Add HashDelegations class

Add HashDelegations class containing the information from the
succinct_hash_delegation dict described in TAP 15.
This allows for easy mypy checks on the types, easy enforcement on
TAP 15 restrictions (as for example that "delegation_hash_prefix_len"
must be between 1 and 32) and support for unrecognized fields
inside succinct_hash_delegation without much of a hassle.

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

tests/test_metadata_serialization.py

@@ -385,12 +385,14 @@ def test_snapshot_serialization(self, test_case_data: str) -> None:
             "terminating": false, "threshold": 1}',
         "ordered keyids": '{"keyids": ["keyid2", "keyid1"], "name": "a", "paths": ["fn1", "fn2"], \
             "terminating": false, "threshold": 1}',
+        # "succinct_hash_delegation with unrecognized fieldse": '{"keyids": ["keyid"], "terminating": false, \
+        #     "threshold": 99, "succinct_hash_delegations": {"delegation_hash_prefix_len": 8, "bin_name_prefix": "foo", "foo": "bar"}}',
     }
 
     @utils.run_sub_tests_with_dataset(valid_delegated_roles)
     def test_delegated_role_serialization(self, test_case_data: str) -> None:
         case_dict = json.loads(test_case_data)
-        deserialized_role = DelegatedRole.from_dict(copy.copy(case_dict))
+        deserialized_role = DelegatedRole.from_dict(copy.deepcopy(case_dict))
         self.assertDictEqual(case_dict, deserialized_role.to_dict())
 
     invalid_delegated_roles: utils.DataSet = {
@@ -413,14 +415,22 @@ def test_delegated_role_serialization(self, test_case_data: str) -> None:
                 "succinct_hash_delegations": {"bin_name_prefix" : "foo"}}',
         "missing bin_name_prefix from succinct_hash_delegations": '{"name": "a", "keyids": ["keyid"], "threshold": 1, "terminating": false, \
                 "succinct_hash_delegations": {"delegation_hash_prefix_len" : 8}}',
+        "succinct_hash_delegation with invalid delegation_hash_prefix_len type": '{"keyids": ["keyid"], "terminating": false, \
+            "threshold": 99, "succinct_hash_delegations": {"delegation_hash_prefix_len": "a", "bin_name_prefix": "foo"}}',
+        "succinct_hash_delegation with invalid bin_name_prefix type": '{"keyids": ["keyid"], "terminating": false, \
+            "threshold": 99, "succinct_hash_delegations": {"delegation_hash_prefix_len": 8, "bin_name_prefix": 1}}',
+        "succinct_hash_delegation with high delegation_hash_prefix_len value": '{"keyids": ["keyid"], "terminating": false, \
+            "threshold": 99, "succinct_hash_delegations": {"delegation_hash_prefix_len": 50, "bin_name_prefix": "foo"}}',
+        "succinct_hash_delegation with low delegation_hash_prefix_len value": '{"keyids": ["keyid"], "terminating": false, \
+            "threshold": 99, "succinct_hash_delegations": {"delegation_hash_prefix_len": 0, "bin_name_prefix": "foo"}}',
     }
 
     @utils.run_sub_tests_with_dataset(invalid_delegated_roles)
     def test_invalid_delegated_role_serialization(
         self, test_case_data: str
     ) -> None:
         case_dict = json.loads(test_case_data)
-        with self.assertRaises(ValueError):
+        with self.assertRaises((ValueError, TypeError)):
             DelegatedRole.from_dict(case_dict)
 
     invalid_delegations: utils.DataSet = {

tuf/api/metadata.py

@@ -1258,6 +1258,64 @@ def to_dict(self) -> Dict[str, Any]:
         return snapshot_dict
 
 
+class HashDelegations:
+    """A container with information about a succinct hash delegation.
+
+    Args:
+        hash_prefix_len: Number of bits between 1 and 32 used to separate the
+            different bins.
+        bin_name_prefix: Prefix of all bin names.
+
+    Raises:
+            ValueError, TypeError: Invalid arguments.
+    """
+
+    def __init__(
+        self,
+        hash_prefix_len: int,
+        bin_name_prefix: str,
+        unrecognized_fields: Optional[Mapping[str, Any]] = None,
+    ) -> None:
+        if hash_prefix_len <= 0 or hash_prefix_len > 32:
+            raise ValueError("hash_prefix_len must be between 1 and 32")
+        if not isinstance(bin_name_prefix, str):
+            raise ValueError("bin_name_prefix must be a string")
+
+        self.hash_prefix_len = hash_prefix_len
+        self.bin_name_prefix = bin_name_prefix
+        self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}
+
+    def __eq__(self, other: Any) -> bool:
+        if not isinstance(other, HashDelegations):
+            return False
+
+        return (
+            self.hash_prefix_len == other.hash_prefix_len
+            and self.bin_name_prefix == other.bin_name_prefix
+            and self.unrecognized_fields == other.unrecognized_fields
+        )
+
+    @classmethod
+    def from_dict(cls, succinct_hash_info: Dict[str, Any]) -> "HashDelegations":
+        """Creates ``HashDelegations`` object from its json/dict representation.
+
+        Raises:
+            ValueError, TypeError: Invalid arguments.
+        """
+        hash_prefix_len = succinct_hash_info.pop("delegation_hash_prefix_len")
+        bin_name_prefix = succinct_hash_info.pop("bin_name_prefix")
+        # All fields left in the succinct_hash_info are unrecognized.
+        return cls(hash_prefix_len, bin_name_prefix, succinct_hash_info)
+
+    def to_dict(self) -> Dict[str, Any]:
+        """Returns the dict representation of self."""
+        return {
+            "delegation_hash_prefix_len": self.hash_prefix_len,
+            "bin_name_prefix": self.bin_name_prefix,
+            **self.unrecognized_fields,
+        }
+
+
 class DelegatedRole(Role):
     """A container with information about a delegated role.
 
@@ -1321,19 +1379,17 @@ def __init__(
             not isinstance(p, str) for p in path_hash_prefixes
         ):
             raise ValueError("Path_hash_prefixes must be strings")
+
+        succinct_info = None
         if succinct_hash_info is not None:
             if not isinstance(succinct_hash_info, Dict):
                 raise ValueError("succinct_hash_info must be a dict")
-            if "delegation_hash_prefix_len" not in succinct_hash_info:
-                raise ValueError(
-                    "succinct_hash_info requires delegation_hash_prefix_len"
-                )
-            if "bin_name_prefix" not in succinct_hash_info:
-                raise ValueError("succinct_hash_info requires bin_name_prefix")
+
+            succinct_info = HashDelegations.from_dict(succinct_hash_info)
 
         self.paths = paths
         self.path_hash_prefixes = path_hash_prefixes
-        self.succinct_hash_info = succinct_hash_info
+        self.succinct_hash_info = succinct_info
 
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, DelegatedRole):
@@ -1377,19 +1433,19 @@ def from_dict(cls, role_dict: Dict[str, Any]) -> "DelegatedRole":
     def to_dict(self) -> Dict[str, Any]:
         """Returns the dict representation of self."""
         base_role_dict = super().to_dict()
-        res_dict: Dict[str, Any] = {
+        res: Dict[str, Any] = {
             "terminating": self.terminating,
             **base_role_dict,
         }
         if self.name is not None:
-            res_dict["name"] = self.name
+            res["name"] = self.name
         if self.paths is not None:
-            res_dict["paths"] = self.paths
+            res["paths"] = self.paths
         elif self.path_hash_prefixes is not None:
-            res_dict["path_hash_prefixes"] = self.path_hash_prefixes
+            res["path_hash_prefixes"] = self.path_hash_prefixes
         elif self.succinct_hash_info is not None:
-            res_dict["succinct_hash_delegations"] = self.succinct_hash_info
-        return res_dict
+            res["succinct_hash_delegations"] = self.succinct_hash_info.to_dict()
+        return res
 
     @staticmethod
     def _is_target_in_pathpattern(targetpath: str, pathpattern: str) -> bool:
@@ -1421,11 +1477,11 @@ def _calculate_delegation(self, hash_bits_representation: str) -> str:
                 "succinct_hash_info must be set to calculate the delegation"
             )
 
-        bit_length = self.succinct_hash_info["delegation_hash_prefix_len"]
+        bit_length = self.succinct_hash_info.hash_prefix_len
         # Get the first bit_length of bits and then cast them to decimal.
         bin_number = int(hash_bits_representation[:bit_length], 2)
 
-        name_prefix = self.succinct_hash_info["bin_name_prefix"]
+        name_prefix = self.succinct_hash_info.bin_name_prefix
         return f"{name_prefix}-{bin_number}"
 
     def find_delegation(self, target_filepath: str) -> Optional[str]: