Mail-Graveyard/backend/04-imap-source.go

package backend

import (
	"crypto/sha256"
	"crypto/tls"
	"errors"
	"fmt"
	"net"
	"net/mail"
	"strconv"
	"strings"
	"time"

	"github.com/emersion/go-imap/v2"
	"github.com/emersion/go-imap/v2/imapclient"
)

// RawMessage ist eine 1:1 aus der Quelle geholte Nachricht: der Rohkoerper
// plus die Metadaten, die den sauberen Umzug ausmachen.
type RawMessage struct {
	MessageID    string    // aus Envelope/Header, Schluessel fuer Idempotenz
	Body         []byte    // kompletter RFC822-Rohtext, wird NICHT umgeschrieben
	Flags        []string  // \Seen \Answered \Flagged ... 1:1 uebernehmen
	InternalDate time.Time // Original-Zeit, per APPEND erhalten
}

// Folder ist ein Ordner der Quelle MIT den Infos, die 11-folders.go fuer die
// Rollen-Zuordnung braucht: dekodierter Name, SPECIAL-USE-Attribute und der
// Hierarchie-Trenner.
type Folder struct {
	Name  string   // dekodiert, z.B. "Geloeschte Objekte" oder "INBOX.Sent"
	Attrs []string // z.B. ["\\Sent"] aus SPECIAL-USE
	Delim string   // "." oder "/" - servereigener Trenner
}

// SourceMailbox abstrahiert Quelle (IMAP primaer, POP3 Fallback), damit
// 07-migrate.go gegen ein Interface arbeitet.
type SourceMailbox interface {
	Folders() ([]Folder, error)                // rekursiver Ordnerbaum
	Fetch(folder string) ([]RawMessage, error) // BODY[] FLAGS INTERNALDATE
	Close() error
}

func OpenIMAPSource(a Account) (SourceMailbox, error) {
	c, err := dialIMAPClient(a.SrcHost, a.SrcPort, a.SrcSecurity, a.SrcInsecure)
	if err != nil {
		return nil, err
	}
	if err := c.Login(a.SrcUser, a.SrcPass).Wait(); err != nil {
		_ = c.Close()
		return nil, err
	}
	return &imapSource{mailbox: &imapClientMailbox{c: c}}, nil
}

type imapSource struct {
	mailbox *imapClientMailbox
}

func (s *imapSource) Folders() ([]Folder, error) {
	boxes, err := s.mailbox.listMailboxes()
	if err != nil {
		return nil, err
	}
	folders := make([]Folder, 0, len(boxes))
	for _, box := range boxes {
		if hasIMAPAttr(box.Attrs, `\Noselect`) {
			continue
		}
		folders = append(folders, Folder{Name: box.Name, Attrs: box.Attrs, Delim: box.Delim})
	}
	if len(folders) == 0 {
		folders = append(folders, Folder{Name: "INBOX", Delim: "/"})
	}
	return folders, nil
}

func (s *imapSource) Fetch(folder string) ([]RawMessage, error) {
	return s.mailbox.fetchAll(folder)
}

func (s *imapSource) Close() error {
	return s.mailbox.Close()
}

type imapClientMailbox struct {
	c *imapclient.Client
}

type imapListMailbox struct {
	Name  string
	Attrs []string
	Delim string
}

func dialIMAPClient(host string, port int, security string, insecure bool) (*imapclient.Client, error) {
	addr := net.JoinHostPort(host, strconv.Itoa(port))
	opts := &imapclient.Options{
		TLSConfig: &tls.Config{
			ServerName:         host,
			MinVersion:         tls.VersionTLS12,
			InsecureSkipVerify: insecure, //nolint:gosec // explicit legacy-provider option
		},
	}
	switch strings.ToLower(strings.TrimSpace(security)) {
	case "tls", "":
		return imapclient.DialTLS(addr, opts)
	case "starttls":
		return imapclient.DialStartTLS(addr, opts)
	case "none":
		return imapclient.DialInsecure(addr, opts)
	default:
		return nil, fmt.Errorf("unbekannter IMAP-Security-Modus %q", security)
	}
}

func (m *imapClientMailbox) listMailboxes() ([]imapListMailbox, error) {
	cmd := m.c.List("", "*", &imap.ListOptions{ReturnSpecialUse: true})
	defer cmd.Close()

	var out []imapListMailbox
	for {
		data := cmd.Next()
		if data == nil {
			break
		}
		out = append(out, imapListMailbox{
			Name:  data.Mailbox,
			Attrs: mailboxAttrsToStrings(data.Attrs),
			Delim: delimString(data.Delim),
		})
	}
	if err := cmd.Close(); err != nil {
		return out, err
	}
	return out, nil
}

func (m *imapClientMailbox) fetchAll(folder string) ([]RawMessage, error) {
	selected, err := m.c.Select(folder, &imap.SelectOptions{ReadOnly: true}).Wait()
	if err != nil {
		return nil, err
	}
	if selected.NumMessages == 0 {
		return nil, nil
	}

	section := &imap.FetchItemBodySection{Peek: true}
	seqSet := imap.SeqSet{}
	seqSet.AddRange(1, 0)
	cmd := m.c.Fetch(seqSet, &imap.FetchOptions{
		Envelope:     true,
		Flags:        true,
		InternalDate: true,
		BodySection:  []*imap.FetchItemBodySection{section},
	})
	defer cmd.Close()

	var out []RawMessage
	for {
		data := cmd.Next()
		if data == nil {
			break
		}
		buf, err := data.Collect()
		if err != nil {
			return out, err
		}
		body := buf.FindBodySection(section)
		if body == nil {
			body = []byte{}
		}
		out = append(out, RawMessage{
			MessageID:    messageIDFromFetch(buf, body),
			Body:         body,
			Flags:        flagsToStrings(sanitizeIMAPFlags(buf.Flags)),
			InternalDate: buf.InternalDate,
		})
	}
	if err := cmd.Close(); err != nil {
		return out, err
	}
	return out, nil
}

func (m *imapClientMailbox) Close() error {
	if m == nil || m.c == nil {
		return nil
	}
	if err := m.c.Logout().Wait(); err != nil {
		return m.c.Close()
	}
	return m.c.Close()
}

func hasIMAPAttr(attrs []string, want string) bool {
	for _, attr := range attrs {
		if strings.EqualFold(attr, want) {
			return true
		}
	}
	return false
}

func mailboxAttrsToStrings(attrs []imap.MailboxAttr) []string {
	out := make([]string, 0, len(attrs))
	for _, attr := range attrs {
		out = append(out, string(attr))
	}
	return out
}

func delimString(delim rune) string {
	if delim == 0 {
		return "/"
	}
	return string(delim)
}

func sanitizeIMAPFlags(in []imap.Flag) []imap.Flag {
	out := make([]imap.Flag, 0, len(in))
	for _, flag := range in {
		switch strings.ToLower(string(flag)) {
		case `\recent`, `\*`:
			continue
		default:
			out = append(out, flag)
		}
	}
	return out
}

func flagsToStrings(flags []imap.Flag) []string {
	out := make([]string, 0, len(flags))
	for _, flag := range flags {
		out = append(out, string(flag))
	}
	return out
}

func stringsToFlags(flags []string) []imap.Flag {
	return sanitizeIMAPFlags(func() []imap.Flag {
		out := make([]imap.Flag, 0, len(flags))
		for _, flag := range flags {
			flag = strings.TrimSpace(flag)
			if flag != "" {
				out = append(out, imap.Flag(flag))
			}
		}
		return out
	}())
}

func messageIDFromFetch(buf *imapclient.FetchMessageBuffer, body []byte) string {
	if buf != nil && buf.Envelope != nil && strings.TrimSpace(buf.Envelope.MessageID) != "" {
		return strings.TrimSpace(buf.Envelope.MessageID)
	}
	return messageID(body)
}

func messageID(body []byte) string {
	msg, err := mail.ReadMessage(strings.NewReader(string(body)))
	if err == nil {
		if id := strings.TrimSpace(msg.Header.Get("Message-ID")); id != "" {
			return id
		}
	}
	return fmt.Sprintf("sha256:%x", sha256.Sum256(body))
}

func isAlreadyExistsError(err error) bool {
	var imapErr *imap.Error
	if errors.As(err, &imapErr) {
		text := strings.ToLower(imapErr.Text)
		return strings.Contains(text, "exist") || strings.Contains(text, "already")
	}
	text := strings.ToLower(err.Error())
	return strings.Contains(text, "exist") || strings.Contains(text, "already")
}