send").WithError(err)
	}
	c.logger.Info("smtp: sent", "subject", msg.Subject, "to", msg.To)
	return nil
}

// buildRawMessage constructs a RFC 5322 MIME message ready for smtp.SendMail.
// BCC recipients are included in the SMTP envelope (via the caller) but are
// intentionally omitted from the message headers.
func buildRawMessage(from string, msg Message) ([]byte, error) {
	var buf bytes.Buffer

	ct := msg.ContentType
	if ct == "" {
		ct = "text/plain"
	}

	if len(msg.Attachments) == 0 {
		writeHeader(&buf, "From", from)
		writeHeader(&buf, "To", strings.Join(msg.To, ", "))
		if len(msg.CC) > 0 {
			writeHeader(&buf, "Cc", strings.Join(msg.CC, ", "))
		}
		if msg.ReplyTo != "" {
			writeHeader(&buf, "Reply-To", msg.ReplyTo)
		}
		writeHeader(&buf, "Subject", msg.Subject)
		writeHeader(&buf, "MIME-Version", "1.0")
		writeHeader(&buf, "Content-Type", ct+"; charset=utf-8")
		buf.WriteString("\r\n")
		buf.WriteString(msg.Body)
		return buf.Bytes(), nil
	}

	// multipart/mixed for messages with attachments.
	// NewWriter is created first to obtain the boundary before writing headers.
	mw := multipart.NewWriter(&buf)

	writeHeader(&buf, "From", from)
	writeHeader(&buf, "To", strings.Join(msg.To, ", "))
	if len(msg.CC) > 0 {
		writeHeader(&buf, "Cc", strings.Join(msg.CC, ", "))
	}
	if msg.ReplyTo != "" {
		writeHeader(&buf, "Reply-To", msg.ReplyTo)
	}
	writeHeader(&buf, "Subject", msg.Subject)
	writeHeader(&buf, "MIME-Version", "1.0")
	writeHeader(&buf, "Content-Type", "multipart/mixed; boundary="+mw.Boundary())
	buf.WriteString("\r\n")

	bodyHeader := textproto.MIMEHeader{}
	bodyHeader.Set("Content-Type", ct+"; charset=utf-8")
	bodyHeader.Set("Content-Transfer-Encoding", "quoted-printable")
	bodyPart, err := mw.CreatePart(bodyHeader)
	if err != nil {
		return nil, err
	}
	qpw := quotedprintable.NewWriter(bodyPart)
	if _, err := io.WriteString(qpw, msg.Body); err != nil {
		return nil, err
	}
	if err := qpw.Close(); err != nil {
		return nil, err
	}

	for _, att := range msg.Attachments {
		attCT := att.ContentType
		if attCT == "" {
			attCT = "application/octet-stream"
		}
		attHeader := textproto.MIMEHeader{}
		attHeader.Set("Content-Type", attCT)
		attHeader.Set("Content-Transfer-Encoding", "base64")
		attHeader.Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s"`, att.Name))
		attPart, err := mw.CreatePart(attHeader)
		if err != nil {
			return nil, err
		}
		enc := base64.NewEncoder(base64.StdEncoding, attPart)
		if _, err := io.Copy(enc, att.Data); err != nil {
			return nil, err
		}
		if err := enc.Close(); err != nil {
			return nil, err
		}
	}

	if err := mw.Close(); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

func writeHeader(buf *bytes.Buffer, key, value string) {
	fmt.Fprintf(buf, "%s: %s\r\n", key, value)
}