Merge pull request #302 from joshuagl/joshuagl/one-filesystem-backend…

…-to-rule-them-all

storage: make FilesystemBackend a singleton

securesystemslib/storage.py

@@ -188,6 +188,20 @@ class FilesystemBackend(StorageBackendInterface):
  local filesystems using Python standard library functions.
  """
 
+ # As FilesystemBackend is effectively a stateless wrapper around various
+ # standard library operations, we only ever need a single instance of it.
+ # That single instance is safe to be (re-)used by all callers. Therefore
+ # implement the singleton pattern to avoid uneccesarily creating multiple
+ # objects.
+ _instance = None
+
+ def __new__(cls, *args, **kwargs):
+ if cls._instance is None:
+ cls._instance = object.__new__(cls, *args, **kwargs)
+ return cls._instance
+
+
+
  class GetFile(object):
  # Implementing get() as a function with the @contextmanager decorator
  # doesn't allow us to cleanly capture exceptions thrown by the underlying

tests/test_storage.py

@@ -100,3 +100,15 @@ def test_folders(self):
  fi.write(leaf.encode('utf-8'))
  found_leaves = self.storage_backend.list_folder(folder)
  self.assertListEqual(leaves, sorted(found_leaves))
+
+
+ def test_singleton(self):
+ # There should only ever be a single instance of FilesystemBackend.
+ # An object's id is unique and constant for the object during its
+ # lifetime. Therefore create more than one instance of FilesystemBackend
+ # and compare their id's
+ fb1 = securesystemslib.storage.FilesystemBackend()
+ fb2 = securesystemslib.storage.FilesystemBackend()
+ self.assertEqual(id(fb1), id(fb2))
+ self.assertEqual(id(self.storage_backend), id(fb1))
+ self.assertEqual(id(fb2), id(self.storage_backend))