motor: protect status update button during mclennan home/jog

[FreddieAkeroyd]

One of the homing modes for the mclennan is to do a jog to a limit, which is done via an SNL homing program. While this is happening the STUP field of the motor record must not be pressed as this aborts the motion. There is a periodic poll of this in db records (https://github.com/ISISComputingGroup/EPICS-motor/blob/master/motorApp/Db/periodic_polling.db) , which we protect from doing such a poll while a jog/home is in progress. I noticed that there is a Status button on the motor details GUI that writes directly to STUP - if somebody was to press this button during a home it would abort the home.

This issue only affects model 1 motor drivers, which we only have the mclennan and linmot. The linmot does not home via a jog, so it is only a mclennan that is affected.

This does not need hardware and can be seen with the maclennan lewis emulator - start a jog and press the status update, the jog aborts

Acceptance criteria

• the "status" button on the motor details display does not action a STUP when a home/jog is in progress, so a jog is not aborted when it is pressed

Approach

This can probably be done by:

• split https://github.com/ISISComputingGroup/EPICS-motor/blob/master/motorApp/Db/periodic_polling.db into two records, a "SAFE_STUP" record that does the existing logic but is Passive scan, and then making the existing record called SCAN run on a POLL_RATE period and forward link to SAFE_STUP.
• Change the Status button on the motor details GUI so it writes to the SAFE_STUP.PROC field instead of motor record .STUP field

To test

• You can initially confirm that the button is actioning SAFE_STUP by setting TPRO to 1 on the record and then looking for a processing message printed to IOC console. It is possible to test this with either a mclennan emulator or real hardware, set a jog going and then press Status. Before the change it should abort jog, after change it should not.