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 theupdateframework#1031) and we should
implement it in the new API.

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

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):