Use custom errors in serializer.json sub-package

Re-raise all errors that happen during de/serialization as custom
De/SerializationError.

Whilelist 'e', which is idiomatic for error, in api/pylintrc, and
inline exempt broad-except, which are okay if re-raised.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

tests/test_api.py

@@ -28,6 +28,10 @@
     Targets
 )
 
+from tuf.api.serialization import (
+    DeserializationError
+)
+
 from tuf.api.serialization.json import (
     JSONSerializer,
     JSONDeserializer,
@@ -118,7 +122,7 @@ def test_generic_read(self):
         with open(bad_metadata_path, 'wb') as f:
             f.write(json.dumps(bad_metadata).encode('utf-8'))
 
-        with self.assertRaises(ValueError):
+        with self.assertRaises(DeserializationError):
             Metadata.from_file(bad_metadata_path)
 
         os.remove(bad_metadata_path)

tuf/api/metadata.py

@@ -117,8 +117,8 @@ def from_file(
 
         Raises:
             securesystemslib.exceptions.StorageError: The file cannot be read.
-            securesystemslib.exceptions.Error, ValueError, KeyError: The
-                metadata cannot be parsed.
+            tuf.api.serialization.DeserializationError:
+                The file cannot be deserialized.
 
         Returns:
             A TUF Metadata object.
@@ -161,6 +161,8 @@ def to_file(self, filename: str, serializer: MetadataSerializer = None,
                 a (local) FilesystemBackend is used.
 
         Raises:
+            tuf.api.serialization.SerializationError:
+                The metadata object cannot be serialized.
             securesystemslib.exceptions.StorageError:
                 The file cannot be written.
 
@@ -191,7 +193,8 @@ def sign(self, key: JsonDict, append: bool = False,
                 CanonicalJSONSerializer is used.
 
         Raises:
-            securesystemslib.exceptions.FormatError: Key argument is malformed.
+            tuf.api.serialization.SerializationError:
+                'signed' cannot be serialized.
             securesystemslib.exceptions.CryptoError, \
                     securesystemslib.exceptions.UnsupportedAlgorithmError:
                 Signing errors.
@@ -230,6 +233,8 @@ def verify(self, key: JsonDict,
             # TODO: Revise exception taxonomy
             tuf.exceptions.Error: None or multiple signatures found for key.
             securesystemslib.exceptions.FormatError: Key argument is malformed.
+            tuf.api.serialization.SerializationError:
+                'signed' cannot be serialized.
             securesystemslib.exceptions.CryptoError, \
                     securesystemslib.exceptions.UnsupportedAlgorithmError:
                 Signing errors.

tuf/api/pylintrc

@@ -1,6 +1,9 @@
 [MESSAGE_CONTROL]
 disable=fixme
 
+[BASIC]
+good-names=e
+
 [FORMAT]
 indent-string="    "
 max-line-length=79

tuf/api/serialization/__init__.py

@@ -13,6 +13,14 @@
 """
 import abc
 
+# TODO: Should these be in tuf.exceptions or inherit from tuf.exceptions.Error?
+class SerializationError(Exception):
+    """Error during serialization. """
+
+class DeserializationError(Exception):
+    """Error during deserialization. """
+
+
 class MetadataDeserializer():
     """Abstract base class for deserialization of Metadata objects. """
     __metaclass__ = abc.ABCMeta

tuf/api/serialization/json.py

@@ -7,6 +7,7 @@
 
 """
 import json
+import six
 
 from securesystemslib.formats import encode_canonical
 
@@ -16,16 +17,22 @@
 from tuf.api.metadata import Metadata, Signed
 from tuf.api.serialization import (MetadataSerializer,
                                    MetadataDeserializer,
-                                   SignedSerializer)
+                                   SignedSerializer,
+                                   SerializationError,
+                                   DeserializationError)
 
 
 class JSONDeserializer(MetadataDeserializer):
     """Provides JSON-to-Metadata deserialize method. """
 
     def deserialize(self, raw_data: bytes) -> Metadata:
         """Deserialize utf-8 encoded JSON bytes into Metadata object. """
-        _dict = json.loads(raw_data.decode("utf-8"))
-        return Metadata.from_dict(_dict)
+        try:
+            _dict = json.loads(raw_data.decode("utf-8"))
+            return Metadata.from_dict(_dict)
+
+        except Exception as e: # pylint: disable=broad-except
+            six.raise_from(DeserializationError, e)
 
 
 class JSONSerializer(MetadataSerializer):
@@ -41,18 +48,26 @@ def __init__(self, compact: bool = False) -> None:
 
     def serialize(self, metadata_obj: Metadata) -> bytes:
         """Serialize Metadata object into utf-8 encoded JSON bytes. """
-        indent = (None if self.compact else 1)
-        separators=((',', ':') if self.compact else (',', ': '))
-        return json.dumps(metadata_obj.to_dict(),
-                          indent=indent,
-                          separators=separators,
-                          sort_keys=True).encode("utf-8")
+        try:
+            indent = (None if self.compact else 1)
+            separators=((',', ':') if self.compact else (',', ': '))
+            return json.dumps(metadata_obj.to_dict(),
+                              indent=indent,
+                              separators=separators,
+                              sort_keys=True).encode("utf-8")
+
+        except Exception as e: # pylint: disable=broad-except
+            six.raise_from(SerializationError, e)
 
 
 class CanonicalJSONSerializer(SignedSerializer):
     """A Signed-to-Canonical JSON 'serialize' method. """
 
     def serialize(self, signed_obj: Signed) -> bytes:
         """Serialize Signed object into utf-8 encoded Canonical JSON bytes. """
-        signed_dict = signed_obj.to_dict()
-        return encode_canonical(signed_dict).encode("utf-8")
+        try:
+            signed_dict = signed_obj.to_dict()
+            return encode_canonical(signed_dict).encode("utf-8")
+
+        except Exception as e: # pylint: disable=broad-except
+            six.raise_from(SerializationError, e)