add Protocol.subdomain_url()

for #512, #548

protocol.py

@@ -1,6 +1,7 @@
 """Base protocol class and common code."""
 import logging
 import threading
+from urllib.parse import urljoin
 
 from cachetools import cached, LRUCache
 from flask import g, request
@@ -106,6 +107,21 @@ class Protocol:
             label = domain.removesuffix(common.SUPERDOMAIN)
             return PROTOCOLS.get(label)
 
+    @classmethod
+    def subdomain_url(cls, path=None):
+        """Returns the URL for a given path on this protocol's subdomain.
+
+        Eg for the path 'foo/bar' on ActivityPub, returns
+        'https://ap.brid.gy/foo/bar'.
+
+        Args:
+          path: str
+
+        Returns:
+          str, URL
+        """
+        return urljoin(f'https://{cls.ABBREV or "fed"}{common.SUPERDOMAIN}/', path)
+
     @classmethod
     def owns_id(cls, id):
         """Returns whether this protocol owns the id, or None if it's unclear.

tests/test_protocol.py

@@ -78,6 +78,11 @@ class ProtocolTest(TestCase):
                 with app.test_request_context('/foo', base_url=url):
                     self.assertEqual(expected, Protocol.for_request(fed=Fake))
 
+    def test_subdomain_url(self):
+        self.assertEqual('https://fa.brid.gy/', Fake.subdomain_url())
+        self.assertEqual('https://fa.brid.gy/foo?bar', Fake.subdomain_url('foo?bar'))
+        self.assertEqual('https://fed.brid.gy/', UIProtocol.subdomain_url())
+
     @patch('requests.get')
     def test_receive_reply_not_feed_not_notification(self, mock_get):
         Follower.get_or_create(to=Fake.get_or_create(id=ACTOR['id']),