uORBDevices_nuttx: Fixed the bug that a publisher had to be started

before an advertiser for multi_pub/subs.

This is achieved using
- A "published"  flag for each uORB device node
- A check before increasing the instance count (basically, a node is
  re-used if nothing has been published on it before which means that it
  has been created by a subscriber.

src/modules/uORB/uORBDevices_nuttx.cpp

@@ -59,6 +59,7 @@ uORB::DeviceNode::DeviceNode
 	_generation(0),
 	_publisher(0),
 	_priority(priority),
+	_published(false),
 	_IsRemoteSubscriberPresent(false),
 	_subscriber_count(0)
 {
@@ -254,6 +255,8 @@ uORB::DeviceNode::write(struct file *filp, const char *buffer, size_t buflen)
 	/* notify any poll waiters */
 	poll_notify(POLLIN);
 
+	_published = true;
+
 	return _meta->o_size;
 }
 
@@ -483,6 +486,14 @@ void uORB::DeviceNode::remove_internal_subscriber()
 	}
 }
 
+
+//-----------------------------------------------------------------------------
+//-----------------------------------------------------------------------------
+bool uORB::DeviceNode::is_published()
+{
+	return _published;
+}
+
 //-----------------------------------------------------------------------------
 //-----------------------------------------------------------------------------
 int16_t uORB::DeviceNode::process_add_subscription(int32_t rateInHz)
@@ -616,9 +627,23 @@ uORB::DeviceMaster::ioctl(struct file *filp, int cmd, unsigned long arg)
 				/* initialise the node - this may fail if e.g. a node with this name already exists */
 				ret = node->init();
 
-				/* if init failed, discard the node and its name */
 				if (ret != OK) {
+					/* if init failed, discard the node */
 					delete node;
+
+					if (ret == -EEXIST) {
+						/* if the node exists already, get the existing one and check if
+						 * something has been published yet. */
+						uORB::DeviceNode *existing_node = GetDeviceNode(devpath);
+
+						if ((existing_node != nullptr) && !(existing_node->is_published())) {
+							/* nothing has been published yet, lets claim it */
+							ret = OK;
+						} else {
+							/* otherwise: data has already been published, keep looking */
+						}
+					}
+					/* also discard the name now */
 					free((void *)objname);
 					free((void *)devpath);