Ensure empty paths raise a value error

Empty paths should not be allowed to be set or
fetched or normalized so check for those and raise
an error when one is encountered.

Also add some basic test conditions that ensure
the normpath method raises when it should.

Change-Id: I7f7e6600f03c67376ba310ab231b2e33cd7528db

taskflow/persistence/backends/impl_memory.py

@@ -74,6 +74,9 @@ class FakeFilesystem(object):
     @classmethod
     def normpath(cls, path):
         """Return a normalized absolutized version of the pathname path."""
+        if not path:
+            raise ValueError("This filesystem can only normalize paths"
+                             " that are not empty")
         if not path.startswith(cls.root_path):
             raise ValueError("This filesystem can only normalize"
                              " paths that start with %s: '%s' is not"

taskflow/tests/unit/persistence/test_memory_persistence.py

@@ -128,6 +128,12 @@ class MemoryFilesystemTest(test.TestCase):
         fs = impl_memory.FakeFilesystem()
         self.assertRaises(exc.NotFound, self._get_item_path, fs, '/c')
 
+    def test_bad_norms(self):
+        fs = impl_memory.FakeFilesystem()
+        self.assertRaises(ValueError, fs.normpath, '')
+        self.assertRaises(ValueError, fs.normpath, 'abc/c')
+        self.assertRaises(ValueError, fs.normpath, '../c')
+
     def test_del_root_not_allowed(self):
         fs = impl_memory.FakeFilesystem()
         self.assertRaises(ValueError, self._del_item_path, fs, '/')