Bugfix: Multiple cancellations may occur, use mutex lock to prevent.

Bugfix: Multiple cancellations may occur, use mutex lock to prevent.

There is an existing check that prevents the cancellation from being called more than once.
What is not being considered is that if the main thread calls cancellation while another cancellation is active then the controller_thread_process_exit() gets called.
The controller_thread_process_exit() function will begin more forcibly shutting things down.

Avoid this by providing a mutex lock to lock the cancellation.
Only once the first cancellation is complete will the second (or more) then return without doing anything.