net: add net.listening boolean property over a getter

Added a listening property into net.Server.prototype indicating
if the server is listening or not for connections.

Other Server constructors that rely on net.Server should also
gain access to this property.

Also included tests for net and http subsystems.

PR-URL: nodejs#4743
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/http.markdown

@@ -560,6 +560,11 @@ parameter is 511 (not 512).
 This function is asynchronous. The last parameter `callback` will be added as
 a listener for the `'listening'` event.  See also [`net.Server.listen(port)`][].
 
+### server.listening
+
+A Boolean indicating whether or not the server is listening for
+connections.
+
 ### server.maxHeadersCount
 
 Limits maximum incoming headers count, equal to 1000 by default. If set to 0 -

doc/api/net.markdown

@@ -202,6 +202,11 @@ server.on('error', (e) => {
 
 (Note: All sockets in Node.js set `SO_REUSEADDR` already)
 
+### server.listening
+
+A Boolean indicating whether or not the server is listening for
+connections.
+
 ### server.maxConnections
 
 Set this property to reject connections when the server's connection count gets

lib/net.js

@@ -1389,6 +1389,14 @@ Server.prototype.listen = function() {
   return self;
 };
 
+Object.defineProperty(Server.prototype, 'listening', {
+  get: function() {
+    return !!this._handle;
+  },
+  configurable: true,
+  enumerable: true
+});
+
 Server.prototype.address = function() {
   if (this._handle && this._handle.getsockname) {
     var out = {};

test/parallel/test-http-listening.js

@@ -0,0 +1,16 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+const server = http.createServer();
+
+assert.strictEqual(server.listening, false);
+
+server.listen(common.PORT, common.mustCall(() => {
+  assert.strictEqual(server.listening, true);
+
+  server.close(common.mustCall(() => {
+    assert.strictEqual(server.listening, false);
+  }));
+}));

test/parallel/test-net-listening.js

@@ -0,0 +1,16 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+
+const server = net.createServer();
+
+assert.strictEqual(server.listening, false);
+
+server.listen(common.PORT, common.mustCall(() => {
+  assert.strictEqual(server.listening, true);
+
+  server.close(common.mustCall(() => {
+    assert.strictEqual(server.listening, false);
+  }));
+}));