child_process: control argv0 for spawned processes

In some cases it useful to control the value of `argv[0]`, c.f.
 - https://github.com/andrewffff/child_process_with_argv0
 - https://github.com/andrep/argv0

This patch adds explicit support for setting the value of `argv[0]`
when spawning a process.

PR-URL: https://github.com/nodejs/node/pull/7696
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.md

@@ -295,6 +295,8 @@ added: v0.1.90
 * `options` {Object}
   * `cwd` {String} Current working directory of the child process
   * `env` {Object} Environment key-value pairs
+  * `argv0` {String} Explicitly set the value of `argv[0]` sent to the child
+    process. This will be set to `command` if not specified.
   * `stdio` {Array|String} Child's stdio configuration. (See
     [`options.stdio`][`stdio`])
   * `detached` {Boolean} Prepare child to run independently of its parent
@@ -397,6 +399,14 @@ child.on('error', (err) => {
 });
 ```
 
+*Note: Certain platforms (OS X, Linux) will use the value of `argv[0]` for the
+process title while others (Windows, SunOS) will use `command`.*
+
+*Note: Node.js currently overwrites `argv[0]` with `process.execPath` on
+startup, so `process.argv[0]` in a Node.js child process will not match the
+`argv0` parameter passed to `spawn` from the parent, retrieve it with the
+`process.argv0` property instead.*
+
 #### options.detached
 <!-- YAML
 added: v0.7.10

lib/child_process.js

@@ -351,7 +351,11 @@ function normalizeSpawnArguments(file /*, args, options*/) {
     }
   }
 
+  if (typeof options.argv0 === 'string') {
+    args.unshift(options.argv0);
+  } else {
     args.unshift(file);
+  }
 
   var env = options.env || process.env;
   var envPairs = [];

test/parallel/test-child-process-spawn-argv0.js

@@ -0,0 +1,18 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+
+// This test spawns itself with an argument to indicate when it is a child to
+// easily and portably print the value of argv[0]
+if (process.argv[2] === 'child') {
+  console.log(process.argv0);
+  return;
+}
+
+const noArgv0 = cp.spawnSync(process.execPath, [__filename, 'child']);
+assert.strictEqual(noArgv0.stdout.toString().trim(), process.execPath);
+
+const withArgv0 = cp.spawnSync(process.execPath, [__filename, 'child'],
+    {argv0: 'withArgv0'});
+assert.strictEqual(withArgv0.stdout.toString().trim(), 'withArgv0');