Merge pull request #302 from joshuagl/joshuagl/one-filesystem-backend-to-rule-them-all

storage: make FilesystemBackend a singleton

Author: joshuagl

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))