dns: support promisified lookup(Service)

doc/api/dns.md

@@ -123,6 +123,10 @@ dns.lookup('example.com', options, (err, addresses) =>
 // addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]
 ```
 
+If this method is invoked as its [`util.promisify()`][]ed version, and `all`
+is not set to `true`, it returns a Promise for an object with `address` and
+`family` properties.
+
 ### Supported getaddrinfo flags
 
 The following flags can be passed as hints to [`dns.lookup()`][].
@@ -163,6 +167,9 @@ dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
 });
 ```
 
+If this method is invoked as its [`util.promisify()`][]ed version, it returns a
+Promise for an object with `hostname` and `service` properties.
+
 ## dns.resolve(hostname[, rrtype], callback)
 <!-- YAML
 added: v0.1.27
@@ -521,3 +528,4 @@ uses. For instance, _they do not use the configuration from `/etc/hosts`_.
 [Implementation considerations section]: #dns_implementation_considerations
 [supported `getaddrinfo` flags]: #dns_supported_getaddrinfo_flags
 [the official libuv documentation]: http://docs.libuv.org/en/latest/threadpool.html
+[`util.promisify()`]: util.html#util_util_promisify_original

lib/dns.js

@@ -26,6 +26,7 @@ const util = require('util');
 const cares = process.binding('cares_wrap');
 const uv = process.binding('uv');
 const internalNet = require('internal/net');
+const { customPromisifyArgs } = require('internal/util');
 
 const GetAddrInfoReqWrap = cares.GetAddrInfoReqWrap;
 const GetNameInfoReqWrap = cares.GetNameInfoReqWrap;
@@ -189,6 +190,9 @@ function lookup(hostname, options, callback) {
   return req;
 }
 
+Object.defineProperty(lookup, customPromisifyArgs,
+                      { value: ['address', 'family'], enumerable: false });
+
 
 function onlookupservice(err, host, service) {
   if (err)
@@ -228,6 +232,9 @@ function lookupService(host, port, callback) {
   return req;
 }
 
+Object.defineProperty(lookupService, customPromisifyArgs,
+                      { value: ['hostname', 'service'], enumerable: false });
+
 
 function onresolve(err, result, ttls) {
   if (ttls && this.ttl)

test/internet/test-dns-ipv4.js

@@ -3,6 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const dns = require('dns');
 const net = require('net');
+const util = require('util');
 const isIPv4 = net.isIPv4;
 
 let running = false;
@@ -184,3 +185,13 @@ TEST(function test_lookupservice_ip_ipv4(done) {
 
   checkWrap(req);
 });
+
+TEST(function test_lookupservice_ip_ipv4_promise(done) {
+  util.promisify(dns.lookupService)('127.0.0.1', 80)
+    .then(common.mustCall(({hostname, service}) => {
+      assert.strictEqual(typeof hostname, 'string');
+      assert(hostname.length > 0);
+      assert(['http', 'www', '80'].includes(service));
+      done();
+    }));
+});

test/internet/test-dns.js

@@ -428,6 +428,32 @@ TEST(function test_lookup_ip_all(done) {
 });
 
 
+TEST(function test_lookup_ip_all_promise(done) {
+  const req = util.promisify(dns.lookup)('127.0.0.1', {all: true})
+    .then(function(ips) {
+      assert.ok(Array.isArray(ips));
+      assert.ok(ips.length > 0);
+      assert.strictEqual(ips[0].address, '127.0.0.1');
+      assert.strictEqual(ips[0].family, 4);
+
+      done();
+    });
+
+  checkWrap(req);
+});
+
+
+TEST(function test_lookup_ip_promise(done) {
+  util.promisify(dns.lookup)('127.0.0.1')
+    .then(function({ address, family }) {
+      assert.strictEqual(address, '127.0.0.1');
+      assert.strictEqual(family, 4);
+
+      done();
+    });
+});
+
+
 TEST(function test_lookup_null_all(done) {
   const req = dns.lookup(null, {all: true}, function(err, ips, family) {
     assert.ifError(err);