Kill child process at the end of promise

Question

I use the following code to create child process, the process is working as expected, what I want to do now is at the end of the process kill it , I try with the last then (which if I put BP it stops there after the process done) my question is how to kill it properly and avoid this error.

getCmd provide the command to run and this is working as expected

var childProcess = Promise.promisify(require('child_process').exec);

....
            .then(getCmd)
            .then(childProcess)
            .spread(function (stdout, stderr) {
                console.log(stdout, stderr);
                return stdout;
            }).then(function(){

                childProcess.kill()
            })

when the line childProcess.kill() is executed I got error :

[TypeError: undefined is not a function]

How to overcome this issue and kill the process at the end

Answer 1

As requested, an example:

var childProcess = require('child_process').exec;
var childPromise = Promise.promisify(childProcess);

....
childPromise
            .then(getCmd)
            .then(childPromise) // not sure if you want the promise or process here
            .spread(function (stdout, stderr) {
                console.log(stdout, stderr);
                return stdout;
            }).then(function(){

                childProcess.kill() // as returned by require(...).exec
            })

Answer 2

As far as I can tell, promisifying require('child_process').exec itself is not correct. At best, you would end up with a function that returns a promise but you would, straightforwardly, be denied a reference to the child-process itself. Some imagination is required.

You can call require('child_process').exec(cmd, callback) in the normal way, inside a new Promise() wrapper and, critically, resolve/reject in such a way that a reference to the child-process is made available down the promise chain.

I'm not sure that Bluebird's .promisify() is quite flexible enough to do the job so here's a manual promisification.

First a couple or requires :

var Promise = require('bluebird');
var child_process = require('child_process');

Now the manual promisifier :

function childProcessPromise (method, cmd) {
    return new Promise(function(resolve, reject) {
        var child = child_process[method](cmd, function(error, stdout, stderr) {
            if(error !== null) {
                error.child = child;//monkeypatch the error with a custom .child property.
                reject(error);
            } else {
                resolve({ 'child':child, 'stdout':stdout, 'stderr':stderr });//all available data bundled into a single object.
            }
        });
    });
}

As written this promisifier is flexible enough to cater for various child_process methods. Just pass (or bind) 'spawn', 'exec', 'execFile' or 'fork' as first parameter.

Your promise chain will look like this :

....
    .then(getCmd)
    .then(childProcessPromise.bind(null, 'exec'))
    .then(function(result) {
        // result.child, result.stdout and result.stderr are available here.
    }, function(error) {
        //error is available here, including its custom .child property.
    });

All untested, so be prepared to adapt/fix.