Use UID-stable target dedup scan

backend/04-imap-source.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net"
 	"net/mail"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -259,6 +260,44 @@ func (m *imapClientMailbox) fetchHeaders(folder string, limit, offset int) ([]Me
 	return out, nil
 }
 
+func (m *imapClientMailbox) fetchAllHeadersByUID(folder string) ([]MessageHeader, 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
+	}
+	uidSet := imap.UIDSet{}
+	uidSet.AddRange(1, 0)
+	cmd := m.c.Fetch(uidSet, &imap.FetchOptions{
+		Envelope:     true,
+		Flags:        true,
+		InternalDate: true,
+		RFC822Size:   true,
+		UID:          true,
+	})
+	defer cmd.Close()
+
+	var out []MessageHeader
+	for {
+		data := cmd.Next()
+		if data == nil {
+			break
+		}
+		buf, err := data.Collect()
+		if err != nil {
+			return out, err
+		}
+		out = append(out, messageHeaderFromFetch(buf))
+	}
+	if err := cmd.Close(); err != nil {
+		return out, err
+	}
+	sortMessageHeadersByUID(out)
+	return out, nil
+}
+
 func (m *imapClientMailbox) fetchOne(folder string, uid uint32) (RawMessage, error) {
 	if uid == 0 {
 		return RawMessage{}, fmt.Errorf("Nachricht ohne UID")
@@ -418,6 +457,12 @@ func reverseHeaders(headers []MessageHeader) {
 	}
 }
 
+func sortMessageHeadersByUID(headers []MessageHeader) {
+	sort.SliceStable(headers, func(i, j int) bool {
+		return headers[i].UID < headers[j].UID
+	})
+}
+
 func messageIDFromFetch(buf *imapclient.FetchMessageBuffer, body []byte) string {
 	if buf != nil && buf.Envelope != nil && strings.TrimSpace(buf.Envelope.MessageID) != "" {
 		return normalizeMessageID(buf.Envelope.MessageID)