Commits on Sep 6, 2018

1. Fix serial sync errors caused by multi thread collision …

   SYMPTOM: Unpredictable errors when calling roboclaw API, manifesting
   in several different forms such as "OSError: 11"
   
   CAUSE: The roboclaw API is not thread-safe, but we have three different
   threads potentially calling into the API simultaneously. When this
   occurs, errors follow.
   1. self.run for our '/odom' publisher thread
   2. self.cmd_vel_callback as  '/cmd_vel' subscriber
   3. self.check_vitals running for diagnostics
   
   FIX: Use a threading.Lock() object to synchronize usage of roboclaw
   API object. Thereby ensuring only one thread will be communicating
   with roboclaw at any given time.
   
   REFERENCES:
   
   ROS Python API Threading Model
   https://answers.ros.org/question/9543/rospy-threading-model/
   
   StackOverflow "Semaphores on Python"
   https://stackoverflow.com/questions/31508574/semaphores-on-python

   

   Roger-random committed Sep 6, 2018
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