allow adding canonical aliases from remote servers

Like is mentioned in the comment, this isn't explicitly required by the
spec, but it's reasonable and what synapse does.

book/changelog.md

@@ -311,3 +311,5 @@ This will be the first release of Grapevine since it was forked from Conduit
     ([!121](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/121))
 25. Add configuration options to tune the value of each cache individually.
     ([!124](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/124))
+26. Allow adding canonical aliases from remote servers.
+    ([!158](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/158))

src/api/client_server/alias.rs

@@ -107,6 +107,8 @@ pub(crate) async fn get_alias_route(
     get_alias_helper(body.body.room_alias).await.map(Ra)
 }
 
+// Can't use `services().rooms.alias.resolve_alias` because we also need the set
+// of servers from the remote get_room_information request.
 pub(crate) async fn get_alias_helper(
     room_alias: OwnedRoomAliasId,
 ) -> Result<get_alias::v3::Response> {

src/api/client_server/state.rs

@@ -212,7 +212,7 @@ async fn send_state_event_for_key_helper(
     let sender_user = sender;
 
     if event_type == &StateEventType::RoomCanonicalAlias {
-        validate_canonical_alias_event(room_id, json.cast_ref())?;
+        validate_canonical_alias_event(room_id, json.cast_ref()).await?;
     }
 
     let room_token = services()
@@ -253,7 +253,7 @@ async fn send_state_event_for_key_helper(
 /// > are already present in the state event.
 ///
 /// [spec]: https://spec.matrix.org/v1.13/client-server-api/#put_matrixclientv3roomsroomidstateeventtypestatekey
-fn validate_canonical_alias_event(
+async fn validate_canonical_alias_event(
     room_id: &RoomId,
     json: &Raw<RoomCanonicalAliasEventContent>,
 ) -> Result<()> {
@@ -311,13 +311,11 @@ fn validate_canonical_alias_event(
                 )
             })?;
 
-            if alias.server_name() != services().globals.server_name()
-                || services()
-                    .rooms
-                    .alias
-                    .resolve_local_alias(&alias)?
-                    .filter(|room| room == room_id)
-                    .is_none()
+            // The spec doesn't say explicitly that we should allow adding new
+            // remote canonical aliases, but it's reasonable behavior and what
+            // synapse does.
+            if services().rooms.alias.resolve_alias(&alias).await?.as_deref()
+                != Some(room_id)
             {
                 return Err(Error::BadRequest(
                     ErrorKind::BadAlias,

src/service/rooms/alias.rs

@@ -1,6 +1,7 @@
+use http::StatusCode;
 use ruma::{
-    api::client::error::ErrorKind, OwnedRoomAliasId, OwnedRoomId, RoomAliasId,
-    RoomId, UserId,
+    api::{client::error::ErrorKind, federation},
+    OwnedRoomAliasId, OwnedRoomId, RoomAliasId, RoomId, UserId,
 };
 
 use crate::{services, Error, Result};
@@ -60,7 +61,7 @@ impl Service {
         self.db.remove_alias(alias)
     }
 
-    /// Looks up the roomid for the given alias.
+    /// Looks up the roomid for the given local alias.
     pub(crate) fn resolve_local_alias(
         &self,
         alias: &RoomAliasId,
@@ -75,4 +76,46 @@ impl Service {
     ) -> Box<dyn Iterator<Item = Result<OwnedRoomAliasId>> + 'a> {
         self.db.local_aliases_for_room(room_id)
     }
+
+    /// Looks up the roomid for the given alias, fetching over federation if
+    /// remote.
+    pub(crate) async fn resolve_alias(
+        &self,
+        alias: &RoomAliasId,
+    ) -> Result<Option<OwnedRoomId>> {
+        if alias.server_name() == services().globals.server_name() {
+            self.resolve_local_alias(alias)
+        } else {
+            self.resolve_remote_alias(alias).await
+        }
+    }
+
+    /// Look up an alias on a remote server over federation.
+    async fn resolve_remote_alias(
+        &self,
+        alias: &RoomAliasId,
+    ) -> Result<Option<OwnedRoomId>> {
+        let result = services()
+            .sending
+            .send_federation_request(
+                alias.server_name(),
+                federation::query::get_room_information::v1::Request {
+                    room_alias: alias.to_owned(),
+                },
+            )
+            .await;
+
+        match result {
+            Ok(response) => Ok(Some(response.room_id)),
+            // The spec only names the 404 status code explicitly, but matching
+            // on M_NOT_FOUND as well seems reasonable.
+            Err(Error::Federation(_, error))
+                if error.status_code == StatusCode::NOT_FOUND
+                    || error.error_kind() == Some(&ErrorKind::NotFound) =>
+            {
+                Ok(None)
+            }
+            Err(error) => Err(error),
+        }
+    }
 }