Make length and hashes optional in Timestamp

As per the specification (v1.0.1) length and hashes fields
in timestamp and snapshot 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

@@ -260,6 +260,18 @@ def test_metadata_snapshot(self):
  snapshot.signed.update('role1', 2, 123, hashes)
  self.assertEqual(snapshot.signed.meta, fileinfo)
 
+ # Update only version. Length and hashes are optional.
+ snapshot.signed.update('role2', 3)
+ fileinfo['role2.json'] = {'version': 3}
+ self.assertEqual(snapshot.signed.meta, fileinfo)
+
+ # Test from_dict and to_dict without hashes and length.
+ snapshot_dict = snapshot.to_dict()
+ test_dict = snapshot_dict['signed'].copy()
+ del test_dict['meta']['role1.json']['length']
+ del test_dict['meta']['role1.json']['hashes']
+ snapshot = Snapshot.from_dict(test_dict)
+ self.assertEqual(snapshot_dict['signed'], snapshot.to_dict())
 
  def test_metadata_timestamp(self):
  timestamp_path = os.path.join(
@@ -295,6 +307,18 @@ def test_metadata_timestamp(self):
  timestamp.signed.update(2, 520, hashes)
  self.assertEqual(timestamp.signed.meta['snapshot.json'], fileinfo)
 
+ # Test from_dict and to_dict without hashes and length.
+ timestamp_dict = timestamp.to_dict()
+ test_dict = timestamp_dict['signed'].copy()
+ del test_dict['meta']['snapshot.json']['length']
+ del test_dict['meta']['snapshot.json']['hashes']
+ timestamp_test = Timestamp.from_dict(test_dict)
+ self.assertEqual(timestamp_dict['signed'], timestamp_test.to_dict())
+
+ # 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_key_class(self):
  keys = {

tuf/api/metadata.py

@@ -606,7 +606,7 @@ class Timestamp(Signed):
  '<HASH ALGO 1>': '<SNAPSHOT METADATA FILE HASH 1>',
  '<HASH ALGO 2>': '<SNAPSHOT METADATA FILE HASH 2>',
  ...
- }
+ } // optional
  }
  }
 
@@ -643,14 +643,19 @@ def to_dict(self) -> Dict[str, Any]:
 
  # Modification.
  def update(
- self, version: int, length: int, hashes: Mapping[str, Any]
+ self,
+ version: int,
+ length: Optional[int] = None,
+ hashes: Optional[Mapping[str, Any]] = 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):