uorb: allow an orb subscription to retrieve data that was published prior to subscribe

src/modules/uORB/uORBDevices.cpp

@@ -146,13 +146,8 @@ uORB::DeviceNode::open(device::file_t *filp)
 			return -ENOMEM;
 		}
 
-		/* If queue size >1, allow the subscriber to read the data in the queue. Otherwise, assume subscriber is up to date.*/
-		if (_queue_size <= 1) {
-			sd->generation = _generation;
-
-		} else {
-			sd->generation = _generation - (_queue_size < _generation ? _queue_size : _generation);
-		}
+		/* If there were any previous publications, allow the subscriber to read them */
+		sd->generation = _generation - (_queue_size < _generation ? _queue_size : _generation);
 
 		/* set priority */
 		sd->set_priority(_priority);

src/modules/uORB/uORBManager.hpp

@@ -186,6 +186,9 @@ public:
 	 * in order to wait for updates to a topic, as well as topic_read,
 	 * orb_check and orb_stat.
 	 *
+	 * If there were any publications of the topic prior to the subscription,
+	 * an orb_check right after orb_subscribe will return true.
+	 *
 	 * Subscription will succeed even if the topic has not been advertised;
 	 * in this case the topic will have a timestamp of zero, it will never
 	 * signal a poll() event, checking will always return false and it cannot
@@ -213,6 +216,9 @@ public:
 	 * in order to wait for updates to a topic, as well as topic_read,
 	 * orb_check and orb_stat.
 	 *
+	 * If there were any publications of the topic prior to the subscription,
+	 * an orb_check right after orb_subscribe_multi will return true.
+	 *
 	 * Subscription will succeed even if the topic has not been advertised;
 	 * in this case the topic will have a timestamp of zero, it will never
 	 * signal a poll() event, checking will always return false and it cannot