void ServiceContext::ClientDeleter::operator()(Client* client) const {
    ServiceContext* const service = client->getServiceContext();
    {
        stdx::lock_guard<stdx::mutex> lk(service->_mutex);
        invariant(service->_clients.erase(client));
        if (service->_clients.empty()) {
            service->_clientsEmptyCondVar.notify_all();
        }
    }
    // The serviceContext may have already been destroyed.
    onDestroy(client, service->_clientObservers);
    delete client;
}