bring back default signing user for shared inbox

for #403, fixes https://console.cloud.google.com/errors/detail/CIfnyvKDx8mDTA;time=P30D?project=bridgy-federated

common.py

@@ -71,7 +71,6 @@ DOMAIN_BLOCKLIST = frozenset((
     'twitter.com',
 ) + DOMAINS)
 
-# currently unused. TODO: remove?
 _DEFAULT_SIGNATURE_USER = None
 
 CACHE_TIME = timedelta(seconds=60)
@@ -88,7 +87,6 @@ def host_url(path_query=None):
   return urllib.parse.urljoin(base, path_query)
 
 
-# currently unused. TODO: remove?
 def default_signature_user():
     global _DEFAULT_SIGNATURE_USER
     if _DEFAULT_SIGNATURE_USER is None:
@@ -101,6 +99,7 @@ def signed_get(url, user, **kwargs):
 
 
 def signed_post(url, user, **kwargs):
+    assert user
     return signed_request(util.requests_post, url, user, **kwargs)
 
 
@@ -117,10 +116,13 @@ def signed_request(fn, url, user, data=None, log_data=True, headers=None, **kwar
 
     Returns: :class:`requests.Response`
     """
-    assert user
     if headers is None:
         headers = {}
 
+    # prepare HTTP Signature and headers
+    if not user:
+        user = default_signature_user()
+
     if data:
         if log_data:
             logging.info(f'Sending AS2 object: {json_dumps(data, indent=2)}')
@@ -200,8 +202,6 @@ def get_as2(url, user=None):
         If we raise a werkzeug HTTPException, it will have an additional
         requests_response attribute with the last requests.Response we received.
     """
-    assert user
-
     def _error(resp):
         msg = f"Couldn't fetch {url} as ActivityStreams 2"
         logger.warning(msg)

tests/test_activitypub.py

@@ -357,7 +357,13 @@ class ActivityPubTest(testutil.TestCase):
         mock_get.assert_not_called()
         mock_post.assert_not_called()
 
-    def test_inbox_create_obj(self, mock_head, mock_get, mock_post):
+    def test_personal_inbox_create_obj(self, *mocks):
+        self._test_inbox_create_obj('/foo.com/inbox', *mocks)
+
+    def test_shared_inbox_create_obj(self, *mocks):
+        self._test_inbox_create_obj('/inbox', *mocks)
+
+    def _test_inbox_create_obj(self, path, mock_head, mock_get, mock_post):
         Follower.get_or_create(ACTOR['id'], 'foo.com')
         Follower.get_or_create('http://other/actor', 'bar.com')
         Follower.get_or_create(ACTOR['id'], 'baz.com')
@@ -367,7 +373,7 @@ class ActivityPubTest(testutil.TestCase):
         mock_post.return_value = requests_response()
 
         with self.client:
-            got = self.client.post('/foo.com/inbox', json=NOTE)
+            got = self.client.post(path, json=NOTE)
             self.assertEqual(200, got.status_code, got.get_data(as_text=True))
             expected_as2 = common.redirect_unwrap({
                 **NOTE,