Nodejs - process hangs on exit (Ctrl+C)

Question

I have a node.js project which does many things, it spawns child processes, it opens an http and socket.io server, etc..

When I run it from the console, closing it with Ctrl+C, it just hangs. From webstorm, stopping the process is a two-step process, first I hit stop, then I need to hit the button again, only the second time the button is a skull icon.

Now, I understand it leaves something open or hanging, but I just can't figure out what, I tried to track all the places where I start a process and made sure I'm killing them properly.

Is there a way to debug this and find out what's making my process hang? Could it be logging that open a write stream and never closes? I'm not even sure what kind of things will make a process hang on SIGINT.

EDIT: I've downloaded pstree to see if any of the child processes that the main process spawns stay alive. It looks like they all terminate properly - the main node process is the only one left.

Answer 1

Scripts are themselves responsible for properly shutting down once they listen to the SIGINT event, as the default handler (killing the process) is disabled then.

Check out this example program:

process.on('SIGINT', function() {
    console.log('SIGINT');
});
console.log('PID: ', process.pid);

var http = require('http'); // HTTP server to keep the script up long enough
http.createServer(function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
}).listen(1337, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1337/');

Execute it and then try killing it: It won't work. The SIGINT signal will always get passed to your custom build signal handler. In order to properly shut the process down you will have to manually call process.exit():

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

var http = require('http');
http.createServer(function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
}).listen(1337, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1337/');

process.exit() will:

1. Set some internal flags
2. Call the process.on('exit') handlers
3. Call process.reallyExit
4. Which will call the C++ exit() function, therefore process.exit() is final and will cause a shutdown (unless you block execution by an endless loop in your on('exit') handler).

---

Long story short: Your code probably listens to SIGINT somewhere. You can fetch a list of those listeners via:

var listeners = process.listeners('SIGINT');

You can even pretty print them on the console:

for (var i = 0; i < listeners.length; i++) {
    console.log(listeners[i].toString());
}

Using the information I gave above you can easily compile yet another SIGINT-handler that will list all the handlers and then cleanly exit the process, hopefully leading your path to the naughty ones:

process.on('SIGINT', function() {
    console.log('Nice SIGINT-handler');
    var listeners = process.listeners('SIGINT');
    for (var i = 0; i < listeners.length; i++) {
        console.log(listeners[i].toString());
    }

    process.exit();
});

---

Complete program for testing:

process.on('SIGINT', function() {
    console.log('Naughty SIGINT-handler');
});
process.on('exit', function () {
    console.log('exit');
});
console.log('PID: ', process.pid);

var http = require('http');
http.createServer(function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
}).listen(1337, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1337/');

process.on('SIGINT', function() {
    console.log('Nice SIGINT-handler');
    var listeners = process.listeners('SIGINT');
    for (var i = 0; i < listeners.length; i++) {
        console.log(listeners[i].toString());
    }

    process.exit();
});