Javascript Node.js TCP/IP服务器队列不工作

我正在编写一个TCP/IP队列，它应该将最大连接用户数限制为3，并让其他任何用户等待，直到用户断开连接。以下是我正在使用的函数：

var maxClients = 3;
var currentClients = 0;
var _pending = [];

function process_pending()
{
  if (_pending.length > 0) {
    var client = _pending.shift();
    currentClients++;
    client(function() 
    {
      currentClients--;
      process.nextTick(process_pending);
    });
  }
}

function client_limit(client) 
{
  if (currentClients < maxClients) {
    currentClients++;
    client(function() 
    {
      currentClients--;
      process.nextTick(process_pending);
    });
  }
  else 
  {
    console.log('Overloaded, queuing clients...');
    _pending.push(client);
  }
}

不能正常工作，我该如何修复它

client(function() 
        {
          currentClients--;
          process.nextTick(process_pending);
        });