Test loading of cached metadata in ngclient

After making a successful update of valid metadata which stores it
in cache and performing a second update with a new updater while
the metadata is already stored in cache, this test verifies that
timestamp, snaphot and targets are loaded from cache and not
downloaded

Fixes theupdateframework#1681

Signed-off-by: Ivana Atanasova <[email protected]>

Author: Ivana Atanasova

tests/test_updater_with_simulator.py

@@ -12,13 +12,13 @@
 import tempfile
 import unittest
 from typing import Optional, Tuple
-from unittest.mock import MagicMock, Mock, patch
+from unittest.mock import call, MagicMock, patch
 
 from tests import utils
 from tests.repository_simulator import RepositorySimulator
-from tuf.api.metadata import SPECIFICATION_VERSION, TargetFile, Targets
+from tuf.api.metadata import SPECIFICATION_VERSION, Targets
 from tuf.exceptions import BadVersionNumberError, UnsignedMetadataError
-from tuf.ngclient import Updater
+from tuf.ngclient import Updater, updater
 
 
 class TestUpdater(unittest.TestCase):
@@ -257,6 +257,44 @@ def test_not_loading_targets_twice(self, wrapped_open: MagicMock) -> None:
         updater.get_targetinfo("somepath")
         wrapped_open.assert_not_called()
 
+    @patch.object(builtins, "open", wraps=builtins.open)
+    def test_update_metadata_to_cache(self, wrapped_open: MagicMock) -> None:
+        updater = self._run_refresh()
+
+        # Add targets to repository
+        self.sim.targets.version += 1
+        self.sim.add_target("targets", b'some content', self.targets_dir)
+        self.sim.update_snapshot()
+
+        # Make a successful update of valid metadata which stores it in cache
+        updater = self._run_refresh()
+
+        # Clean up calls to fetch during refresh()
+        wrapped_open.reset_mock()
+
+        updater.get_targetinfo(self.targets_dir)
+
+        # Create a new updater and perform a second update while
+        # the metadata is already stored in cache (metadata dir)
+        updater = Updater(
+            self.metadata_dir,
+            "https://example.com/metadata/",
+            self.targets_dir,
+            "https://example.com/targets/",
+            self.sim,
+        )
+        updater.get_targetinfo(self.targets_dir)
+
+        # Test that timestamp, snaphot and targets
+        # are loaded from cache and not downloaded
+        wrapped_open.assert_has_calls([
+            call(os.path.join(self.metadata_dir, "root.json"), "rb"),
+            call(os.path.join(self.metadata_dir, "timestamp.json"), "rb"),
+            call(os.path.join(self.metadata_dir, "snapshot.json"), "rb"),
+            call(os.path.join(self.metadata_dir, "targets.json"), "rb"),
+        ])
+        wrapped_open.reset_mock()
+
 
 if __name__ == "__main__":
     if "--dump" in sys.argv: