From 89e2faaa8e046ceaf62771eea3f531223f353086 Mon Sep 17 00:00:00 2001
From: nexy7574 <git@nexy7574.co.uk>
Date: Tue, 8 Jul 2025 17:07:50 +0100
Subject: [PATCH] feat: Ask remote servers for individual unknown events

---
 src/api/client/room/event.rs           |   7 +-
 src/service/rooms/timeline/backfill.rs | 114 ++++++++++++++++++++++++-
 2 files changed, 112 insertions(+), 9 deletions(-)

diff --git a/src/api/client/room/event.rs b/src/api/client/room/event.rs
index 47228d67..75ae0758 100644
--- a/src/api/client/room/event.rs
+++ b/src/api/client/room/event.rs
@@ -18,7 +18,7 @@ pub(crate) async fn get_room_event_route(
 	let event = services
 		.rooms
 		.timeline
-		.get_pdu(event_id)
+		.get_remote_pdu(room_id, event_id)
 		.map_err(|_| err!(Request(NotFound("Event {} not found.", event_id))));
 
 	let visible = services
@@ -33,11 +33,6 @@ pub(crate) async fn get_room_event_route(
 		return Err!(Request(Forbidden("You don't have permission to view this event.")));
 	}
 
-	debug_assert!(
-		event.event_id() == event_id && event.room_id() == room_id,
-		"Fetched PDU must match requested"
-	);
-
 	event.add_age().ok();
 
 	Ok(get_room_event::v3::Response { event: event.into_format() })
diff --git a/src/service/rooms/timeline/backfill.rs b/src/service/rooms/timeline/backfill.rs
index e976981e..dd613afd 100644
--- a/src/service/rooms/timeline/backfill.rs
+++ b/src/service/rooms/timeline/backfill.rs
@@ -1,5 +1,6 @@
 use std::iter::once;
 
+use conduwuit::{Err, PduEvent};
 use conduwuit_core::{
 	Result, debug, debug_warn, implement, info,
 	matrix::{
@@ -11,7 +12,7 @@ use conduwuit_core::{
 };
 use futures::{FutureExt, StreamExt};
 use ruma::{
-	RoomId, ServerName,
+	EventId, RoomId, ServerName,
 	api::federation,
 	events::{
 		StateEventType, TimelineEventType, room::power_levels::RoomPowerLevelsEventContent,
@@ -100,7 +101,7 @@ pub async fn backfill_if_required(&self, room_id: &RoomId, from: PduCount) -> Re
 		.boxed();
 
 	while let Some(ref backfill_server) = servers.next().await {
-		info!("Asking {backfill_server} for backfill");
+		info!("Asking {backfill_server} for backfill in {room_id}");
 		let response = self
 			.services
 			.sending
@@ -128,10 +129,117 @@ pub async fn backfill_if_required(&self, room_id: &RoomId, from: PduCount) -> Re
 		}
 	}
 
-	info!("No servers could backfill, but backfill was needed in room {room_id}");
+	warn!("No servers could backfill, but backfill was needed in room {room_id}");
 	Ok(())
 }
 
+#[implement(super::Service)]
+#[tracing::instrument(name = "get_remote_pdu", level = "debug", skip(self))]
+pub async fn get_remote_pdu(&self, room_id: &RoomId, event_id: &EventId) -> Result<PduEvent> {
+	let local = self.get_pdu(event_id).await;
+	if local.is_ok() {
+		// We already have this PDU, no need to backfill
+		debug!("We already have {event_id} in {room_id}, no need to backfill.");
+		return local;
+	}
+	debug!("Preparing to fetch event {event_id} in room {room_id} from remote servers.");
+	// Similar to backfill_if_required, but only for a single PDU
+	// Fetch a list of servers to try
+	if self
+		.services
+		.state_cache
+		.room_joined_count(room_id)
+		.await
+		.is_ok_and(|count| count <= 1)
+		&& !self
+			.services
+			.state_accessor
+			.is_world_readable(room_id)
+			.await
+	{
+		// Room is empty (1 user or none), there is no one that can backfill
+		return Err!(Request(NotFound("No one can backfill this PDU, room is empty.")));
+	}
+
+	let power_levels: RoomPowerLevelsEventContent = self
+		.services
+		.state_accessor
+		.room_state_get_content(room_id, &StateEventType::RoomPowerLevels, "")
+		.await
+		.unwrap_or_default();
+
+	let room_mods = power_levels.users.iter().filter_map(|(user_id, level)| {
+		if level > &power_levels.users_default && !self.services.globals.user_is_local(user_id) {
+			Some(user_id.server_name())
+		} else {
+			None
+		}
+	});
+
+	let canonical_room_alias_server = once(
+		self.services
+			.state_accessor
+			.get_canonical_alias(room_id)
+			.await,
+	)
+	.filter_map(Result::ok)
+	.map(|alias| alias.server_name().to_owned())
+	.stream();
+	let mut servers = room_mods
+		.stream()
+		.map(ToOwned::to_owned)
+		.chain(canonical_room_alias_server)
+		.chain(
+			self.services
+				.server
+				.config
+				.trusted_servers
+				.iter()
+				.map(ToOwned::to_owned)
+				.stream(),
+		)
+		.ready_filter(|server_name| !self.services.globals.server_is_ours(server_name))
+		.filter_map(|server_name| async move {
+			self.services
+				.state_cache
+				.server_in_room(&server_name, room_id)
+				.await
+				.then_some(server_name)
+		})
+		.boxed();
+
+	while let Some(ref backfill_server) = servers.next().await {
+		info!("Asking {backfill_server} for event {}", event_id);
+		let response = self
+			.services
+			.sending
+			.send_federation_request(backfill_server, federation::event::get_event::v1::Request {
+				event_id: event_id.to_owned(),
+				include_unredacted_content: Some(false),
+			})
+			.await;
+		let pdu = match response {
+			| Ok(response) => {
+				self.backfill_pdu(backfill_server, response.pdu)
+					.boxed()
+					.await?;
+				debug!("Successfully backfilled {event_id} from {backfill_server}");
+				Some(self.get_pdu(event_id).await)
+			},
+			| Err(e) => {
+				warn!("{backfill_server} failed to provide backfill for room {room_id}: {e}");
+				None
+			},
+		};
+		if let Some(pdu) = pdu {
+			debug!("Fetched {event_id} from {backfill_server}");
+			return pdu;
+		}
+	}
+
+	Err!("No servers could be used to fetch {} in {}.", room_id, event_id)
+}
+
 #[implement(super::Service)]
 #[tracing::instrument(skip(self, pdu), level = "debug")]
 pub async fn backfill_pdu(&self, origin: &ServerName, pdu: Box<RawJsonValue>) -> Result<()> {