dgram: close() should accept a callback

Like net, http, and https server.close, and socket.end(), etc.

PR-URL: #217
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/dgram.markdown

@@ -82,7 +82,7 @@ are created.
 
 ### Event: 'close'
 
-Emitted when a socket is closed with `close()`.  No new `message` events will be emitted
+Emitted after a socket is closed with `close()`.  No new `message` events will be emitted
 on this socket.
 
 ### Event: 'error'
@@ -228,9 +228,10 @@ shown below.
     });
 
 
-### socket.close()
+### socket.close([callback])
 
-Close the underlying socket and stop listening for data on it.
+Close the underlying socket and stop listening for data on it. If a callback is
+provided, it is added as a listener for the ['close'](#dgram_event_close) event.
 
 ### socket.address()
 

lib/dgram.js

@@ -352,7 +352,9 @@ function afterSend(err) {
 }
 
 
-Socket.prototype.close = function() {
+Socket.prototype.close = function(callback) {
+  if (callback)
+    this.on('close', callback);
   this._healthCheck();
   this._stopReceiving();
   this._handle.close();

test/parallel/test-dgram-close.js

@@ -32,9 +32,13 @@ buf.fill(42);
 var socket = dgram.createSocket('udp4');
 var handle = socket._handle;
 var closeEvents = 0;
+var closeCallbacks = 0;
 socket.send(buf, 0, buf.length, common.PORT, 'localhost');
-assert.strictEqual(socket.close(), socket);
+assert.strictEqual(socket.close(function() {
+  ++closeCallbacks;
+}), socket);
 socket.on('close', function() {
+  assert.equal(closeCallbacks, 1);
   ++closeEvents;
 });
 socket = null;
@@ -48,4 +52,5 @@ setImmediate(function() {
 
 process.on('exit', function() {
   assert.equal(closeEvents, 1);
+  assert.equal(closeCallbacks, 1);
 });