[minor] Throw an error on invalid method usage

Throw an error if `WebSocketServer.prototype.completeUpgrade()` is
called more than once with the same socket argument.

lib/websocket-server.js

@@ -7,7 +7,7 @@ const { createServer, STATUS_CODES } = require('http');
 const PerMessageDeflate = require('./permessage-deflate');
 const WebSocket = require('./websocket');
 const { format, parse } = require('./extension');
-const { GUID } = require('./constants');
+const { GUID, kWebSocket } = require('./constants');
 
 const keyRegex = /^[+/0-9A-Za-z]{22}==$/;
 
@@ -254,6 +254,7 @@ class WebSocketServer extends EventEmitter {
    * @param {net.Socket} socket The network socket between the server and client
    * @param {Buffer} head The first packet of the upgraded stream
    * @param {Function} cb Callback
+   * @throws {Error} If called more than once with the same socket
    * @private
    */
   completeUpgrade(key, extensions, req, socket, head, cb) {
@@ -262,6 +263,13 @@ class WebSocketServer extends EventEmitter {
     //
     if (!socket.readable || !socket.writable) return socket.destroy();
 
+    if (socket[kWebSocket]) {
+      throw new Error(
+        'server.handleUpgrade() was called more than once with the same ' +
+          'socket, possibly due to a misconfiguration'
+      );
+    }
+
     const digest = createHash('sha1')
       .update(key + GUID)
       .digest('base64');

test/websocket-server.test.js

@@ -11,6 +11,7 @@ const fs = require('fs');
 
 const Sender = require('../lib/sender');
 const WebSocket = require('..');
+const { NOOP } = require('../lib/constants');
 
 describe('WebSocketServer', () => {
   describe('#ctor', () => {
@@ -383,6 +384,42 @@ describe('WebSocketServer', () => {
     });
   });
 
+  describe('#completeUpgrade', () => {
+    it('throws an error if called twice with the same socket', (done) => {
+      const server = http.createServer();
+
+      server.listen(0, () => {
+        const wss = new WebSocket.Server({ noServer: true });
+
+        server.on('upgrade', (req, socket, head) => {
+          wss.handleUpgrade(req, socket, head, (ws) => {
+            ws.close();
+          });
+          assert.throws(
+            () => wss.handleUpgrade(req, socket, head, NOOP),
+            (err) => {
+              assert.ok(err instanceof Error);
+              assert.strictEqual(
+                err.message,
+                'server.handleUpgrade() was called more than once with the ' +
+                  'same socket, possibly due to a misconfiguration'
+              );
+              return true;
+            }
+          );
+        });
+
+        const ws = new WebSocket(`ws://localhost:${server.address().port}`);
+
+        ws.on('open', () => {
+          ws.on('close', () => {
+            server.close(done);
+          });
+        });
+      });
+    });
+  });
+
   describe('Connection establishing', () => {
     it('fails if the Sec-WebSocket-Key header is invalid (1/2)', (done) => {
       const wss = new WebSocket.Server({ port: 0 }, () => {