activitypub.postprocess_as2: for Create/Update, put object's to/cc into activity

for possible Pleroma/Akkoma interop:
https://git.pleroma.social/pleroma/pleroma/-/issues/3206#note_108296
https://indieweb.social/@diego@lounge.collabfc.com/112977955332152430

activitypub.py

@@ -796,6 +796,14 @@ def postprocess_as2(activity, orig_obj=None, wrap=True):
             for recip in as1.get_objects(orig_obj, field):
                 add(cc, util.get_url(recip) or recip.get('id'))
 
+    # for CRUD activities, Pleroma (and Akkoma?) seem to crash if the activity's
+    # to and cc aren't exactly the same as the object's. (I think?)
+    # https://indieweb.social/@diego@lounge.collabfc.com/112977955332152430
+    # https://git.pleroma.social/pleroma/pleroma/-/issues/3206#note_108296
+    if type in ('Create', 'Update'):
+        activity['to'] = util.get_list(obj, 'to')
+        activity['cc'] = util.get_list(obj, 'cc')
+
     # WARNING: activity here is AS2, but we're using as1.is_dm. right now the
     # logic is effectively the same for our purposes, but watch out here if that
     # ever changes.

tests/test_activitypub.py

@@ -2459,6 +2459,19 @@ class ActivityPubUtilsTest(TestCase):
         self.assertEqual(['https://masto.foo/@other'],
                          postprocess_as2(obj)['cc'])
 
+    def test_postprocess_as2_object_to_cc_into_activity(self):
+        got = postprocess_as2({
+            '@context': 'https://www.w3.org/ns/activitystreams',
+            'type': 'Create',
+            'object': {
+                'to': ['abc'],
+                'cc': ['def', 'xyz'],
+            },
+        })
+        self.assertEqual(['abc', 'https://www.w3.org/ns/activitystreams#Public'],
+                         got['to'])
+        self.assertEqual(['def', 'xyz'], got['cc'])
+
     def test_postprocess_as2_dm_note(self):
         dm = {
             'objectType': 'note',

tests/test_integrations.py

@@ -158,6 +158,7 @@ class IntegrationTests(TestCase):
                 'cc': ['http://inst/bob'],
             },
             'to': ['https://www.w3.org/ns/activitystreams#Public'],
+            'cc': ['http://inst/bob'],
         })
 
 

tests/test_web.py

@@ -277,6 +277,12 @@ AS2_CREATE = {
         }],
     },
     'to': [as2.PUBLIC_AUDIENCE],
+    'cc': [
+        'https://mas.to/author',
+        'https://mas.to/bystander',
+        'https://mas.to/recipient',
+        as2.PUBLIC_AUDIENCE,
+    ],
 }
 AS2_UPDATE = copy.deepcopy(AS2_CREATE)
 AS2_UPDATE.update({