child_process: make .fork()'d child auto-exit

A child process created with .fork() needed to call `process.exit()` explicitly
because the communication channel with the parent kept the event loop alive.

Fix that by only ref'ing the channel when there are 'message' event listeners.

Fixes #3799.

lib/child_process.js

@@ -452,7 +452,15 @@ exports._forkChild = function(fd) {
   // set process.send()
   var p = createPipe(true);
   p.open(fd);
+  p.unref();
   setupChannel(process, p);
+
+  process.on('newListener', function(name) {
+    if (name == 'message' && this.listeners('message').length == 0) p.ref();
+  });
+  process.on('removeListener', function(name) {
+    if (name == 'message' && this.listeners('message').length == 0) p.unref();
+  });
 };
 
 

test/fixtures/child-process-spawn-node.js

@@ -1,10 +1,10 @@
 var assert = require('assert');
 
-process.on('message', function(m) {
+function onmessage(m) {
   console.log('CHILD got message:', m);
   assert.ok(m.hello);
-  // Note that we have to force exit.
-  process.exit();
-});
+  process.removeListener('message', onmessage);
+}
 
+process.on('message', onmessage);
 process.send({ foo: 'bar' });

test/simple/test-child-process-fork3.js

@@ -0,0 +1,25 @@
+// 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 common = require('../common');
+var child_process = require('child_process');
+
+child_process.fork(common.fixturesDir + '/empty.js'); // should not hang