activitypub.postprocess_as2: hashtags: use activity id's domain for href

for https://github.com/snarfed/bridgy-fed/issues/1013

activitypub.py

@@ -731,8 +731,8 @@ def postprocess_as2(activity, orig_obj=None, wrap=True):
         name = tag.get('name')
         if name and tag.get('type', 'Tag') == 'Tag':
             tag['type'] = 'Hashtag'
-            tag.setdefault('href', common.host_url(
-                f'hashtag/{quote_plus(name.removeprefix("#"))}'))
+            url_path = f'/hashtag/{quote_plus(name.removeprefix("#"))}'
+            tag.setdefault('href', urljoin(activity['id'], url_path))
             if not name.startswith('#'):
                 tag['name'] = f'#{name}'
 

tests/test_activitypub.py

@@ -1997,7 +1997,7 @@ class ActivityPubUtilsTest(TestCase):
         self.assert_equals({
             'tag': [
                 {'type': 'Hashtag', 'name': '#bar', 'href': 'bar'},
-                {'type': 'Hashtag', 'name': '#baz', 'href': 'http://localhost/hashtag/baz'},
+                {'type': 'Hashtag', 'name': '#baz', 'href': '/hashtag/baz'},
                 {'type': 'Mention', 'href': 'foo'},
             ],
             'to': [as2.PUBLIC_AUDIENCE],