Protocol.load: return copies of cached objects, not the originals

...so that modifications aren't durable in memory until we put() them. for #558

protocol.py

@@ -625,7 +625,12 @@ class Protocol:
             with objects_cache_lock:
                 cached = objects_cache.get(id)
                 if cached:
-                    return cached
+                    # make a copy so that if the client modifies this entity in
+                    # memory, those modifications aren't applied to the cache
+                    # until they explicitly put() the modified entity.
+                    return Object(id=cached.key.id(), **cached.to_dict(
+                        # computed properties
+                        exclude=['as1', 'expire', 'object_ids', 'type']))
 
         obj = orig_as1 = None
         if local:

tests/test_models.py

@@ -11,6 +11,7 @@ from .testutil import Fake, TestCase
 
 from models import AtpNode, Follower, Object, OBJECT_EXPIRE_AGE
 import protocol
+from protocol import Protocol
 from web import Web
 
 from .test_activitypub import ACTOR

tests/test_protocol.py

@@ -196,11 +196,16 @@ class ProtocolTest(TestCase):
         self.assertEqual([], Fake.fetched)
 
     def test_load_cached(self):
-        obj = Object(our_as1={'x': 'y'})
+        obj = Object(id='foo', our_as1={'x': 'y'})
         protocol.objects_cache['foo'] = obj
         loaded = Fake.load('foo')
         self.assert_entities_equal(obj, loaded)
 
+        # check that it's a separate copy of the entity in the cache
+        # https://github.com/snarfed/bridgy-fed/issues/558#issuecomment-1603203927
+        loaded.our_as1 = {'a': 'b'}
+        self.assertEqual({'x': 'y'}, Protocol.load('foo').our_as1)
+
     def test_load_remote_true_existing_empty(self):
         Fake.objects['foo'] = {'x': 'y'}
         Object(id='foo').put()