cluster: cluster.disconnect() should check status

Workers that are already disconnected but not yet exited should not be
disconnected, trying to do so raises exceptions.

Signed-off-by: Julien Gilli <julien.gilli@joyent.com>

lib/cluster.js

@@ -407,6 +407,7 @@ function masterInit() {
     } else {
       for (var key in workers) {
         key = workers[key];
+        if (cluster.workers[key].isConnected())
           cluster.workers[key].disconnect();
       }
     }

test/simple/test-cluster-disconnect-before-exit.js

@@ -0,0 +1,35 @@
+// 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 cluster = require('cluster');
+
+if (cluster.isMaster) {
+  var worker = cluster.fork().on('online', disconnect);
+
+  function disconnect() {
+    worker.disconnect();
+    // The worker remains in cluster.workers until both disconnect AND exit.
+    // Disconnect is supposed to disconnect all workers, but not workers that
+    // are already disconnected, since calling disconnect() on an already
+    // disconnected worker would error.
+    worker.on('disconnect', cluster.disconnect);
+  }
+}