child_process: preserve argument type

A previous fix for a `maxBuffer` bug resulted in a change to the
argument type for the `data` event on `child.stdin` and `child.stdout`
when using `child_process.exec()`.

This fixes the `maxBuffer` bug in a way that does not have that side
effect.

Fixes: nodejs#7342
Refs: nodejs#1901

lib/child_process.js

@@ -249,7 +249,7 @@ exports.execFile = function(file /*, args, options, callback*/) {
       child.stdout.setEncoding(encoding);
 
     child.stdout.addListener('data', function(chunk) {
-      stdoutLen += chunk.length;
+      stdoutLen += encoding ? Buffer.byteLength(chunk, encoding) : chunk.length;
 
       if (stdoutLen > options.maxBuffer) {
         ex = new Error('stdout maxBuffer exceeded');
@@ -268,7 +268,7 @@ exports.execFile = function(file /*, args, options, callback*/) {
       child.stderr.setEncoding(encoding);
 
     child.stderr.addListener('data', function(chunk) {
-      stderrLen += chunk.length;
+      stderrLen += encoding ? Buffer.byteLength(chunk, encoding) : chunk.length;
 
       if (stderrLen > options.maxBuffer) {
         ex = new Error('stderr maxBuffer exceeded');

test/parallel/test-child-process-exec-maxBuffer.js

@@ -0,0 +1,27 @@
+'use strict';
+// Refs: https://github.com/nodejs/node/issues/1901
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+const unicode = '中文测试'; // Length = 4, Byte length = 13
+
+if (process.argv[2] === 'stdout') {
+  console.log(unicode);
+  return;
+}
+
+if (process.argv[2] === 'stderr') {
+  console.error(unicode);
+  return;
+}
+
+const cmd = `${process.execPath} ${__filename}`;
+const outputs = ['stdout', 'stderr'];
+
+outputs.forEach((stream) => {
+  const cb = common.mustCall((err, stdout, stderr) => {
+    assert.strictEqual(err.message, `${stream} maxBuffer exceeded`);
+  });
+
+  cp.exec(`${cmd} ${stream}`, {maxBuffer: 10}, cb);
+});

test/parallel/test-child-process-exec-stdout-stderr-data-string.js

@@ -0,0 +1,22 @@
+'use strict';
+// Refs: https://github.com/nodejs/node/issues/7342
+const common = require('../common');
+const assert = require('assert');
+const exec = require('child_process').exec;
+
+var stdoutIsString = false;
+var stderrIsString = false;
+
+const command = common.isWindows ? 'dir' : 'ls';
+exec(command).stdout.on('data', (data) => {
+  stdoutIsString = (typeof data === 'string');
+});
+
+exec('fhqwhgads').stderr.on('data', (data) => {
+  stderrIsString = (typeof data === 'string');
+});
+
+process.on('exit', () => {
+  assert(stdoutIsString);
+  assert(stderrIsString);
+});