feat: handle backfill requests

Based on https://gitlab.com/famedly/conduit/-/merge_requests/421

src/api/server_server.rs

@@ -12,6 +12,7 @@
         client::error::{Error as RumaError, ErrorKind},
         federation::{
             authorization::get_event_authorization,
+            backfill::get_backfill,
             device::get_devices::{self, v1::UserDevice},
             directory::{get_public_rooms, get_public_rooms_filtered},
             discovery::{get_server_keys, get_server_version, ServerSigningKeys, VerifyKey},
@@ -42,8 +43,9 @@
     },
     serde::{Base64, JsonObject, Raw},
     to_device::DeviceIdOrAllDevices,
-    CanonicalJsonObject, CanonicalJsonValue, EventId, MilliSecondsSinceUnixEpoch, OwnedEventId,
-    OwnedRoomId, OwnedServerName, OwnedServerSigningKeyId, OwnedUserId, RoomId, ServerName,
+    uint, user_id, CanonicalJsonObject, CanonicalJsonValue, EventId, MilliSecondsSinceUnixEpoch,
+    OwnedEventId, OwnedRoomId, OwnedServerName, OwnedServerSigningKeyId, OwnedUserId, RoomId,
+    ServerName,
 };
 use serde_json::value::{to_raw_value, RawValue as RawJsonValue};
 use std::{
@@ -950,6 +952,83 @@ pub async fn get_event_route(
     })
 }
 
+/// # `GET /_matrix/federation/v1/backfill/<room_id>`
+///
+/// Retrieves events from before the sender joined the room, if the room's
+/// history visibility allows.
+pub async fn get_backfill_route(
+    body: Ruma<get_backfill::v1::Request>,
+) -> Result<get_backfill::v1::Response> {
+    if !services().globals.allow_federation() {
+        return Err(Error::bad_config("Federation is disabled."));
+    }
+
+    let sender_servername = body
+        .sender_servername
+        .as_ref()
+        .expect("server is authenticated");
+
+    info!("Got backfill request from: {}", sender_servername);
+
+    if !services()
+        .rooms
+        .state_cache
+        .server_in_room(sender_servername, &body.room_id)?
+    {
+        return Err(Error::BadRequest(
+            ErrorKind::Forbidden,
+            "Server is not in room.",
+        ));
+    }
+
+    services()
+        .rooms
+        .event_handler
+        .acl_check(sender_servername, &body.room_id)?;
+
+    let until = body
+        .v
+        .iter()
+        .map(|eventid| services().rooms.timeline.get_pdu_count(eventid))
+        .filter_map(|r| r.ok().flatten())
+        .max()
+        .ok_or(Error::BadRequest(
+            ErrorKind::InvalidParam,
+            "No known eventid in v",
+        ))?;
+
+    let limit = body.limit.min(uint!(100));
+
+    let all_events = services()
+        .rooms
+        .timeline
+        .pdus_until(&user_id!("@doesntmatter:conduit.rs"), &body.room_id, until)?
+        .take(limit.try_into().unwrap());
+
+    let events = all_events
+        .filter_map(|r| r.ok())
+        .filter(|(_, e)| {
+            matches!(
+                services().rooms.state_accessor.server_can_see_event(
+                    sender_servername,
+                    &e.room_id,
+                    &e.event_id,
+                ),
+                Ok(true),
+            )
+        })
+        .map(|(pdu_id, _)| services().rooms.timeline.get_pdu_json_from_id(&pdu_id))
+        .filter_map(|r| r.ok().flatten())
+        .map(|pdu| PduEvent::convert_to_outgoing_federation_event(pdu))
+        .collect();
+
+    Ok(get_backfill::v1::Response {
+        origin: services().globals.server_name().to_owned(),
+        origin_server_ts: MilliSecondsSinceUnixEpoch::now(),
+        pdus: events,
+    })
+}
+
 /// # `POST /_matrix/federation/v1/get_missing_events/{roomId}`
 ///
 /// Retrieves events that the sender is missing.

src/main.rs

@@ -390,6 +390,7 @@ fn routes() -> Router {
         .ruma_route(server_server::get_public_rooms_filtered_route)
         .ruma_route(server_server::send_transaction_message_route)
         .ruma_route(server_server::get_event_route)
+        .ruma_route(server_server::get_backfill_route)
         .ruma_route(server_server::get_missing_events_route)
         .ruma_route(server_server::get_event_authorization_route)
         .ruma_route(server_server::get_room_state_route)

src/service/mod.rs

@@ -77,7 +77,12 @@ pub fn build<
                 search: rooms::search::Service { db },
                 short: rooms::short::Service { db },
                 state: rooms::state::Service { db },
-                state_accessor: rooms::state_accessor::Service { db },
+                state_accessor: rooms::state_accessor::Service {
+                    db,
+                    server_visibility_cache: Mutex::new(LruCache::new(
+                        (100.0 * config.conduit_cache_capacity_modifier) as usize,
+                    )),
+                },
                 state_cache: rooms::state_cache::Service { db },
                 state_compressor: rooms::state_compressor::Service {
                     db,

src/service/rooms/state_accessor/mod.rs

 mod data;
-use std::{collections::HashMap, sync::Arc};
+use std::{
+    collections::HashMap,
+    sync::{Arc, Mutex},
+};
 
 pub use data::Data;
-use ruma::{events::StateEventType, EventId, RoomId};
+use lru_cache::LruCache;
+use ruma::{
+    events::{
+        room::{
+            history_visibility::{HistoryVisibility, RoomHistoryVisibilityEventContent},
+            member::{MembershipState, RoomMemberEventContent},
+        },
+        StateEventType,
+    },
+    EventId, OwnedServerName, OwnedUserId, RoomId, ServerName, UserId,
+};
+use tracing::error;
 
-use crate::{PduEvent, Result};
+use crate::{services, Error, PduEvent, Result};
 
 pub struct Service {
     pub db: &'static dyn Data,
+    pub server_visibility_cache: Mutex<LruCache<(OwnedServerName, u64), bool>>,
 }
 
 impl Service {
@@ -46,6 +61,99 @@ pub fn state_get(
         self.db.state_get(shortstatehash, event_type, state_key)
     }
 
+    /// Get membership for given user in state
+    fn user_membership(&self, shortstatehash: u64, user_id: &UserId) -> Result<MembershipState> {
+        self.state_get(
+            shortstatehash,
+            &StateEventType::RoomMember,
+            user_id.as_str(),
+        )?
+        .map_or(Ok(MembershipState::Leave), |s| {
+            serde_json::from_str(s.content.get())
+                .map(|c: RoomMemberEventContent| c.membership)
+                .map_err(|_| Error::bad_database("Invalid room membership event in database."))
+        })
+    }
+
+    /// The user was a joined member at this state (potentially in the past)
+    fn user_was_joined(&self, shortstatehash: u64, user_id: &UserId) -> bool {
+        self.user_membership(shortstatehash, user_id)
+            .map(|s| s == MembershipState::Join)
+            .unwrap_or_default() // Return sensible default, i.e. false
+    }
+
+    /// The user was an invited or joined room member at this state (potentially
+    /// in the past)
+    fn user_was_invited(&self, shortstatehash: u64, user_id: &UserId) -> bool {
+        self.user_membership(shortstatehash, user_id)
+            .map(|s| s == MembershipState::Join || s == MembershipState::Invite)
+            .unwrap_or_default() // Return sensible default, i.e. false
+    }
+
+    /// Whether a server is allowed to see an event through federation, based on
+    /// the room's history_visibility at that event's state.
+    #[tracing::instrument(skip(self))]
+    pub fn server_can_see_event(
+        &self,
+        origin: &ServerName,
+        room_id: &RoomId,
+        event_id: &EventId,
+    ) -> Result<bool> {
+        let shortstatehash = match self.pdu_shortstatehash(event_id)? {
+            Some(shortstatehash) => shortstatehash,
+            None => return Ok(false),
+        };
+
+        if let Some(visibility) = self
+            .server_visibility_cache
+            .lock()
+            .unwrap()
+            .get_mut(&(origin.to_owned(), shortstatehash))
+        {
+            return Ok(*visibility);
+        }
+
+        let history_visibility = self
+            .state_get(shortstatehash, &StateEventType::RoomHistoryVisibility, "")?
+            .map_or(Ok(HistoryVisibility::Shared), |s| {
+                serde_json::from_str(s.content.get())
+                    .map(|c: RoomHistoryVisibilityEventContent| c.history_visibility)
+                    .map_err(|_| {
+                        Error::bad_database("Invalid history visibility event in database.")
+                    })
+            })?;
+
+        let mut current_server_members = services()
+            .rooms
+            .state_cache
+            .room_members(room_id)
+            .filter_map(|r| r.ok())
+            .filter(|member| member.server_name() == origin);
+
+        let visibility = match history_visibility {
+            HistoryVisibility::WorldReadable | HistoryVisibility::Shared => true,
+            HistoryVisibility::Invited => {
+                // Allow if any member on requesting server was AT LEAST invited, else deny
+                current_server_members.any(|member| self.user_was_invited(shortstatehash, &member))
+            }
+            HistoryVisibility::Joined => {
+                // Allow if any member on requested server was joined, else deny
+                current_server_members.any(|member| self.user_was_joined(shortstatehash, &member))
+            }
+            _ => {
+                error!("Unknown history visibility {history_visibility}");
+                false
+            }
+        };
+
+        self.server_visibility_cache
+            .lock()
+            .unwrap()
+            .insert((origin.to_owned(), shortstatehash), visibility);
+
+        Ok(visibility)
+    }
+
     /// Returns the state hash for this pdu.
     pub fn pdu_shortstatehash(&self, event_id: &EventId) -> Result<Option<u64>> {
         self.db.pdu_shortstatehash(event_id)