email_agent/internal/mailops/smtp.go

// SMTP send path for Phase 5 replies (FR-8, FR-15).
//
// Gmail SMTP is reached over IPv4 only, exactly like the IMAP read path
// (AGENTS.md §9): IPv6 to Gmail is broken from .13. The connection upgrades with
// STARTTLS on port 587 and authenticates with the same app password as IMAP.
//
// Reply identity (FR-15): the From header is the address the mail arrived at
// (X-Pm-Original-To). When that is unknown, it falls back to Sam Rolfe at the
// Gmail account. The SMTP envelope sender is always the authenticated Gmail
// account, which is what Gmail expects; the From header carries the alias.
package mailops

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

	"gitea.lab.audasmedia.com.au/sam/email_agent/internal/mailfetch"
	"gitea.lab.audasmedia.com.au/sam/email_agent/internal/store"
)

// ReplyFromName is the display name used when the arrived-at address is unknown
// (AGENTS.md §13.1: "Otherwise send as Sam Rolfe").
const ReplyFromName = "Sam Rolfe"

// OutgoingMessage is one reply to send.
type OutgoingMessage struct {
	// To is the recipient (the original sender).
	To string
	// From is the From header value, normally "Sam Rolfe <arrived-at@address>".
	From string
	// Subject is the reply subject (already prefixed with Re: when appropriate).
	Subject string
	// InReplyTo and References carry the original Message-ID so Gmail threads
	// the reply.
	InReplyTo  string
	References string
	// Body is the plain-text reply body.
	Body string
}

// Sender sends a reply. The live implementation is *Client; tests substitute a
// fake.
type Sender interface {
	Send(ctx context.Context, msg OutgoingMessage) error
}

// SMTPConfig is the connection detail for the Gmail SMTP submission endpoint.
type SMTPConfig struct {
	Host     string
	Port     int
	User     string
	Password string
}

// smtpDialFunc opens a TCP connection. It is a field so tests can assert the
// IPv4-only network is used.
type smtpDialFunc func(ctx context.Context, host string, port int, timeout time.Duration) (net.Conn, error)

// dialSMTP4 forces IPv4, matching mailfetch.Dial on the read path.
func dialSMTP4(ctx context.Context, host string, port int, timeout time.Duration) (net.Conn, error) {
	d := net.Dialer{Timeout: timeout}
	return d.DialContext(ctx, "tcp4", net.JoinHostPort(host, fmt.Sprintf("%d", port)))
}

// ReplyIdentity returns the From header for a reply (FR-15): the address the
// mail arrived at, otherwise Sam Rolfe at the Gmail account.
func ReplyIdentity(msg store.Message, gmailUser string) string {
	addr := strings.TrimSpace(msg.SourceAddress)
	if addr == "" {
		addr = strings.TrimSpace(gmailUser)
	}
	if addr == "" {
		return ReplyFromName
	}
	return (&mail.Address{Name: ReplyFromName, Address: addr}).String()
}

// ReplyTo returns the recipient of a reply: the original sender.
func ReplyTo(msg store.Message) string {
	addr := strings.TrimSpace(msg.FromAddr)
	if addr == "" {
		return ""
	}
	if parsed, err := mail.ParseAddress(addr); err == nil {
		return parsed.Address
	}
	return addr
}

// ReplySubject prefixes the original subject with "Re: " once.
func ReplySubject(subject string) string {
	s := strings.TrimSpace(subject)
	if s == "" {
		return "Re: (no subject)"
	}
	if strings.HasPrefix(strings.ToLower(s), "re:") {
		return s
	}
	return "Re: " + s
}

// CheckSMTP verifies the submission path without sending mail: it dials Gmail
// over IPv4, upgrades with STARTTLS, and authenticates with the app password.
// It stops before MAIL FROM, so no message is sent. It is the safe part of the
// Phase 5 send-path verification.
func (c *Client) CheckSMTP(ctx context.Context) error {
	if !c.cfg.GmailConfigured() {
		return ErrNotConfigured
	}
	host := c.cfg.SMTPHost
	port := c.cfg.SMTPPort

	conn, err := c.smtpDial(ctx, host, port, mailfetch.DefaultTimeout)
	if err != nil {
		return fmt.Errorf("dial smtp %s:%d (ipv4): %w", host, port, err)
	}
	client, err := smtp.NewClient(conn, host)
	if err != nil {
		_ = conn.Close()
		return fmt.Errorf("smtp handshake: %w", err)
	}
	defer func() { _ = client.Close() }()

	if ok, _ := client.Extension("STARTTLS"); ok {
		if err := client.StartTLS(&tls.Config{ServerName: host, MinVersion: tls.VersionTLS12}); err != nil {
			return fmt.Errorf("smtp starttls: %w", err)
		}
	}
	if err := client.Auth(smtp.PlainAuth("", c.cfg.GmailUser, c.cfg.GmailAppPassword, host)); err != nil {
		return fmt.Errorf("smtp auth: %w", err)
	}
	c.logger.Info("smtp check ok", "host", host, "port", port, "user", c.cfg.GmailUser)
	return client.Quit()
}

// Send builds and sends one reply over IPv4 + STARTTLS + the app password.
func (c *Client) Send(ctx context.Context, msg OutgoingMessage) error {
	if !c.cfg.GmailConfigured() {
		return ErrNotConfigured
	}
	if strings.TrimSpace(msg.To) == "" {
		return fmt.Errorf("reply has no recipient")
	}
	host := c.cfg.SMTPHost
	port := c.cfg.SMTPPort

	conn, err := c.smtpDial(ctx, host, port, mailfetch.DefaultTimeout)
	if err != nil {
		return fmt.Errorf("dial smtp %s:%d (ipv4): %w", host, port, err)
	}
	client, err := smtp.NewClient(conn, host)
	if err != nil {
		_ = conn.Close()
		return fmt.Errorf("smtp handshake: %w", err)
	}
	defer func() { _ = client.Close() }()

	if ok, _ := client.Extension("STARTTLS"); ok {
		if err := client.StartTLS(&tls.Config{ServerName: host, MinVersion: tls.VersionTLS12}); err != nil {
			return fmt.Errorf("smtp starttls: %w", err)
		}
	}
	if err := client.Auth(smtp.PlainAuth("", c.cfg.GmailUser, c.cfg.GmailAppPassword, host)); err != nil {
		return fmt.Errorf("smtp auth: %w", err)
	}
	// The envelope sender is the authenticated account; the From header carries
	// the arrived-at alias (FR-15).
	if err := client.Mail(c.cfg.GmailUser); err != nil {
		return fmt.Errorf("smtp mail from: %w", err)
	}
	if err := client.Rcpt(msg.To); err != nil {
		return fmt.Errorf("smtp rcpt to %s: %w", msg.To, err)
	}
	w, err := client.Data()
	if err != nil {
		return fmt.Errorf("smtp data: %w", err)
	}
	raw := BuildRFC822(msg, c.cfg.GmailUser, time.Now().UTC())
	if _, err := w.Write(raw); err != nil {
		_ = w.Close()
		return fmt.Errorf("write reply: %w", err)
	}
	if err := w.Close(); err != nil {
		return fmt.Errorf("finish reply: %w", err)
	}
	if err := client.Quit(); err != nil {
		return fmt.Errorf("smtp quit: %w", err)
	}
	c.logger.Info("reply sent", "to", msg.To, "from", msg.From, "subject", msg.Subject)
	return nil
}

// BuildRFC822 renders the reply as an RFC822 message. The Message-ID is
// generated and the headers are CRLF-terminated for SMTP. It is exported so the
// wire format can be tested without a live server.
func BuildRFC822(msg OutgoingMessage, gmailUser string, now time.Time) []byte {
	var b strings.Builder
	writeHeader(&b, "From", msg.From)
	writeHeader(&b, "To", msg.To)
	writeHeader(&b, "Subject", msg.Subject)
	writeHeader(&b, "Date", now.Format(time.RFC1123Z))
	writeHeader(&b, "Message-ID", generateMessageID(now, gmailUser))
	if v := strings.TrimSpace(msg.InReplyTo); v != "" {
		writeHeader(&b, "In-Reply-To", ensureAngle(v))
	}
	if v := strings.TrimSpace(msg.References); v != "" {
		writeHeader(&b, "References", ensureAngle(v))
	}
	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")
	b.WriteString(strings.ReplaceAll(strings.ReplaceAll(msg.Body, "\r\n", "\n"), "\n", "\r\n"))
	if !strings.HasSuffix(b.String(), "\r\n") {
		b.WriteString("\r\n")
	}
	return []byte(b.String())
}

// writeHeader encodes a header value when it contains non-ASCII text.
func writeHeader(b *strings.Builder, key, value string) {
	value = strings.TrimSpace(value)
	for _, r := range value {
		if r > 127 {
			value = mime.QEncoding.Encode("UTF-8", value)
			break
		}
	}
	b.WriteString(key)
	b.WriteString(": ")
	b.WriteString(value)
	b.WriteString("\r\n")
}

// generateMessageID builds a unique Message-ID for an outgoing reply.
func generateMessageID(now time.Time, user string) string {
	domain := "email-agent.local"
	if user != "" {
		if i := strings.LastIndex(user, "@"); i >= 0 && i < len(user)-1 {
			domain = user[i+1:]
		}
	}
	buf := make([]byte, 8)
	_, _ = rand.Read(buf)
	return fmt.Sprintf("<%d.%s@%s>", now.UnixNano(), hex.EncodeToString(buf), domain)
}

// ensureAngle wraps a Message-ID in angle brackets when it is not already.
func ensureAngle(id string) string {
	id = strings.TrimSpace(id)
	if strings.HasPrefix(id, "<") && strings.HasSuffix(id, ">") {
		return id
	}
	return "<" + strings.Trim(id, "<>") + ">"
}