doc: make socket IPC examples more robust

This commit aims to improve the documentation examples that send
sockets over IPC channels. Specifically, pauseOnConnect is added
to a server that inspects the socket before sending and a
'message' handler adds a check that the socket still exists.

PR-URL: nodejs/node#13196
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

Author: cjihrig

doc/api/child_process.md

@@ -1050,8 +1050,9 @@ const { fork } = require('child_process');
 const normal = fork('subprocess.js', ['normal']);
 const special = fork('subprocess.js', ['special']);
 
-// Open up the server and send sockets to child
-const server = require('net').createServer();
+// Open up the server and send sockets to child. Use pauseOnConnect to prevent
+// the sockets from being read before they are sent to the child process.
+const server = require('net').createServer({ pauseOnConnect: true });
 server.on('connection', (socket) => {
 
   // If this is special priority
@@ -1071,7 +1072,12 @@ passed to the event callback function:
 ```js
 process.on('message', (m, socket) => {
   if (m === 'socket') {
-    socket.end(`Request handled with ${process.argv[2]} priority`);
+    if (socket) {
+      // Check that the client socket exists.
+      // It is possible for the socket to be closed between the time it is
+      // sent and the time it is received in the child process.
+      socket.end(`Request handled with ${process.argv[2]} priority`);
+    }
   }
 });
 ```
@@ -1081,6 +1087,10 @@ tracking when the socket is destroyed. To indicate this, the `.connections`
 property becomes `null`. It is recommended not to use `.maxConnections` when
 this occurs.
 
+It is also recommended that any `'message'` handlers in the child process
+verify that `socket` exists, as the connection may have been closed during the
+time it takes to send the connection to the child.
+
 *Note: this function uses [`JSON.stringify()`][] internally to serialize the
 `message`.*