tests: do not hardcode service name in test-dns.

Instead of hard-coding http service name in test-dns, retrieve it from
/etc/services. This is not ideal, but it's still better than hard-coding
it.

Fixes nodejs#8047.

test/common.js

@@ -219,3 +219,58 @@ exports.checkSpawnSyncRet = function(ret) {
   assert.strictEqual(ret.status, 0);
   assert.strictEqual(ret.error, undefined);
 };
+
+var etcServicesFileName = path.join('/etc', 'services');
+if (process.platform === 'win32') {
+  etcServicesFileName = path.join('/Windows', 'System32', 'drivers', 'etc',
+    'services');
+}
+
+/*
+ * Returns a string that represents the service name associated
+ * to the service bound to port "port" and using protocol "protocol".
+ *
+ * If the service is not defined in the services file, it returns
+ * the port number as a string.
+ *
+ * Returns undefined if /etc/services (or its equivalent on non-UNIX
+ * platforms) can't be read.
+ */
+exports.getServiceName = function getServiceName(port, protocol) {
+  if (port == null) {
+    throw new Error("Missing port number");
+  }
+
+  if (typeof protocol !== 'string') {
+    throw new Error("Protocol must be a string");
+  }
+
+  /*
+   * By default, if a service can't be found in /etc/services,
+   * its name is considered to be its port number.
+   */
+  var serviceName = port.toString();
+
+  try {
+    /*
+     * I'm not a big fan of readFileSync, but reading /etc/services asynchronously
+     * here would require implementing a simple line parser, which seems overkill
+     * for a simple utility function that is not running concurrently with any
+     * other one.
+     */
+    var servicesContent = fs.readFileSync(etcServicesFileName,
+      { encoding: 'utf8'});
+    var regexp = util.format('^(\\w+)\\s+\\s%d/%s\\s', port, protocol);
+    var re = new RegExp(regexp, 'm');
+
+    var matches = re.exec(servicesContent);
+    if (matches && matches.length > 1) {
+      serviceName = matches[1];
+    }
+  } catch(e) {
+    console.error('Cannot find /etc/services file!');
+    return undefined;
+  }
+
+  return serviceName;
+}

test/internet/test-dns.js

@@ -495,7 +495,23 @@ TEST(function test_lookupservice_ip_ipv4(done) {
   var req = dns.lookupService('127.0.0.1', 80, function(err, host, service) {
     if (err) throw err;
     assert.strictEqual(host, 'localhost');
-    assert.strictEqual(service, 'http');
+
+    /*
+     * Retrieve the actual HTTP service name as setup on the host currently
+     * running the test by reading it from /etc/services. This is not ideal,
+     * as the service name lookup could use another mechanism (e.g nscd), but
+     * it's already better than hardcoding it.
+     */
+    var httpServiceName = common.getServiceName(80, 'tcp');
+    if (!httpServiceName) {
+      /*
+       * Couldn't find service name, reverting to the most sensible default
+       * for port 80.
+       */
+      httpServiceName = 'http';
+    }
+
+    assert.strictEqual(service, httpServiceName);
 
     done();
   });
@@ -515,7 +531,23 @@ TEST(function test_lookupservice_ip_ipv6(done) {
      * that most sane platforms use either one of these two by default.
      */
     assert(host === 'localhost' || host === 'ip6-localhost');
-    assert.strictEqual(service, 'http');
+
+    /*
+     * Retrieve the actual HTTP service name as setup on the host currently
+     * running the test by reading it from /etc/services. This is not ideal,
+     * as the service name lookup could use another mechanism (e.g nscd), but
+     * it's already better than hardcoding it.
+     */
+    var httpServiceName = common.getServiceName(80, 'tcp');
+    if (!httpServiceName) {
+      /*
+       * Couldn't find service name, reverting to the most sensible default
+       * for port 80.
+       */
+      httpServiceName = 'http';
+    }
+
+    assert.strictEqual(service, httpServiceName);
 
     done();
   });