AP_Arming uses AP_Notify to retrieve failsafe state

[rmackay9]

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AP_Notify is meant only to provide notification of the vehicle state to the user so it should be theoretically possible to remove the AP_Notify library and have the vehicle operate normally (i.e. no real functional change beyond the loss of status reporting to the user). The AP_Arming library breaks this though by using AP_Notify to capture the vehicle's radio (aka throttle) failsafe and battery failsafe status. bad! https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_Arming/AP_Arming.cpp#L371

[peterbarker]

Easiest fix here is probably to move those checks to AP_Arming_Copter where the arming library has access to the failsafe object.

[enjoybeta]

https://github.com/ArduPilot/ardupilot/blob/86e495f415d4ddcf7046e7c2bece3d68a1b2c970/libraries/AP_Arming/AP_Arming.cpp#L371

[WickedShell]

I'm handling the battery one already. (There is an expected follow up PR to the two stage failsafe one that I'm sending in shortly that consolidates the battery information elsewhere).

[IamPete1]

Battery case is fixed but the RC failsafe still remains,

https://github.com/ArduPilot/ardupilot/blob/0b0ca6a9e99e5f22621af1d0199f008ce1c786d0/libraries/AP_Arming/AP_Arming.cpp#L669

The fix is probably to add a pure virtual AP_Vehicle method to check rc fail safe state.

[peterbarker]

A PR exists to remove the line from AP_Arming now: https://github.com/ArduPilot/ardupilot/pull/18358

There was some resistance at the DevCall where it was brought up for merge.