Change multi-thread MujocoModelGen to n-length queue

Create an 10-length queue in MujocoModelGenerator to store the
mujoco_models.

rllab/dynamics_randomization/mujoco_model_gen.py

@@ -1,5 +1,5 @@
 import atexit
-import sys
+from queue import Queue
 from threading import Event
 from threading import Thread
 
@@ -32,6 +32,8 @@ def __init__(self, file_path, variations):
         """
         self._variations = variations
         self._file_path = file_path
+        # Synchronized queue to store mujoco_models
+        self._models = Queue(maxsize=10)
         # Worker Thread
         self._worker_thread = Thread(
             target=self._generator_routine, daemon=True, name="Worker-Thread")
@@ -60,24 +62,20 @@ def get_model(self):
             user in this class.
         """
         if not self._worker_thread.is_alive():
-            # If worker thread is dead because of an error, raise an error in main thread
+            # If worker thread terminates because of an error, terminates main thread
             raise ChildProcessError("Error raised in Worker-Thread")
 
-        if not self._model_ready.is_set():
-            # If the model is not ready yet, wait for it to be finished.
-            self._model_ready.wait()
-        # Cleat the event flag for the next iteration
-        self._model_ready.clear()
-        # Request a new model to the worker thread.
-        self._model_requested.set()
-        return self._mujoco_model
+        return self._models.get()
 
     def stop(self):
         """
         Stops the worker thread. This method has to be called when the corresponding
         randomized environment is terminated or when the training is interrupted.
         """
         if self._worker_thread.is_alive():
+            while not self._models.empty():
+                self._models.get()
+
             self._model_requested.set()
             self._stop_event.set()
             self._worker_thread.join()
@@ -126,9 +124,4 @@ def _generator_routine(self):
 
             model_xml = etree.tostring(parsed_model.getroot()).decode("ascii")
             self._mujoco_model = load_model_from_xml(model_xml)
-
-            # Wake up the calling thread if it was waiting
-            self._model_ready.set()
-            # Go to idle mode (wait for event)
-            self._model_requested.wait()
-            self._model_requested.clear()
+            self._models.put(self._mujoco_model)