✨ feat(email): pkg/email + Mailpit docker-compose service (ADR-0029 Phase A.1) (#59)

Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

pkg/email/smtp_sender.go

@@ -0,0 +1,155 @@
+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())
+}