You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: backend.go
    path of file: ./repos/vault/builtin/logical/rabbitmq
    
    the code of the file until where you have to start completion: // Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: BUSL-1.1

package rabbitmq

import (
	"context"
	"s
    
func (b *backend) Client(ctx context.Context, s logical.Storage) (*rabbithole.Client, error) {
	b.lock.RLock()

	// If we already have a client, return it
	if b.client != nil {
		b.lock.RUnlock()
		return b.client, nil
	}

	b.lock.RUnlock()

	// Otherwise, attempt to make connection
	connConfig, err := readConfig(ctx, s)
	if err != nil {
		return nil, err
	}

	b.lock.Lock()
	defer b.lock.Unlock()

	// If the client was created during the lock switch, return it
	if b.client != nil {
		return b.client, nil
	}

	b.client, err = rabbithole.NewClient(connConfig.URI, connConfig.Username, connConfig.Password)
	if err != nil {
		return nil, err
	}
	// Use a default pooled transport so there would be no leaked file descriptors
	b.client.SetTransport(cleanhttp.DefaultPooledTransport())

	return b.client, nil
}