node zeromq problem demonstration for https://github.com/JustinTulloss/zeromq.node/issues/523

client.js

var zmq = require('zmq');
var socket = zmq.socket('dealer');

socket.connect('tcp://127.0.0.1:3333');

socket.on('message', function(payload) {
    console.info('Received:', payload.toString());
});

console.info('Sending...');
socket.send(["msg1"]);
socket.send(["msg2"]);

setInterval(function() {
    console.info('Calling _flushReads');
    socket._flushReads();
}, 3000)

info.txt

$ npm version
{ test: '1.0.0',
  npm: '3.8.6',
  ares: '1.10.1-DEV',
  http_parser: '2.7.0',
  icu: '56.1',
  modules: '47',
  node: '5.11.0',
  openssl: '1.0.2g',
  uv: '1.8.0',
  v8: '4.6.85.31',
  zlib: '1.2.8' }
$ cat /etc/debian_version 
8.4
$ uname -a
Linux messai7 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08) x86_64 GNU/Linux
$ dpkg -l | grep zmq
ii  libzmq3:amd64                          4.0.5+dfsg-2+deb8u1                             amd64        lightweight messaging kernel (shared library)
ii  libzmq3-dev:amd64                      4.0.5+dfsg-2+deb8u1                             amd64        lightweight messaging kernel (development files)
ii  python-zmq                             14.4.0-1                                        amd64        Python bindings for 0MQ library
ii  python3-zmq                            14.4.0-1                                        amd64        Python3 bindings for 0MQ library

output.txt

$ ./server.py >/dev/null &
[1] 5868

# this is how it should be - response is received immediately:
$ npm run-script client

> [email protected] client /home/messa/code/pokusy/bug-node-zmq
> node client.js

Sending...
Received: msg1
Received: msg2
Calling _flushReads
^C

# but sometimes, responses are received only after explicitly calling _flushReads:
$ npm run-script client

> [email protected] client /home/messa/code/pokusy/bug-node-zmq
> node client.js

Sending...
Calling _flushReads
Received: msg1
Received: msg2
Calling _flushReads
^C

package.json

{
  "name": "test",
  "version": "1.0.0",
  "description": "",
  "dependencies": {
    "zmq": "^2.15.0"
  },
  "devDependencies": {},
  "scripts": {
    "client": "node client.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC"
}

server.py

#!/usr/bin/env python3

import zmq

zsock = zmq.Context().socket(zmq.ROUTER)
zsock.bind('tcp://127.0.0.1:3333')
while True:
    parts = zsock.recv_multipart()
    zsock.send_multipart(parts)
    print(parts)