为什么 parent 进程在运行分离 child 后不会自动退出？

Question

Node.js 8.9.1，Linux 版本 4.10.0-42-generic

Parent

const { fork } = require('child_process');

const forked = fork('child.js', {
  detached: true,
  stdio: 'ignore'
});

const config = {
  name: 'trex',
  interval: 2000
};

forked.send(config);
forked.unref();

for (let i = 0; i < 3; i++) {
  console.log('do staff');
}

Child

const work = function() {
  let sum = 0;
  for (let i = 0; i < 1e10; i++) {
    sum += i;
  }
};

const start = function(config) {
  setTimeout(function run() {
    work();
    setTimeout(run, config.interval);
  }, config.interval);
};

process.on('message', function(config) {
  start(config);
});

我需要 parent 来启动 child 并正常退出。现在，如果我执行 node parent.js，我会看到 parent 仍然是运行ning.

trex@beast-cave:~/dev/$ ps aux | grep -e "parent\|child" | grep node
trex      5134  0.0  0.1 874016 29460 pts/11   Sl+  10:44   0:00 node parent.js
trex      5140 86.3  0.1 874108 30252 ?        Rsl  10:44   4:59 /home/trex/.nvm/versions/node/v8.9.1/bin/node child.js

我知道有process.exit()。但是为什么不能正常退出呢？在我的应用程序中 parent 在 setTimeout 循环中，有很多逻辑，它必须在一个时间间隔内只运行一次。

Answer 1

来自 https://nodejs.org/api/child_process.html 文档：

subprocess.disconnect()# Added in: v0.7.2 Closes the IPC channel between parent and child, allowing the child to exit gracefully once there are no other connections keeping it alive. After calling this method the subprocess.connected and process.connected properties in both the parent and child (respectively) will be set to false, and it will be no longer possible to pass messages between the processes.

The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling subprocess.disconnect().

Note that when the child process is a Node.js instance (e.g. spawned using child_process.fork()), the process.disconnect() method can be invoked within the child process to close the IPC channel as well.

const work = function() {
  let sum = 0;
  for (let i = 0; i < 1e10; i++) {
    sum += i;
  }
};

const start = function(config) {
  setTimeout(function run() {
    work();
    setTimeout(run, config.interval);
  }, config.interval);
};

process.on('message', function(config) {
  start(config);
  process.disconnect();
});

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