Add generic Metadata.read_from_json class method

Add generic read from json class method that returns a Metadata
object with a signed field that contains the appropriate Signed
subclass, based on the signed._type field of the read metadata.

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

tests/test_api.py

@@ -6,6 +6,7 @@
 
 """
 
+import json
 import sys
 import logging
 import os
@@ -27,8 +28,10 @@ def setUpModule():
 # Since setUpModule is called after imports we need to import conditionally.
 if IS_PY_VERSION_SUPPORTED:
     from tuf.api.metadata import (
+        Metadata,
         Snapshot,
         Timestamp,
+        Targets
     )
 
 
@@ -90,6 +93,37 @@ def tearDownClass(cls):
     #     threshold = Threshold(1, 5)
     #     return KeyRing(threshold=threshold, keys=key_list)
 
+    def test_generic_read(self):
+        for metadata, inner_metadata_cls in [
+                ("snapshot", Snapshot),
+                ("timestamp", Timestamp),
+                ("targets", Targets)]:
+
+            path = os.path.join(self.repo_dir, 'metadata', metadata + '.json')
+            metadata_obj = Metadata.read_from_json(path)
+
+            # Assert that generic method ...
+            # ... instantiates the right inner class for each metadata type
+            self.assertTrue(
+                    isinstance(metadata_obj.signed, inner_metadata_cls))
+            # ... and reads the same metadata file as the corresponding method
+            # on the inner class would do (compare their dict representation)
+            self.assertDictEqual(
+                    metadata_obj.as_dict(),
+                    inner_metadata_cls.read_from_json(path).as_dict())
+
+        # Assert that it chokes correctly on an unknown metadata type
+        bad_metadata_path = 'bad-metadata.json'
+        bad_metadata = {'signed': {'_type': 'bad-metadata'}}
+        with open(bad_metadata_path, 'wb') as f:
+            f.write(json.dumps(bad_metadata).encode('utf-8'))
+
+        with self.assertRaises(ValueError):
+            Metadata.read_from_json(bad_metadata_path)
+
+        os.remove(bad_metadata_path)
+
+
     def test_metadata_base(self):
         # Use of Snapshot is arbitrary, we're just testing the base class features
         # with real data

tuf/api/metadata.py

@@ -23,10 +23,6 @@
      disable check as there might be a justified reason to write WIP
      metadata to json.
 
- * It might be nice to have a generic Metadata.read_from_json that
-   can load any TUF role metadata and instantiate the appropriate object based
-   on the json '_type' field.
-
  * Add Root metadata class
 
 """
@@ -116,6 +112,50 @@ def as_dict(self) -> JsonDict:
     #                     break
 
     #     return len(verified_keyids) >= key_ring.threshold.least
+    @classmethod
+    def read_from_json(
+            cls, filename: str,
+            storage_backend: Optional[StorageBackendInterface] = None
+            ) -> 'Metadata':
+        """Loads JSON-formatted TUF metadata from a file storage.
+
+        Arguments:
+            filename: The path to read the file from.
+            storage_backend: An object that implements
+                securesystemslib.storage.StorageBackendInterface. Per default
+                a (local) FilesystemBackend is used.
+
+        Raises:
+            securesystemslib.exceptions.StorageError: The file cannot be read.
+            securesystemslib.exceptions.Error, ValueError: The metadata cannot
+                be parsed.
+
+        Returns:
+            A TUF Metadata object.
+
+        """
+        signable = load_json_file(filename, storage_backend)
+
+        # TODO: Should we use constants?
+        # And/or maybe a dispatch table? (<-- maybe too much magic)
+        _type = signable['signed']['_type']
+
+        if _type == 'targets':
+            inner_cls = Targets
+        elif _type == 'snapshot':
+            inner_cls = Snapshot
+        elif _type == 'timestamp':
+            inner_cls = Timestamp
+        elif _type == 'root':
+            # TODO: implement Root class
+            raise NotImplementedError('Root not yet implemented')
+        else:
+            raise ValueError(f'unrecognized metadata type "{_type}"')
+
+        return Metadata(
+                signed=inner_cls(**signable['signed']),
+                signatures=signable['signatures'])
+
 
     def write_to_json(
             self, filename: str,