Problem: zmq 4.1.6 cannot pub msg to a zmq 2.x

[laplaceyang]

In some of my code, my server use the zmq 4.1.6 with PUB mode, and another old server with SUB mode build with zmq 2.2.0.

Then I cannot receive anything in the sub server.

Here is the example:
https://gist.github.com/laplaceyang/8cf1c004832f4ef618c69ceb868ef33b
build the PUB server with zmq 4.1.6 and SUB server with zmq 2.x

but if i build the server with zmq 3.x, it works.

It seems that change in stream_engine.cpp lead to this problem

---

In zmq 3.x, we have function

we inject a phantom subscription message and process identity msg in the same function

but in zmq 4.x, we separate it to two function

identity process func

inject phantom subscription message function

This change lead that,we should trigger two socket in event to finish subscribe.

but I dump the tcp package,
The SUB server just send one package to PUB.

After this no more package exchanges

[laplaceyang]

well , I create a pull request just now. pr#2256