Method ModifyMessageLabels

internal/identity/store.go:2631–2690

ModifyMessageLabels applies a delta — add then remove — to a message's labels[] in a single atomic statement. Returns the updated labels (deduplicated, sorted) so the caller can echo them back in the response without a second round-trip. Inputs are assumed already normalized (lowercased, charset-va

(ctx context.Context, messageID, agentID string, add, remove []string)

// The whole thing runs as one UPDATE so a concurrent PATCH from a
// second client can't observe a partial state.
func (s *Store) ModifyMessageLabels(ctx context.Context, messageID, agentID string, add, remove []string) ([]string, error) {
	// Pre-check the post-add length against the cap. Done as a
	// dedicated SELECT-then-UPDATE so we can return a specific error
	// rather than a generic constraint violation — the handler maps
	// ErrLabelLimitExceeded to 400 with a useful message.
	tx, err := s.pool.Begin(ctx)
	if err != nil {
		return nil, err
	}
	defer tx.Rollback(ctx)

	var current []string
	err = tx.QueryRow(ctx,
		`SELECT labels FROM messages WHERE id = $1 AND agent_id = $2 AND expires_at > now() AND NOT (direction = 'inbound' AND status IN (`+heldInboundStatuses+`)) FOR UPDATE`,
		messageID, agentID,
	).Scan(&current)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return nil, ErrMessageNotFound
		}
		return nil, err
	}

	// Apply the delta in-memory so the cap check is exact. The set
	// semantics here mirror what the SQL UPDATE below does:
	// labels' = (labels ∪ add) \ remove.
	labelSet := map[string]struct{}{}
	for _, l := range current {
		labelSet[l] = struct{}{}
	}
	for _, l := range add {
		labelSet[l] = struct{}{}
	}
	for _, l := range remove {
		delete(labelSet, l)
	}
	if len(labelSet) > MaxLabelsPerMessage {
		return nil, ErrLabelLimitExceeded
	}

	final := make([]string, 0, len(labelSet))
	for l := range labelSet {
		final = append(final, l)
	}
	sort.Strings(final)

	if _, err := tx.Exec(ctx,
		`UPDATE messages SET labels = $1 WHERE id = $2 AND agent_id = $3 AND NOT (direction = 'inbound' AND status IN (`+heldInboundStatuses+`))`,
		final, messageID, agentID,
	); err != nil {
		return nil, err
	}
	if err := tx.Commit(ctx); err != nil {
		return nil, err
	}
	if final == nil {
		final = []string{}
	}