when an unbridged user replies to a bridged user, DM them a prompt

one time only, per user per protocol!

for #1205

protocol.py

@@ -1373,22 +1373,33 @@ class Protocol:
 
         # process direct targets
         for id in sorted(target_uris):
-            protocol = Protocol.for_id(id)
-            if not protocol:
+            target_proto = Protocol.for_id(id)
+            if not target_proto:
                 logger.info(f"Can't determine protocol for {id}")
                 continue
-            elif protocol.is_blocklisted(id):
+            elif target_proto.is_blocklisted(id):
                 logger.info(f'{id} is blocklisted')
                 continue
-            elif (protocol not in to_protocols
-                  and obj.source_protocol != protocol.LABEL):
-                continue
 
-            orig_obj = protocol.load(id)
+            orig_obj = target_proto.load(id)
             if not orig_obj or not orig_obj.as1:
                 logger.info(f"Couldn't load {id}")
                 continue
 
+            target_author_key = target_proto.actor_key(orig_obj)
+            if (target_proto not in to_protocols
+                  and obj.source_protocol != target_proto.LABEL):
+                # if author isn't bridged and inReplyTo author is, DM a prompt
+                if id in in_reply_tos:
+                    if target_author := target_author_key.get():
+                        if target_author.is_enabled(from_cls):
+                            target_proto.maybe_bot_dm(to_user=from_user,
+                                                      type='replied_to_bridged_user',
+                                                      text=f"""\
+    Hi! You <a href="{obj.as1.get('url') or obj.key.id()}">recently replied</a> to {obj.actor_link(image=False)}, who's bridged here from {target_proto.PHRASE}. If you want them to see your replies in the future, you can bridge your account into {target_proto.PHRASE} by following this account. <a href="https://fed.brid.gy/docs">See the docs</a> for more information.""")
+
+                continue
+
             # deliver self-replies to followers
             # https://github.com/snarfed/bridgy-fed/issues/639
             if id in in_reply_tos and owner == as1.get_owner(orig_obj.as1):
@@ -1404,11 +1415,11 @@ class Protocol:
                         logger.info(f'Adding target {target} for copy {copy.uri} of original {id}')
                         targets[Target(protocol=copy.protocol, uri=target)] = orig_obj
 
-            if protocol == from_cls and from_cls.LABEL != 'fake':
+            if target_proto == from_cls and from_cls.LABEL != 'fake':
                 logger.info(f'Skipping same-protocol target {id}')
                 continue
 
-            target = protocol.target_for(orig_obj)
+            target = target_proto.target_for(orig_obj)
             if not target:
                 # TODO: surface errors like this somehow?
                 logger.error(f"Can't find delivery target for {id}")
@@ -1417,14 +1428,13 @@ class Protocol:
             logger.info(f'Target for {id} is {target}')
             # only use orig_obj for inReplyTos and repost objects
             # https://github.com/snarfed/bridgy-fed/issues/1237
-            targets[Target(protocol=protocol.LABEL, uri=target)] = (
+            targets[Target(protocol=target_proto.LABEL, uri=target)] = (
                 orig_obj if id in in_reply_tos or id in as1.get_ids(obj.as1, 'object')
                 else None)
 
-            orig_user = protocol.actor_key(orig_obj)
-            if orig_user:
-                logger.info(f'Recipient is {orig_user}')
-                obj.add('notify', orig_user)
+            if target_author_key:
+                logger.info(f'Recipient is {target_author_key}')
+                obj.add('notify', target_author_key)
 
         if obj.type == 'undo':
             logger.info('Object is an undo; adding targets for inner object')

tests/test_protocol.py

@@ -1055,6 +1055,8 @@ class ProtocolReceiveTest(TestCase):
         self.assertEqual(0, mock_send.call_count)
 
     def test_reply_to_non_bridged_post_skips_enabled_protocol_with_followers(self):
+        self.make_user(id='fa.brid.gy', cls=Web)
+
         # should skip even if it's enabled and we have followers there
         self.user.enabled_protocols = ['eefake']
         self.user.put()
@@ -1076,6 +1078,40 @@ class ProtocolReceiveTest(TestCase):
         self.assertEqual(202, code)
         self.assertEqual([], ExplicitEnableFake.sent)
 
+    def test_reply_from_non_bridged_post_isnt_bridged_but_gets_dm_prompt(self):
+        self.make_user(id='fa.brid.gy', cls=Web)
+        self.user.enabled_protocols = ['eefake']
+        self.user.put()
+
+        eve = self.make_user('eefake:eve', cls=ExplicitEnableFake, obj_as1={
+            'id': 'eefake:eve',
+        })
+
+        self.store_object(id='fake:post', source_protocol='fake', our_as1={
+            'id': 'fake:post',
+            'objectType': 'note',
+            'author': 'fake:alice',
+        })
+
+        # with self.assertRaises(NoContent):
+        _, code = ExplicitEnableFake.receive_as1({
+            'id': 'eefake:reply',
+            'objectType': 'note',
+            'actor': 'eefake:eve',
+            'inReplyTo': 'fake:post',
+        })
+        self.assertEqual(204, code)
+
+        self.assertEqual([], Fake.sent)
+        self.assertEqual([
+            ('https://fa.brid.gy/#replied_to_bridged_user-dm-eefake:eve-2022-01-02T03:04:05+00:00',
+             'eefake:eve:target'),
+        ], ExplicitEnableFake.sent)
+
+        eve = eve.key.get()
+        self.assertEqual([DM(protocol='fake', type='replied_to_bridged_user')],
+                         eve.sent_dms)
+
     @patch.object(ATProto, 'send', return_value=True)
     def test_repost_of_non_bridged_account_skips_atproto(self, mock_send):
         user = self.make_user('eefake:user', cls=ExplicitEnableFake,