cluster: fix premature 'disconnect' event

Don't emit the 'disconnect' event until all workers have gone away.
Before this commit, the event was emitted when all open handles were
closed, which usually - but not always - amounts to the same thing.

Fixes #6346.

lib/cluster.js

@@ -323,7 +323,8 @@ function masterInit() {
       var handle = handles[key];
       if (handle.remove(worker)) delete handles[key];
     }
-    if (Object.keys(handles).length === 0) {
+    if (Object.keys(cluster.workers).length === 0) {
+      assert(Object.keys(handles).length === 0, 'Resource leak detected.');
       intercom.emit('disconnect');
     }
   });

test/simple/test-cluster-disconnect-idle-worker.js

@@ -0,0 +1,34 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var cluster = require('cluster');
+
+if (cluster.isMaster) {
+  cluster.fork();
+  cluster.fork();
+  cluster.disconnect(common.mustCall(function() {
+    assert.deepEqual(Object.keys(cluster.workers), []);
+  }));
+} else {
+  setTimeout(assert.fail, 1e6);
+}