Raise if none of paths and path_hash_prefixes is set

The specification does not state clearly what is the
behaviour when none of delegation's "paths" and
"path_hash_prefixes" is set. See #1497.

Until this issue is clarified, copy current
Updater which raises an error in such case.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

tests/test_metadata_serialization.py

@@ -242,10 +242,9 @@ def test_snapshot_serialization(self, test_case_data: str):
         "no path attribute":
             '{"keyids": ["keyid"], "name": "a", "terminating": false, \
             "path_hash_prefixes": ["h1", "h2"], "threshold": 99}',
-        "no hash or path prefix":
-            '{"keyids": ["keyid"], "name": "a", "terminating": true, "threshold": 3}',
         "unrecognized field":
-            '{"keyids": ["keyid"], "name": "a", "terminating": true, "threshold": 3, "foo": "bar"}',
+            '{"keyids": ["keyid"], "name": "a", "paths": ["fn1", "fn2"], \
+            "terminating": true, "threshold": 3, "foo": "bar"}',
     }
 
     @run_sub_tests_with_dataset(valid_delegated_roles)
@@ -257,10 +256,10 @@ def test_delegated_role_serialization(self, test_case_data: str):
 
     valid_delegations: DataSet = {
         "all": '{"keys": {"keyid" : {"keytype": "rsa", "scheme": "rsassa-pss-sha256", "keyval": {"public": "foo"}}}, \
-            "roles": [ {"keyids": ["keyid"], "name": "a", "terminating": true, "threshold": 3} ]}',
+            "roles": [ {"keyids": ["keyid"], "name": "a", "paths": ["fn1", "fn2"], "terminating": true, "threshold": 3} ]}',
         "unrecognized field":
             '{"keys": {"keyid" : {"keytype": "rsa", "scheme": "rsassa-pss-sha256", "keyval": {"public": "foo"}}}, \
-            "roles": [ {"keyids": ["keyid"], "name": "a", "terminating": true, "threshold": 3} ], \
+            "roles": [ {"keyids": ["keyid"], "name": "a", "paths": ["fn1", "fn2"], "terminating": true, "threshold": 3} ], \
             "foo": "bar"}',
     }
 
@@ -305,13 +304,13 @@ def test_targetfile_serialization(self, test_case_data: str):
             "targets": { "file.txt": {"length": 12, "hashes": {"sha256" : "abc"} } }, \
             "delegations": {"keys": {"keyid" : {"keytype": "rsa", \
                     "scheme": "rsassa-pss-sha256", "keyval": {"public": "foo"} }}, \
-                "roles": [ {"keyids": ["keyid"], "name": "a", "terminating": true, "threshold": 3} ]} \
+                "roles": [ {"keyids": ["keyid"], "name": "a", "paths": ["fn1", "fn2"], "terminating": true, "threshold": 3} ]} \
             }',
         "empty targets": '{"_type": "targets", "spec_version": "1.0.0", "version": 1, "expires": "2030-01-01T00:00:00Z", \
             "targets": {}, \
             "delegations": {"keys": {"keyid" : {"keytype": "rsa", \
                     "scheme": "rsassa-pss-sha256", "keyval": {"public": "foo"} }}, \
-                "roles": [ {"keyids": ["keyid"], "name": "a", "terminating": true, "threshold": 3} ]} \
+                "roles": [ {"keyids": ["keyid"], "name": "a", "paths": ["fn1", "fn2"], "terminating": true, "threshold": 3} ]} \
             }',
         "no delegations": '{"_type": "targets", "spec_version": "1.0.0", "version": 1, "expires": "2030-01-01T00:00:00Z", \
             "targets":  { "file.txt": {"length": 12, "hashes": {"sha256" : "abc"} } } \

tuf/api/metadata.py

@@ -962,12 +962,12 @@ def update(self, rolename: str, role_info: MetaFile) -> None:
 class DelegatedRole(Role):
     """A container with information about a delegated role.
 
-    A delegation can happen in three ways:
-        - paths is None and path_hash_prefixes is None: delegates all targets
+    A delegation can happen in two ways:
         - paths is set: delegates targets matching any path pattern in paths
         - path_hash_prefixes is set: delegates targets whose target path hash
             starts with any of the prefixes in path_hash_prefixes
-    paths and path_hash_prefixes are mutually exclusive: both cannot be set.
+    paths and path_hash_prefixes are mutually exclusive: both cannot be set,
+        at least one of them must be set.
 
     Attributes:
         name: A string giving the name of the delegated role.
@@ -996,6 +996,13 @@ def __init__(
                 "Only one of the attributes 'paths' and"
                 "'path_hash_prefixes' can be set!"
             )
+
+        if paths is None and path_hash_prefixes is None:
+            raise ValueError(
+                "At least one of the attributes 'paths' and"
+                "'path_hash_prefixes' must be set!"
+            )
+
         self.paths = paths
         self.path_hash_prefixes = path_hash_prefixes