cluster: return worker reference from disconnect()

Changes disconnect() to return a refererence to the worker.
This will enable method chaining such as

    worker.disconnect().once('disconnect', doThis);

PR-URL: https://github.com/nodejs/node/pull/10019
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

doc/api/cluster.md

@@ -262,6 +262,8 @@ It is not emitted in the worker.
 added: v0.7.7
 -->
 
+* Returns: {Worker} A reference to `worker`.
+
 In a worker, this function will close all servers, wait for the `'close'` event on
 those servers, and then disconnect the IPC channel.
 

lib/cluster.js

@@ -429,6 +429,7 @@ function masterInit() {
     send(this, { act: 'disconnect' });
     removeHandlesForWorker(this);
     removeWorker(this);
+    return this;
   };
 
   Worker.prototype.destroy = function(signo) {
@@ -686,6 +687,7 @@ function workerInit() {
 
   Worker.prototype.disconnect = function() {
     _disconnect.call(this);
+    return this;
   };
 
   Worker.prototype.destroy = function() {

test/parallel/test-cluster-worker-destroy.js

@@ -8,6 +8,7 @@
  */
 
 const common = require('../common');
+const assert = require('assert');
 const cluster = require('cluster');
 let worker1, worker2;
 
@@ -26,7 +27,8 @@ if (cluster.isMaster) {
       cluster.worker.destroy();
     });
 
-    cluster.worker.disconnect();
+    const w = cluster.worker.disconnect();
+    assert.strictEqual(w, cluster.worker, 'did not return a reference');
   } else {
     // Call destroy when worker is not disconnected yet
     cluster.worker.destroy();

test/parallel/test-cluster-worker-disconnect.js

@@ -40,7 +40,8 @@ if (cluster.isWorker) {
 
   // Disconnect worker when it is ready
   worker.once('listening', common.mustCall(() => {
-    worker.disconnect();
+    const w = worker.disconnect();
+    assert.strictEqual(worker, w, 'did not return a reference');
   }));
 
   // Check cluster events

test/parallel/test-cluster-worker-init.js

@@ -13,7 +13,8 @@ if (cluster.isMaster) {
 
   worker.on('message', common.mustCall((message) => {
     assert.strictEqual(message, true, 'did not receive expected message');
-    worker.disconnect();
+    const w = worker.disconnect();
+    assert.strictEqual(worker, w, 'did not return a reference');
   }));
 
   worker.on('online', () => {