archivmail/pkg/mailparser/parser.go

package mailparser

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

// Attachment represents a MIME attachment in a parsed email.
type Attachment struct {
	Filename    string
	ContentType string
	Data        []byte
	Size        int
}

// ParsedMail holds the structured content of a parsed email message.
type ParsedMail struct {
	From        string
	To          []string
	CC          []string
	Subject     string
	MessageID   string
	InReplyTo   string   // In-Reply-To header (single message-id, no angle brackets)
	References  []string // References header (list of message-ids, no angle brackets)
	TextBody    string
	HTMLBody    string
	Date        time.Time
	Attachments []Attachment
	Raw         []byte
}

// Parse parses a raw RFC 2822 / MIME email and returns a ParsedMail.
func Parse(raw []byte) (*ParsedMail, error) {
	msg, err := mail.ReadMessage(bytes.NewReader(raw))
	if err != nil {
		return nil, fmt.Errorf("mailparser: read message: %w", err)
	}

	pm := &ParsedMail{Raw: raw}

	// From
	if from := msg.Header.Get("From"); from != "" {
		addrs, err := mail.ParseAddressList(from)
		if err == nil && len(addrs) > 0 {
			pm.From = addrs[0].Address
		} else {
			pm.From = from
		}
	}

	// To
	if to := msg.Header.Get("To"); to != "" {
		addrs, err := mail.ParseAddressList(to)
		if err == nil {
			for _, a := range addrs {
				pm.To = append(pm.To, a.Address)
			}
		}
	}

	// CC
	if cc := msg.Header.Get("Cc"); cc != "" {
		addrs, err := mail.ParseAddressList(cc)
		if err == nil {
			for _, a := range addrs {
				pm.CC = append(pm.CC, a.Address)
			}
		}
	}

	// Subject - decode MIME encoded-words
	pm.Subject = decodeMIMEHeader(msg.Header.Get("Subject"))

	// Message-ID - strip angle brackets
	msgID := msg.Header.Get("Message-Id")
	pm.MessageID = strings.Trim(msgID, "<>")

	// In-Reply-To - strip angle brackets
	if irt := msg.Header.Get("In-Reply-To"); irt != "" {
		pm.InReplyTo = strings.Trim(strings.TrimSpace(irt), "<>")
	}

	// References - space-separated list of message-ids
	if refs := msg.Header.Get("References"); refs != "" {
		for _, r := range strings.Fields(refs) {
			r = strings.Trim(r, "<>")
			if r != "" {
				pm.References = append(pm.References, r)
			}
		}
	}

	// Date — try go-message parser first, then fallback formats, then zero
	if d, err := msg.Header.Date(); err == nil {
		pm.Date = d
	} else {
		// Some MUAs emit non-standard variants (e.g. "+0100 (CET)" suffix).
		// Try common RFC 2822 / non-standard formats before giving up.
		raw := strings.TrimSpace(msg.Header.Get("Date"))
		// Strip parenthesised timezone comment: "... +0100 (CET)" → "... +0100"
		if idx := strings.LastIndex(raw, "("); idx > 0 {
			raw = strings.TrimSpace(raw[:idx])
		}
		parsed := false
		for _, layout := range []string{
			"Mon, 2 Jan 2006 15:04:05 -0700",
			"Mon, 02 Jan 2006 15:04:05 -0700",
			"2 Jan 2006 15:04:05 -0700",
			"02 Jan 2006 15:04:05 -0700",
			"Mon, 2 Jan 2006 15:04:05 MST",
			"Mon, 02 Jan 2006 15:04:05 MST",
			// Colon in timezone offset (e.g. "+02:00") used by some MTA versions
			"Mon, 2 Jan 2006 15:04:05 -07:00",
			"Mon, 02 Jan 2006 15:04:05 -07:00",
			"2 Jan 2006 15:04:05 -07:00",
			"02 Jan 2006 15:04:05 -07:00",
			// Without seconds
			"Mon, 2 Jan 2006 15:04 -0700",
			"Mon, 02 Jan 2006 15:04 -0700",
			"2 Jan 2006 15:04 -0700",
			// Go stdlib aliases
			time.RFC1123Z,
			time.RFC1123,
		} {
			if t, err := time.Parse(layout, raw); err == nil {
				pm.Date = t
				parsed = true
				break
			}
		}
		if !parsed {
			// Leave pm.Date as zero — storage will use DB DEFAULT NOW()
			pm.Date = time.Time{}
		}
	}

	// Parse body / MIME parts
	contentType := msg.Header.Get("Content-Type")
	mediaType, params, err := mime.ParseMediaType(contentType)
	if err != nil {
		// No content-type or parse error: treat as plain text
		body, _ := io.ReadAll(msg.Body)
		pm.TextBody = string(body)
		return pm, nil
	}

	if strings.HasPrefix(mediaType, "multipart/") {
		boundary := params["boundary"]
		if err := parseMultipart(pm, msg.Body, boundary); err != nil {
			return nil, fmt.Errorf("mailparser: multipart: %w", err)
		}
	} else {
		body, _ := io.ReadAll(msg.Body)
		decoded := decodeBody(body, msg.Header.Get("Content-Transfer-Encoding"))
		if strings.Contains(mediaType, "html") {
			pm.HTMLBody = string(decoded)
		} else {
			pm.TextBody = string(decoded)
		}
	}

	return pm, nil
}

// parseMultipart walks MIME parts and fills text, html, and attachments.
func parseMultipart(pm *ParsedMail, body io.Reader, boundary string) error {
	mr := multipart.NewReader(body, boundary)
	for {
		part, err := mr.NextPart()
		if err == io.EOF {
			break
		}
		if err != nil {
			return err
		}

		ct := part.Header.Get("Content-Type")
		mediaType, params, err := mime.ParseMediaType(ct)
		if err != nil {
			mediaType = "application/octet-stream"
			params = map[string]string{}
		}

		data, _ := io.ReadAll(part)
		cte := part.Header.Get("Content-Transfer-Encoding")
		decoded := decodeBody(data, cte)

		// Check disposition for attachment
		disp := part.Header.Get("Content-Disposition")
		dispType, dispParams, _ := mime.ParseMediaType(disp)
		filename := dispParams["filename"]
		if filename == "" {
			filename = params["name"]
		}
		filename = decodeMIMEHeader(filename)

		if strings.HasPrefix(dispType, "attachment") || filename != "" {
			pm.Attachments = append(pm.Attachments, Attachment{
				Filename:    filename,
				ContentType: mediaType,
				Data:        decoded,
				Size:        len(decoded),
			})
			continue
		}

		// Nested multipart
		if strings.HasPrefix(mediaType, "multipart/") {
			if err := parseMultipart(pm, bytes.NewReader(decoded), params["boundary"]); err != nil {
				return err
			}
			continue
		}

		switch {
		case strings.Contains(mediaType, "text/plain"):
			pm.TextBody += string(decoded)
		case strings.Contains(mediaType, "text/html"):
			pm.HTMLBody += string(decoded)
		}
	}
	return nil
}

// decodeBody decodes Content-Transfer-Encoding if needed.
func decodeBody(data []byte, cte string) []byte {
	switch strings.ToLower(strings.TrimSpace(cte)) {
	case "quoted-printable":
		decoded, err := io.ReadAll(quotedprintable.NewReader(bytes.NewReader(data)))
		if err == nil {
			return decoded
		}
	case "base64":
		clean := bytes.ReplaceAll(data, []byte("\r\n"), []byte{})
		clean = bytes.ReplaceAll(clean, []byte("\n"), []byte{})
		clean = bytes.ReplaceAll(clean, []byte("\r"), []byte{})
		decoded := make([]byte, base64.StdEncoding.DecodedLen(len(clean)))
		n, err := base64.StdEncoding.Decode(decoded, clean)
		if err == nil {
			return decoded[:n]
		}
	}
	return data
}

// decodeMIMEHeader decodes RFC 2047 encoded-word headers.
func decodeMIMEHeader(s string) string {
	dec := new(mime.WordDecoder)
	decoded, err := dec.DecodeHeader(s)
	if err != nil {
		return s
	}
	return decoded
}