lib: fix cluster handle leak

It is possible to cause a resource leak in SharedHandle if a worker is
added after all other workers have been removed. This commit fixes the
leak.

Fixes: nodejs#2510
PR-URL: nodejs#3510

lib/cluster.js

@@ -345,7 +345,10 @@ function masterInit() {
        * if it has disconnected, otherwise we might
        * still want to access it.
        */
-      if (!worker.isConnected()) removeWorker(worker);
+      if (!worker.isConnected()) {
+        removeHandlesForWorker(worker);
+        removeWorker(worker);
+      }
 
       worker.suicide = !!worker.suicide;
       worker.state = 'dead';

test/parallel/test-cluster-shared-worker-added-after-disconnect.js

@@ -0,0 +1,41 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+const cluster = require('cluster');
+cluster.schedulingPolicy = cluster.SCHED_NONE;
+
+if (cluster.isMaster) {
+  var worker1, worker2;
+
+  worker1 = cluster.fork();
+  worker1.on('message', common.mustCall(function() {
+    worker2 = cluster.fork();
+    const c = net.connect(common.PORT, common.mustCall(function() {
+      c.unref();
+      worker1.send('die');
+      worker2.send('die');
+    }));
+    c.on('error', function(e) {
+      // ECONNRESET is OK
+      if (e.code !== 'ECONNRESET')
+        throw e;
+    });
+  }));
+
+  return;
+}
+
+var server = net.createServer({}, function(c) {
+  c.end('bye');
+});
+
+server.listen(common.PORT, function() {
+  process.send('listening');
+});
+
+process.on('message', function listener() {
+  server.close(function() {
+    process.exit();
+  });
+});