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: #4056

doc/api/net.md

@@ -367,6 +367,10 @@ 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

@@ -1343,6 +1343,7 @@ function listenInCluster(server, address, port, addressType,
     addressType: addressType,
     fd: fd,
     flags,
+    backlog,
   };
 
   // Get the master's server handle, and listen on it

test/parallel/test-cluster-net-listen-backlog.js

@@ -0,0 +1,42 @@
+'use strict';
+
+const common = require('../common');
+const Countdown = require('../common/countdown');
+const assert = require('assert');
+// Monkey-patch `net.Server._listen2`
+const net = require('net');
+const cluster = require('cluster');
+
+// Ensures that the `backlog` is used to create a `net.Server`.
+const kExpectedBacklog = 127;
+if (cluster.isMaster) {
+  let worker
+
+  const nativeListen = net.Server.prototype._listen2;
+  const countdown = new Countdown(2, () => {
+    worker.disconnect();
+  });
+
+  net.Server.prototype._listen2 = common.mustCall(
+    function(address, port, addressType, backlog, fd, flags) {
+      assert(backlog, kExpectedBacklog);
+      countdown.dec();
+      return nativeListen.call(this, address, port, addressType, backlog, fd, flags);
+    }
+  );
+
+  worker = cluster.fork();
+  worker.on('message', () => {
+    countdown.dec();
+  });
+} else {
+  const server = net.createServer()
+
+  server.listen({
+    host: common.localhostIPv4,
+    port: 0,
+    backlog: kExpectedBacklog,
+  }, common.mustCall(() => {
+    process.send(true);
+  }));
+}