add Mention tag for Mastodon replies

fixes #34. thanks @swentel!

common.py

@@ -254,6 +254,21 @@ def postprocess_as2(activity, target=None, key=None):
                     'Only using the first: %s' % in_reply_tos[0])
             activity['inReplyTo'] = in_reply_to[0]
 
+        # Mastodon evidently requires a Mention tag for replies to generate a
+        # notification to the original post's author. not required for likes,
+        # reposts, etc. details:
+        # https://github.com/snarfed/bridgy-fed/issues/34
+        to = target.get('actor') or target.get('attributedTo')
+        if to:
+            if isinstance(to, dict):
+                to = to.get('url') or to.get('id')
+            if to:
+                activity.setdefault('tag', []).append({
+                    'type': 'Mention',
+                    'href': to,
+                })
+
+
     # activity objects (for Like, Announce, etc): prefer id over url
     obj = activity.get('object', {})
     if obj:

tests/test_webmention.py

@@ -175,6 +175,10 @@ class WebmentionTest(testutil.TestCase):
                     'preferredUsername': 'orig',
                     'name': 'Ms. ☕ Baz',
                 }],
+                'tag': [{
+                    'type': 'Mention',
+                    'href': 'http://orig/author',
+                }],
             },
         }
         self.as2_update = copy.deepcopy(self.as2_create)
@@ -503,4 +507,3 @@ class WebmentionTest(testutil.TestCase):
         self.assertIn('Target post http://orig/url has no Atom link', got.body)
 
         self.assertIsNone(Response.get_by_id('http://a/reply http://orig/post'))
-