don't check POLLOUT for single-socket proxies

src/proxy.cpp

@@ -142,10 +142,14 @@ int zmq::proxy (
             return -1;
 
         //  Get the pollout separately because when combining this with pollin it maxes the CPU
-        //  because pollout shall most of the time return directly
-        rc = zmq_poll (&itemsout [0], 2, 0);
-        if (unlikely (rc < 0))
-            return -1;
+        //  because pollout shall most of the time return directly.
+        //  POLLOUT is only checked when frontend and backend sockets are not the same.
+        if (frontend_ != backend_) {
+            rc = zmq_poll (&itemsout [0], 2, 0);
+            if (unlikely (rc < 0)) {
+                return -1;
+            }
+        }
 
         //  Process a control command if any
         if (control_ && items [2].revents & ZMQ_POLLIN) {
@@ -180,13 +184,14 @@ int zmq::proxy (
         //  Process a request
         if (state == active
         &&  items [0].revents & ZMQ_POLLIN
-        &&  itemsout [1].revents & ZMQ_POLLOUT) {
+        &&  (frontend_ == backend_ || itemsout [1].revents & ZMQ_POLLOUT)) {
             rc = forward(frontend_, backend_, capture_,msg);
             if (unlikely (rc < 0))
                 return -1;
         }
         //  Process a reply
         if (state == active
+        &&  frontend_ != backend_
         &&  items [1].revents & ZMQ_POLLIN
         &&  itemsout [0].revents & ZMQ_POLLOUT) {
             rc = forward(backend_, frontend_, capture_,msg);