Test JSONSerializer.serialize with validate=True

Add a simple test verifying that by calling JSONSerializer.serialize()
on JSONSerializer with validate = True will fail if an attribute has a
not allowed value.

Comprehensive testing is not needed here as the validation used inside
JSONSerializer.serialize() relies on the validation that is done during
metadata objects initialization which is tested in detail.

Additionally, I added validation when calling JSONSerializer.serialize()
in the modify_metadata() inside the test_trusted_metadata_set.py module
as this function modifies an existing object and I wanted to make
sure it does that without compromising it.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tests/test_api.py

@@ -183,6 +183,13 @@ def test_md__eq__(self) -> None:
             md_obj = Metadata.from_file(f"{self.repo_dir}/metadata/{role}.json")
             self.assertEqualRecursively(md_obj)
 
+    def test_serialize_with_validate(self) -> None:
+        # Assert that by changing one required attribute validation will fail.
+        root = Metadata.from_file(f"{self.repo_dir}/metadata/root.json")
+        root.signed.version = 0
+        with self.assertRaises(ValueError):
+            root.to_bytes(JSONSerializer(validate=True))
+
     def test_to_from_bytes(self) -> None:
         for metadata in TOP_LEVEL_ROLE_NAMES:
             path = os.path.join(self.repo_dir, "metadata", metadata + ".json")

tests/test_trusted_metadata_set.py

@@ -22,6 +22,7 @@
     Targets,
     Timestamp,
 )
+from tuf.api.serialization.json import JSONSerializer
 from tuf.ngclient._internal.trusted_metadata_set import TrustedMetadataSet
 
 logger = logging.getLogger(__name__)
@@ -49,7 +50,7 @@ def modify_metadata(
         metadata = Metadata.from_bytes(cls.metadata[rolename])
         modification_func(metadata.signed)
         metadata.sign(cls.keystore[rolename])
-        return metadata.to_bytes()
+        return metadata.to_bytes(JSONSerializer(validate=True))
 
     @classmethod
     def setUpClass(cls) -> None: