Make parallelism options more well defined.

volatility/framework/constants/__init__.py

@@ -56,5 +56,8 @@ MAC_BANNERS_PATH = os.path.join(CACHE_PATH, "mac_banners.cache")
 
 ProgressCallback = Optional[Callable[[float, str], None]]
 
-# Options are 'multiprocessing', 'threading' or None
-PARALLELISM = 'multiprocessing'
+PARALLELISM_OFF = 0
+PARALLELISM_THREADING = 1
+PARALLELISM_MULTIPROCESSING = 2
+
+PARALLELISM = PARALLELISM_MULTIPROCESSING

volatility/framework/layers/physical.py

@@ -104,7 +104,7 @@ class FileLayer(interfaces.layers.DataLayerInterface):
         self._file_ = None  # type: Optional[IO[Any]]
         self._size = None  # type: Optional[int]
         # Construct the lock now (shared if made before threading) in case we ever need it
-        if constants.PARALLELISM == 'threading':
+        if constants.PARALLELISM == constants.PARALLELISM_THREADING:
             self._lock = threading.Lock()
         else:
             # We don't need a lock for multiprocessing because child threads can't inherit file descriptors by default