[major] Call the callback with an error if the server is closed

Match the behavior of Node.js core `net.Server` and call the callback
of `WebSocketServer.prototype.close()` with an error if the server is
already closed.

lib/websocket-server.js

@@ -145,13 +145,19 @@ class WebSocketServer extends EventEmitter {
    * @public
    */
   close(cb) {
-    if (cb) this.once('close', cb);
-
     if (this._state === CLOSED) {
+      if (cb) {
+        this.once('close', () => {
+          cb(new Error('The server is not running'));
+        });
+      }
+
       process.nextTick(emitClose, this);
       return;
     }
 
+    if (cb) this.once('close', cb);
+
     if (this._state === CLOSING) return;
     this._state = CLOSING;
 

test/websocket-server.test.js

@@ -326,11 +326,21 @@ describe('WebSocketServer', () => {
     });
 
     it("emits the 'close' event if the server is already closed", (done) => {
+      let callbackCalled = false;
       const wss = new WebSocket.Server({ port: 0 }, () => {
         wss.close(() => {
           assert.strictEqual(wss._state, 2);
-          wss.on('close', done);
-          wss.close();
+
+          wss.on('close', () => {
+            callbackCalled = true;
+          });
+
+          wss.close((err) => {
+            assert.ok(callbackCalled);
+            assert.ok(err instanceof Error);
+            assert.strictEqual(err.message, 'The server is not running');
+            done();
+          });
         });
       });
     });