web follow bug fix: error gracefully on BF user addresses

fixes https://console.cloud.google.com/errors/detail/CLSnttKfy4v90wE;time=P30D?project=bridgy-federated

follow.py

@@ -106,8 +106,8 @@ class FollowCallback(indieauth.Callback):
                 flash(f"{addr} isn't a native fediverse account")
                 return redirect(user.user_page_path('following'))
             as2_url = webfinger.fetch_actor_url(addr)
-            if not as2_url:
-                flash(f"Couldn't find ActivityPub profile link for {addr}")
+            if ActivityPub.owns_id(as2_url) is False:
+                flash(f"{addr} isn't a native fediverse account")
                 return redirect(user.user_page_path('following'))
 
         # TODO(#512): follower will always be Web here, but we should generalize

tests/test_follow.py

@@ -197,7 +197,7 @@ class FollowTest(TestCase):
                           source_protocol='activitypub')
 
         mock_get.side_effect = (
-            requests_response(''),
+            requests_response(''),  # indieauth https://alice.com fetch for user json
         )
         mock_post.side_effect = (
             requests_response('me=https://alice.com'),
@@ -229,7 +229,7 @@ class FollowTest(TestCase):
             }],
         }
         mock_get.side_effect = (
-            requests_response(''),
+            requests_response(''),  # indieauth https://alice.com fetch for user json
             self.as2_resp(followee),
             self.as2_resp(followee),
         )
@@ -256,6 +256,32 @@ class FollowTest(TestCase):
         self.assertEqual(["@example.com@web.brid.gy isn't a native fediverse account"],
                          get_flashed_messages())
 
+    def test_callback_upgraded_bridged_account_error(self, mock_get, mock_post):
+        mock_post.return_value = requests_response('me=https://alice.com')
+        mock_get.side_effect = [
+            requests_response(''),  # indieauth https://alice.com fetch for user json
+            requests_response({     # webfinger
+                'subject': 'acct:bob.com@bob.com',
+                'aliases': ['https://bob.com/'],
+                'links': [{
+                    'rel': 'self',
+                    'type': as2.CONTENT_TYPE,
+                    'href': 'https://web.brid.gy/bob.com',
+                }],
+            }),
+        ]
+
+        bob = self.make_user('bob.com', cls=Web, obj_id='https://bob.com/')
+
+        self.state['state'] = '@bob.com@bob.com'
+        state = util.encode_oauth_state(self.state)
+        resp = self.client.get(f'/follow/callback?code=my_code&state={state}')
+
+        self.assertEqual(302, resp.status_code)
+        self.assertEqual('/web/alice.com/following', resp.headers['Location'])
+        self.assertEqual(["@bob.com@bob.com isn't a native fediverse account"],
+                         get_flashed_messages())
+
     def check(self, input, resp, expected_follow, mock_get, mock_post,
               fetched_followee=True):
         self.assertEqual(302, resp.status_code)