How to execute code after a spawned, detached, child exits from NodeJS - node.js

Question

My objective is to have some code execute after a detached, unreferenced, child process is spawned from a NodeJS app. Here is the code that I have:
var child_options = {
cwd : prj
, env : {
PATH: cmd_directory
}
, detatched : true
, stdio : 'ignore'
};
//Spawn a child process with myapp with the options and command line params
child = spawn('myapp', params_array, child_options, function(err, stdout, stderr){
if (err) {
console.log("\t\tProblem executing myapp =>\n\t\t" + err);
} else {
console.log("\t\tLaunched myapp successfully!")
}
});
//Handle the child processes exiting. Maybe send an email?
child.on('exit', function(data) {
fs.writeFile(path.resolve("/Users/me/Desktop/myapp-child.log"), "Finished with child process!");
});
//Let the child process run in its own session without parent
child.unref();
So the function inside the exit handler does not seem to get executed when the child process finishes. Is there any way at all to have code execute after the child process exits even when it's detached and when calling the .unref() method?
Note that if I change the 'stdio' key value in the child_options object from 'ignore' to 'inherit' then the exit handler does execute.
Any ideas?
UPDATE PART 1
So, I still can not figure this one out. I went back to the NodeJS docs on spawn, and noticed the example about spawning "long-running processes". In one example, they redirect the child process' output to files instead of just using 'ignore' for the 'stdio' option. So I changed the 'stdio' key within the child_options object as in the following, but alas I am still not able to execute the code within the 'close' or 'exit' event:
var out_log = fs.openSync(path.resolve(os.tmpdir(), "stdout.log"), 'a'),
err_log = fs.openSync(path.resolve(os.tmpdir(), "stderr.log"), 'a');
var child_options = {
cwd : prj
, env : {
PATH: cmd_directory
}
, detatched : true
, stdio : ['ignore', out_log, err_log]
};
So, the stdout.log file does get the stdout from the child process—so I know it gets redirected. However, the code in the close or exit event still does not execute. Then I thought I would be able to detect when the writing to the out_log file was finished, in which case I would be able to execute code at that point. However, I cannot figure out how to do that. Any suggestions?

Answer 1

You can add listener to 'close' event, e.g. replace 'exit' with 'close'. It worked on my side even with 'ignore' stdio. Also, input parameter in callback is exit code number or null.
According to nodejs documentation difference between exit and close events:
The 'close' event is emitted when the stdio streams of a child process
have been closed. This is distinct from the 'exit' event, since
multiple processes might share the same stdio streams.
Hope it helps.