Add poller_t::wait (#680)

* Add poller_t::wait

* Update poller_t::wait to return an optional which is nullopt on a timeout

* Update poller_t::wait to return std::optional since the trivial optional implementation can only handle trivial types

Author: cacharle

tests/poller.cpp

@@ -178,6 +178,31 @@ TEST_CASE("poller remove registered non empty", "[poller]")
 
 const std::string hi_str = "Hi";
 
+#if CPPZMQ_HAS_OPTIONAL
+TEST_CASE("poller wait", "[poller]")
+{
+    common_server_client_setup s;
+    CHECK_NOTHROW(s.client.send(zmq::message_t{hi_str}, zmq::send_flags::none));
+    zmq::poller_t<int> poller;
+    int i = 42;
+    CHECK_NOTHROW(poller.add(s.server, zmq::event_flags::pollin, &i));
+    auto event = poller.wait();
+    CHECK(event.has_value());
+    CHECK(s.server == event.value().socket);
+    CHECK(&i == event.value().user_data);
+}
+
+TEST_CASE("poller wait timeout", "[poller]")
+{
+    common_server_client_setup s;
+    // No message sent so it will timeout
+    zmq::poller_t<int> poller;
+    CHECK_NOTHROW(poller.add(s.server, zmq::event_flags::pollin, nullptr));
+    auto event = poller.wait(std::chrono::milliseconds{3});
+    CHECK(!event.has_value());
+}
+#endif
+
 TEST_CASE("poller poll basic", "[poller]")
 {
     common_server_client_setup s;

zmq.hpp

@@ -2781,6 +2781,24 @@ template<typename T = no_user_data> class poller_t
         }
     }
 
+#if CPPZMQ_HAS_OPTIONAL
+    std::optional<event_type>
+    wait(std::chrono::milliseconds timeout = std::chrono::milliseconds{-1})
+    {
+        event_type event;
+        int rc = zmq_poller_wait(poller_ptr.get(),
+                                 reinterpret_cast<zmq_poller_event_t *>(&event),
+                                 static_cast<long>(timeout.count()));
+        if (rc == -1) {
+            if (zmq_errno() == EAGAIN)
+                return {};
+            else
+                throw error_t();
+        }
+        return event;
+    }
+#endif
+
     template<typename Sequence>
     size_t wait_all(Sequence &poller_events, const std::chrono::milliseconds timeout)
     {