Why the node child spawn process detached from parent process and start running independently ? - node.js

Question

I have a node child spawn process which is continuously write in to the file using writestream on every "data event" received. The script is run under ssh and facing an edge case problem.
Consider multiple terminals open with same ssh host and the script is start to run in one terminal. Someone accidentally close the ssh terminal during it's execution and doesn't want to stop child process execution. while using a command ps -ef | grep "command name" the child process still running with different parent process id (it shows 1) but the writestream in the child process stops writing to the file. It seems like child process become zombie process eventhough i detached the process from parent. You can find the script below:
var execSpawn = require('child_process').spawn;
var Promise = require('bluebird');
var spawnAction = function(path, cmd, cb){
return function(resolve, reject, onCancel){
cmdExec = execSpawn(path, cmd, {detached: true});
//cmdExec = execSpawn(path, cmd, {detached: true}).unref();
var fileData = {}
var count = 0;
var stream = fs.createWriteStream('filepath');
cmdExec.stdout.setEncoding('utf8');
cmdExec.stdout.on('data', function(data){
//Certain actions with filedata and count;
stream.write(data);
});
cmdExec.stderr.on('data', function(data){
//some actions
stream.write("error");
});
cmdExec.on('close', function(){
stream.end();
if(cb){
resolve(cb(fileData));
}else{
resolve(count);
}
});
}
}
This script is running properly when it is allow to run completely without any interruption. When the script execution terminal closes the child process stop the writestream to the file. If i try with detach along with unref() it throws an error like it couldn't figure out the event stdout.on over the child process.
Cannot read property 'stdout' of undefined
More information during the script running. This is taken in the same host in different terminal
ps -ef | grep command_name
root 19904 19191 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sdb
root 19905 19191 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sdc
root 19906 19191 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sdd
root 19907 19191 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sde
root 23101 13105 0 20:16 pts/0 00:00:00 grep --color=auto command_name
After closing the script running terminal before it finishes. I am getting this:
ps -ef | grep command_name
root 19904 1 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sdb
root 19905 1 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sdc
root 19906 1 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sdd
root 19907 1 0 20:16 ? 00:00:00 cli command_name pfitzner7 /dev/sde
root 23163 13105 0 20:16 pts/0 00:00:00 grep --color=auto command_name
I am trying to figure out why it's happening this issue and will it be there any possible ways to do in different way. Why did the parent process id changed to 1 even if it's detached child process? How can a child spawn process can run independently from the parent process?
Please let me know your suggestions on this approach or the reason for the error.
Thanks in advance.

Answer 1

I think, you don't need to detach your childrens. The childrens are spawned asynchronously, so you may manipulate with a few child's stdio at the same time