Preventing tread collisions

[MRAB]

On 2012-12-12 20:11, Wanderer wrote:
> I have a program that has a main GUI and a camera. In the main GUI, you can manipulate the images taken by the camera. You can also use the menu to check the camera's settings. Images are taken by the camera in a separate thread, so the long exposures don't block the GUI. I block conflicts between the camera snapshot thread and the main thread by setting a flag called self.cameraActive. I check to see if the cameraActive flag is false and set the cameraActive to True just before starting the thread. I generate an event on exiting the thread which sets the cameraActive flag to False. I also check and set and reset the flag in all the menu commands that access the camera. Like this.
>
>      def onProperties(self, event):
>          """ Display a message window with the camera properties
>          event -- The camera properties menu event
>          """
>          # Update the temperature
>          if not self.cameraActive:
>              self.cameraActive = True
>              self.camera.getTemperature()
>              camDict = self.camera.getPropertyDict()
>              self.cameraActive = False
>          else:
>              camDict = {'Error': 'Camera Busy'}
>          dictMessage(camDict, 'Camera Properties')
>
> This works but my question is, is there a better way using semaphores, locks or something else to prevent collisions between threads?
>
That suffers from a race condition in that self.cameraActive might be
False when it's checked in the 'if' condition but set to True just
afterwards by the other thread.

You could try a non-blocking semaphore:

def __init__(self):
     self.cameraActive = Semaphore()

def onProperties(self, event):
     """ Display a message window with the camera properties
     event -- The camera properties menu event
     """
     # Update the temperature
     if self.cameraActive.acquire(False): # Non-blocking
         # Successfully acquired the semaphore, so the camera wasn't active
         self.camera.getTemperature()
         camDict = self.camera.getPropertyDict()
         self.cameraActive.release()
     else:
         camDict = {'Error': 'Camera Busy'}