fix(cluster): respect the backlog from workers

Currently, the master process would ignore `backlog` from worker
processes and use the default value instead. This commit will respect
the first `backlog` passed to the master process for a specific handle.

Refs: nodejs#4056

doc/api/net.md

@@ -285,6 +285,10 @@ server.listen({
 });
 ```
 
+When `exclusive` is `true` and the underlying handle is shared, it's
+possible that several workers query a handle with different backlogs.
+In this case, the first `backlog` passed to the master process will be used.
+
 Starting an IPC server as root may cause the server path to be inaccessible for
 unprivileged users. Using `readableAll` and `writableAll` will make the server
 accessible for all users.

lib/internal/cluster/round_robin_handle.js

@@ -13,7 +13,7 @@ const { constants } = internalBinding('tcp_wrap');
 
 module.exports = RoundRobinHandle;
 
-function RoundRobinHandle(key, address, { port, fd, flags }) {
+function RoundRobinHandle(key, address, { port, fd, flags, backlog }) {
   this.key = key;
   this.all = new Map();
   this.free = new Map();
@@ -22,16 +22,17 @@ function RoundRobinHandle(key, address, { port, fd, flags }) {
   this.server = net.createServer(assert.fail);
 
   if (fd >= 0)
-    this.server.listen({ fd });
+    this.server.listen({ fd, backlog });
   else if (port >= 0) {
     this.server.listen({
       port,
       host: address,
       // Currently, net module only supports `ipv6Only` option in `flags`.
       ipv6Only: Boolean(flags & constants.UV_TCP_IPV6ONLY),
+      backlog,
     });
   } else
-    this.server.listen(address);  // UNIX socket path.
+    this.server.listen(address, backlog);  // UNIX socket path.
 
   this.server.once('listening', () => {
     this.handle = this.server._handle;

lib/net.js

@@ -1335,6 +1335,7 @@ function listenInCluster(server, address, port, addressType,
     addressType: addressType,
     fd: fd,
     flags,
+    backlog,
   };
 
   // Get the master's server handle, and listen on it