logger: only add data message to the log if orb_copy succeeds, when adding a new instance

src/modules/logger/logger.cpp

@@ -441,7 +441,7 @@ bool Logger::copy_if_updated_multi(LoggerSubscription &sub, int multi_instance,
 		if (OK == orb_exists(sub.metadata, multi_instance)) {
 			handle = orb_subscribe_multi(sub.metadata, multi_instance);
 
-			//PX4_INFO("subscribed to instance %d of topic %s", multi_instance, topic->o_name);
+			//PX4_INFO("subscribed to instance %d of topic %s", multi_instance, sub.metadata->o_name);
 
 			/* copy first data */
 			if (handle >= 0) {
@@ -454,8 +454,11 @@ bool Logger::copy_if_updated_multi(LoggerSubscription &sub, int multi_instance,
 					orb_set_interval(handle, interval);
 				}
 
-				orb_copy(sub.metadata, handle, buffer);
-				updated = true;
+				/* It can happen that orb_exists returns true, even if there is no publisher (but another subscriber).
+				 * We catch this here, because orb_copy will fail in this case. */
+				if (orb_copy(sub.metadata, handle, buffer) == PX4_OK) {
+					updated = true;
+				}
 			}
 		}
 

src/modules/uORB/uORBManager.hpp

@@ -295,7 +295,8 @@ public:
 	int  orb_stat(int handle, uint64_t *time) ;
 
 	/**
-	 * Check if a topic has already been created.
+	 * Check if a topic has already been created (a publisher or a subscriber exists with
+	 * the given instance).
 	 *
 	 * @param meta    ORB topic metadata.
 	 * @param instance  ORB instance