Activity.target_for: fix infinite loop when author is object id

fixes #603 and https://console.cloud.google.com/errors/detail/CLPmoOqegtfm0AE;time=P30D?project=bridgy-federated

activitypub.py

@@ -133,7 +133,9 @@ class ActivityPub(User, Protocol):
             for field in 'actor', 'author', 'attributedTo':
                 inner_obj = as1.get_object(obj.as1, field)
                 inner_id = inner_obj.get('id') or as1.get_url(inner_obj)
-                if not inner_id:
+                if (not inner_id
+                        or inner_id == obj.as1.get('id')
+                        or (obj.key and inner_id == obj.key.id())):
                     continue
 
                 # TODO: need a "soft" kwarg for load to suppress errors?

tests/test_activitypub.py

@@ -1932,6 +1932,14 @@ class ActivityPubUtilsTest(TestCase):
         self.assertEqual('http://mas.to/inbox', ActivityPub.target_for(obj))
         mock_get.assert_has_calls([self.as2_req('http://the/author')])
 
+    @patch('requests.get')
+    def test_target_for_author_is_object_id(self, mock_get):
+        obj = self.store_object(id='http://the/author', our_as1={
+            'author': 'http://the/author',
+        })
+        # test is that we short circuit out instead of infinite recursion
+        self.assertIsNone(ActivityPub.target_for(obj))
+
     @patch('requests.post')
     def test_send_blocklisted(self, mock_post):
         self.assertFalse(ActivityPub.send(Object(as2=NOTE),