dns: send lookup c-ares errors to callback

Calling dns.lookup with arguments that generate an error from c-ares
previously sent those errors back to the callback. This commit restores
the ca9eb71 behavior.

Fixes #7731.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Signed-off-by: Fedor Indutny <fedor@indutny.com>

lib/dns.js

@@ -132,8 +132,12 @@ exports.lookup = function(hostname, family, callback) {
     hostname: hostname,
     oncomplete: onlookup
   };
+
   var err = cares.getaddrinfo(req, hostname, family);
-  if (err) throw errnoException(err, 'getaddrinfo', hostname);
+  if (err) {
+    callback(errnoException(err, 'getaddrinfo', hostname));
+    return {};
+  }
 
   callback.immediately = true;
   return req;

test/simple/test-dns-lookup-cb-error.js

@@ -0,0 +1,44 @@
+// 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 cares = process.binding('cares_wrap');
+
+var dns = require('dns');
+
+// Stub `getaddrinfo` to *always* error.
+cares.getaddrinfo = function() {
+  return process.binding('uv').UV_ENOENT;
+};
+
+assert.doesNotThrow(function() {
+  var tickValue = 0;
+
+  dns.lookup('example.com', function(error, result, addressType) {
+    assert.equal(tickValue, 1);
+    assert.equal(error.code, 'ENOENT');
+  });
+
+  // Make sure that the error callback is called
+  // on next tick.
+  tickValue = 1;
+});