New API: Timestamp - make length and hashes optional

As per the specification (v1.0.1) length and hashes fields
in timestamp metadata are optional.
We have implement this in the older API
(see #1031) and we should
implement it in the new API.

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

tests/test_api.py

@@ -232,6 +232,10 @@ def test_metadata_snapshot(self):
         self.assertNotEqual(snapshot.signed.meta, fileinfo)
         snapshot.signed.update('role1', 2, 123, hashes)
         self.assertEqual(snapshot.signed.meta, fileinfo)
+        # Update only version. Length and hashes are optional.
+        snapshot.signed.update('role1', 3)
+        fileinfo['role1.json'] = {'version': 3}
+        self.assertEqual(snapshot.signed.meta, fileinfo)
 
 
     def test_metadata_timestamp(self):
@@ -267,6 +271,10 @@ def test_metadata_timestamp(self):
         self.assertNotEqual(timestamp.signed.meta['snapshot.json'], fileinfo)
         timestamp.signed.update(2, 520, hashes)
         self.assertEqual(timestamp.signed.meta['snapshot.json'], fileinfo)
+        # Update only version. Length and hashes are optional.
+        timestamp.signed.update(3)
+        fileinfo = {'version': 3}
+        self.assertEqual(timestamp.signed.meta['snapshot.json'], fileinfo)
 
 
     def test_metadata_root(self):

tuf/api/metadata.py

@@ -431,7 +431,7 @@ class Timestamp(Signed):
                         '<HASH ALGO 1>': '<SNAPSHOT METADATA FILE HASH 1>',
                         '<HASH ALGO 2>': '<SNAPSHOT METADATA FILE HASH 2>',
                         ...
-                    }
+                    } // optional
                 }
             }
 
@@ -455,13 +455,16 @@ def to_dict(self) -> JsonDict:
 
 
     # Modification.
-    def update(self, version: int, length: int, hashes: JsonDict) -> None:
+    def update(self, version: int, length: Optional[int] = None,
+            hashes: Optional[JsonDict] = None) -> None:
         """Assigns passed info about snapshot metadata to meta dict. """
-        self.meta['snapshot.json'] = {
-            'version': version,
-            'length': length,
-            'hashes': hashes
-        }
+
+        self.meta['snapshot.json'] = {'version': version}
+        if length is not None:
+            self.meta['snapshot.json']['length'] = length
+
+        if hashes is not None:
+            self.meta['snapshot.json']['hashes'] = hashes
 
 
 class Snapshot(Signed):