Graceful kill off a detached node.js spawned child process in Windows

Question

Not a duplicate of this : I can kill process fine, I want to know how to detect, within the process, that it's being killed, and gracefully shutdown.

Overview:

I have a CLI tool to spawn and kill a child node.js process. The demo code is contained in these three files:

spawn.js -- Will spawn the child.js script, detached. For simplicity, I pipe the child's stdio to an out.log file

child.js -- A simple counter which writes to a file, uses the readline method to detect an emulated SIGINT in Windows

kill.js -- Invokes a process.kill() on the child process, using it's PID

---

Code:

spawn.js

'use strict';

var spawn = require('child_process').spawn;
var fs = require('fs');
var path = require('path');

var childFilePath = path.resolve(__dirname, 'child.js');

var out = fs.openSync('./out.log', 'a');
var err = fs.openSync('./out.log', 'a');

var options = {
  detached: true,
  stdio: ['ignore', out, err],
};

var child = spawn(process.execPath, [childFilePath], options);
child.unref();

child.js

'use strict';

var fs = require('fs');
var path = require('path');

if (process.platform === 'win32') {
  console.log('win32 true');
  var rl = require('readline').createInterface({
    input: process.stdin,
    output: process.stdout,
  });

  rl.on('SIGINT', function() {
    process.emit('SIGINT');
  });
}

process.on('SIGINT', function() {
  console.log('SIGINT');
  process.exit();
});

var filepath = path.resolve(__dirname, 'pid.txt');

fs.writeFile(filepath, process.pid);

var i = 0;
setInterval(function () {
  console.log(i++);
}, 1000);

kill.js

'use strict';

var fs = require('fs');
var path = require('path');


var pidPath = path.resolve(__dirname, 'pid.txt');


fs.readFile(pidPath, 'utf8', function (err, data) {
  if (err) {
    return console.log(err);
  }
  process.kill(data, 'SIGINT');
});

---

Problem:

When sending the process.kill(PID, 'SIGINT') , it doesn't actually detect it as a SIGINT in windows. I can run child.js manually and use CTRL+C to kill the process to trigger a SIGINT , so I know that the readline code is working (or maybe not since SIGINT will trigger without the readline code, but it will n.netheless trigger a SIGINT )

Does process.kill() not send the type of signal to a detached process? How do I detect that that a separate script is trying to kill my child process and shutdown gracefully?

Answer 1

I have exactly the same problem. I noticed the rl.on("SIGINT") is not working but rl.on("close") works!

var rl = require('readline').createInterface({
    input: process.stdin,
    output: process.stdout,
})

rl.on('close', function() {
    process.emit('SIGINT')
})

Here is my debug output

Sat Feb 17 2018 11:43:28 GMT+0800 (China Standard Time) process started (pid=6920)
Sat Feb 17 2018 11:43:28 GMT+0800 (China Standard Time) SIGINT captured! cleanup and then call process.exit(0)
Sat Feb 17 2018 11:43:28 GMT+0800 (China Standard Time) byebye! (code=0, signal=undefined)

Note that I'm not pretty sure that adding extra code to a subprocess is a good idea. Think about forking a non nodejs process, it might not be able to use such trick (eg redis-server, kdb+). I am still looking for a way to let spawn.js to kill subprocess gracefully.

Update 1: Here is my initial problem reported to PM2 community https://github.com/Unitech/pm2/issues/3467

Answer 2

There is a library kill-with-style to do this and works with detached process too. It has various options to set signal, timeout and retry options.