AP_SmartRTL: give warning buffer is about to fill-up

libraries/AP_SmartRTL/AP_SmartRTL.cpp

@@ -350,6 +350,13 @@ void AP_SmartRTL::run_background_cleanup()
 
     // perform routine cleanup which removes 10 to 50 points if possible
     routine_cleanup(path_points_count, path_points_completed_limit);
+
+    // warn if buffer is about to be filled
+    if ((path_points_count >0) && (path_points_count >= _path_points_max - 9) && (AP_HAL::millis() - _last_low_space_notify_ms > 10000)) {
+        gcs().send_text(MAV_SEVERITY_INFO, "SmartRTL Low on space!");
+       _last_low_space_notify_ms = now;
+    }
+
 }
 
 // routine cleanup is called regularly from run_background_cleanup

libraries/AP_SmartRTL/AP_SmartRTL.h

@@ -173,6 +173,7 @@ private:
     uint32_t _last_position_save_ms;    // the system time a position was saved to the path (used for timeout)
     uint32_t _thorough_clean_request_ms;// the last system time the thorough cleanup was requested (set by thorough_cleanup method, used by background cleanup)
     uint32_t _thorough_clean_complete_ms; // set to _thorough_clean_request_ms when the background thread completes the thorough cleanup
+    uint32_t _last_low_space_notify_ms; //last time low on SmartRTL space was notified on Mavlink. Minimum time is required before re-notification to avoid nagging.
     ThoroughCleanupType _thorough_clean_type;   // used by example sketch to test simplify and prune separately
 
     // path variables