Embed modification function helpers

Instead of using general abstract modification functions embed smaller
modification functions inside each test where it's needed and
create modify_metadata function that does all of the common stuff like:
- instantiating a metadata object
- calling the modification function
- signing the modified object
- serializing back to bytes.

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

Author: MVrachev

tests/test_trusted_metadata_set.py

@@ -1,12 +1,12 @@
 import logging
-from typing import Optional, Union
+from typing import Optional, Union, Callable
 import os
 import sys
 import unittest
 from datetime import datetime
 
 from tuf import exceptions
-from tuf.api.metadata import Metadata, MetaFile
+from tuf.api.metadata import Metadata, Signed, Timestamp, Snapshot, MetaFile
 from tuf.ngclient._internal.trusted_metadata_set import TrustedMetadataSet
 
 from securesystemslib.signer import SSlibSigner
@@ -84,41 +84,15 @@ def _update_all_besides_targets(
         snapshot_bytes = snapshot_bytes or self.metadata["snapshot"]
         self.trusted_set.update_snapshot(snapshot_bytes)
 
-    def _modify_timestamp_meta(self, version: Optional[int] = 1):
-        """Remove hashes and length from timestamp.meta["snapshot.json"].
-        Create a timestamp.meta["snapshot.json"] containing only version.
-
-        Args:
-            version:
-                Version used when instantiating MetaFile for timestamp.meta.
-
-        """
-        timestamp = Metadata.from_bytes(self.metadata["timestamp"])
-        timestamp.signed.meta["snapshot.json"] = MetaFile(version)
-        timestamp.sign(self.keystore["timestamp"])
-        return timestamp.to_bytes()
-
-    def _modify_snapshot_meta(
-            self, version: Union[int, None] = 1, length: Optional[int]= None
-        ):
-        """Modify hashes and length from snapshot_meta.meta["snapshot.json"].
-        If version and length is None, then snapshot meta will be an empty dictionary.
-
-        Args:
-            version:
-                Version used when instantiating MetaFile for snapshot.meta.
-            length:
-                Length used when instantiating MetaFile for snapshot.meta.
-
-        """
-        snapshot = Metadata.from_bytes(self.metadata["snapshot"])
-        if version is None and length is None:
-            snapshot.signed.meta = {}
-        else:
-            for metafile_path in snapshot.signed.meta:
-                snapshot.signed.meta[metafile_path] = MetaFile(version, length)
-        snapshot.sign(self.keystore["snapshot"])
-        return snapshot.to_bytes()
+    def modify_metadata(
+        self, rolename: str, modification_func: Callable[["Signed"], None]
+    ):
+        """Instantiate metadata from rolename type, call modification_func and
+        sign it again with self.keystore[rolename] signer."""
+        metadata = Metadata.from_bytes(self.metadata[rolename])
+        modification_func(metadata.signed)
+        metadata.sign(self.keystore[rolename])
+        return metadata.to_bytes()
 
     def test_update(self):
         self.trusted_set.root_update_finished()
@@ -255,7 +229,10 @@ def test_update_timestamp_new_timestamp_ver_below_trusted_ver(self):
             self.trusted_set.update_timestamp(self.metadata["timestamp"])
 
     def test_update_timestamp_snapshot_ver_below_trusted_snapshot_ver(self):
-        modified_timestamp = self._modify_timestamp_meta(version=3)
+        def version_modifier(timestamp: Timestamp):
+            timestamp.version = 3
+
+        modified_timestamp = self.modify_metadata("timestamp", version_modifier)
         self._root_updated_and_update_timestamp(modified_timestamp)
         # new_timestamp.snapshot.version < trusted_timestamp.snapshot.version
         with self.assertRaises(exceptions.ReplayedMetadataError):
@@ -272,16 +249,27 @@ def test_update_timestamp_expired(self):
 
 
     def test_update_snapshot_cannot_verify_snapshot_with_threshold(self):
-        modified_timestamp = self._modify_timestamp_meta()
-        self._root_updated_and_update_timestamp(modified_timestamp)
+        def hashes_length_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+
+        timestamp = self.modify_metadata("timestamp", hashes_length_modifier)
+        self._root_updated_and_update_timestamp(timestamp)
         snapshot = Metadata.from_bytes(self.metadata["snapshot"])
         snapshot.signatures.clear()
         with self.assertRaises(exceptions.UnsignedMetadataError):
             self.trusted_set.update_snapshot(snapshot.to_bytes())
 
     def test_update_snapshot_version_different_timestamp_snapshot_version(self):
-        modified_timestamp = self._modify_timestamp_meta(version=2)
-        self._root_updated_and_update_timestamp(modified_timestamp)
+        def hashes_length_version_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+            timestamp.meta["snapshot.json"].version = 2
+
+        timestamp = self.modify_metadata(
+            "timestamp", hashes_length_version_modifier
+        )
+        self._root_updated_and_update_timestamp(timestamp)
         # new_snapshot.version != trusted timestamp.meta["snapshot"].version
         snapshot = Metadata.from_bytes(self.metadata["snapshot"])
         snapshot.signed.version = 3
@@ -290,8 +278,12 @@ def test_update_snapshot_version_different_timestamp_snapshot_version(self):
             self.trusted_set.update_snapshot(snapshot.to_bytes())
 
     def test_update_snapshot_after_successful_update_new_snapshot_no_meta(self):
-        modified_timestamp = self._modify_timestamp_meta()
-        self._update_all_besides_targets(modified_timestamp)
+        def hashes_length_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+
+        timestamp = self.modify_metadata("timestamp", hashes_length_modifier)
+        self._update_all_besides_targets(timestamp)
         # Test removing a meta_file in new_snapshot compared to the old snapshot
         snapshot = Metadata.from_bytes(self.metadata["snapshot"])
         snapshot.signed.meta = {}
@@ -300,8 +292,12 @@ def test_update_snapshot_after_successful_update_new_snapshot_no_meta(self):
             self.trusted_set.update_snapshot(snapshot.to_bytes())
 
     def test_update_snapshot_after_succesfull_update_new_snapshot_meta_version_different(self):
-        modified_timestamp = self._modify_timestamp_meta()
-        self._root_updated_and_update_timestamp(modified_timestamp)
+        def hashes_length_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+
+        timestamp = self.modify_metadata("timestamp", hashes_length_modifier)
+        self._root_updated_and_update_timestamp(timestamp)
         # snapshot.meta["project1"].version != new_snapshot.meta["project1"].version
         snapshot = Metadata.from_bytes(self.metadata["snapshot"])
         for metafile_path in snapshot.signed.meta.keys():
@@ -312,8 +308,12 @@ def test_update_snapshot_after_succesfull_update_new_snapshot_meta_version_diffe
             self.trusted_set.update_snapshot(self.metadata["snapshot"])
 
     def test_update_snapshot_expired_new_snapshot(self):
-        modified_timestamp = self._modify_timestamp_meta()
-        self._root_updated_and_update_timestamp(modified_timestamp)
+        def hashes_length_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+
+        timestamp = self.modify_metadata("timestamp", hashes_length_modifier)
+        self._root_updated_and_update_timestamp(timestamp)
         # new_snapshot has expired
         snapshot = Metadata.from_bytes(self.metadata["snapshot"])
         snapshot.signed.expires = datetime(1970, 1, 1)
@@ -323,34 +323,48 @@ def test_update_snapshot_expired_new_snapshot(self):
 
 
     def test_update_targets_no_meta_in_snapshot(self):
-        modified_timestamp = self._modify_timestamp_meta()
-        modified_snapshot = self._modify_snapshot_meta(version=None)
-        self._update_all_besides_targets(
-            timestamp_bytes = modified_timestamp,
-            snapshot_bytes= modified_snapshot
-        )
+        def hashes_length_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+
+        timestamp = self.modify_metadata("timestamp", hashes_length_modifier)
+        def no_meta_modifier(snapshot: Snapshot):
+            snapshot.meta = {}
+
+        snapshot = self.modify_metadata("snapshot", no_meta_modifier)
+        self._update_all_besides_targets(timestamp, snapshot)
         # remove meta information with information about targets from snapshot
         with self.assertRaises(exceptions.RepositoryError):
             self.trusted_set.update_targets(self.metadata["targets"])
 
     def test_update_targets_hash_different_than_snapshot_meta_hash(self):
-        modified_timestamp = self._modify_timestamp_meta()
-        modified_snapshot = self._modify_snapshot_meta(version=1, length=1)
-        self._update_all_besides_targets(
-                timestamp_bytes = modified_timestamp,
-                snapshot_bytes= modified_snapshot
-        )
+        def hashes_length_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+
+        timestamp = self.modify_metadata("timestamp", hashes_length_modifier)
+        def meta_length_modifier(snapshot: Snapshot):
+            for metafile_path in snapshot.meta:
+                snapshot.meta[metafile_path] = MetaFile(version=1, length=1)
+
+        snapshot = self.modify_metadata("snapshot", meta_length_modifier)
+        self._update_all_besides_targets(timestamp, snapshot)
         # observed_hash != stored hash in snapshot meta for targets
         with self.assertRaises(exceptions.RepositoryError):
             self.trusted_set.update_targets(self.metadata["targets"])
 
     def test_update_targets_version_different_snapshot_meta_version(self):
-        modified_timestamp = self._modify_timestamp_meta()
-        modified_snapshot = self._modify_snapshot_meta(version=2)
-        self._update_all_besides_targets(
-                timestamp_bytes = modified_timestamp,
-                snapshot_bytes= modified_snapshot
-        )
+        def hashes_length_modifier(timestamp: Timestamp):
+            timestamp.meta["snapshot.json"].hashes = None
+            timestamp.meta["snapshot.json"].length = None
+
+        timestamp = self.modify_metadata("timestamp", hashes_length_modifier)
+        def meta_modifier(snapshot: Snapshot):
+            for metafile_path in snapshot.meta:
+                snapshot.meta[metafile_path] = MetaFile(version=2)
+
+        snapshot = self.modify_metadata("snapshot", meta_modifier)
+        self._update_all_besides_targets(timestamp, snapshot)
         # new_delegate.signed.version != meta.version stored in snapshot
         with self.assertRaises(exceptions.BadVersionNumberError):
             self.trusted_set.update_targets(self.metadata["targets"])