[minor] Throw an error on invalid usage

Throw an error if the same `server` option is used for multiple
WebSocket servers at the same time.

lib/websocket-server.js

@@ -10,6 +10,7 @@ const { format, parse } = require('./extension');
 const { GUID } = require('./constants');
 
 const keyRegex = /^[+/0-9A-Za-z]{22}==$/;
+const kUsedByWebSocketServer = Symbol('kUsedByWebSocketServer');
 
 /**
  * Class representing a WebSocket server.
@@ -82,6 +83,14 @@ class WebSocketServer extends EventEmitter {
  }
 
  if (this._server) {
+ if (this._server[kUsedByWebSocketServer]) {
+ throw new Error(
+ 'The HTTP/S server is already being used by another WebSocket server'
+ );
+ } else {
+ this._server[kUsedByWebSocketServer] = true;
+ }
+
  this._removeListeners = addListeners(this._server, {
  listening: this.emit.bind(this, 'listening'),
  error: this.emit.bind(this, 'error'),
@@ -135,6 +144,8 @@ class WebSocketServer extends EventEmitter {
  const server = this._server;
 
  if (server) {
+ server[kUsedByWebSocketServer] = false;
+
  this._removeListeners();
  this._removeListeners = this._server = null;
 

test/websocket-server.test.js

@@ -17,11 +17,22 @@ describe('WebSocketServer', () => {
  assert.throws(() => new WebSocket.Server());
  });
 
- it('throws an error if no port or server is specified', () => {
- assert.throws(() => new WebSocket.Server({}));
- });
-
  describe('options', () => {
+ it('throws an error if no `port` or `server` option is specified', () => {
+ assert.throws(() => new WebSocket.Server({}));
+ });
+
+ it('throws an error if the server is already being used', () => {
+ const server = http.createServer();
+
+ new WebSocket.Server({ server });
+
+ assert.throws(
+ () => new WebSocket.Server({ server }),
+ /^Error: The HTTP\/S server is already being used by another WebSocket server$/
+ );
+ });
+
  it('exposes options passed to constructor', (done) => {
  const wss = new WebSocket.Server({ port: 0 }, () => {
  assert.strictEqual(wss.options.port, 0);
@@ -225,15 +236,21 @@ describe('WebSocketServer', () => {
 
  it('cleans event handlers on precreated server', (done) => {
  const server = http.createServer();
- const wss = new WebSocket.Server({ server });
+ const wss1 = new WebSocket.Server({ server });
 
  server.listen(0, () => {
- wss.close(() => {
+ wss1.close(() => {
  assert.strictEqual(server.listenerCount('listening'), 0);
  assert.strictEqual(server.listenerCount('upgrade'), 0);
  assert.strictEqual(server.listenerCount('error'), 0);
 
- server.close(done);
+ // Check that no error is thrown if `server` is resued now as there
+ // are no other `WebSocketServer`s using it.
+ const wss2 = new WebSocket.Server({ server });
+
+ wss2.close(() => {
+ server.close(done);
+ });
  });
  });
  });