zmq_poll: account for the fact that ZMQ_FD is edge-triggered

src/zmq.cpp

@@ -403,6 +403,7 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
         }
     }
 
+    bool first_pass = true;
     int timeout = timeout_ > 0 ? timeout_ / 1000 : -1;
     int nevents = 0;
 
@@ -410,7 +411,7 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
 
         //  Wait for events. Ignore interrupts if there's infinite timeout.
         while (true) {
-            int rc = poll (pollfds, nitems_, timeout);
+            int rc = poll (pollfds, nitems_, first_pass ? 0 : timeout);
             if (rc == -1 && errno == EINTR) {
                 if (timeout_ < 0)
                     continue;
@@ -431,7 +432,7 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
 
             //  The poll item is a 0MQ socket. Retrieve pending events
             //  using the ZMQ_EVENTS socket option.
-            if (items_ [i].socket && (pollfds [i].revents & POLLIN)) {
+            if (items_ [i].socket) {
                 size_t zmq_events_size = sizeof (uint32_t);
                 uint32_t zmq_events;
                 if (zmq_getsockopt (items_ [i].socket, ZMQ_EVENTS, &zmq_events,
@@ -461,6 +462,13 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
                 nevents++;
         }
 
+        //  If there are no events from the first pass (the one with no
+        //  timout), do at least the second pass so that we wait.
+        if (first_pass && nevents == 0 && timeout_ != 0) {
+            first_pass = false;
+            continue;
+        }
+
         //  If timeout is set to infinite and we have to events to return
         //  we can restart the polling.
         if (timeout == -1 && nevents == 0)
@@ -514,6 +522,8 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
         }
     }
 
+    bool first_pass = true;
+    timeval zero_timeout = {0, 0};
     timeval timeout = {timeout_ / 1000000, timeout_ % 1000000};
     int nevents = 0;
     fd_set inset, outset, errset;
@@ -526,7 +536,7 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
             memcpy (&outset, &pollset_out, sizeof (fd_set));
             memcpy (&errset, &pollset_err, sizeof (fd_set));
             int rc = select (maxfd, &inset, &outset, &errset,
-                (timeout_ < 0) ? NULL : &timeout);
+                first_pass ? &zero_timeout : (timeout_ < 0 ? NULL : &timeout));
 #if defined ZMQ_HAVE_WINDOWS
             wsa_assert (rc != SOCKET_ERROR);
 #else
@@ -584,6 +594,13 @@ int zmq_poll (zmq_pollitem_t *items_, int nitems_, long timeout_)
                 nevents++;
         }
 
+        //  If there are no events from the first pass (the one with no
+        //  timout), do at least the second pass so that we wait.
+        if (first_pass && nevents == 0 && timeout_ != 0) {
+            first_pass = false;
+            continue;
+        }
+
         //  If timeout is set to infinite and we have to events to return
         //  we can restart the polling.
         if (timeout_ < 0 && nevents == 0)