Child Process: fork stdio option doesn't support the String variant that spawn does

[SEUH]

Version: v7.4.0
Platform: Windows 64 bit

The following code forks a script but all it's stdio objects are null

index.js

var child = childProcess.fork('./userapp/test.js', [], {
  stdio: 'pipe'
});
console.log('stdio', child.stdio);

test.js

var count = 0;
setInterval(function () {
  if (count == 3) process.exit(1);
  console.log('test: ' + count);
  count++;
}, 1000, 0);

console output:

stdio [ null, null, null, null ]
test: 0
test: 1
test: 2

Also i've tried to set the childs stdio with stdio: [stream, stream, stream] but this didn't work and the child used the parents stream to output

[sam-github]

Inconsistent and annoying, but documented, for what that's worth:

stdio Supports the array version of child_process.spawn()'s stdio option.

• https://nodejs.org/api/child_process.html#child_process_child_process_fork_modulepath_args_options

[SEUH]

@sam-github I see what you mean but this doesn't solve the problem.
Even with the array version provided and 'ipc', it doesn't work

var child = childProcess.fork('./userapp/test.js', config.project.args, {
  stdio: [0,1,2,'ipc']
});
console.log('stdio', child.stdio);

still the same output as before. Also replacing 0,1,2 with writeable streams doesn't work

[sam-github]

@SEUH as-expected.

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe'.

• https://nodejs.org/api/child_process.html#child_process_child_stdio

[SEUH]

@sam-github I still don't get it. Am i missing something? If i set stdio: ['pipe','pipe','pipe','ipc'] stdio's are still null

[sam-github]

And to be clear: not-a-problem

child.stdio exists so that the parent and read and write data to the child's stdio.

It isn't possible to write child to a child's input, if the child is getting input from a tty, or an explicit fd, or anything other than if the child is reading input from a pipe connected to the parent.

Same for output: the data the child writes to its stdout or stderr can be read by the parent only if the child's stdout or stderr is connected via a pipe to the parent.

So the docs don't just describe an arbitary limitation, bug, or missing feature. They describe a natural behaviour.

Note that your original description attempted to pipe the child's stdio using 'pipe'. That is a missing feature, because in spawn, that string value is a short-hand for ['pipe', 'pipe', 'pipe']. Your example above isn't equivalent, its the equivalent of what spawn would do with stdio: 'inherit' (also a feature missing from fork(), of course), and very different from 'pipe'.

[sam-github]

> child_process.fork('./f.js', {stdio: ['pipe', 'pipe', 'pipe', 'ipc']}).stdio[0]
Socket {
  connecting: false,
.... etc.

Works for me. Provide a standalone runnable example if you don't think it works.

[SEUH]

@sam-github Found the problem...i provided empty args

var child = childProcess.fork('./userapp/test.js', config.project.args, {
  stdio: ['pipe','pipe','pipe','ipc']
});
console.log('stdio', child.stdio);

config.project.args was null.

[sam-github]

Now that is a bug, IMO:

> child_process.fork('./f.js', null, {stdio: ['pipe', 'pipe', 'pipe', 'ipc']}).stdio
[ null, null, null, null ]

Providing null or undefined as the arg array should either type-error, or just mean "no args" - the latter makes more sense to me, but I would have to check the other array args in the child_process API, and make sure that the handling was consistent.

I suspect it is assuming there is no options if the args are null/undefined ATM, but haven't looked at the code.

[SEUH]

@sam-github Right...do you open an issue for this?

[sam-github]

If you did I would really appreciate it. Otherwise I will do so later.

[sam-github]

And if you have interest, both the feature and the bug are quite easy to implement, good first contributions.

[trendsetter37]

@sam-github I can take a stab a this

[sam-github]

@trendsetter37 Can you report the bug, so the bug and the feature are tracked as individual issues? Then say which of the two (both?) you are going to take a stab at. Thanks!

[trendsetter37]

Sure, I'll report the bug and take a stab at both.