dgram: make send cb act as "error" event handler

This allows users to provide a callback that handles potential
errors resulting from a `socket.send` call. The original behavior
of emitting the error event on the socket is preserved.

Fixes: https://github.com/joyent/node/issues/4846
PR-URL: https://github.com/joyent/node/pull/7738
PR-URL: https://github.com/nodejs/io.js/pull/1796
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/dgram.markdown

@@ -114,7 +114,9 @@ assigned a random port number and is bound to the "all interfaces" address
 An optional callback may be specified to detect DNS errors or for determining
 when it's safe to reuse the `buf` object.  Note that DNS lookups delay the time
 to send for at least one tick.  The only way to know for sure that the datagram
-has been sent is by using a callback.
+has been sent is by using a callback. If an error occurs and a callback is given,
+the error will be the first argument to the callback. If a callback is not given,
+the error is emitted as an `'error'` event on the `socket` object.
 
 With consideration for multi-byte characters, `offset` and `length` will
 be calculated with respect to

lib/dgram.js

@@ -300,7 +300,15 @@ Socket.prototype.send = function(buffer,
 
   self._handle.lookup(address, function(ex, ip) {
     if (ex) {
-      if (callback) callback(ex);
+      if (callback) {
+        callback(ex);
+
+        if (self.listeners('error').length)
+          self.emit('error', ex);
+
+        return;
+      }
+
       self.emit('error', ex);
     } else if (self._handle) {
       var req = new SendWrap();

test/simple/test-dgram-send-cb-quelches-error.js

@@ -0,0 +1,54 @@
+// 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 dgram = require('dgram');
+var dns = require('dns');
+
+var socket = dgram.createSocket('udp4');
+var buffer = new Buffer('gary busey');
+var times = 0;
+
+dns.setServers([]);
+
+// assert that:
+// * callbacks act as "error" listeners if given.
+// * callbacks and error events are emitted in a certain order.
+socket.send(buffer, 0, buffer.length, 100, 'dne.example.com', callbackOnly);
+
+function callbackOnly(err) {
+  assert.ok(err);
+  socket.once('error', onEvent);
+  socket.send(buffer, 0, buffer.length, 100, 'dne.example.com', onCallback);
+}
+
+function onEvent(err) {
+  assert.ok(err);
+  assert.equal(times, 0); 
+  times++;
+}
+
+function onCallback(err) {
+  assert.ok(err);
+  assert.equal(times, 1); 
+  socket.close();
+}