tests: Test client max metadata length config

Fixes #1730

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

tests/test_updater_top_level_update.py

@@ -18,6 +18,7 @@
 from tests.repository_simulator import RepositorySimulator
 from tuf.api.exceptions import (
     BadVersionNumberError,
+    DownloadLengthMismatchError,
     ExpiredMetadataError,
     LengthOrHashMismatchError,
     UnsignedMetadataError,
@@ -777,6 +778,30 @@ def test_expired_metadata(self, mock_time: Mock) -> None:
             md = Metadata.from_file(f"{self.metadata_dir}/{role}.json")
             self.assertEqual(md.signed.version, 2)
 
+    def test_max_metadata_lengths(self) -> None:
+        """Test that clients configured max metadata lengths are respected"""
+
+        # client has root v1 already: create a new one available for download
+        self.sim.root.version += 1
+        self.sim.publish_root()
+
+        config_vars = [
+            "root_max_length",
+            "timestamp_max_length",
+            "snapshot_max_length",
+            "targets_max_length",
+        ]
+        # make sure going over any length limit raises DownloadLengthMismatchError
+        for var_name in config_vars:
+            updater = self._init_updater()
+            setattr(updater.config, var_name, 100)
+            with self.assertRaises(DownloadLengthMismatchError):
+                updater.refresh()
+
+        # All good with normal length limits
+        updater = self._init_updater()
+        updater.refresh()
+
 
 if __name__ == "__main__":
     if "--dump" in sys.argv: