doc: correct info in child_process.md

[vsemozhetbyt]

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, child_process

child.stderr, child.stdin, and child.stdout are null, not undefined, if the relevant stdio properties are set to anything other than 'pipe':

console.log(require('child_process').spawn('echo',{stdio: ['ignore','ignore','ignore']}));

ChildProcess {
domain: null,
_events: {},
_eventsCount: 0,
_maxListeners: undefined,
_closesNeeded: 1,
_closesGot: 0,
connected: false,
signalCode: null,
exitCode: null,
killed: false,
spawnfile: 'echo',
_handle: Process { owner: [Circular], onexit: [Function], pid: 4252 },
spawnargs: [ 'echo' ],
pid: 4252,
stdin: null,
stdout: null,
stderr: null,
stdio: [ null, null, null ] }

The relevant test checks null values.

[jasnell]

Landed in 76279cb

[sam-github]

@vsemozhetbyt does this need backporting? At what time did it change? Are there unit tests asserting this behaviour?

[vsemozhetbyt]

@sam-github

console.log(`// v8 ${process.versions.node}`);constchild=require('child_process').spawn('echo',{stdio: ['ignore','ignore','ignore']});console.log(child.stdin,child.stdout,child.stderr);

// v8 4.8.1
null null null
// v8 6.10.1
null null null
// v8 7.7.4
null null null
// v8 8.0.0-nightly20170321ab2d49bcac
null null null

There is a test checking the null. It seems it is rather old.

[sam-github]

OK, thanks, the docs can be (should be?) backported then, I labelled.