`RuntimeError: Shutdown must be called from main thread` when interrupting server.py

[mearlboro]

It seems JVM requires to be stopped from the same thread that started it. We need to find a way to implement a graceful exit that correctly shuts down the JVM.

Trace:

Traceback (most recent call last):
  File "/home/pi/.local/lib/python3.7/site-packages/flask/app.py", line 2095, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/pi/.local/lib/python3.7/site-packages/flask/app.py", line 2080, in wsgi_app
    response = self.handle_exception(e)
  File "/home/pi/.local/lib/python3.7/site-packages/flask/app.py", line 2077, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/pi/.local/lib/python3.7/site-packages/flask/app.py", line 1525, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/pi/.local/lib/python3.7/site-packages/flask/app.py", line 1523, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/pi/.local/lib/python3.7/site-packages/flask/app.py", line 1509, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
  File "camera/server/server.py", line 68, in stop_tracking
    proc.stop()
  File "/home/pi/camera/server/video.py", line 477, in stop
    self.calc.exit()
  File "/home/pi/camera/core/emergence.py", line 189, in exit
    jp.shutdownJVM()
  File "/home/pi/.local/lib/python3.7/site-packages/jpype/_core.py", line 329, in shutdownJVM
    raise RuntimeError("Shutdown must be called from main thread")
RuntimeError: Shutdown must be called from main thread