test: add X-GM-EXT-1 round-trip tests and fix capability assertions

Covers the previously-untested X-GM-LABELS extension surface:

- imap/command: STORE/FETCH/SEARCH parser tests, incl. the bare-label
  (Python imaplib) form and NOT X-GM-LABELS — the exact wire forms
  Paperless-NGX emits.
- internal/response: itemGmailLabels formatting (empty/quoting/spaces).
- connector dummy: store labels as non-exclusive label mailboxes so
  STORE/FETCH/SEARCH round-trip end-to-end without touching folder
  membership; previous methods were no-op stubs.
- tests: STORE -> FETCH -> STORE remove round trip, multi-message,
  labels with spaces, and SEARCH / NOT X-GM-LABELS (the Paperless
  dedupe query).
- Fix TestCapability/TestCapabilityAuthenticateDisabled/login/
  authenticate which now must advertise X-GM-EXT-1 (sorted last);
  these were broken when the feature commit started advertising it.

Author: B. Olausson

connector/dummy.go

@@ -276,17 +276,57 @@ func (conn *Dummy) MarkMessagesForwarded(ctx context.Context, cache IMAPStateWri
 	return nil
 }
 
-func (conn *Dummy) MarkMessagesWithGmailLabels(_ context.Context, _ IMAPStateWrite, _ []imap.MessageID, _ []string, _ bool) error {
-	// Dummy connector: no-op for Gmail label operations.
+// MarkMessagesWithGmailLabels applies or removes Gmail-style labels. Each label
+// is backed by a non-exclusive mailbox; (un)labelling only (un)links the message
+// from that mailbox and never touches folder membership (e.g. INBOX), matching
+// the real Bridge connector contract.
+func (conn *Dummy) MarkMessagesWithGmailLabels(_ context.Context, _ IMAPStateWrite, messageIDs []imap.MessageID, labels []string, add bool) error {
+	for _, label := range labels {
+		var mboxID imap.MailboxID
+
+		if add {
+			id, mbox, created := conn.state.getOrCreateLabelMailbox(label)
+			mboxID = id
+
+			if created {
+				conn.pushUpdate(imap.NewMailboxCreated(mbox))
+			}
+		} else {
+			id, ok := conn.state.getLabelMailboxID(label)
+			if !ok {
+				// Removing a label that was never applied is a no-op.
+				continue
+			}
+
+			mboxID = id
+		}
+
+		for _, messageID := range messageIDs {
+			if add {
+				conn.state.addGmailLabel(messageID, label)
+				conn.state.addMessageToMailbox(messageID, mboxID)
+			} else {
+				conn.state.removeGmailLabel(messageID, label)
+				conn.state.removeMessageFromMailbox(messageID, mboxID)
+			}
+
+			conn.pushUpdate(imap.NewMessageMailboxesUpdated(
+				messageID,
+				conn.state.getMailboxIDs(messageID),
+				conn.state.getMessageFlags(messageID),
+			))
+		}
+	}
+
 	return nil
 }
 
-func (conn *Dummy) GetGmailLabels(_ context.Context, _ imap.MessageID) ([]string, error) {
-	return nil, nil
+func (conn *Dummy) GetGmailLabels(_ context.Context, messageID imap.MessageID) ([]string, error) {
+	return conn.state.getGmailLabels(messageID), nil
 }
 
-func (conn *Dummy) GetGmailLabelMailboxID(_ context.Context, _ string) (imap.MailboxID, bool) {
-	return "", false
+func (conn *Dummy) GetGmailLabelMailboxID(_ context.Context, label string) (imap.MailboxID, bool) {
+	return conn.state.getLabelMailboxID(label)
 }
 
 func (conn *Dummy) Sync(ctx context.Context) error {

connector/dummy_state.go

@@ -2,6 +2,7 @@ package connector
 
 import (
 	"context"
+	"sort"
 	"sync"
 	"time"
 
@@ -18,6 +19,11 @@ type dummyState struct {
 	mailboxes  map[imap.MailboxID]*dummyMailbox
 	lastIMAPID imap.IMAPID
 
+	// labelMailboxes maps a Gmail label name to the (non-exclusive) mailbox
+	// that backs it. Used by the X-GM-EXT-1 extension: applying a label adds
+	// the message to this mailbox without touching folder membership.
+	labelMailboxes map[string]imap.MailboxID
+
 	lock sync.RWMutex
 }
 
@@ -36,16 +42,20 @@ type dummyMessage struct {
 	flags     imap.FlagSet
 
 	mboxIDs map[imap.MailboxID]struct{}
+
+	// labels holds the Gmail-style labels (X-GM-LABELS) applied to this message.
+	labels map[string]struct{}
 }
 
 func newDummyState(flags, permFlags, attrs imap.FlagSet) *dummyState {
 	return &dummyState{
-		flags:      flags,
-		permFlags:  permFlags,
-		attrs:      attrs,
-		messages:   make(map[imap.MessageID]*dummyMessage),
-		mailboxes:  make(map[imap.MailboxID]*dummyMailbox),
-		lastIMAPID: imap.NewIMAPID(),
+		flags:          flags,
+		permFlags:      permFlags,
+		attrs:          attrs,
+		messages:       make(map[imap.MessageID]*dummyMessage),
+		mailboxes:      make(map[imap.MailboxID]*dummyMailbox),
+		lastIMAPID:     imap.NewIMAPID(),
+		labelMailboxes: make(map[string]imap.MailboxID),
 	}
 }
 
@@ -222,6 +232,7 @@ func (state *dummyState) createMessage(
 		flags:   otherFlags,
 		date:    date,
 		mboxIDs: map[imap.MailboxID]struct{}{mboxID: {}},
+		labels:  make(map[string]struct{}),
 	}
 
 	return state.toMessage(messageID)
@@ -245,6 +256,79 @@ func (state *dummyState) removeMessageFromMailbox(messageID imap.MessageID, mbox
 	delete(state.messages[messageID].mboxIDs, mboxID)
 }
 
+// getOrCreateLabelMailbox returns the (non-exclusive) mailbox backing the given
+// Gmail label, creating it on first use. The returned bool reports whether the
+// mailbox was newly created so the caller can emit a MailboxCreated update.
+func (state *dummyState) getOrCreateLabelMailbox(label string) (imap.MailboxID, imap.Mailbox, bool) {
+	state.lock.Lock()
+	defer state.lock.Unlock()
+
+	if mboxID, ok := state.labelMailboxes[label]; ok {
+		return mboxID, state.toMailbox(mboxID), false
+	}
+
+	mboxID := imap.MailboxID(uuid.NewString())
+
+	// Labels are non-exclusive: applying one must not evict the message from
+	// its folder (e.g. INBOX), matching real Bridge behaviour.
+	state.mailboxes[mboxID] = &dummyMailbox{
+		mboxName:  []string{label},
+		exclusive: false,
+	}
+	state.labelMailboxes[label] = mboxID
+
+	return mboxID, state.toMailbox(mboxID), true
+}
+
+func (state *dummyState) getLabelMailboxID(label string) (imap.MailboxID, bool) {
+	state.lock.Lock()
+	defer state.lock.Unlock()
+
+	mboxID, ok := state.labelMailboxes[label]
+
+	return mboxID, ok
+}
+
+func (state *dummyState) addGmailLabel(messageID imap.MessageID, label string) {
+	state.lock.Lock()
+	defer state.lock.Unlock()
+
+	msg := state.messages[messageID]
+	if msg.labels == nil {
+		msg.labels = make(map[string]struct{})
+	}
+
+	msg.labels[label] = struct{}{}
+}
+
+func (state *dummyState) removeGmailLabel(messageID imap.MessageID, label string) {
+	state.lock.Lock()
+	defer state.lock.Unlock()
+
+	if msg := state.messages[messageID]; msg.labels != nil {
+		delete(msg.labels, label)
+	}
+}
+
+func (state *dummyState) getGmailLabels(messageID imap.MessageID) []string {
+	state.lock.Lock()
+	defer state.lock.Unlock()
+
+	msg, ok := state.messages[messageID]
+	if !ok {
+		return nil
+	}
+
+	labels := make([]string, 0, len(msg.labels))
+	for label := range msg.labels {
+		labels = append(labels, label)
+	}
+
+	sort.Strings(labels)
+
+	return labels
+}
+
 func (state *dummyState) setSeen(messageID imap.MessageID, seen bool) {
 	state.lock.Lock()
 	defer state.lock.Unlock()