child_process: make fork() execPath configurable

Allows for arbitrary path to executable spawned using `fork`. This
fixes some issues around running multiple versions of node with workers
and allows arbitrary IPC with compatible executables.

Fixes #3248.

Author: bmeck

doc/api/child_process.markdown

@@ -516,6 +516,7 @@ leaner than `child_process.exec`. It has the same options.
   * `cwd` {String} Current working directory of the child process
   * `env` {Object} Environment key-value pairs
   * `encoding` {String} (Default: 'utf8')
+  * `execPath` {String} Executable used to create the child process
 * Return: ChildProcess object
 
 This is a special case of the `spawn()` functionality for spawning Node
@@ -534,4 +535,10 @@ These child Nodes are still whole new instances of V8. Assume at least 30ms
 startup and 10mb memory for each new Node. That is, you cannot create many
 thousands of them.
 
+The `execPath` property in the `options` object allows for a process to be
+created for the child rather than the current `node` executable. This should be
+done with care and by default will talk over the fd represented an
+environmental variable `NODE_CHANNEL_FD` on the child process. The input and
+output on this fd is expected to be line delimited JSON objects.
+
 [EventEmitter]: events.html#events_class_events_eventemitter

lib/child_process.js

@@ -441,7 +441,9 @@ exports.fork = function(modulePath /*, args, options*/) {
   options.stdio = options.silent ? ['pipe', 'pipe', 'pipe', 'ipc'] :
       [0, 1, 2, 'ipc'];
 
-  return spawn(process.execPath, args, options);
+  options.execPath = options.execPath || process.execPath;
+
+  return spawn(options.execPath, args, options);
 };
 
 

test/simple/test-child-process-fork-exec-path.js

@@ -0,0 +1,57 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var assert = require('assert');
+var cp = require('child_process');
+var fs = require('fs');
+var path = require('path');
+var common = require('../common');
+var msg = {test: 'this'};
+var nodePath = process.execPath;
+var symlinkPath = path.join(common.tmpDir, 'node-symlink');
+
+if (process.env.FORK) {
+  assert(process.send);
+  assert.equal(process.argv[0], symlinkPath);
+  process.send(msg);
+  process.exit();
+}
+else {
+  try {
+    fs.unlinkSync(symlinkPath);
+  }
+  catch (e) {
+    if (e.code !== 'ENOENT') throw e;
+  }
+  fs.symlinkSync(nodePath, symlinkPath);
+
+  var child = require('child_process').fork(__filename, {
+    execPath: symlinkPath,
+    env: { FORK: 'true' }
+  });
+  child.on('message', common.mustCall(function(recv) {
+    assert.deepEqual(msg, recv);
+  }));
+  child.on('exit', common.mustCall(function(code) {
+    fs.unlinkSync(symlinkPath);
+    assert.equal(code, 0);
+  }));
+}