Add tests for invalid delegated role metadata

Extend TestDelegationsGraphs with a test case for
unsigned metadata.

Signed-off-by: Teodora Sechkova <[email protected]>

Author: sechkova

tests/test_updater_delegation_graphs.py

@@ -22,6 +22,7 @@
     DelegatedRole,
     Targets,
 )
+from tuf.exceptions import UnsignedMetadataError
 from tuf.ngclient import Updater
 
 
@@ -287,6 +288,47 @@ def test_graph_traversal(self, test_data: DelegationsTestCase) -> None:
         finally:
             self.teardown_subtest()
 
+    invalid_metadata: utils.DataSet = {
+        "unsigned delegated role": DelegationsTestCase(
+            delegations=[
+                TestDelegation("targets", "A"),
+                TestDelegation("targets", "B"),
+                TestDelegation("A", "C"),
+            ],
+            # The traversal stops after visiting an invalid role
+            visited_order=["A"],
+        )
+    }
+
+    @utils.run_sub_tests_with_dataset(invalid_metadata)
+    def test_invalid_metadata(self, test_data: DelegationsTestCase) -> None:
+        try:
+            self._init_repo(test_data)
+            # The invalid role is the last visited
+            invalid_role = test_data.visited_order[-1]
+            self.sim.signers[invalid_role].clear()
+
+            self.setup_subtest()
+            # The invalid role metadata must not be persisted
+            exp_files = [*TOP_LEVEL_ROLE_NAMES, *test_data.visited_order[:-1]]
+            exp_calls = [call(role, 1) for role in test_data.visited_order]
+
+            updater = self._init_updater()
+            # Call explicitly refresh to simplify the expected_calls list
+            updater.refresh()
+
+            with patch.object(
+                self.sim, "_fetch_metadata", wraps=self.sim._fetch_metadata
+            ) as wrapped_fetch:
+
+                with self.assertRaises(UnsignedMetadataError):
+                    updater.get_targetinfo("missingpath")
+
+                self.assertListEqual(wrapped_fetch.call_args_list, exp_calls)
+                self._assert_files_exist(exp_files)
+        finally:
+            self.teardown_subtest()
+
 
 class TestTargetFileSearch(TestDelegations):
     """