Can't kill child process on Windows

Question

The following will never exit

var child_process = require('child_process');

var ps = child_process.spawn('.\\node_modules\\.bin\\babel.cmd', ['input.js', '--out-file', 'output.js', '--watch']);

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

ps.on('close', function() {
  console.log('close');
});

setTimeout(function () {
  ps.kill();
}, 2000);

What's going on here? also what is the correct thing to do here? The only way to make this process actually close is to kill the parent process. I suspect it's waiting for stdio to flush or something like that?

It does die if it is given an 'ignore' stdio configuration, but I need the streams.

var ps = child_process.spawn('.\\node_modules\\.bin\\babel.cmd', ['test.js', '--out-file', 'output.js', '--watch'], {
    stdio: 'ignore'
});

Answer 1

I have the same issue on Windows (Win 10 64x). I can't terminate a process of a spawned child process.

I start a service (custom HTTP server service.windows) using child_process.spawn():

const { spawn } = require('child_process');
let cp = spawn('"C:\\Users\\user\\app\\service.windows"', [], { shell: true,  });

cp.stderr.on('data', (data) => {
    console.log(`stderr: ${data}`);
    console.log('cp.connected', cp.connected);
    console.log('process.pid', process.pid); // 6632 <<= main node.js PID
    console.log('cp.pid', cp.pid); // 9424 <<= child PID

    // All these are useless, they just kill `cp`
    cp.kill('SIGINT'); // Doesn't terminate service.windows
    cp.kill('SIGKILL'); // Doesn't terminate service.windows
    cp.kill('SIGTERM'); // Doesn't terminate service.windows
});

And I want to terminate my HTTP server service (service.windows). And it is not possible on Windows using cp.kill('ANY SIGNAL'). Node.js kills its child process (cp) but my HTTP server (service.windows) still runs fine.

When I check it in other terminal, I see my server is running just fine:

$ netstat -ano | findstr :9090
  TCP    0.0.0.0:9090           0.0.0.0:0              LISTENING       1340
  TCP    [::]:9090              [::]:0                 LISTENING       1340

I try manually kill my server by PID but with T flag, not F. The difference:

T terminate all child processes along with the parent process, commonly known as a tree kill.

F process(es) be forcefully terminated.

$ taskkill -T -PID 1340
ERROR: The process with PID 1340 (child process of PID 9424) could not be terminated.
Reason: This process can only be terminated forcefully (with /F option).

And it exactly says that my server 1340 is a child of that cp - PID 9424.

OK, I try to terminate that cp using T flag again. And boom, not possible. cp is a child of my main node.js process.pid 6632:

$ taskkill -T -PID 9424
ERROR: The process with PID 1340 (child process of PID 9424) could not be terminated.
Reason: This process can only be terminated forcefully (with /F option).
ERROR: The process with PID 9424 (child process of PID 6632) could not be terminated.
Reason: One or more child processes of this process were still running.

I can only kill it forcefully with F flag:

$ taskkill -F -T -PID 9424
SUCCESS: The process with PID 1340 (child process of PID 9424) has been terminated.
SUCCESS: The process with PID 9424 (child process of PID 6632) has been terminated.

Most disappointing thing is that Node.js docs doesn't say jack ship about how to deal with this issue. They only say "yes, we know the issue exist and we just inform you about it".

The only option I see on Windows is to use taskkill -F -T -PID 9424 in spawned process:

exec('taskkill -F -T -PID 9424');

Answer 2

You are spawning child process cmd.exe by using babel.cmd. This process then starts another grandchild process node and runs babel script. When you do ps.kill() it only kills the cmd.exe but not the child processes created by it.

Although cmd.exe is closed, the stdio stream of the parent process is still shared with the other processes in the tree. So the parent process is waiting for the stream to be closed. Using stdio: 'ignore' will only stop sharing the stdio stream with other processes in the tree but those grandchild processes will still continue running.

---

Update:

After discussing with OP, I tested with all three stdio options: pipe(default), inherit, ignore in iojs 3.3.0, Windows 7 (32bit). None of them kills the grandchild process.

With inherit and ignore, both parent process and child process(cmd.exe) are killed but the grand child process still floats around and doesn't belong to any process tree.

With pipe, only the child process is terminated but both parent and grand child process continue running. The reason that parent process is not exiting is most likely because the stdio of parent node.exe is still shared with the other processes as mentioned in original answer.

child.unref() doesn't have any effect on killing grand child process. It only removes the child process from parent's event loop so that the parent process can exit normally.

---

I could think of couple of solutions:

1. Invoke the babel.js directly without that cmd script:

   var ps = child_process.spawn('node', ['.\\node_modules\\babel\\bin\\babel.js', 'input.js', '--out-file', 'output.js', '--watch'])
   

2. Use windows taskkill command with \T flag to kill the process and all the child processes started by it:

   os = require('os');
   if(os.platform() === 'win32'){
       child_process.exec('taskkill /pid ' + ps.pid + ' /T /F')
   }else{
       ps.kill();  
   }
   

   There are some npm modules that can handle killing child processes in both Unix and Windows e.g. tree-kill. For windows it uses taskkill or tasklist.