ngclient: Fail gracefully on missing role

If role is delegated but missing from snapshot, we currently raise a
undocumented KeyError: a generic RepositoryError seems better as callers
are expected to handle it (and adding a more specific error seems
useless as this is a repository software bug, not just expired metadata or
something).

The same check is also done later in TrustedMetadataSet but I think
keeping the check in both is clearest.

Fixes #2195

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tests/test_updater_fetch_target.py

@@ -16,6 +16,7 @@
 from tests import utils
 from tests.repository_simulator import RepositorySimulator
 from tuf.api.exceptions import RepositoryError
+from tuf.api.metadata import DelegatedRole, Delegations
 from tuf.ngclient import Updater
 
 
@@ -209,6 +210,19 @@ def test_invalid_target_cache(self) -> None:
         with open(path, "rb") as f:
             self.assertEqual(f.read(), target.content)
 
+    def test_meta_missing_delegated_role(self) -> None:
+        """Test a delegation where the role is not part of the snapshot"""
+
+        # Add new delegation, update snapshot. Do not add the actual role
+        role = DelegatedRole("role1", [], 1, True, ["*"])
+        self.sim.targets.delegations = Delegations({}, roles={role.name: role})
+        self.sim.update_snapshot()
+
+        # assert that RepositoryError is raised when role1 is needed
+        updater = self._init_updater()
+        with self.assertRaises(RepositoryError):
+            updater.get_targetinfo("")
+
 
 if __name__ == "__main__":
     if "--dump" in sys.argv:

tuf/ngclient/updater.py

@@ -389,7 +389,14 @@ def _load_targets(self, role: str, parent_role: str) -> Metadata[Targets]:
             logger.debug("Failed to load local %s: %s", role, e)
 
             assert self._trusted_set.snapshot is not None  # nosec
-            metainfo = self._trusted_set.snapshot.signed.meta[f"{role}.json"]
+
+            snapshot = self._trusted_set.snapshot.signed
+            metainfo = snapshot.meta.get(f"{role}.json")
+            if metainfo is None:
+                raise exceptions.RepositoryError(
+                    f"Role {role} was delegated but is not part of snapshot"
+                )
+
             length = metainfo.length or self.config.targets_max_length
             version = None
             if self._trusted_set.root.signed.consistent_snapshot: