Plane: fixed terrain RTL with rally points

this fixes a bug where if the terrain database cache does not have the
tile for the location of a rally point then RTL to the rally point
with TERRAIN_FOLLOW=1 will not track terrain

The underlying issue is that Location::loc.change_alt_frame() will
return false if the location is not in the terrain memory cache. We
can't just extrapolate as the rally point could be in a totally
different terrain area to the current location. So instead we set it
as terrain_following_pending and fix it as soon as the terrain cache
is filled.

fixes https://github.com/ArduPilot/ardupilot/issues/25157

ArduPlane/Plane.h

@@ -735,6 +735,9 @@ private:
         // are we trying to follow terrain?
         bool terrain_following;
 
+        // are we waiting to load terrain data to init terrain following
+        bool terrain_following_pending;
+
         // target altitude above terrain in cm, valid if terrain_following
         // is set
         int32_t terrain_alt_cm;
@@ -855,7 +858,7 @@ private:
     void reset_offset_altitude(void);
     void set_offset_altitude_location(const Location &start_loc, const Location &destination_loc);
     bool above_location_current(const Location &loc);
-    void setup_terrain_target_alt(Location &loc) const;
+    void setup_terrain_target_alt(Location &loc);
     int32_t adjusted_altitude_cm(void);
     int32_t adjusted_relative_altitude_cm(void);
     float mission_alt_offset(void);

ArduPlane/altitude.cpp

@@ -212,6 +212,12 @@ void Plane::set_target_altitude_location(const Location &loc)
         target_altitude.amsl_cm += home.alt;
     }
 #if AP_TERRAIN_AVAILABLE
+    if (target_altitude.terrain_following_pending) {
+        /* we didn't get terrain data to init when we started on this
+           target, retry
+        */
+        setup_terrain_target_alt(next_WP_loc);
+    }
     /*
       if this location has the terrain_alt flag set and we know the
       terrain altitude of our current location then treat it as a
@@ -469,12 +475,16 @@ bool Plane::above_location_current(const Location &loc)
   modify a destination to be setup for terrain following if
   TERRAIN_FOLLOW is enabled
  */
-void Plane::setup_terrain_target_alt(Location &loc) const
+void Plane::setup_terrain_target_alt(Location &loc)
 {
 #if AP_TERRAIN_AVAILABLE
     if (terrain_enabled_in_current_mode()) {
-        loc.change_alt_frame(Location::AltFrame::ABOVE_TERRAIN);
+        if (!loc.change_alt_frame(Location::AltFrame::ABOVE_TERRAIN)) {
+            target_altitude.terrain_following_pending = true;
+            return;
+        }
     }
+    target_altitude.terrain_following_pending = false;
 #endif
 }
 

ArduPlane/commands_logic.cpp

@@ -8,7 +8,11 @@ bool Plane::start_command(const AP_Mission::Mission_Command& cmd)
     // default to non-VTOL loiter
     auto_state.vtol_loiter = false;
 
-        // log when new commands start
+#if AP_TERRAIN_AVAILABLE
+    plane.target_altitude.terrain_following_pending = false;
+#endif
+
+    // log when new commands start
     if (should_log(MASK_LOG_CMD)) {
         logger.Write_Mission_Cmd(mission, cmd);
     }

ArduPlane/mode.cpp

@@ -97,6 +97,10 @@ bool Mode::enter()
     quadplane.mode_enter();
 #endif
 
+#if AP_TERRAIN_AVAILABLE
+    plane.target_altitude.terrain_following_pending = false;
+#endif
+
     bool enter_result = _enter();
 
     if (enter_result) {