process: avoid using the same fd for ipc and stdio

There is already a check in place to prevent stdin and the IPC
channel from sharing a file descriptor. This commit adds a
similar check to stdout and stderr.

Refs: libuv/libuv#1851
Refs: libuv/libuv#1897
PR-URL: #21466
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/process/stdio.js

@@ -182,11 +182,24 @@ function createWritableStdioStream(fd) {
     case 'PIPE':
     case 'TCP':
       var net = require('net');
-      stream = new net.Socket({
-        fd: fd,
-        readable: false,
-        writable: true
-      });
+
+      // If fd is already being used for the IPC channel, libuv will return
+      // an error when trying to use it again. In that case, create the socket
+      // using the existing handle instead of the fd.
+      if (process.channel && process.channel.fd === fd) {
+        stream = new net.Socket({
+          handle: process.channel,
+          readable: false,
+          writable: true
+        });
+      } else {
+        stream = new net.Socket({
+          fd,
+          readable: false,
+          writable: true
+        });
+      }
+
       stream._type = 'pipe';
       break;