LIBZMQ-497 there could be unsent bytes in encoder

When we send a large message, the message can be splitted into two chunks.
One is in the encoder buffer and the other is the zero-copy pointer.
The session could get the term before the last chunk is sent.

src/encoder.hpp

@@ -126,6 +126,11 @@ namespace zmq
             *size_ = pos;
         }
 
+        inline bool has_data ()
+        {
+            return to_write > 0;
+        }
+
     protected:
 
         //  Prototype of state machine action.

src/i_encoder.hpp

@@ -47,6 +47,7 @@ namespace zmq
         virtual void get_data (unsigned char **data_, size_t *size_,
             int *offset_ = NULL) = 0;
 
+        virtual bool has_data () = 0;
     };
 
 }

src/stream_engine.cpp

@@ -65,6 +65,7 @@ zmq::stream_engine_t::stream_engine_t (fd_t fd_, const options_t &options_, cons
     options (options_),
     endpoint (endpoint_),
     plugged (false),
+    terminating (false),
     socket (NULL)
 {
     //  Put the socket into non-blocking mode.
@@ -188,6 +189,11 @@ void zmq::stream_engine_t::unplug ()
 
 void zmq::stream_engine_t::terminate ()
 {
+    if (!terminating && encoder && encoder->has_data ()) {
+        //  Give io_thread a chance to send in the buffer
+        terminating = true;
+        return;
+    }
     unplug ();
     delete this;
 }
@@ -298,6 +304,8 @@ void zmq::stream_engine_t::out_event ()
     //  this is necessary to prevent losing incomming messages.
     if (nbytes == -1) {
         reset_pollout (handle);
+        if (unlikely (terminating))
+            terminate ();
         return;
     }
 
@@ -309,6 +317,10 @@ void zmq::stream_engine_t::out_event ()
     if (unlikely (handshaking))
         if (outsize == 0)
             reset_pollout (handle);
+
+    if (unlikely (terminating))
+        if (outsize == 0)
+            terminate ();
 }
 
 void zmq::stream_engine_t::activate_out ()

src/stream_engine.hpp

@@ -135,6 +135,7 @@ namespace zmq
         std::string endpoint;
 
         bool plugged;
+        bool terminating;
 
         // Socket
         zmq::socket_base_t *socket;