child_process: add keepOpen option to send()

This option allows an instance of net.Socket to be kept open in
the sending process.

Fixes: https://github.com/nodejs/node/issues/4271
PR-URL: https://github.com/nodejs/node/pull/5283
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/child_process.markdown

@@ -803,7 +803,12 @@ receive the object as the second argument passed to the callback function
 registered on the `process.on('message')` event.
 
 The `options` argument, if present, is an object used to parameterize the
-sending of certain types of handles.
+sending of certain types of handles. `options` supports the following
+properties:
+
+  * `keepOpen` - A Boolean value that can be used when passing instances of
+    `net.Socket`. When `true`, the socket is kept open in the sending process.
+    Defaults to `false`.
 
 The optional `callback` is a function that is invoked after the message is
 sent but before the child may have received it.  The function is called with a

lib/internal/child_process.js

@@ -78,21 +78,25 @@ const handleConversion = {
         if (firstTime) socket.server._setupSlave(socketList);
 
         // Act like socket is detached
-        socket.server._connections--;
+        if (!options.keepOpen)
+          socket.server._connections--;
       }
 
+      var handle = socket._handle;
+
       // remove handle from socket object, it will be closed when the socket
       // will be sent
-      var handle = socket._handle;
-      handle.onread = function() {};
-      socket._handle = null;
+      if (!options.keepOpen) {
+        handle.onread = function() {};
+        socket._handle = null;
+      }
 
       return handle;
     },
 
     postSend: function(handle, options) {
       // Close the Socket handle after sending it
-      if (handle)
+      if (handle && !options.keepOpen)
         handle.close();
     },
 

test/parallel/test-child-process-send-keep-open.js

@@ -0,0 +1,52 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+const net = require('net');
+
+if (process.argv[2] !== 'child') {
+  // The parent process forks a child process, starts a TCP server, and connects
+  // to the server. The accepted connection is passed to the child process,
+  // where the socket is written. Then, the child signals the parent process to
+  // write to the same socket.
+  let result = '';
+
+  process.on('exit', () => {
+    assert.strictEqual(result, 'childparent');
+  });
+
+  const child = cp.fork(__filename, ['child']);
+
+  // Verify that the child exits successfully
+  child.on('exit', common.mustCall((exitCode, signalCode) => {
+    assert.strictEqual(exitCode, 0);
+    assert.strictEqual(signalCode, null);
+  }));
+
+  const server = net.createServer((socket) => {
+    child.on('message', common.mustCall((msg) => {
+      assert.strictEqual(msg, 'child_done');
+      socket.end('parent', () => {
+        server.close();
+        child.disconnect();
+      });
+    }));
+
+    child.send('socket', socket, {keepOpen: true}, common.mustCall((err) => {
+      assert.ifError(err);
+    }));
+  });
+
+  server.listen(common.PORT, () => {
+    const socket = net.connect(common.PORT, common.localhostIPv4);
+    socket.setEncoding('utf8');
+    socket.on('data', (data) => result += data);
+  });
+} else {
+  // The child process receives the socket from the parent, writes data to
+  // the socket, then signals the parent process to write
+  process.on('message', common.mustCall((msg, socket) => {
+    assert.strictEqual(msg, 'socket');
+    socket.write('child', () => process.send('child_done'));
+  }));
+}