Fix exception handling in read message path

[jkryl]

The problem is if exception is thrown from handler of message event on zmq socket:

• error event on zmq socket is emitted even though it's not a socket error but just an exception in event's handler (in user's code) and propagation of exception from user's code is stopped
• the socket stops emitting message events after it, because reading of messages did not finish and if new messages arrive the state does not change

I have put together two simple test programs to demonstrate the problem. Pubber emits event every second and subber receives it and throws exception if event number is 10.

// pubber.js
const zmq = require('zmq');
const sock = zmq.socket('pub');

sock.bindSync('tcp://127.0.0.1:3000');
console.log('Publisher bound to port 3000');

var i = 1;

setInterval(function() {
    console.log('sending message ' + i);
    sock.send(['event', (i++).toString()]);
}, 1000);

// subber.js
const zmq = require('zmq');
const sock = zmq.socket('sub');

sock.connect('tcp://127.0.0.1:3000');
sock.subscribe('event');
console.log('Subscriber connected to port 3000');

sock.on('message', function(name, data) {
    data = parseInt(data);
    console.log('received', name.toString(), data);

    if (data == 10) {
        throw Error('Exception from message handler');
    }
});

sock.on('error', function(err) {
    console.log('In socket error handler:', err.toString());
});

process.on('uncaughtException', function(err) {
    console.log('Uncaught exception:', err.toString());
});

The fix is to distinguish between exceptions thrown from _zmq.readv() (from zmq bindings) and exceptions thrown from user provided callbacks. For the former case we behave like before. For the later case we reschedule reading from socket and re-throw exception so that it can get handled as normally.

[ronkorving]

We've been here in the past, and it was the source of all kinds of trouble iirc. Allow me some time to review this in more depth.