Protocol.check_supported: allow empty content if there's a link attachment

for #2658

[deploy]

Author: snarfed

protocol.py

@@ -2467,15 +2467,21 @@ def check_supported(cls, obj, direction):
                 and inner_type not in cls.SUPPORTED_AS1_TYPES)):
             error(f"Bridgy Fed for {cls.LABEL} doesn't support {obj.type} {inner_type} yet", status=204)
 
-        # don't allow posts with blank content and no image/video/audio
+        # don't allow posts with blank content and no image/video/audio/link
         crud_obj = (as1.get_object(obj.as1) if obj.type in ('post', 'update')
                     else obj.as1)
+        links = [att for att in as1.get_objects(crud_obj, 'attachments')
+                 if att.get('objectType') == 'link']
         if (crud_obj.get('objectType') in as1.POST_TYPES
                 and not util.get_url(crud_obj, key='image')
-                and not any(util.get_urls(crud_obj, 'attachments', inner_key='stream'))
+                and not util.get_urls(crud_obj, 'attachments', inner_key='stream')
+                # link attachments get moved into content, eg in
+                # activitypub.postprocess_as2
+                # https://github.com/snarfed/bridgy-fed/issues/2658
+                and not any(util.get_url(l) for l in links)
                 # TODO: handle articles with displayName but not content
                 and not source.html_to_text(crud_obj.get('content')).strip()):
-            error('Blank content and no image or video or audio', status=204)
+            error('Blank content and no image or video or audio or link', status=204)
 
         # receiving DMs is only allowed to protocol bot accounts
         if direction == 'receive':

tests/test_protocol.py

@@ -1515,10 +1515,31 @@ def test_check_supported(self):
                  'object': {'objectType': 'note'}},
                 {'objectType': 'activity', 'verb': 'update',
                  'object': {'objectType': 'note'}},
+                # link attachment with no url
+                {'objectType': 'note',
+                 'attachments': [{'objectType': 'link', 'displayName': 'foo'}]},
         ):
             with self.subTest(obj=obj), self.assertRaises(NoContent):
                 Fake.check_supported(Object(our_as1=obj), 'receive')
 
+        # blank content but link attachment
+        # https://github.com/snarfed/bridgy-fed/issues/2658
+        for obj in ({
+                'objectType': 'note',
+                'attachments': [{'objectType': 'link', 'url': 'http://a/link'}],
+        }, {
+            'objectType': 'activity', 'verb': 'post',
+            'object': {
+                'objectType': 'note',
+                'attachments': [{
+                    'objectType': 'link',
+                    'url': 'http://a/link',
+                }],
+            },
+        }):
+            with self.subTest(obj=obj):
+                Fake.check_supported(Object(our_as1=obj), 'receive')
+
         # from and to a copy id of a protocol bot user
         self.make_user(cls=Web, id='ap.brid.gy',
                        copies=[Target(protocol='fake', uri='fake:ap-bot')])