Problem: Cannot get thread safe socket fd

Solution: Add a method to get the zmq_poller's signaler fd. Then we can
associate a poller instance with every thread safe socket and use its
fd.

include/zmq.h

@@ -709,6 +709,7 @@ ZMQ_EXPORT int zmq_poller_wait_all (void *poller,
                                     zmq_poller_event_t *events,
                                     int n_events,
                                     long timeout);
+ZMQ_EXPORT int zmq_poller_fd (void *poller);
 
 #if defined _WIN32
 ZMQ_EXPORT int

src/socket_poller.cpp

@@ -86,6 +86,17 @@ bool zmq::socket_poller_t::check_tag ()
     return _tag == 0xCAFEBABE;
 }
 
+int zmq::socket_poller_t::signaler_fd ()
+{
+    if (_signaler) {
+        return _signaler->get_fd ();
+    } else {
+        // Only thread-safe socket types are guaranteed to have a signaler.
+        errno = EINVAL;
+        return -1;
+    }
+}
+
 int zmq::socket_poller_t::add (socket_base_t *socket_,
                                void *user_data_,
                                short events_)

src/socket_poller.hpp

@@ -75,6 +75,8 @@ class socket_poller_t
     int add_fd (fd_t fd_, void *user_data_, short events_);
     int modify_fd (fd_t fd_, short events_);
     int remove_fd (fd_t fd_);
+    // Returns the signaler's fd if there is one, otherwise errors.
+    int signaler_fd ();
 
     int wait (event_t *event_, int n_events_, long timeout_);
 

src/zmq.cpp

@@ -1278,6 +1278,11 @@ int zmq_poller_wait_all (void *poller_,
     return rc;
 }
 
+int zmq_poller_fd (void *poller_)
+{
+    return static_cast<zmq::socket_poller_t *> (poller_)->signaler_fd ();
+}
+
 //  Peer-specific state
 
 int zmq_socket_get_peer_state (void *s_,

src/zmq_draft.h

@@ -105,6 +105,7 @@ int zmq_poller_wait_all (void *poller_,
                          zmq_poller_event_t *events_,
                          int n_events_,
                          long timeout_);
+int zmq_poller_fd (void *poller_);
 
 #if defined _WIN32
 int zmq_poller_add_fd (void *poller_,