CMake: Add compile check to test for correct ZeroMQ C++ headers

[t-b]

We are relying on the presence of convertability from zmq::socket_t to void *.

Add a cmake check so that we know that we have the correct version.

Close #273.

@Ingvord.

[t-b]

@Ingvord @bourtemb How do I locally debug the travis test failures? I have docker but it's not clear for me what scripts I have to run.

And I'm not just doing it with trial and error with travis as that is way too slow for debugging.

[Ingvord]

@t-b , well I do not run travis locally, I run the build process and tests (all in one run, or just a single one) from CLion. Taking into account that I am on Debian 8 build process finishes fine. Which is not the case for this particular PR for non-Debian 8 configurations.

As I see cmake complains about cppzmq:

-- Looking for C++ include zmq.hpp - found
CMake Error at configure/CMakeLists.txt:161 (message):
  Please use a compatible version of cppzmq

On Debian 9&7 where we build our own version of cppzmq (on Debian 8 we use apt-get to fetch libzmq).

The error message is added in this PR so the answer is easy - what we build is not compatible or maybe the reason is that cmake compiles against wrong version of cppzmq :)

Fetching cppzmq code is in .travis.yml and the build is in install_cppzmq.sh

Hope this helps

[Ingvord]

I guess we need to avoid ${ZMQ_PKG_XXX} here and use ${CPPZMQ_BASE} everywhere. As it is now - cmake compiles against cppzmq from pkg-config (from apt-get) which is indeed incompatible on Debian 9&7

[bourtemb]

@Ingvord @bourtemb How do I locally debug the travis test failures? I have docker but it's not clear for me what scripts I have to run.

Hi @t-b,

The scripts to run the tests are actually described in https://github.com/tango-controls/cppTango/blob/tango-9-lts/INSTALL.md file and on the wiki for running the tests from CLion: https://github.com/tango-controls/cppTango/wiki/Contribution-Guide

[mliszcz]

Why can't we simply use explicit conversions to void* to compile Tango with libzmq3-dev from Ubuntu 16 repos?

diff --git a/cppapi/client/zmqeventconsumer.cpp b/cppapi/client/zmqeventconsumer.cpp
index 25128a25..2b0f676a 100644
--- a/cppapi/client/zmqeventconsumer.cpp
+++ b/cppapi/client/zmqeventconsumer.cpp
@@ -181,9 +181,9 @@ void *ZmqEventConsumer::run_undetached(TANGO_UNUSED(void *arg))
        zmq::pollitem_t *items = new zmq::pollitem_t [MAX_SOCKET_SUB];
        int nb_poll_item = 3;

-       items[0].socket = *control_sock;
-       items[1].socket = *heartbeat_sub_sock;
-       items[2].socket = *event_sub_sock;
+       items[0].socket = static_cast<void*>(*control_sock);
+       items[1].socket = static_cast<void*>(*heartbeat_sub_sock);
+       items[2].socket = static_cast<void*>(*event_sub_sock);

        for (int loop = 0;loop < nb_poll_item;loop++)
        {
@@ -1063,7 +1063,7 @@ bool ZmqEventConsumer::process_ctrl(zmq::message_t &received_ctrl,zmq::pollitem_
 // Update poll item list
 //

-                poll_list[old_poll_nb].socket = *tmp_sock;
+                poll_list[old_poll_nb].socket = static_cast<void*>(*tmp_sock);
                 poll_list[old_poll_nb].fd = 0;
                 poll_list[old_poll_nb].events = ZMQ_POLLIN;
                 poll_list[old_poll_nb].revents = 0;
diff --git a/cppapi/server/zmqeventsupplier.cpp b/cppapi/server/zmqeventsupplier.cpp
index b1752101..f5bfb11e 100644
--- a/cppapi/server/zmqeventsupplier.cpp
+++ b/cppapi/server/zmqeventsupplier.cpp
@@ -686,7 +686,7 @@ void ZmqEventSupplier::create_mcast_socket(string &mcast_data,int rate,McastSock
 // Bind the publisher socket to the specified port
 //

-    if (zmq_bind(*(ms.pub_socket),ms.endpoint.c_str()) != 0)
+    if (zmq_bind(static_cast<void*>(*ms.pub_socket),ms.endpoint.c_str()) != 0)
     {
         TangoSys_OMemStream o;
         o << "Can't bind ZMQ socket with endpoint ";

In some older implementations of zmq.hpp, void* conversion is explicit in C++11 (and above):

#if __cplusplus >= 201103L
#define ZMQ_CPP11
#define ZMQ_NOTHROW noexcept
#define ZMQ_EXPLICIT explicit
#else
    #define ZMQ_CPP03
    #define ZMQ_NOTHROW
    #define ZMQ_EXPLICIT
#endif

// ...

        //  Be careful with this, it's probably only useful for
        //  using the C api together with an existing C++ api.
        //  Normally you should never need to use this.
        inline ZMQ_EXPLICIT operator void* () ZMQ_NOTHROW
        {
            return ptr;
        }

// ...

[t-b]

@mliszcz Good point.

I think we should do something along these lines.

Looking at https://github.com/zeromq/cppzmq/blob/d25c58a05dc981a63a92f8b1f7ce848e7f21d008/zmq.hpp#L1272

also shows that operator void* is gone so we have to find different solutions for different zmq versions

How about something like (from the top of my head):

template<typename T>
void* ConvertToZMQPlainSocket(const T &s)
{
#if CPPZMQ_VERSION >= ZMQ_MAKE_VERSION(A, B, C)
    return s.handle();
#elif CPPZMQ_VERSION >= ZMQ_MAKE_VERSION(D, E, F)
    return static_cast<void*>(s);
#else
    return s;
#endif
}

With that wrapper we have a chance of getting rid of the code again at some point in the future.

[mliszcz]

Looks good to me. Two comments:

• zmq::socket_base is a new thing starting from 4.3.1. The wrapper needs to accept zmq::socket_t.
• instead of checking for particular zmq version in preprocessor, SFINAE could be used to detect if socket provides handle() method (but it may be tricky and verbose without c++11)

[t-b]

@bourtemb @mliszcz So I nearly finished the code to detect all the different cppzmq variants. But looking closer into the latest version of cppzmq showed that socket_t is still available and it also has a implicit operator *void() again (used to be implicit, now explicit, then implicit again). So I think Michal's solution is the way to go. I'll prepare a PR.