libdm: enhance error message

libdm/libdm-deptree.c

@@ -1503,8 +1503,37 @@ static int _thin_pool_node_message(struct dm_tree_node *dnode, struct thin_messa
 	}
 
 	if (!_node_message(dnode->info.major, dnode->info.minor,
-			   tm->expected_errno, buf))
+			   tm->expected_errno, buf)) {
+		switch (m->type) {
+		case DM_THIN_MESSAGE_CREATE_SNAP:
+		case DM_THIN_MESSAGE_CREATE_THIN:
+			if (errno == EEXIST) {
+				/*
+				 * ATM errno from ioctl() is preserved through code error path chain
+				 * If this would ever change, another way need to be used to
+				 * obtain result from failed DM message
+				 */
+				log_error("Thin pool %s already contain thin device with device_id %u.",
+					  _node_name(dnode), m->u.m_create_snap.device_id);
+				/*
+				 * TODO:
+				 *
+				 * Give some useful advice how to solve this problem,
+				 * until lvconvert --repair can handle this automatically
+				 */
+				log_error("Manual intervention may be required to remove device dev_id=%u in thin pool metadata.",
+					  m->u.m_create_snap.device_id);
+				log_error("Optionally new thin volume with device_id=%u can be manually added into a volume group.",
+					  m->u.m_create_snap.device_id);
+				log_warn("WARNING: When uncertain how to do this, contact support!");
+				return 0;
+			}
+			/* fall through */
+		default:
 			return_0;
+		}
+
+	}
 
 	return 1;
 }