sgmail-skill/internal/parser/parser.go

package parser

import (
	"bytes"
	"encoding/base64"
	"fmt"
	"io"
	"mime"
	"mime/multipart"
	"mime/quotedprintable"
	"net/mail"
	"path/filepath"
	"strings"
	"time"

	"golang.org/x/net/html/charset"

	"internal-mail-skill/internal/model"
)

func Parse(raw []byte) (model.ParsedEmail, error) {
	msg, err := mail.ReadMessage(bytes.NewReader(raw))
	if err != nil {
		return model.ParsedEmail{}, fmt.Errorf("read message: %w", err)
	}

	parsed := model.ParsedEmail{
		MessageIDHeader: strings.TrimSpace(msg.Header.Get("Message-ID")),
		InReplyTo:       strings.TrimSpace(msg.Header.Get("In-Reply-To")),
		References:      strings.TrimSpace(msg.Header.Get("References")),
		Subject:         decodeHeader(msg.Header.Get("Subject")),
		DateHeader:      strings.TrimSpace(msg.Header.Get("Date")),
	}
	if sentAt, err := mail.ParseDate(parsed.DateHeader); err == nil {
		parsed.SentAt = &sentAt
	}
	if from, err := mail.ParseAddress(msg.Header.Get("From")); err == nil {
		parsed.SenderName = decodeHeader(from.Name)
		parsed.SenderEmail = strings.ToLower(from.Address)
	}
	parsed.To = parseAddressList(msg.Header.Get("To"))
	parsed.Cc = parseAddressList(msg.Header.Get("Cc"))

	if err := parsePart(mail.Header(msg.Header), msg.Body, &parsed); err != nil {
		return model.ParsedEmail{}, err
	}
	return parsed, nil
}

func parsePart(header mail.Header, body io.Reader, out *model.ParsedEmail) error {
	mediaType, params, err := mime.ParseMediaType(header.Get("Content-Type"))
	if err != nil || mediaType == "" {
		mediaType = "text/plain"
	}
	mediaType = strings.ToLower(mediaType)

	if strings.HasPrefix(mediaType, "multipart/") {
		boundary := params["boundary"]
		if boundary == "" {
			return fmt.Errorf("multipart message missing boundary")
		}
		mr := multipart.NewReader(body, boundary)
		for {
			part, err := mr.NextPart()
			if err == io.EOF {
				return nil
			}
			if err != nil {
				return fmt.Errorf("read multipart: %w", err)
			}
			if err := parsePart(mail.Header(part.Header), part, out); err != nil {
				return err
			}
		}
	}

	decoded := transferDecodedReader(header, body)
	filename := partFilename(header)
	disposition := strings.ToLower(header.Get("Content-Disposition"))
	isAttachment := filename != "" || strings.Contains(disposition, "attachment")

	if !isAttachment && strings.HasPrefix(mediaType, "text/") {
		text, err := readText(decoded, params["charset"])
		if err != nil {
			return fmt.Errorf("read text part: %w", err)
		}
		switch mediaType {
		case "text/html":
			appendText(&out.BodyHTML, text)
		default:
			appendText(&out.BodyText, text)
		}
		return nil
	}

	data, err := io.ReadAll(decoded)
	if err != nil {
		return fmt.Errorf("read attachment: %w", err)
	}
	if filename == "" {
		filename = "attachment.bin"
	}
	out.Attachments = append(out.Attachments, model.ParsedAttachment{
		Filename:    sanitizeFilename(filename),
		ContentType: mediaType,
		ContentID:   trimAngle(header.Get("Content-ID")),
		SizeBytes:   int64(len(data)),
		Data:        data,
	})
	return nil
}

func decodeHeader(value string) string {
	value = strings.TrimSpace(value)
	if value == "" {
		return ""
	}
	decoder := &mime.WordDecoder{CharsetReader: charset.NewReaderLabel}
	decoded, err := decoder.DecodeHeader(value)
	if err != nil {
		return value
	}
	return decoded
}

func parseAddressList(value string) []model.Address {
	value = strings.TrimSpace(value)
	if value == "" {
		return nil
	}
	list, err := mail.ParseAddressList(value)
	if err != nil {
		decoded := decodeHeader(value)
		list, err = mail.ParseAddressList(decoded)
		if err != nil {
			return nil
		}
	}
	out := make([]model.Address, 0, len(list))
	for _, addr := range list {
		out = append(out, model.Address{
			Name:  decodeHeader(addr.Name),
			Email: strings.ToLower(addr.Address),
		})
	}
	return out
}

func transferDecodedReader(header mail.Header, r io.Reader) io.Reader {
	switch strings.ToLower(strings.TrimSpace(header.Get("Content-Transfer-Encoding"))) {
	case "base64":
		return base64.NewDecoder(base64.StdEncoding, r)
	case "quoted-printable":
		return quotedprintable.NewReader(r)
	default:
		return r
	}
}

func readText(r io.Reader, label string) (string, error) {
	if label != "" {
		cr, err := charset.NewReaderLabel(label, r)
		if err == nil {
			r = cr
		}
	}
	data, err := io.ReadAll(r)
	if err != nil {
		return "", err
	}
	return string(data), nil
}

func partFilename(header mail.Header) string {
	if disp := header.Get("Content-Disposition"); disp != "" {
		_, params, err := mime.ParseMediaType(disp)
		if err == nil && params["filename"] != "" {
			return decodeHeader(params["filename"])
		}
	}
	if ct := header.Get("Content-Type"); ct != "" {
		_, params, err := mime.ParseMediaType(ct)
		if err == nil && params["name"] != "" {
			return decodeHeader(params["name"])
		}
	}
	return ""
}

func appendText(dst *string, value string) {
	if strings.TrimSpace(value) == "" {
		return
	}
	if strings.TrimSpace(*dst) == "" {
		*dst = value
		return
	}
	*dst += "\n" + value
}

func sanitizeFilename(name string) string {
	name = filepath.Base(strings.TrimSpace(name))
	name = strings.ReplaceAll(name, "\x00", "")
	name = strings.ReplaceAll(name, "/", "_")
	name = strings.ReplaceAll(name, "\\", "_")
	if name == "." || name == "" {
		return "attachment.bin"
	}
	return name
}

func trimAngle(value string) string {
	value = strings.TrimSpace(value)
	value = strings.TrimPrefix(value, "<")
	value = strings.TrimSuffix(value, ">")
	return value
}

func ParseTimeRFC3339(value string) *time.Time {
	if value == "" {
		return nil
	}
	t, err := time.Parse(time.RFC3339Nano, value)
	if err != nil {
		return nil
	}
	return &t
}