uorb: fix several race conditions during topic initialization

Possible race conditions (they all happen between the check of existence
of a topic and trying to create the node):
- single instance, with multiple advertisers during the first advertise:
  both advertisers see the topic as non-existent and try to advertise it.
  One of them will fail, leading to an error message.
  This is the cause for telemetry_status advert failure seen in SITL in
  rare cases.
- multi-instance: subscription to non-existing instance -> px4_open fails,
  and the subscriber tries to create the node. If during that time a
  publisher publishes that instance, the subscriber will get (instance+1)
  (or fails if the max number of instances is exceeded).
  This is a race that goes pretty much unnoticed.
- multi-instance: 2 publishers can get the same instance (if is_published()
  is false in case both have not published data yet).
  This can also go unnoticed.
  Therefore the patch changes where _advertised is set: it is now set
  directly during the advertisement instead of during publication.

src/modules/uORB/uORBDeviceMaster.cpp

@@ -55,7 +55,7 @@ uORB::DeviceMaster::~DeviceMaster()
 }
 
 int
-uORB::DeviceMaster::advertise(const struct orb_metadata *meta, int *instance, int priority)
+uORB::DeviceMaster::advertise(const struct orb_metadata *meta, bool is_advertiser, int *instance, int priority)
 {
 	int ret = PX4_ERROR;
 
@@ -119,22 +119,45 @@ uORB::DeviceMaster::advertise(const struct orb_metadata *meta, int *instance, in
 			delete node;
 
 			if (ret == -EEXIST) {
-				/* if the node exists already, get the existing one and check if
-				 * something has been published yet. */
+				/* if the node exists already, get the existing one and check if it's advertised. */
 				uORB::DeviceNode *existing_node = getDeviceNodeLocked(meta, group_tries);
 
-				if (existing_node != nullptr && !existing_node->is_advertised()) {
-					/* nothing has been published yet, lets claim it */
-					existing_node->set_priority(priority);
+				/*
+				 * We can claim an existing node in these cases:
+				 * - The node is not advertised (yet). It means there is already one or more subscribers or it was
+				 *   unadvertised.
+				 * - We are a single-instance advertiser requesting the first instance.
+				 *   (Usually we don't end up here, but we might in case of a race condition between 2
+				 *   advertisers).
+				 * - We are a subscriber requesting a certain instance.
+				 *   (Also we usually don't end up in that case, but we might in case of a race condtion
+				 *   between an advertiser and subscriber).
+				 */
+				bool is_single_instance_advertiser = is_advertiser && !instance;
+
+				if (existing_node != nullptr &&
+				    (!existing_node->is_advertised() || is_single_instance_advertiser || !is_advertiser)) {
+					if (is_advertiser) {
+						existing_node->set_priority(priority);
+						/* Set as advertised to avoid race conditions (otherwise 2 multi-instance advertisers
+						 * could get the same instance).
+						 */
+						existing_node->mark_as_advertised();
+					}
+
 					ret = PX4_OK;
 
 				} else {
-					/* otherwise: data has already been published, keep looking */
+					/* otherwise: already advertised, keep looking */
 				}
 			}
 
 		} else {
-			// add to the node map;.
+			if (is_advertiser) {
+				node->mark_as_advertised();
+			}
+
+			// add to the node map.
 			_node_list.add(node);
 		}