child_process: support stdio option in fork()

This commit allows child_process.fork() to pass stdio options
to spawn(). This allows fork() to more easily take advantage of
additional stdio channels.

Refs: https://github.com/nodejs/node-v0.x-archive/issues/5727
PR-URL: https://github.com/nodejs/node/pull/7811
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

doc/api/child_process.md

@@ -253,6 +253,8 @@ added: v0.5.0
     piped to the parent, otherwise they will be inherited from the parent, see
     the `'pipe'` and `'inherit'` options for [`child_process.spawn()`][]'s
     [`stdio`][] for more details (Default: `false`)
+  * `stdio` {Array} Supports the array version of [`child_process.spawn()`][]'s
+    [`stdio`][] option. When this option is provided, it overrides `silent`.
   * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
 * Return: {ChildProcess}

lib/child_process.js

@@ -49,10 +49,14 @@ exports.fork = function(modulePath /*, args, options*/) {
 
   args = execArgv.concat([modulePath], args);
 
-  // Leave stdin open for the IPC channel. stdout and stderr should be the
-  // same as the parent's if silent isn't set.
-  options.stdio = options.silent ? ['pipe', 'pipe', 'pipe', 'ipc'] :
-      [0, 1, 2, 'ipc'];
+  if (!Array.isArray(options.stdio)) {
+    // Leave stdin open for the IPC channel. stdout and stderr should be the
+    // same as the parent's if silent isn't set.
+    options.stdio = options.silent ? ['pipe', 'pipe', 'pipe', 'ipc'] :
+        [0, 1, 2, 'ipc'];
+  } else if (options.stdio.indexOf('ipc') === -1) {
+    throw new TypeError('Forked processes must have an IPC channel');
+  }
 
   options.execPath = options.execPath || process.execPath;
 

test/parallel/test-child-process-fork-stdio.js

@@ -0,0 +1,54 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+const net = require('net');
+
+if (process.argv[2] === 'child') {
+  process.stdout.write('this should be ignored');
+  process.stderr.write('this should not be ignored');
+
+  const pipe = new net.Socket({ fd: 4 });
+
+  process.on('disconnect', () => {
+    pipe.unref();
+  });
+
+  pipe.setEncoding('utf8');
+  pipe.on('data', (data) => {
+    process.send(data);
+  });
+} else {
+  assert.throws(() => {
+    cp.fork(__filename, {stdio: ['pipe', 'pipe', 'pipe', 'pipe']});
+  }, /Forked processes must have an IPC channel/);
+
+  let ipc = '';
+  let stderr = '';
+  const buf = Buffer.from('data to send via pipe');
+  const child = cp.fork(__filename, ['child'], {
+    stdio: [0, 'ignore', 'pipe', 'ipc', 'pipe']
+  });
+
+  assert.strictEqual(child.stdout, null);
+
+  child.on('message', (msg) => {
+    ipc += msg;
+
+    if (ipc === buf.toString()) {
+      child.disconnect();
+    }
+  });
+
+  child.stderr.on('data', (chunk) => {
+    stderr += chunk;
+  });
+
+  child.on('exit', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+    assert.strictEqual(stderr, 'this should not be ignored');
+  }));
+
+  child.stdio[4].write(buf);
+}