AC_WPNav: remove unassigned _track_desired

[chobitsfan]

_track_desired is never assigned but is used in shift_wp_origin_to_current_pos(). It may be a little dangerous

[khancyr]

That is an important bug ! Nice catch.
_track_desired will be default to 0 and thus we will never pass the first check on shift_wp_origin_to_current_pos() ... that is an issue

[khancyr]

Should be safe to merge. it will be similar to the other shift functions so that isn't an issue

[rmackay9]

Great thank, I'll merge in a moment.

I don't think this will make a functional change because, as @khancyr says, _track_desired will default to zero so it should never cause AC_WPNav::shift_wp_origin_to_current_pos() to return early.

[chobitsfan]

Hi @rmackay9 @khancyr I am sorry but I may found a bigger possible problem.

I trace the git log and found the _track_desired assignment is removed when #15896 merged. Before #15896 merged, shift_wp_origin_to_current_pos() will return early if vehicle is not takeoff from origin (for exmaple, vehicle startup here and moved to other place). This will cause a horizontal drift when copter takeoff. I notice this problem when doing indoor ext nav flying test

[rmackay9]

@chobitsfan,

OK, thanks. I've added an issue to the Copter-4.1 issues list (see #16478). If you have logs of the issue that would be great.

[chobitsfan]

Hi @rmackay9 Thank you
log 20 2021-4-8 am 09-41-18.zip

[chobitsfan]

I create #17132 to fix this issue