代码如下：

const cluster = require('cluster');
const http = require('http');
const fs = require('fs');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });

} else {
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8080, function() {
    fs.open('./test.txt', 'a', (err, fd) => {
      console.log(err, fd);
    })
  });

  console.log(`Worker ${process.pid} started`);
}

结果：

文件 fd 是相同的，请问是怎么做到的？