Update expired metadata tests logic

This change improves the logic of expired metadata tests, so that
it is explicitly visible what the expiry time and the versions are
and when update/refresh is called in that period

Signed-off-by: Ivana Atanasova <iyovcheva@vmware.com>

Author: ivanayov

tests/test_updater_top_level_update.py

@@ -310,43 +310,78 @@ def test_new_timestamp_unsigned(self) -> None:
 
     @patch.object(datetime, "datetime", wraps=datetime.datetime)
     def test_expired_timestamp_version_rollback(self, mock_time: Mock) -> None:
+        """Verifies that local timestamp is used in rollback checks even if it is expired.
+
+        The timestamp updates and rollback checks are performed
+        with the following timing:
+         - Timestamp v1 expiry set to day 7
+         - First updater refresh performed on day 0
+         - Repository publishes timestamp v2 on day 0
+         - Timestamp v2 expiry set to day 21
+         - Second updater refresh performed on day 18:
+           assert that rollback check uses expired timestamp v1"""
+
+        now = datetime.datetime.utcnow()
+        self.sim.timestamp.expires = now + datetime.timedelta(days=7)
+
+        self.sim.timestamp.version = 2
+
+        # Make a successful update of valid metadata which stores it in cache
         self._run_refresh()
 
+        self.sim.timestamp.expires = now + datetime.timedelta(days=21)
+
+        self.sim.timestamp.version = 1
+
         mock_time.utcnow.return_value = (
-            datetime.datetime.utcnow() + datetime.timedelta(seconds=1)
+            datetime.datetime.utcnow() + datetime.timedelta(days=18)
         )
         with patch("datetime.datetime", mock_time):
-            # Check for a rollback attack
-            self.sim.timestamp.version = 2
-            self._run_refresh()
-
-            self.sim.timestamp.version = 1
+            # Check that a rollback protection is performed even if
+            # local timestamp has expired
             with self.assertRaises(BadVersionNumberError):
                 self._run_refresh()
 
-            self._assert_version_equals(Timestamp.type, 2)
+        self._assert_version_equals(Timestamp.type, 2)
 
     @patch.object(datetime, "datetime", wraps=datetime.datetime)
     def test_expired_timestamp_snapshot_rollback(self, mock_time: Mock) -> None:
+        """Verifies that rollback protection is done even if local timestamp has expired.
+
+        The snapshot updates and rollback protection checks are performed
+        with the following timing:
+         - Timestamp v1 expiry set to day 7
+         - Repository bumps snapshot to v3 on day 0
+         - First updater refresh performed on day 0
+         - Timestamp v2 expiry set to day 21
+         - Second updater refresh performed on day 18:
+           assert that rollback protection is done with expired timestamp v1"""
+
+        now = datetime.datetime.utcnow()
+        self.sim.timestamp.expires = now + datetime.timedelta(days=7)
+
+        # Bump the snapshot version number to 3
+        self.sim.update_snapshot()
+        self.sim.update_snapshot()
+
+        # Make a successful update of valid metadata which stores it in cache
         self._run_refresh()
 
+        self.sim.snapshot.version = 1
+        # Snapshot version number is set to 2, which is still less than 3
+        self.sim.update_snapshot()
+        self.sim.timestamp.expires = now + datetime.timedelta(days=21)
+
         mock_time.utcnow.return_value = (
-            datetime.datetime.utcnow() + datetime.timedelta(seconds=1)
+            datetime.datetime.utcnow() + datetime.timedelta(days=18)
         )
         with patch("datetime.datetime", mock_time):
-            # Check for a rollback attack.
-            self.sim.snapshot.version = 2
-            self.sim.update_timestamp()  # timestamp v2
-            self._run_refresh()
-
-            # Snapshot meta version is smaller than previous
-            self.sim.timestamp.snapshot_meta.version = 1
-            self.sim.timestamp.version += 1  # timestamp v3
-
+            # Assert that rollback protection is done even if
+            # local timestamp has expired
             with self.assertRaises(BadVersionNumberError):
                 self._run_refresh()
 
-            self._assert_version_equals(Timestamp.type, 2)
+        self._assert_version_equals(Timestamp.type, 3)
 
     def test_new_timestamp_version_rollback(self) -> None:
         # Check for a rollback attack
@@ -706,22 +741,34 @@ def test_load_metadata_from_cache(self, wrapped_open: MagicMock) -> None:
 
     @patch.object(datetime, "datetime", wraps=datetime.datetime)
     def test_expired_metadata(self, mock_time: Mock) -> None:
-        # Test that expired local timestamp/snapshot can be used for updating
-        # from remote
+        """Verifies that expired local timestamp/snapshot can be used for
+        updating from remote.
+
+        The updates and verifications are performed with the following timing:
+         - Timestamp v1 expiry set to day 7
+         - First updater refresh performed on day 0
+         - Repository bumps snapshot and targets to v2 on day 0
+         - Timestamp v2 expiry set to day 21
+         - Second updater refresh performed on day 18,
+           it is successful and timestamp/snaphot final versions are v2"""
+
+        now = datetime.datetime.utcnow()
+        self.sim.timestamp.expires = now + datetime.timedelta(days=7)
 
         # Make a successful update of valid metadata which stores it in cache
         self._run_refresh()
 
-        # Simulate expired local metadata by mocking system time one second ahead
+        self.sim.targets.version += 1
+        self.sim.update_snapshot()
+        self.sim.timestamp.expires = now + datetime.timedelta(days=21)
+
+        # Mocking time so that local timestam has expired
+        # but the new timestamp has not
         mock_time.utcnow.return_value = (
-            datetime.datetime.utcnow() + datetime.timedelta(seconds=1)
+            datetime.datetime.utcnow() + datetime.timedelta(days=18)
         )
         with patch("datetime.datetime", mock_time):
-            self.sim.targets.version += 1
-            self.sim.update_snapshot()
-        # Create a new updater and perform a second update while
-        # the metadata is already stored in cache (metadata dir)
-        self._run_refresh()
+            self._run_refresh()
 
         # Assert that the final version of timestamp/snapshot is version 2
         # which means a successful refresh is performed