feat: Add backfill_missing_events helper

src/service/rooms/event_handler/fetch_and_handle_outliers.rs

@@ -1,19 +1,149 @@
 use std::{
-	collections::{BTreeMap, HashSet, VecDeque, hash_map},
+	cmp::min,
+	collections::{BTreeMap, HashMap, HashSet, VecDeque, hash_map},
 	time::Instant,
 };
 
 use conduwuit::{
-	Event, PduEvent, debug, debug_warn, implement, matrix::event::gen_event_id_canonical_json,
-	trace, utils::continue_exponential_backoff_secs, warn,
+	Err, Event, PduEvent, debug, debug_info, debug_warn, err,
+	matrix::event::gen_event_id_canonical_json, trace, utils::continue_exponential_backoff_secs,
+	warn,
 };
+use futures::StreamExt;
 use ruma::{
-	CanonicalJsonValue, EventId, OwnedEventId, RoomId, ServerName,
-	api::federation::event::get_event,
+	CanonicalJsonValue, EventId, OwnedEventId, OwnedRoomId, OwnedServerName, RoomId, ServerName,
+	api::federation::event::{get_event, get_missing_events},
 };
 
 use super::get_room_version_rules;
 
+impl super::Service {
+	/// Uses `/_matrix/federation/v1/get_missing_events` to fill gaps in the
+	/// DAG.
+	///
+	/// When this function is called, the "earliest events" (current forward
+	/// extremities) will be collected, and the function will loop with an
+	/// exponentially incrementing limit (up to 100 per request) until it has
+	/// filled the gap, i.e. when the remote says there's no more events.
+	///
+	/// This function does not persist the events. The caller is responsible for
+	/// passing them through handle_incoming_pdu.
+	pub(super) async fn backfill_missing_events(
+		&self,
+		room_id: OwnedRoomId,
+		latest_events: Vec<OwnedEventId>,
+		via: OwnedServerName,
+	) -> conduwuit::Result<HashMap<OwnedEventId, PduEvent>> {
+		if latest_events.is_empty() {
+			return Ok(HashMap::new());
+		}
+		let earliest_events = self
+			.services
+			.state
+			.get_forward_extremities(&room_id)
+			.collect::<Vec<_>>()
+			.await;
+		// TODO: min_depth is probably necessary to avoid fetching the entire room
+		// history if there are very long gaps
+		let mut latest_events = latest_events;
+		let mut loop_count: u64 = 3;
+		// Start with a base number of 3 so that we fetch 10, 16, 25, 36, etc
+		// instead of 1, 2, 4, 9, so on.
+		let mut backfilled_events = HashMap::with_capacity(10);
+
+		while !latest_events.is_empty() {
+			let mut request = get_missing_events::v1::Request::new(
+				room_id.clone(),
+				earliest_events.clone(),
+				latest_events.clone(),
+			);
+			request.limit = min(loop_count.saturating_pow(2), 100)
+				.try_into()
+				.expect("limit cannot be greater than 100, which fits into UInt");
+			if backfilled_events.len() > 1000 {
+				warn!(
+					"Received {} missing events, refusing to fetch more (infinite loop?)",
+					backfilled_events.len()
+				);
+				break;
+			}
+
+			debug_info!(
+				backfilled=%backfilled_events.len(),
+				%loop_count,
+				"Asking {via} for up to {} missing events",
+				request.limit
+			);
+			trace!(
+				?latest_events,
+				?earliest_events,
+				%via,
+				limit=%request.limit,
+				"Requesting missing events"
+			);
+			let response: get_missing_events::v1::Response = self
+				.services
+				.sending
+				.send_federation_request(&via, request)
+				.await?;
+			loop_count = loop_count.saturating_add(1);
+
+			// Some buggy servers (including old continuwuity) may return the same events
+			// multiple times, which can cause this to be an infinite loop.
+			// In order to break this loop, if we see no new events from this response (i.e.
+			// all events in the response are already in backfilled_events), we stop,
+			// with a warning.
+			let mut unseen: usize = 0;
+			let chunk_len = response.events.len();
+			if response.events.is_empty() {
+				debug_info!("No more missing events found");
+				break;
+			}
+			for event in response.events {
+				let (incoming_room_id, event_id, pdu_json) =
+					self.parse_incoming_pdu(&event).await.map_err(|e| {
+						err!(BadServerResponse("{via} returned an invalid event: {e:?}"))
+					})?;
+				if incoming_room_id != room_id {
+					return Err!(BadServerResponse(
+						"{via} returned {event_id} in missing events which belongs to \
+						 {incoming_room_id}, not {room_id}"
+					));
+				}
+				if backfilled_events.contains_key(&event_id) {
+					debug_warn!(%via, %event_id, "Remote retransmitted event");
+					continue;
+				}
+				// TODO: Should this be scoped to the GME session? We might end up incorrectly
+				// assuming we've caught up if we do this
+				if self.services.timeline.pdu_exists(&event_id).await {
+					debug!(%via, %event_id, "Already seen event in database");
+					continue;
+				}
+				unseen = unseen.saturating_add(1);
+				let parsed = PduEvent::from_id_val(&event_id, pdu_json)
+					.map_err(|e| err!(BadServerResponse("Unable to parse {event_id}: {e}")))?;
+				backfilled_events.insert(event_id, parsed.clone());
+				for prev_event_id in parsed.prev_events() {
+					if backfilled_events.contains_key(prev_event_id)
+						|| self.services.timeline.pdu_exists(prev_event_id).await
+					{
+						continue;
+					}
+					latest_events.push(prev_event_id.to_owned());
+				}
+			}
+			latest_events.retain(|e| !backfilled_events.contains_key(e));
+			debug!(
+				chunk=%chunk_len,
+				remaining=%latest_events.len(),
+				"Got missing events"
+			);
+		}
+
+		Ok(backfilled_events)
+	}
+
 	/// Find the event and auth it. Once the event is validated (steps 1 - 8)
 	/// it is appended to the outliers Tree.
 	///
@@ -23,7 +153,6 @@ use super::get_room_version_rules;
 	/// b. Look at outlier pdu tree
 	/// c. Ask origin server over federation
 	/// d. TODO: Ask other servers over federation?
-#[implement(super::Service)]
 	pub(super) async fn fetch_and_handle_outliers<'a, Pdu, Events>(
 		&self,
 		origin: &'a ServerName,
@@ -131,8 +260,8 @@ where
 
 						if calculated_event_id != *next_id {
 							warn!(
-							"Server didn't return event id we requested: requested: {next_id}, \
-							 we got {calculated_event_id}. Event: {:?}",
+								"Server didn't return event id we requested: requested: \
+								 {next_id}, we got {calculated_event_id}. Event: {:?}",
 								&res.pdu
 							);
 						}
@@ -147,7 +276,8 @@ where
 								) {
 									| Ok(auth_event) => {
 										trace!(
-										"Found auth event id {auth_event} for event {next_id}"
+											"Found auth event id {auth_event} for event \
+											 {next_id}"
 										);
 										todo_auth_events.push_back(auth_event);
 									},
@@ -231,3 +361,4 @@ where
 		trace!("Fetched and handled {} outlier pdus", pdus.len());
 		pdus
 	}
+}

src/service/rooms/event_handler/parse_incoming_pdu.rs

@@ -56,7 +56,10 @@ fn extract_room_id(event_type: &str, pdu: &CanonicalJsonObject) -> Result<OwnedR
 
 /// Parses every entry in an array as an event ID, returning an error if any
 /// step fails.
-fn expect_event_id_array(value: &CanonicalJsonObject, field: &str) -> Result<Vec<OwnedEventId>> {
+pub(super) fn expect_event_id_array(
+	value: &CanonicalJsonObject,
+	field: &str,
+) -> Result<Vec<OwnedEventId>> {
 	value
 		.get(field)
 		.ok_or_else(|| err!(Request(BadJson("missing field `{field}` on PDU"))))?