Worker dies when you pass an error object to next() inside MIDDLEWARE_HANDSHAKE_WS

According to the documentation for MIDDLEWARE_HANDSHAKE_WS, you can pass an error object to next() to block a connection before the underlying WebSocket is created. However, doing so causes the SocketCluster worker to crash:

$ node server
   [Busy] Launching SocketCluster
   !! The sc-hot-reboot plugin is watching for code changes in the /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp directory
   >> Broker PID: 15257
   >> WorkerCluster PID: 15258
   >> Worker PID: 15259
   [Active] SocketCluster started
            Version: 14.3.2
            Environment: dev
            WebSocket engine: ws
            Port: 8000
            Master PID: 15256
            Worker count: 1
            Broker count: 1

1542668941065 - Origin: Worker (PID 15259)
   [Warning] Error: This is an error object, not a string
    at Object.<anonymous> (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/worker.js:20:14)
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:228:33
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:1105:9
    at replenish (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:982:17)
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:986:9
    at _asyncMap (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:1103:5)
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:1189:16
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:1016:16
    at Object.<anonymous> (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:227:20)
    at Object.<anonymous> (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:89:12)
1542668941065 - Origin: Worker (PID 15259)
   [Error] TypeError [ERR_INVALID_ARG_TYPE]: The "string" argument must be one of type string, Buffer, or ArrayBuffer. Received type object
    at Function.byteLength (buffer.js:514:11)
    at abortHandshake (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/ws/lib/websocket-server.js:342:33)
    at options.verifyClient (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/ws/lib/websocket-server.js:208:33)
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/socketcluster-server/scserver.js:661:13
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:1110:9
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:460:16
    at iterateeCallback (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:962:17)
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:944:16
    at /Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/node_modules/async/dist/async.js:1107:13
    at Object.<anonymous> (/Users/maxwellcarey/projects/socketcluster-middleware-handshake-ws-obj-not-string/myApp/worker.js:20:9)
1542668941071 - Worker 0 exited - Exit code: 1
   >> Worker PID: 15262
1542668941343 - Worker 0 was respawned

Here's the worker.js that generated the above output:

var SCWorker = require('socketcluster/scworker');
var express = require('express');
var serveStatic = require('serve-static');
var path = require('path');

class Worker extends SCWorker {
  run() {
    console.log('   >> Worker PID:', process.pid);
    var environment = this.options.environment;

    var httpServer = this.httpServer;
    var scServer = this.scServer;

    var app = express();
    app.use(serveStatic(path.resolve(__dirname, 'public')));

    httpServer.on('request', app);

    scServer.addMiddleware(scServer.MIDDLEWARE_HANDSHAKE_WS, function(req, next) {
        next(new Error('This is an error object, not a string'));
    });
  }
}

new Worker();

This happens because the WebSocket backend's verifyClient callback expects the third argument to be a string, but SocketCluster is passing the error object. See the docs for ws verifyClient:

if verifyClient is provided with two arguments then those are:

• info {Object} Same as above.
• cb {Function} A callback that must be called by the user upon inspection of the info fields. Arguments in this callback are:
  • result {Boolean} Whether or not to accept the handshake.
  • code {Number} When result is false this field determines the HTTP error status code to be sent to the client.
  • name {String} When result is false this field determines the HTTP reason phrase.
  • headers {Object} When result is false this field determines additional HTTP headers to be sent to the client. For example, { 'Retry-After': 120 }.

It can be fixed in socketcluster-server as follows (although it would be better to check whether err is actually an object with a message property):

--- scserver.js	2018-10-11 01:11:20.000000000 -0500
+++ scserver.js.new	2018-11-19 17:27:44.000000000 -0600
@@ -658,7 +658,7 @@ SCServer.prototype.verifyHandshake = fun
             } else if (self.middlewareEmitWarnings) {
               self.emit('warning', err);
             }
-            cb(false, 401, err);
+            cb(false, 401, err.message);
           } else {
             cb(true);
           }

Start in scserver.js at SCServer.prototype.verifyHandshake, where the ws verifyClient callback receives the middleware error, and use the worker.js reproduction to trigger the failure. Compare the callback arguments with the ws documentation. Done means passing an error object to next() rejects the handshake without causing the worker to exit.