Improve the reliability of auto-joining remote rooms (#10237)

If a room is remote and we don't have a user in it, always try to join it. It might fail if the room is invite-only, but we don't have a user to invite with, so at this point it's the best we can do.

Fixes #10233 (at least to some extent)

3 files changed, 96 insertions, 17 deletions

diff --git a/changelog.d/10237.misc b/changelog.d/10237.misc
new file mode 100644
index 0000000000..d76c119a41
--- /dev/null
+++ b/changelog.d/10237.misc
@@ -0,0 +1 @@
+Improve the reliability of auto-joining remote rooms.
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index ca1ed6a5c0..4b4b579741 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -386,11 +386,32 @@ class RegistrationHandler(BaseHandler):
                 room_alias = RoomAlias.from_string(r)
 
                 if self.hs.hostname != room_alias.domain:
-                    logger.warning(
-                        "Cannot create room alias %s, "
-                        "it does not match server domain",
+                    # If the alias is remote, try to join the room. This might fail
+                    # because the room might be invite only, but we don't have any local
+                    # user in the room to invite this one with, so at this point that's
+                    # the best we can do.
+                    logger.info(
+                        "Cannot automatically create room with alias %s as it isn't"
+                        " local, trying to join the room instead",
                         r,
                     )
+
+                    (
+                        room,
+                        remote_room_hosts,
+                    ) = await room_member_handler.lookup_room_alias(room_alias)
+                    room_id = room.to_string()
+
+                    await room_member_handler.update_membership(
+                        requester=create_requester(
+                            user_id, authenticated_entity=self._server_name
+                        ),
+                        target=UserID.from_string(user_id),
+                        room_id=room_id,
+                        remote_room_hosts=remote_room_hosts,
+                        action="join",
+                        ratelimit=False,
+                    )
                 else:
                     # A shallow copy is OK here since the only key that is
                     # modified is room_alias_name.
@@ -448,22 +469,32 @@ class RegistrationHandler(BaseHandler):
                     )
 
                 # Calculate whether the room requires an invite or can be
-                # joined directly. Note that unless a join rule of public exists,
-                # it is treated as requiring an invite.
-                requires_invite = True
-
-                state = await self.store.get_filtered_current_state_ids(
-                    room_id, StateFilter.from_types([(EventTypes.JoinRules, "")])
+                # joined directly. By default, we consider the room as requiring an
+                # invite if the homeserver is in the room (unless told otherwise by the
+                # join rules). Otherwise we consider it as being joinable, at the risk of
+                # failing to join, but in this case there's little more we can do since
+                # we don't have a local user in the room to craft up an invite with.
+                requires_invite = await self.store.is_host_joined(
+                    room_id,
+                    self.server_name,
                 )
 
-                event_id = state.get((EventTypes.JoinRules, ""))
-                if event_id:
-                    join_rules_event = await self.store.get_event(
-                        event_id, allow_none=True
+                if requires_invite:
+                    # If the server is in the room, check if the room is public.
+                    state = await self.store.get_filtered_current_state_ids(
+                        room_id, StateFilter.from_types([(EventTypes.JoinRules, "")])
                     )
-                    if join_rules_event:
-                        join_rule = join_rules_event.content.get("join_rule", None)
-                        requires_invite = join_rule and join_rule != JoinRules.PUBLIC
+
+                    event_id = state.get((EventTypes.JoinRules, ""))
+                    if event_id:
+                        join_rules_event = await self.store.get_event(
+                            event_id, allow_none=True
+                        )
+                        if join_rules_event:
+                            join_rule = join_rules_event.content.get("join_rule", None)
+                            requires_invite = (
+                                join_rule and join_rule != JoinRules.PUBLIC
+                            )
 
                 # Send the invite, if necessary.
                 if requires_invite:
diff --git a/tests/handlers/test_register.py b/tests/handlers/test_register.py
index a9fd3036dc..c901003225 100644
--- a/tests/handlers/test_register.py
+++ b/tests/handlers/test_register.py
@@ -18,7 +18,7 @@ from synapse.api.auth import Auth
 from synapse.api.constants import UserTypes
 from synapse.api.errors import Codes, ResourceLimitError, SynapseError
 from synapse.spam_checker_api import RegistrationBehaviour
-from synapse.types import RoomAlias, UserID, create_requester
+from synapse.types import RoomAlias, RoomID, UserID, create_requester
 
 from tests.test_utils import make_awaitable
 from tests.unittest import override_config
@@ -643,3 +643,50 @@ class RegistrationTestCase(unittest.HomeserverTestCase):
             )
 
         return user_id, token
+
+
+class RemoteAutoJoinTestCase(unittest.HomeserverTestCase):
+    """Tests auto-join on remote rooms."""
+
+    def make_homeserver(self, reactor, clock):
+        self.room_id = "!roomid:remotetest"
+
+        async def update_membership(*args, **kwargs):
+            pass
+
+        async def lookup_room_alias(*args, **kwargs):
+            return RoomID.from_string(self.room_id), ["remotetest"]
+
+        self.room_member_handler = Mock(spec=["update_membership", "lookup_room_alias"])
+        self.room_member_handler.update_membership.side_effect = update_membership
+        self.room_member_handler.lookup_room_alias.side_effect = lookup_room_alias
+
+        hs = self.setup_test_homeserver(room_member_handler=self.room_member_handler)
+        return hs
+
+    def prepare(self, reactor, clock, hs):
+        self.handler = self.hs.get_registration_handler()
+        self.store = self.hs.get_datastore()
+
+    @override_config({"auto_join_rooms": ["#room:remotetest"]})
+    def test_auto_create_auto_join_remote_room(self):
+        """Tests that we don't attempt to create remote rooms, and that we don't attempt
+        to invite ourselves to rooms we're not in."""
+
+        # Register a first user; this should call _create_and_join_rooms
+        self.get_success(self.handler.register_user(localpart="jeff"))
+
+        _, kwargs = self.room_member_handler.update_membership.call_args
+
+        self.assertEqual(kwargs["room_id"], self.room_id)
+        self.assertEqual(kwargs["action"], "join")
+        self.assertEqual(kwargs["remote_room_hosts"], ["remotetest"])
+
+        # Register a second user; this should call _join_rooms
+        self.get_success(self.handler.register_user(localpart="jeff2"))
+
+        _, kwargs = self.room_member_handler.update_membership.call_args
+
+        self.assertEqual(kwargs["room_id"], self.room_id)
+        self.assertEqual(kwargs["action"], "join")
+        self.assertEqual(kwargs["remote_room_hosts"], ["remotetest"])