child_process: acknowledge sent handles

Fix race-condition when multiple handles are sent and SCM_RIGHTS
messages are gets merged by OS by avoiding sending multiple handles at
once!

fix #4885

lib/child_process.js

@@ -24,6 +24,7 @@ var EventEmitter = require('events').EventEmitter;
 var net = require('net');
 var dgram = require('dgram');
 var Process = process.binding('process_wrap').Process;
+var assert = require('assert');
 var util = require('util');
 var constants; // if (!constants) constants = process.binding('constants');
 
@@ -321,6 +322,7 @@ function handleMessage(target, message, handle) {
 
 function setupChannel(target, channel) {
   target._channel = channel;
+  target._handleQueue = null;
 
   var decoder = new StringDecoder('utf8');
   var jsonBuffer = '';
@@ -358,8 +360,22 @@ function setupChannel(target, channel) {
 
   // handlers will go through this
   target.on('internalMessage', function(message, handle) {
+    // Once acknowledged - continue sending handles.
+    if (message.cmd === 'NODE_HANDLE_ACK') {
+      assert(Array.isArray(target._handleQueue));
+      var queue = target._handleQueue;
+      target._handleQueue = null;
+      queue.forEach(function(args) {
+        target.send(args.message, args.handle);
+      });
+      return;
+    }
+
     if (message.cmd !== 'NODE_HANDLE') return;
 
+    // Acknowledge handle receival.
+    target.send({ cmd: 'NODE_HANDLE_ACK' });
+
     var obj = handleConversion[message.type];
 
     // Update simultaneous accepts on Windows
@@ -389,6 +405,7 @@ function setupChannel(target, channel) {
       // this message will be handled by an internalMessage event handler
       message = {
         cmd: 'NODE_HANDLE',
+        type: null,
         msg: message
       };
 
@@ -407,6 +424,12 @@ function setupChannel(target, channel) {
         throw new TypeError("This handle type can't be sent");
       }
 
+      // Queue-up message and handle if we haven't received ACK yet.
+      if (this._handleQueue) {
+        this._handleQueue.push({ message: message.msg, handle: handle });
+        return;
+      }
+
       var obj = handleConversion[message.type];
 
       // convert TCP object to native handle object
@@ -416,6 +439,10 @@ function setupChannel(target, channel) {
       if (obj.simultaneousAccepts) {
         net._setSimultaneousAccepts(handle);
       }
+    } else if (this._handleQueue) {
+      // Queue request anyway to avoid out-of-order messages.
+      this._handleQueue.push({ message: message, handle: null });
+      return;
     }
 
     var string = JSON.stringify(message) + '\n';
@@ -426,6 +453,8 @@ function setupChannel(target, channel) {
                               'write',
                               'cannot write to IPC channel.');
       this.emit('error', er);
+    } else if (handle && !this._handleQueue) {
+      this._handleQueue = [];
     }
 
     if (obj && obj.postSend) {