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
snaphot and targets are loaded from cache and not downloaded and
that timestamp is loaded from cache and used to verify the new
timestamp

Fixes #1681

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

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 +257,42 @@ def test_not_loading_targets_twice(self, wrapped_open: MagicMock) -> None:
         updater.get_targetinfo("somepath")
         wrapped_open.assert_not_called()
 
+    def test_update_metadata_to_cache(self) -> 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()
+
+        with patch.object(
+                self.sim, "_fetch_metadata", wraps=self.sim._fetch_metadata
+            ) as wrapped_fetch:
+
+            # Make a successful update of valid metadata which stores it in cache
+            updater = self._run_refresh()
+
+            # Clean up calls to fetch during refresh()
+            wrapped_fetch.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 snaphot and targets are loaded from cache and not downloaded
+            # and that timestamp is loaded from cache and used to verify the new timestamp
+            wrapped_fetch.assert_has_calls([call("root", 2), call("timestamp", None)])
+            wrapped_fetch.reset_mock()
+
 
 if __name__ == "__main__":
     if "--dump" in sys.argv: