RTL: fix corner case where vehicle loiters in place instead of doing the RTL to Home

[sfuhrer]

Solved Problem

With RTL_TYPE=0 and RTL_APPR_FORCE=1, the logic in RTL::findRtlDestination() does not check if the mission is valid, only if the mission has a valid land item. If the mission is not valid, it will result in anyway switching into the Mission RTL, only to then find that the mission is invalid and loiter at place instead of returning.

Solution

Instead of adding an additional check for the mission validity there, the proposal here is to not return hasMissionLandStart() as true.

Changelog Entry

For release notes:

Bugfix: RTL: fix corner case where vehicle loiters in place instead of doing the RTL to Home.

Alternatives

Add a check for mission validity in

PX4-Autopilot/src/modules/navigator/rtl.cpp

Lines 402 to 403 in 4d0a88f

	if (((_param_rtl_type.get() == 1) || (_param_rtl_type.get() == 3) || (fabsf(FLT_MAX - min_dist) < FLT_EPSILON))
	&& hasMissionLandStart()) {

[KonradRudin]

The changes do not fix the issue. The RTL has another function here https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/navigator/rtl.cpp#L618-L621 You would need to fix this function. But both the RTL and the mission_base should have the same logic in the function. Maybe a library function would be better here.

[KonradRudin]

LGTM. Current behavior as i tested it:

• If a mission was evaluated invalid, it wont choose the mission landing. If there is no valid RTL point available, it will fly back to takeoff
• If the mission is valid and the RTL_type right, it will use the mission landing
• If the mission is still evaluated valid although it is invalid (e.g. through mission checker parameter change) It will use the mission landing because the mission is not evaluated anymore (For a mission though it will enter mission mode, but then loiter as it wont execute the mission).
  All the above seems ok for me, as a fix for the last point would need some restructuring on how the mission checker is performed.

[sfuhrer]

If the mission is still evaluated valid although it is invalid (e.g. through mission checker parameter change) It will use the mission landing because the mission is not evaluated anymore (For a mission though it will enter mission mode, but then loiter as it wont execute the mission).

Corner case though is that when you try to start an invalid mission it will then keep the mission invalidated, meaning that it won't use if for the RTL neither.
But as you said, to cleanly solve that we need to restructure a lot, and basically split up the landing part of a mission from the main mission body.