style: Remove redundant length check for dag build

Tracking down weird soft-fails

src/api/client/membership/invite.rs

@@ -221,7 +221,7 @@ pub(crate) async fn invite_helper(
 		let pdu_id = services
 			.rooms
 			.event_handler
-			.handle_incoming_pdu(recipient_user.server_name(), room_id, &event_id, value, true)
+			.handle_incoming_pdu(recipient_user.server_name(), room_id, &event_id, value, false)
 			.boxed()
 			.await?
 			.ok_or_else(|| {

src/api/server/send.rs

@@ -241,7 +241,7 @@ async fn handle(
 	pdus: impl Stream<Item = Pdu> + Send,
 	edus: impl Stream<Item = Edu> + Send,
 ) -> std::result::Result<ResolvedMap, TransactionError> {
-	// group pdus by room
+	// Group PDUs by room for parallel processing
 	let pdus = pdus
 		.collect()
 		.map(|mut pdus: Vec<_>| {
@@ -252,7 +252,7 @@ async fn handle(
 		})
 		.await;
 
-	// we can evaluate rooms concurrently
+	// Evaluate rooms in parallel
 	let results: ResolvedMap = pdus
 		.into_iter()
 		.try_stream()
@@ -266,7 +266,7 @@ async fn handle(
 		.boxed()
 		.await?;
 
-	// evaluate edus after pdus, at least for now.
+	// Evaluate EDUs after PDUs in case some of the PDUs then forbid some EDUs.
 	edus.for_each_concurrent(automatic_width(), |edu| handle_edu(services, client, origin, edu))
 		.boxed()
 		.await;
@@ -296,20 +296,12 @@ async fn handle_room(
 	// Try to sort PDUs by their dependencies, but fall back to arbitrary order on
 	// failure (e.g., cycles). This is best-effort; proper ordering is the sender's
 	// responsibility.
-	let sorted_event_ids = if pdu_map.len() >= 2 {
-		let refmap = pdu_map
-			.iter()
-			.map(|(event_id, obj)| (event_id.clone(), obj))
-			.collect();
-		build_local_dag(&refmap, DagBuilderTree::PrevEvents)
-			.await
-			.unwrap_or_else(|e| {
-				debug_warn!("Failed to build local DAG for room {room_id}: {e}");
-				pdu_map.keys().cloned().collect()
-			})
-	} else {
-		pdu_map.keys().cloned().collect()
-	};
+	let sorted_event_ids = build_local_dag(&pdu_map, DagBuilderTree::PrevEvents)
+		.await
+		.unwrap_or_else(|e| {
+			debug_warn!("Failed to build local DAG for room {room_id}: {e}");
+			pdu_map.keys().cloned().collect()
+		});
 	let mut results = Vec::with_capacity(sorted_event_ids.len());
 	for event_id in sorted_event_ids {
 		let value = pdu_map
@@ -322,7 +314,7 @@ async fn handle_room(
 		let result = services
 			.rooms
 			.event_handler
-			.handle_incoming_pdu(origin, room_id, &event_id, value.clone(), true)
+			.handle_incoming_pdu(origin, room_id, &event_id, value, false)
 			.boxed()
 			.await
 			.map(|_| ());

src/api/server/send_join.rs

@@ -200,7 +200,7 @@ async fn create_join_event(
 	let pdu_id = services
 		.rooms
 		.event_handler
-		.handle_incoming_pdu(sender.server_name(), room_id, &event_id, value.clone(), true)
+		.handle_incoming_pdu(sender.server_name(), room_id, &event_id, value.clone(), false)
 		.boxed()
 		.await?
 		.ok_or_else(|| err!(Request(InvalidParam("Could not accept as timeline event."))))?;

src/api/server/send_knock.rs

@@ -166,7 +166,7 @@ pub(crate) async fn create_knock_event_v1_route(
 	let pdu_id = services
 		.rooms
 		.event_handler
-		.handle_incoming_pdu(sender.server_name(), &body.room_id, &event_id, value.clone(), true)
+		.handle_incoming_pdu(sender.server_name(), &body.room_id, &event_id, value.clone(), false)
 		.boxed()
 		.await?
 		.ok_or_else(|| err!(Request(InvalidParam("Could not accept as timeline event."))))?;

src/api/server/send_leave.rs

@@ -153,7 +153,7 @@ async fn create_leave_event(
 	let pdu_id = services
 		.rooms
 		.event_handler
-		.handle_incoming_pdu(origin, room_id, &event_id, value, true)
+		.handle_incoming_pdu(origin, room_id, &event_id, value, false)
 		.boxed()
 		.await?
 		.ok_or_else(|| err!(Request(InvalidParam("Could not accept as timeline event."))))?;

src/core/error/err.rs

@@ -149,6 +149,14 @@ macro_rules! err_log {
 #[macro_export]
 #[collapse_debuginfo(yes)]
 macro_rules! err_lev {
+	(debug_info) => {
+		if $crate::debug::logging() {
+			$crate::tracing::Level::INFO
+		} else {
+			$crate::tracing::Level::DEBUG
+		}
+	};
+
 	(debug_warn) => {
 		if $crate::debug::logging() {
 			$crate::tracing::Level::WARN

src/service/rooms/event_handler/acl_check.rs

@@ -14,6 +14,7 @@ pub async fn acl_check(&self, server_name: &ServerName, room_id: &RoomId) -> Res
 			.await
 			.map(|c: RoomServerAclEventContent| c)
 		else {
+			trace!("Room has no ACL, allowing");
 			return Ok(());
 		};
 

src/service/rooms/event_handler/fetch_and_handle_outliers.rs

@@ -1,5 +1,6 @@
 use std::{
 	collections::{HashMap, HashSet, VecDeque},
+	fmt::{Display, Formatter},
 	time::Instant,
 };
 
@@ -33,6 +34,20 @@ pub enum DagBuilderTree {
 	AuthEvents,
 }
 
+impl DagBuilderTree {
+	#[must_use]
+	pub fn as_str(&self) -> &str {
+		match self {
+			| Self::AuthEvents => "auth_events",
+			| Self::PrevEvents => "prev_events",
+		}
+	}
+}
+
+impl Display for DagBuilderTree {
+	fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result { f.write_str(self.as_str()) }
+}
+
 /// Attempts to build a localised directed acyclic graph out of the given PDUs,
 /// returning them in a topologically sorted order.
 ///
@@ -42,23 +57,22 @@ pub enum DagBuilderTree {
 /// not account for power levels or other tie breaks.
 #[allow(clippy::implicit_hasher)]
 pub async fn build_local_dag(
-	pdu_map: &HashMap<OwnedEventId, &CanonicalJsonObject>,
+	pdu_map: &HashMap<OwnedEventId, CanonicalJsonObject>,
 	tree: DagBuilderTree,
 ) -> Result<Vec<OwnedEventId>> {
-	debug_assert!(pdu_map.len() >= 2, "needless call to build_local_dag with less than 2 PDUs");
+	if pdu_map.len() <= 1 {
+		return Ok(pdu_map.keys().cloned().collect());
+	}
+
 	let mut dag: HashMap<OwnedEventId, HashSet<OwnedEventId>> =
 		HashMap::with_capacity(pdu_map.len());
 	let mut id_origin_ts: HashMap<OwnedEventId, _> = HashMap::with_capacity(pdu_map.len());
-	let tree = match tree {
-		| DagBuilderTree::AuthEvents => "auth_events",
-		| DagBuilderTree::PrevEvents => "prev_events",
-	};
 
 	for (event_id, value) in pdu_map {
 		// Parse all prev events as event IDs - if they are missing, return an error (we
 		// can't sanely continue in this case), otherwise skip invalid prev events.
 		let prev_events = value
-			.get(tree)
+			.get(tree.as_str())
 			.and_then(CanonicalJsonValue::as_array)
 			.ok_or_else(|| err!(Request(BadJson("event JSON for {event_id} is missing {tree}"))))?
 			.iter()
@@ -370,9 +384,8 @@ async fn fetch_event_via(
 			.send_federation_request(&remote, get_event::v1::Request::new(event_id.clone()))
 			.await?;
 
-		let (calculated_event_id, value) = self
-			.parse_incoming_pdu_with_known_room(&res.pdu, room_version_rules)
-			.await?;
+		let (calculated_event_id, value) =
+			Self::parse_incoming_pdu_with_known_room(&res.pdu, room_version_rules)?;
 
 		if calculated_event_id != event_id {
 			Err!(Request(BadJson(warn!(
@@ -538,11 +551,7 @@ pub(super) async fn fetch_and_handle_auth_events<Pdu>(
 			}
 		}
 
-		let refmap: HashMap<OwnedEventId, &CanonicalJsonObject> = discovered_events
-			.iter()
-			.map(|(id, data)| (id.clone(), data))
-			.collect();
-		let seeded_ordered = build_local_dag(&refmap, DagBuilderTree::AuthEvents)
+		let seeded_ordered = build_local_dag(&discovered_events, DagBuilderTree::AuthEvents)
 			.await
 			.expect("failed to build local DAG");
 		let mut pdus = HashMap::with_capacity(seeded_ordered.len());

src/service/rooms/event_handler/fetch_auth.rs

@@ -1,9 +1,8 @@
 use std::collections::{HashMap, hash_map};
 
-use conduwuit::{Err, Event, EventTypeExt, PduEvent, Result, err};
+use conduwuit::{Err, Event, EventTypeExt, PduEvent, Result, err, warn};
 use ruma::{
-	CanonicalJsonObject, OwnedEventId, ServerName,
-	api::federation::authorization::get_event_authorization,
+	OwnedEventId, ServerName, api::federation::authorization::get_event_authorization,
 	room_version_rules::RoomVersionRules,
 };
 use tokio::join;
@@ -50,16 +49,49 @@ pub(super) async fn fetch_and_persist_event_auth<Pdu>(
 
 		for auth_pdu_json in event_auth.auth_chain {
 			let (auth_event_room_id, auth_event_id, auth_pdu_json) =
-				self.parse_incoming_pdu(&auth_pdu_json).await?;
+				match self.parse_incoming_pdu(&auth_pdu_json).await {
+					| Ok(parsed) => parsed,
+					| Err(e) => {
+						warn!(error=?e, "Dropping auth chain event as it could not be parsed");
+						continue;
+					},
+				};
+			if let Err(e) = Self::pdu_format_check_1(
+				&auth_pdu_json,
+				room_version_rules,
+				create_event.event_id(),
+			) {
+				// drop this PDU
+				warn!(%auth_event_id, error=?e, "Dropping auth chain event as it violates the room event format");
+				continue;
+			}
+			let auth_pdu_json = match self
+				.signature_hash_check_2_3(auth_pdu_json, room_version_rules)
+				.await
+			{
+				| Ok(pdu_json) => pdu_json,
+				| Err(e) => {
+					// drop this PDU
+					warn!(
+						%auth_event_id,
+						error=?e,
+						"Dropping auth chain event as it has an invalid signature"
+					);
+					continue;
+				},
+			};
+
+			// PDU check 4 is done when we've finished aggregating
 			if auth_event_room_id != incoming_event.room_id_or_hash() {
 				return Err!(Request(Forbidden(
-					"Auth event {auth_event_id} is in {auth_event_room_id}, not {}.",
+					"Auth chain event {auth_event_id} is in {auth_event_room_id}, not {}.",
 					incoming_event.room_id_or_hash()
 				)));
 			}
 			let auth_pdu = PduEvent::from_id_val(&auth_event_id, auth_pdu_json).map_err(|e| {
 				err!(Request(BadJson("Invalid PDU {auth_event_id} in auth chain: {e}")))
 			})?;
+
 			if auth_pdu.state_key().is_none() {
 				return Err!(Request(BadJson(
 					"Invalid PDU {auth_event_id} in auth_chain: not a state event"
@@ -97,17 +129,11 @@ async fn authorise_remote_auth_chain<Pdu>(
 	where
 		Pdu: Event + Send + Sync,
 	{
-		// TODO(nex): build_local_dag's signature needs changing because all callsites
-		// seem to do this refmap hack.
 		let pdu_objects = auth_chain_map
 			.iter()
-			.map(|(event_id, pdu)| (event_id, pdu.to_canonical_object()))
+			.map(|(event_id, pdu)| (event_id.clone(), pdu.to_canonical_object()))
 			.collect::<HashMap<_, _>>();
-		let refmap: HashMap<OwnedEventId, &CanonicalJsonObject> = pdu_objects
-			.iter()
-			.map(|(id, data)| ((*id).to_owned(), data))
-			.collect();
-		let auth_chain_topo = build_local_dag(&refmap, DagBuilderTree::AuthEvents)
+		let auth_chain_topo = build_local_dag(&pdu_objects, DagBuilderTree::AuthEvents)
 			.await?
 			.into_iter()
 			.map(|event_id| (event_id.clone(), auth_chain_map.get(&event_id).unwrap().to_owned()))
@@ -127,6 +153,12 @@ async fn authorise_remote_auth_chain<Pdu>(
 				continue;
 			}
 
+			// IMPORTANT: We can't use the handy dandy `handle_outlier_pdu` function here
+			// because it may then try to fetch missing auth events, resulting in deep
+			// recursion. We will do the minimum required steps to validate the PDU here.
+			// Checks 1-3 were already done before this function is called, so we only need
+			// to do check 4.
+
 			let mut auth_events_by_key: HashMap<_, _> =
 				HashMap::with_capacity(pdu.auth_events.len());
 
@@ -150,24 +182,21 @@ async fn authorise_remote_auth_chain<Pdu>(
 					},
 					| hash_map::Entry::Occupied(_) => {
 						// Duplicate auth events by key are not allowed.
-						self.services
-							.pdu_metadata
-							.mark_event_rejected(auth_event_id);
-						self.services.pdu_metadata.mark_event_rejected(&event_id);
+						self.reject_and_persist(&event_id, &pdu.to_canonical_object());
 						continue 'outer;
 					},
 				}
 			}
 			if !self
-				.is_event_self_authorised(
+				.auth_state_check_4(
 					&pdu,
 					room_version_rules,
 					create_event.as_pdu(),
 					&auth_events_by_key,
 				)
-				.await
+				.await?
 			{
-				self.services.pdu_metadata.mark_event_rejected(&event_id);
+				self.reject_and_persist(&event_id, &pdu.to_canonical_object());
 			}
 		}
 

src/service/rooms/event_handler/fetch_prev.rs

@@ -5,13 +5,13 @@
 	utils::{BoolExt, IterStream, stream::BroadbandExt},
 };
 use futures::StreamExt;
-use ruma::{CanonicalJsonObject, MilliSecondsSinceUnixEpoch, OwnedEventId, RoomId, ServerName};
+use ruma::{MilliSecondsSinceUnixEpoch, RoomId, ServerName};
 
 use crate::rooms::event_handler::{build_local_dag, fetch_and_handle_outliers::DagBuilderTree};
 
 impl super::Service {
 	/// Fetches any missing prev_events for this event and persists them before
-	/// returning.
+	/// returning. The caller is responsible for then handling the incoming PDU.
 	pub(super) async fn fetch_prevs(
 		&self,
 		room_id: &RoomId,
@@ -84,13 +84,7 @@ pub(super) async fn fetch_prevs(
 			})
 			.collect::<HashMap<_, _>>();
 
-		let to_persist = if mapped.len() <= 1 {
-			mapped.keys().map(ToOwned::to_owned).collect()
-		} else {
-			let refmap: HashMap<OwnedEventId, &CanonicalJsonObject> =
-				mapped.iter().map(|(id, data)| (id.clone(), data)).collect();
-			build_local_dag(&refmap, DagBuilderTree::PrevEvents).await?
-		};
+		let to_persist = build_local_dag(&mapped, DagBuilderTree::PrevEvents).await?;
 
 		let job_start = Instant::now();
 		trace!("Starting to persist {} prev events", to_persist.len());

src/service/rooms/event_handler/handle_incoming_pdu.rs

@@ -4,46 +4,37 @@
 };
 
 use conduwuit::{
-	Err, Event, Result, debug, debug_error, debug_warn, defer, err, error, info,
-	matrix::PartialPdu, result::DebugInspect, trace, utils::time::jitter, warn,
+	Err, Event, Result, debug, debug_error, debug_warn, defer, matrix::PartialPdu, trace,
+	utils::time::jitter,
 };
-use futures::{
-	FutureExt, StreamExt,
-	future::{OptionFuture, try_join4},
-};
-use ruma::{CanonicalJsonValue, EventId, OwnedUserId, RoomId, ServerName, UserId};
+use futures::{FutureExt, StreamExt, future::try_join3};
+use ruma::{CanonicalJsonValue, EventId, RoomId, ServerName, UserId};
 use tokio::sync::mpsc;
 
-use crate::rooms::timeline::{RawPduId, pdu_fits};
+use crate::rooms::timeline::RawPduId;
 
 impl super::Service {
-	/// When receiving an event one needs to:
-	/// 0. Check the server is in the room
-	/// 1. Skip the PDU if we already know about it
-	/// 1.1. Remove unsigned field
-	/// 2. Check signatures, otherwise drop
-	/// 3. Check content hash, redact if doesn't match
-	/// 4. Fetch any missing auth events doing all checks listed here starting
-	///    at 1. These are not timeline events
-	/// 5. Reject "due to auth events" if can't get all the auth events or some
-	///    of the auth events are also rejected "due to auth events"
-	/// 6. Reject "due to auth events" if the event doesn't pass auth based on
-	///    the auth events
-	/// 7. Persist this event as an outlier
-	/// 8. If not timeline event: stop
-	/// 9. Fetch any missing prev events doing all checks listed here starting
-	///    at 1. These are timeline events
-	/// 10. Fetch missing state and auth chain events by calling `/state_ids` at
-	///     backwards extremities doing all the checks in this list starting at
-	///     1. These are not timeline events
-	/// 11. Check the auth of the event passes based on the state of the event
-	/// 12. Ensure that the state is derived from the previous current state
-	///     (i.e. we calculated by doing state res where one of the inputs was a
-	///     previously trusted set of state, don't just trust a set of state we
-	///     got from a remote)
-	/// 13. Use state resolution to find new room state
-	/// 14. Check if the event passes auth based on the "current state" of the
-	///     room, if not soft fail it
+	/// Handles an incoming PDU from federation.
+	///
+	/// First checks that we want to receive this PDU. If we already have it as
+	/// a timeline PDU, or we don't want to receive the PDU (e.g. origin ACL'd,
+	/// room disabled/unknown), abort.
+	///
+	/// The PDU is then handled as an outlier event, which performs [PDU checks]
+	/// 1 through 4. See: `handle_outlier_pdu`.
+	///
+	/// Once handled as an outlier, any missing prev events are fetched, and
+	/// then the PDU will be promoted/upgraded from an outlier to a timeline
+	/// event clients can see. See: `upgrade_outlier_to_timeline_pdu`. After
+	/// this finishes, the PDU is either accepted or left as an outlier.
+	///
+	/// If the PDU is successfully upgraded, the remaining extremity count of
+	/// the room is checked. If there are a potentially problematic number of
+	/// forward extremities, a squasher task is started with a debounce period,
+	/// which will eventually send a dummy event that ties up as many DAG forks
+	/// as possible.
+	///
+	/// [PDU checks]: https://spec.matrix.org/v1.19/server-server-api/#checks-performed-on-receipt-of-a-pdu
 	#[tracing::instrument(
 		name = "pdu",
 		skip_all,
@@ -55,51 +46,33 @@ pub async fn handle_incoming_pdu<'a>(
 		room_id: &'a RoomId,
 		event_id: &'a EventId,
 		value: BTreeMap<String, CanonicalJsonValue>,
-		is_timeline_event: bool,
+		is_backfilled_event: bool,
 	) -> Result<Option<RawPduId>> {
-		// 1. Skip the PDU if we already have it as a timeline event
+		// Skip the PDU if we already have it as a timeline event. We still re-process
+		// outliers in this scenario.
 		if let Ok(pdu_id) = self.services.timeline.get_pdu_id(event_id).await {
+			debug!("Database hit for incoming PDU, skipping processing");
 			return Ok(Some(pdu_id));
 		}
-		if !pdu_fits(&mut value.clone()) {
-			warn!(
-				"dropping incoming PDU {event_id} in room {room_id} from {origin} because it \
-				 exceeds 65535 bytes or is otherwise too large."
-			);
-			return Err!(Request(TooLarge("PDU is too large")));
-		}
 		trace!(
 			"processing incoming PDU from {origin} for room {room_id} with event id {event_id}"
 		);
 
-		// 1.1 Check we even know about the room
-		let meta_exists = self.services.metadata.exists(room_id).map(Ok);
+		// If this is a membership state event for a local user, we will be interested
+		// in this event even if we aren't in the room. This allows us to process things
+		// like revoking invites over federation, without being in the room.
+		let is_interesting_member_event = value.get("type").and_then(|t| t.as_str())
+			== Some("m.room.member")
+			&& value
+				.get("state_key")
+				.and_then(|s| s.as_str())
+				.and_then(|s| UserId::parse(s).ok())
+				.is_some_and(|u| self.services.globals.user_is_local(&u));
 
-		// 1.2 Check if the room is disabled
-		let is_disabled = self.services.metadata.is_disabled(room_id).map(Ok);
-
-		// 1.3.1 Check room ACL on origin field/server
-		let origin_acl_check = self.acl_check(origin, room_id);
-
-		// 1.3.2 Check room ACL on sender's server name
-		let sender: OwnedUserId = value
-			.get("sender")
-			.and_then(|v| v.as_str())
-			.ok_or_else(|| err!("No sender in object"))
-			.and_then(|v| Ok(UserId::parse(v)?))
-			.map_err(|e| err!(Request(BadJson("PDU does not have a valid sender key: {e}"))))?;
-
-		let sender_acl_check: OptionFuture<_> = sender
-			.server_name()
-			.ne(origin)
-			.then(|| self.acl_check(sender.server_name(), room_id))
-			.into();
-
-		let (meta_exists, is_disabled, (), ()) = try_join4(
-			meta_exists,
-			is_disabled,
-			origin_acl_check,
-			sender_acl_check.map(|o| o.unwrap_or(Ok(()))),
+		let (room_exists, is_disabled, ()) = try_join3(
+			self.services.metadata.exists(room_id).map(Ok),
+			self.services.metadata.is_disabled(room_id).map(Ok),
+			self.acl_check(origin, room_id),
 		)
 		.await
 		.inspect_err(
@@ -112,21 +85,11 @@ pub async fn handle_incoming_pdu<'a>(
 			)));
 		}
 
-		if !self
-			.services
-			.state_cache
-			.server_in_room(self.services.globals.server_name(), room_id)
-			.await
-		{
-			info!(
-				%origin,
-				%room_id,
-				"Dropping inbound PDU for room we aren't participating in"
-			);
-			return Err!(Request(NotFound("This server is not participating in that room.")));
-		}
-
-		if !meta_exists {
+		if !room_exists {
+			if is_interesting_member_event {
+				// TODO: handle interesting membership events where we aren't in
+				// the room
+			}
 			return Err!(Request(NotFound("Room is unknown to this server")));
 		}
 
@@ -152,8 +115,10 @@ pub async fn handle_incoming_pdu<'a>(
 			.handle_outlier_pdu(origin, create_event, event_id, room_id, value)
 			.await?;
 
-		// 8. if not timeline event: stop
-		if !is_timeline_event {
+		// If this event is being processed as part of backfill, we don't want to end up
+		// *appending* it during the upgrade process, so we return early.
+		if is_backfilled_event {
+			debug!("Not promoting incoming event as it is being backfilled");
 			return Ok(None);
 		}
 
@@ -166,11 +131,16 @@ pub async fn handle_incoming_pdu<'a>(
 			.await?
 			.origin_server_ts();
 		if incoming_pdu.origin_server_ts() < first_ts_in_room {
+			debug_warn!(
+				"Not promoting incoming event as it is sent before we joined the room (but was \
+				 not backfilled)"
+			);
 			return Ok(None);
 		}
 
-		// 9. Fetch any missing prev events doing all checks listed here starting at 1.
-		//    These are timeline events
+		// Fetch any missing prev events doing all checks listed here starting at 1.
+		// These are timeline events.
+		// TODO: This part needs to be done in a background queue somewhere.
 
 		debug!("Fetching and persisting any missing prev events");
 		Box::pin(self.fetch_prevs(
@@ -181,13 +151,14 @@ pub async fn handle_incoming_pdu<'a>(
 			first_ts_in_room,
 		))
 		.await
-		.debug_inspect_err(|e| {
-			error!("Failed to fetch and persist incoming event's prev_events: {e:?}");
+		.inspect_err(|e| {
+			debug_error!("Failed to fetch and persist incoming event's prev_events: {e:?}");
 		})?;
 
-		let is_dummy_event = incoming_pdu.event_type().to_string() == "org.matrix.dummy_event";
+		let is_dummy_event = incoming_pdu.event_type().to_string() == "org.matrix.dummy_event"
+			&& incoming_pdu.state_key().is_none();
 
-		// Done with prev events, now handling the incoming event
+		// Done with prev events, now we can handle promoting the PDU
 		let pdu_id = Box::pin(self.upgrade_outlier_to_timeline_pdu(
 			incoming_pdu,
 			val,

src/service/rooms/event_handler/handle_outlier_pdu.rs

@@ -1,23 +1,22 @@
 use std::collections::{BTreeMap, HashMap, hash_map};
 
 use conduwuit::{
-	Err, Event, EventTypeExt, PduEvent, Result, debug, debug_info, debug_warn, err, info,
-	matrix::StateKey, state_res::auth_check, trace, warn,
-};
-use futures::future::ready;
-use ruma::{
-	CanonicalJsonObject, CanonicalJsonValue, EventId, OwnedEventId, RoomId, ServerName,
-	canonical_json::redact, events::StateEventType, room_version_rules::RoomVersionRules,
+	Err, Event, EventTypeExt, PduEvent, Result, debug, debug_warn, err, info, trace,
 };
+use ruma::{CanonicalJsonObject, CanonicalJsonValue, EventId, OwnedEventId, RoomId, ServerName};
 
-use super::{check_room_id, get_room_version_rules};
-use crate::rooms::timeline::pdu_fits;
+use super::get_room_version_rules;
 
 impl super::Service {
 	/// Handles a PDU as an outlier, performing basic checks like signatures and
 	/// hashes, proclaimed event auth, and then adding it to the outlier tree.
+	///
+	/// This performs steps 1 through 4 of [S-S section 5.1][spec], returning
+	/// the parsed PDU and modified JSON object.
+	///
+	/// [spec]: https://spec.matrix.org/v1.19/server-server-api/#checks-performed-on-receipt-of-a-pdu
 	#[allow(clippy::too_many_arguments)]
-	#[tracing::instrument(name="handle_outlier", skip_all, fields(%event_id))]
+	#[tracing::instrument(name = "handle_outlier", skip_all)]
 	pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 		&self,
 		origin: &'a ServerName,
@@ -29,61 +28,39 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 	where
 		Pdu: Event + Send + Sync,
 	{
-		if !pdu_fits(&mut value.clone()) {
-			warn!(
-				"dropping incoming PDU {event_id} in room {room_id} from {origin} because it \
-				 exceeds 65535 bytes or is otherwise too large."
+		// Skip outlier handling if we already have this event as either a timeline or
+		// outlier PDU.
+		if let Ok(pdu_event) = self.services.timeline.get_pdu(event_id).await {
+			debug!(
+				"Database hit for {event_id} (event is either an outlier or already promoted), \
+				 skipping outlier handling"
 			);
-			return Err!(Request(TooLarge("PDU is too large")));
+			value.insert(
+				"event_id".to_owned(),
+				CanonicalJsonValue::String(event_id.as_str().to_owned()),
+			);
+			return Ok((pdu_event, value));
 		}
-		// 1. Remove unsigned field
-		value.remove("unsigned");
 
-		// 2. Check signatures, otherwise drop
-		// 3. check content hash, redact if doesn't match
+		// 1. Check that the PDU follows the format for the room version
+		// (in this case, just size check)
 		let room_version_rules = get_room_version_rules(create_event)?;
-		let mut incoming_pdu = match self
-			.services
-			.server_keys
-			.verify_event(&value, &room_version_rules)
-			.await
-		{
-			| Ok(ruma::signatures::Verified::All) => {
-				if let Ok(pdu_event) = self.services.timeline.get_pdu(event_id).await {
-					debug!(
-						"Already have event {event_id} as an outlier or timeline event, not \
-						 re-processing"
-					);
-					value.insert(
-						"event_id".to_owned(),
-						CanonicalJsonValue::String(event_id.as_str().to_owned()),
-					);
-					check_room_id(room_id, &pdu_event)?;
-					return Ok((pdu_event, value));
-				}
-				value
-			},
-			| Ok(ruma::signatures::Verified::Signatures) => {
-				if let Ok(pdu_event) = self.services.timeline.get_pdu(event_id).await {
-					debug!(
-						"Received a redacted copy of {event_id}, but we already knew about it. \
-						 Re-using known content instead."
-					);
-					check_room_id(room_id, &pdu_event)?;
-					let obj = pdu_event.to_canonical_object();
-					return Ok((pdu_event, obj));
-				}
+		Self::pdu_format_check_1(&value, &room_version_rules, create_event.event_id())
+			.inspect_err(|e| {
+				info!(
+					err=?e,
+					"Dropping incoming PDU from {origin} because it violates the room event format"
+				);
+			})?;
 
-				debug_info!("Calculated hash does not match (redaction): {event_id}");
-				redact(value, &room_version_rules.redaction, None)
-					.map_err(|e| err!(Request(BadJson("Failed to redact {event_id}: {e}"))))?
-			},
-			| Err(e) => {
-				return Err!(Request(Forbidden(debug_error!(
-					"Signature verification failed for {event_id}: {e}"
-				))));
-			},
-		};
+		value.remove("unsigned");
+		let room_version_rules = get_room_version_rules(create_event)?;
+
+		// 2. Check signatures, otherwise drop.
+		// 3. Check content hash, redacting the event if it fails.
+		let mut incoming_pdu = self
+			.signature_hash_check_2_3(value, &room_version_rules)
+			.await?;
 
 		// Now that we have checked the signature and hashes we can add the eventID and
 		// convert to our PduEvent type
@@ -91,14 +68,11 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 			"event_id".to_owned(),
 			CanonicalJsonValue::String(event_id.as_str().to_owned()),
 		);
-
 		let pdu_event = serde_json::from_value::<PduEvent>(
 			serde_json::to_value(&incoming_pdu).expect("CanonicalJsonObj is a valid JsonValue"),
 		)
 		.map_err(|e| err!(Request(BadJson(debug_warn!("Event is not a valid PDU: {e}")))))?;
 
-		check_room_id(room_id, &pdu_event)?;
-
 		// TODO(nex): From hereon the event is not dropped, and thus always added as an
 		// outlier. However, we only do that at the end of this function, which means
 		// several duplicated calls to add_pdu_outlier. Shouldn't we just do it here
@@ -116,7 +90,6 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 
 		for auth_event_id in pdu_event.auth_events() {
 			if let Ok(auth_event) = self.services.timeline.get_pdu(auth_event_id).await {
-				check_room_id(room_id, &auth_event)?;
 				trace!("Found auth event {auth_event_id} for outlier event {event_id} locally");
 				auth_events.insert(auth_event_id.to_owned(), auth_event);
 			} else {
@@ -154,7 +127,7 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 		}
 
 		// Ensure none of the auth events are rejected - if they are, reject too.
-		for auth_event_id in auth_events.keys() {
+		for (auth_event_id, auth_event) in &auth_events {
 			if self
 				.services
 				.pdu_metadata
@@ -166,15 +139,27 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 					 {auth_event_id}",
 					event_id,
 				);
-				self.services.pdu_metadata.mark_event_rejected(event_id);
+				self.reject_and_persist(event_id, &incoming_pdu);
 				return Err!(Request(Forbidden(
 					"Event has rejected auth event: {auth_event_id}"
 				)));
 			}
+			if auth_event.room_id_or_hash() != room_id {
+				debug_warn!(
+					%auth_event_id,
+					auth_event_room_id=%auth_event.room_id_or_hash(),
+					expected_room_id=%room_id,
+					"Rejecting incoming event which depends on an auth event in another room.",
+				);
+				self.reject_and_persist(event_id, &incoming_pdu);
+				return Err!(Request(Forbidden(
+					"Event depends on a cross-room auth event: {auth_event_id}"
+				)));
+			}
 		}
 
-		// 6. Reject "due to auth events" if the event doesn't pass auth based on the
-		//    auth events
+		// 4. Reject "due to auth events" if the event doesn't pass auth based on the
+		//    claimed auth events
 		debug!("Checking based on auth events");
 		let mut auth_events_by_key: HashMap<_, _> = HashMap::with_capacity(auth_events.len());
 		// Build map of auth events
@@ -184,8 +169,6 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 				.expect("we just checked that we have all auth events")
 				.to_owned();
 
-			check_room_id(room_id, &auth_event)?;
-
 			let key = auth_event.kind().with_state_key(
 				auth_event
 					.state_key
@@ -197,98 +180,45 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 					v.insert(auth_event);
 				},
 				| hash_map::Entry::Occupied(_) => {
-					self.services
-						.outlier
-						.add_pdu_outlier(pdu_event.event_id(), &incoming_pdu);
-					self.services.pdu_metadata.mark_event_rejected(event_id);
-					return Err!(Request(Forbidden(
+					self.reject_and_persist(event_id, &incoming_pdu);
+					return Err!(Request(Forbidden(debug_warn!(
 						"Auth event's type and state_key combination exists multiple times: {}, \
 						 {}",
 						auth_event.kind,
 						auth_event.state_key().unwrap_or("")
-					)));
+					))));
 				},
 			}
 		}
 
 		if !self
-			.is_event_self_authorised(
+			.auth_state_check_4(
 				&pdu_event,
 				&room_version_rules,
 				create_event.as_pdu(),
 				&auth_events_by_key,
 			)
-			.await
+			.await?
 		{
-			self.services.pdu_metadata.mark_event_rejected(event_id);
-			self.services
-				.outlier
-				.add_pdu_outlier(pdu_event.event_id(), &incoming_pdu);
-			return Err!(Request(Forbidden(
+			self.reject_and_persist(event_id, &incoming_pdu);
+			return Err!(Request(Forbidden(debug_warn!(
 				"Event authorisation fails based on event's claimed auth events"
-			)));
+			))));
 		}
 
-		trace!("Validation successful.");
-
 		// 7. Persist the event as an outlier.
 		self.services
 			.outlier
 			.add_pdu_outlier(pdu_event.event_id(), &incoming_pdu);
 
-		trace!("Added pdu as outlier.");
+		debug!("PDU passed checks and has been persisted as an outlier");
 
 		Ok((pdu_event, incoming_pdu))
 	}
 
-	/// Helper method that turns the return value of `is_event_self_authorised`
-	/// into a `Result` depending on the value.
-	///
-	/// If the event is not authorised, a Forbidden error is returned.
-	/// Otherwise, an empty `Ok`.
-	pub(super) async fn is_event_self_authorised(
-		&self,
-		pdu: &PduEvent,
-		room_version_rules: &RoomVersionRules,
-		create_event: &PduEvent,
-		auth_events_by_key: &HashMap<(StateEventType, StateKey), PduEvent>,
-	) -> bool {
-		self.expect_event_is_self_authorised(
-			pdu,
-			room_version_rules,
-			create_event,
-			auth_events_by_key,
-		)
-		.await
-		.is_ok()
-	}
-
-	/// Checks PDU check 4: Passes authorisation rules based on the event's auth
-	/// events ([spec]).
-	///
-	/// If the auth check fails, false is returned, otherwise true.
-	///
-	/// [spec]: https://spec.matrix.org/v1.19/server-server-api/#checks-performed-on-receipt-of-a-pdu
-	pub(super) async fn expect_event_is_self_authorised(
-		&self,
-		pdu: &PduEvent,
-		room_version_rules: &RoomVersionRules,
-		create_event: &PduEvent,
-		auth_events_by_key: &HashMap<(StateEventType, StateKey), PduEvent>,
-	) -> Result<bool> {
-		let state_fetch = |ty: &StateEventType, sk: &str| {
-			let key = (ty.to_owned(), sk.into());
-			ready(auth_events_by_key.get(&key).map(ToOwned::to_owned))
-		};
-
-		auth_check(
-			room_version_rules,
-			pdu,
-			None, // TODO: third party invite
-			state_fetch,
-			create_event,
-		)
-		.await
-		.map_err(|e| err!("Event self-authentication failed: {e:?}"))
+	/// Marks the event as rejected and then saves it as an outlier.
+	pub(super) fn reject_and_persist(&self, event_id: &EventId, pdu: &CanonicalJsonObject) {
+		self.services.pdu_metadata.mark_event_rejected(event_id);
+		self.services.outlier.add_pdu_outlier(event_id, pdu);
 	}
 }

src/service/rooms/event_handler/mod.rs

@@ -6,6 +6,7 @@
 mod handle_incoming_pdu;
 mod handle_outlier_pdu;
 mod parse_incoming_pdu;
+pub mod pdu_checks;
 mod policy_server;
 mod resolve_state;
 mod state_at_incoming;
@@ -19,7 +20,7 @@
 	DagBuilderTree, GET_MISSING_EVENTS_MAX_BATCH_SIZE, build_local_dag,
 };
 use ruma::{
-	OwnedEventId, OwnedRoomId, RoomId, events::room::create::RoomCreateEventContent,
+	OwnedEventId, OwnedRoomId, events::room::create::RoomCreateEventContent,
 	room_version_rules::RoomVersionRules,
 };
 use tokio::sync::{Notify, mpsc};
@@ -114,22 +115,6 @@ async fn event_fetch(&self, event_id: OwnedEventId) -> Option<PduEvent> {
 	}
 }
 
-fn check_room_id<Pdu: Event>(room_id: &RoomId, pdu: &Pdu) -> Result {
-	if pdu
-		.room_id()
-		.is_some_and(|claimed_room_id| claimed_room_id != room_id)
-	{
-		return Err!(Request(InvalidParam(error!(
-			pdu_event_id = %pdu.event_id(),
-			pdu_room_id = pdu.room_id().map(tracing::field::display),
-			%room_id,
-			"Found event from room in room",
-		))));
-	}
-
-	Ok(())
-}
-
 fn get_room_version_rules<Pdu: Event>(create_event: &Pdu) -> Result<RoomVersionRules> {
 	let content: RoomCreateEventContent = create_event.get_content()?;
 	let Some(room_version_rules) = content.room_version.rules() else {

src/service/rooms/event_handler/parse_incoming_pdu.rs

@@ -23,7 +23,12 @@ fn extract_room_id(event_type: &str, pdu: &CanonicalJsonObject) -> Result<OwnedR
 			.map_err(|e| err!(Request(BadJson("Invalid room_id {room_id:?} in pdu: {e}"))));
 	}
 	// If there's no room ID, and this is not a create event, it is illegal.
-	if event_type != "m.room.create" || pdu.get("state_key").is_none() {
+	let is_create = event_type == "m.room.create"
+		&& pdu
+			.get("state_key")
+			.and_then(|v| v.as_str())
+			.is_some_and(str::is_empty);
+	if !is_create {
 		return Err!(Request(BadJson("Missing room_id in pdu")));
 	}
 
@@ -42,10 +47,7 @@ fn extract_room_id(event_type: &str, pdu: &CanonicalJsonObject) -> Result<OwnedR
 		return Err!(Request(BadJson("Unknown room version in pdu")));
 	};
 
-	if !room_version_rules
-		.authorization
-		.room_create_event_id_as_room_id
-	{
+	if room_version_rules.event_format.require_room_create_room_id {
 		return Err!(Request(BadJson("Missing room_id in pdu")));
 	}
 
@@ -80,33 +82,10 @@ pub(super) fn expect_event_id_array(
 }
 
 impl super::Service {
-	/// Performs some basic validation on the PDU to make sure it's not
-	/// obviously malformed. This is not a full validation, but guards against
-	/// extreme errors.
-	///
-	/// Currently, this just validates that prev/auth events are within
-	/// acceptable ranges. Other servers do some additional things like
-	/// checking depth range, but serde will do that later when converting the
-	/// object to a PduEvent.
-	pub fn validate_pdu(&self, pdu: &CanonicalJsonObject) -> Result {
-		// Since v3:
-		// `event_id` should not be present on the PDU.
-		// NOTE: The above is ignored since technically it's still allowed to be
-		// included, but should be ignored instead.
-		// `auth_events` and `prev_events` must be an array of event IDs
-		let auth_events = expect_event_id_array(pdu, "auth_events")?;
-		if auth_events.len() > 10 {
-			return Err!(Request(BadJson("PDU has too many auth events")));
-		}
-		let prev_events = expect_event_id_array(pdu, "prev_events")?;
-		if prev_events.len() > 20 {
-			return Err!(Request(BadJson("PDU has too many prev events")));
-		}
-		Ok(())
-	}
-
-	pub async fn parse_incoming_pdu_with_known_room(
-		&self,
+	/// Parses an incoming PDU JSON object, generating an event ID for it. Does
+	/// not insert the event ID into the returned object. Does not discover the
+	/// room ID.
+	pub(super) fn parse_incoming_pdu_with_known_room(
 		pdu: &RawJsonValue,
 		room_version_rules: &RoomVersionRules,
 	) -> Result<(OwnedEventId, CanonicalJsonObject)> {
@@ -114,10 +93,13 @@ pub async fn parse_incoming_pdu_with_known_room(
 			gen_event_id_canonical_json(pdu, room_version_rules).map_err(|e| {
 				err!(Request(InvalidParam("Could not convert event to canonical json: {e}")))
 			})?;
-		self.validate_pdu(&value)?;
+		// NOTE: validation checks are now performed by `pdu_format_check_1`.
 		Ok((event_id, value))
 	}
 
+	/// Parses an incoming PDU JSON object, generating an event ID for it and
+	/// attempts to discover the associated room ID. Does not insert the event
+	/// ID into the returned object.
 	pub async fn parse_incoming_pdu(&self, pdu: &RawJsonValue) -> Result<Parsed> {
 		let value = serde_json::from_str::<CanonicalJsonObject>(pdu.get()).map_err(|e| {
 			err!(BadServerResponse(debug_warn!("Error parsing incoming event {e:?}")))
@@ -142,7 +124,7 @@ pub async fn parse_incoming_pdu(&self, pdu: &RawJsonValue) -> Result<Parsed> {
 			gen_event_id_canonical_json(pdu, &room_version_rules).map_err(|e| {
 				err!(Request(InvalidParam("Could not convert event to canonical json: {e}")))
 			})?;
-		self.validate_pdu(&value)?;
+		// NOTE: validation checks are now performed by `pdu_format_check_1`.
 		Ok((room_id, event_id, value))
 	}
 }

src/service/rooms/event_handler/pdu_checks.rs

@@ -0,0 +1,289 @@
+use std::collections::HashMap;
+
+use conduwuit::{
+	Err, Event, EventTypeExt, PduEvent, Result, debug, debug::DebugInspect, debug_error,
+	debug_info, err, info, matrix::StateKey, state_res, trace,
+};
+use futures::future::ready;
+use ruma::{
+	CanonicalJsonObject, EventId, OwnedEventId, ServerName, api::error::ErrorKind,
+	canonical_json::redact, events::StateEventType, room_version_rules::RoomVersionRules,
+};
+
+use crate::rooms::{
+	event_handler::parse_incoming_pdu::expect_event_id_array, timeline::pdu_fits,
+};
+
+impl super::Service {
+	/// Checks that the PDU conforms to the PDU format (check 1). This is
+	/// already mostly done during deserialisation, so this function just checks
+	/// that the PDU isn't a too large.
+	pub(super) fn pdu_format_check_1(
+		pdu_json: &CanonicalJsonObject,
+		room_version_rules: &RoomVersionRules,
+		create_event_id: &EventId,
+	) -> Result<()> {
+		let event_format = &room_version_rules.event_format;
+		// NOTE: if we do any more validation outside of deserialisation, it has to be
+		// done here.
+
+		if !pdu_fits(pdu_json) {
+			return Err!(Request(TooLarge("PDU is too large")));
+		}
+
+		if event_format.require_room_create_room_id {
+			if pdu_json.get("room_id").is_none() {
+				return Err!(Request(BadJson("Missing required PDU field: `room_id`")));
+			}
+		}
+
+		let auth_events = expect_event_id_array(pdu_json, "auth_events")?;
+		if auth_events.len() > 10 {
+			return Err!(Request(BadJson("PDU has too many auth events")));
+		}
+
+		let create_event_in_auth_events = auth_events.iter().any(|id| id == create_event_id);
+		if !event_format.allow_room_create_in_auth_events && create_event_in_auth_events {
+			return Err!(Request(BadJson("PDU references a create event")));
+		}
+
+		let prev_events = expect_event_id_array(pdu_json, "prev_events")?;
+		if prev_events.len() > 20 {
+			return Err!(Request(BadJson("PDU has too many prev events")));
+		}
+
+		Ok(())
+	}
+
+	/// Checks that the PDU has a valid signature (check 2), and redacts it if
+	/// the content hash verification fails (check 3), returning the
+	/// potentially modified JSON. Returns an error if the PDU cannot be
+	/// redacted, or fails signature verification.
+	pub(super) async fn signature_hash_check_2_3(
+		&self,
+		pdu_json: CanonicalJsonObject,
+		room_version_rules: &RoomVersionRules,
+	) -> Result<CanonicalJsonObject> {
+		match self
+			.services
+			.server_keys
+			.verify_event(&pdu_json, room_version_rules)
+			.await
+		{
+			| Ok(ruma::signatures::Verified::All) => {
+				trace!("Signatures and hashes verified successfully");
+				Ok(pdu_json)
+			},
+			| Ok(ruma::signatures::Verified::Signatures) => {
+				debug_info!("Content hash mismatch, redacting event and continuing");
+				let redacted = redact(pdu_json, &room_version_rules.redaction, None)
+					.map_err(|e| err!(Request(BadJson("Unable to redact event: {e}"))))?;
+				Ok(redacted)
+			},
+			| Err(e) => {
+				Err!(Request(Forbidden(debug_error!("Signature verification failed: {e}"))))
+			},
+		}
+	}
+
+	/// Checks PDU check 4: Passes authorisation rules based on the event's auth
+	/// events ([spec]).
+	///
+	/// If the auth check fails, false is returned, otherwise true.
+	///
+	/// [spec]: https://spec.matrix.org/v1.19/server-server-api/#checks-performed-on-receipt-of-a-pdu
+	pub(super) async fn auth_state_check_4(
+		&self,
+		pdu: &PduEvent,
+		room_version_rules: &RoomVersionRules,
+		create_event: &PduEvent,
+		auth_events_by_key: &HashMap<(StateEventType, StateKey), PduEvent>,
+	) -> Result<bool> {
+		let state_fetch = |ty: &StateEventType, sk: &str| {
+			let key = (ty.to_owned(), sk.into());
+			ready(auth_events_by_key.get(&key).map(ToOwned::to_owned))
+		};
+
+		state_res::event_auth::auth_check(
+			room_version_rules,
+			pdu,
+			None, // TODO: third party invite
+			state_fetch,
+			create_event,
+		)
+		.await
+		.map_err(|e| err!("Event self-authentication failed: {e:?}"))
+	}
+
+	/// Checks that the event passes PDU check 5, which ensures that the event
+	/// is authorised based on the state before the event (which is the resolved
+	/// state across all prev events).
+	///
+	/// Returns a boolean indicating whether the event is authorised, and also
+	/// the resolved state before the event for later use. Returns an error if
+	/// state fetching or auth checking fails.
+	pub(super) async fn state_before_check_5(
+		&self,
+		incoming_pdu: &PduEvent,
+		room_version_rules: &RoomVersionRules,
+		create_event: &PduEvent,
+		origin: &ServerName,
+	) -> Result<(bool, HashMap<u64, OwnedEventId>)> {
+		debug!(
+			event_id = %incoming_pdu.event_id,
+			"Resolving state at event"
+		);
+		let room_id = incoming_pdu.room_id_or_hash();
+
+		// If the incoming event only has one prev event, we can just use the state at
+		// that event, but otherwise we have to resolve across each fork. If we're
+		// missing even one of the prev events, we have to ask a remote server for help.
+		//
+		// TODO: this can be optimised by only loading auth chain events into memory,
+		// rather than the entire state.
+		let state_before = self
+			.state_before_incoming(&incoming_pdu, room_version_rules)
+			.await?;
+		let state_before = match state_before {
+			| Some(s) => s,
+			| None => {
+				trace!("Could not calculate incoming state, asking remote {origin} for it");
+				self.fetch_state(origin, create_event, &room_id, incoming_pdu.event_id())
+					.await
+					.inspect_err(|e| {
+						debug_error!("Could not fetch state from {origin}: {e}");
+					})?
+			},
+		};
+
+		if state_before.is_empty()
+			&& *incoming_pdu.event_type() != StateEventType::RoomCreate.into()
+		{
+			// This can happen if the remote sends an event but cannot be reached to fetch
+			// the state at it, and all other servers in the room (which might just be the
+			// unreachable server) are unable to provide required info.
+			// returning an error here allows the upgrade to be attempted at another time.
+			return Err!(Request(Forbidden("Could not resolve incoming state before event")));
+		}
+		trace!(state_events = state_before.len(), "Calculated incoming state");
+
+		let state_fetch_state = &state_before;
+		let state_fetch = |k: StateEventType, s: StateKey| async move {
+			let shortstatekey = self.services.short.get_shortstatekey(&k, &s).await.ok()?;
+
+			let event_id = state_fetch_state.get(&shortstatekey)?;
+			self.services.timeline.get_pdu(event_id).await.ok()
+		};
+
+		debug!(
+			event_id = %incoming_pdu.event_id,
+			"Running state-before auth check"
+		);
+
+		// PDU check: 5
+		let auth_check = state_res::event_auth::auth_check(
+			room_version_rules,
+			incoming_pdu,
+			None, // TODO: third party invite
+			|ty, sk| state_fetch(ty.clone(), sk.into()),
+			create_event.as_pdu(),
+		)
+		.await
+		.map_err(|e| err!(Request(Forbidden("Auth check failed: {e:?}"))))?;
+		Ok((auth_check, state_before))
+	}
+
+	/// Checks that the event passes PDU check 6, which ensures that the event
+	/// is authorised based on the room's current state (which is the resolved
+	/// state across all current forward extremities).
+	///
+	/// Returns a boolean indicating whether the event is authorised, or an
+	/// error if the auth check fails.
+	pub(super) async fn current_state_check_6(
+		&self,
+		incoming_pdu: &PduEvent,
+		room_version_rules: &RoomVersionRules,
+		create_event: &PduEvent,
+	) -> Result<bool> {
+		debug!(
+			event_id = %incoming_pdu.event_id,
+			"Gathering auth events"
+		);
+		let auth_events = self
+			.services
+			.state
+			.get_auth_events(
+				&incoming_pdu.room_id_or_hash(),
+				incoming_pdu.kind(),
+				incoming_pdu.sender(),
+				incoming_pdu.state_key(),
+				incoming_pdu.content(),
+				room_version_rules,
+			)
+			.await?;
+
+		let state_fetch = |k: &StateEventType, s: &str| {
+			let key = k.with_state_key(s);
+			ready(auth_events.get(&key).map(ToOwned::to_owned))
+		};
+
+		debug!(
+			event_id = %incoming_pdu.event_id,
+			"Running current state auth check"
+		);
+		state_res::event_auth::auth_check(
+			room_version_rules,
+			incoming_pdu,
+			None, // third-party invite
+			state_fetch,
+			create_event.as_pdu(),
+		)
+		.await
+		.map_err(|e| err!(Request(Forbidden("Auth check failed: {e:?}"))))
+	}
+
+	/// Performs PDU check 7 - does the policy server allow this event.
+	///
+	/// If the policy server forbids the event, false is returned. If there is a
+	/// problem contacting the policy server, or it returns an unrecognised
+	/// response, an appropriate error is returned.
+	pub(super) async fn policy_server_check_7(
+		&self,
+		incoming_pdu: &PduEvent,
+		pdu_json: &mut CanonicalJsonObject,
+		room_version_rules: &RoomVersionRules,
+	) -> Result<bool> {
+		let event_id = pdu_json
+			.remove("event_id")
+			.expect("event_id should be present in pdu_json at this stage");
+		if let Err(e) = self
+			.policy_server_allows_event(
+				incoming_pdu,
+				pdu_json,
+				&incoming_pdu.room_id_or_hash(),
+				room_version_rules,
+				true,
+			)
+			.await
+			.debug_inspect(|()| {
+				debug!(
+					event_id = %incoming_pdu.event_id,
+					"Event has passed policy server check."
+				);
+			}) {
+			return if matches!(e.kind(), ErrorKind::Forbidden) {
+				info!(
+					event_id = %incoming_pdu.event_id,
+					error = %e,
+					"Event has been marked as spam by policy server: {}",
+					e.message(),
+				);
+				Ok(false)
+			} else {
+				Err(e)
+			};
+		}
+		pdu_json.insert("event_id".to_owned(), event_id);
+		Ok(true)
+	}
+}

src/service/rooms/event_handler/state_at_incoming.rs

@@ -16,10 +16,31 @@
 use crate::rooms::short::ShortStateHash;
 
 impl super::Service {
-	// TODO: if we know the prev_events of the incoming event we can avoid the
-	// request and build the state from a known point and resolve if > 1 prev_event
+	/// Resolves the state before the incoming event.
+	///
+	/// If we do not know enough information to resolve the state, `Ok(None)` is
+	/// returned, and the caller will have to figure it out some other way (e.g.
+	/// by fetching the state from a remote server).
+	pub(super) async fn state_before_incoming<Pdu>(
+		&self,
+		incoming_pdu: &Pdu,
+		room_version_rules: &RoomVersionRules,
+	) -> Result<Option<HashMap<u64, OwnedEventId>>>
+	where
+		Pdu: Event + Send + Sync,
+	{
+		if incoming_pdu.prev_events().count() == 1 {
+			self.state_before_incoming_degree_one(incoming_pdu).await
+		} else {
+			self.state_before_incoming_resolved(incoming_pdu, room_version_rules)
+				.await
+		}
+	}
+
+	/// Determines the state before the incoming pdu, when it has only one prev
+	/// event. This is a special case that does not require state resolution.
 	#[tracing::instrument(name = "state", level = "debug", skip_all)]
-	pub(super) async fn state_at_incoming_degree_one<Pdu>(
+	async fn state_before_incoming_degree_one<Pdu>(
 		&self,
 		incoming_pdu: &Pdu,
 	) -> Result<Option<HashMap<u64, OwnedEventId>>>
@@ -66,7 +87,7 @@ pub(super) async fn state_at_incoming_degree_one<Pdu>(
 				.await;
 
 			state.insert(shortstatekey, prev_event.to_owned());
-			// Now it's the state after the pdu
+			// Now it's the state at the pdu
 		}
 
 		debug_assert!(!state.is_empty(), "should be returning None for empty HashMap result");
@@ -74,11 +95,14 @@ pub(super) async fn state_at_incoming_degree_one<Pdu>(
 		Ok(Some(state))
 	}
 
+	/// Resolves the state before the incoming pdu across all of its prev
+	/// events. If we do not know enough information to resolve the state,
+	/// `Ok(None)` is returned, and the caller will have to figure it out some
+	/// other way (e.g. by fetching the state from a remote server).
 	#[tracing::instrument(name = "state", level = "debug", skip_all)]
-	pub(super) async fn state_at_incoming_resolved<Pdu>(
+	async fn state_before_incoming_resolved<Pdu>(
 		&self,
 		incoming_pdu: &Pdu,
-		room_id: &RoomId,
 		room_version_rules: &RoomVersionRules,
 	) -> Result<Option<HashMap<u64, OwnedEventId>>>
 	where
@@ -108,6 +132,7 @@ pub(super) async fn state_at_incoming_resolved<Pdu>(
 		};
 
 		trace!("Calculating fork states...");
+		let room_id = &incoming_pdu.room_id_or_hash();
 		let (fork_states, auth_chain_sets): (Vec<StateMap<_>>, Vec<HashSet<_>>) =
 			extremity_sstatehashes
 				.into_iter()
@@ -145,6 +170,8 @@ pub(super) async fn state_at_incoming_resolved<Pdu>(
 			.await
 	}
 
+	/// Determines the state at an incoming fork (aka the state at a prev
+	/// event), returning the resolved state and its associated auth chain.
 	async fn state_at_incoming_fork<Pdu>(
 		&self,
 		room_id: &RoomId,

src/service/rooms/event_handler/upgrade_outlier_pdu.rs

@@ -1,27 +1,15 @@
-use std::{borrow::Borrow, sync::Arc, time::Instant};
+use std::time::Instant;
 
 use conduwuit::{
-	Err, Result, debug, debug_error, debug_info, err, info, is_equal_to,
-	matrix::{Event, EventTypeExt, PduEvent, StateKey, state_res},
-	result::DebugInspect,
+	Err, Result, debug, debug_info, debug_warn, is_true,
+	matrix::{Event, PduEvent},
 	trace,
-	utils::{
-		IterStream,
-		stream::{BroadbandExt, ReadyExt},
-	},
-	warn,
-};
-use futures::{FutureExt, StreamExt, future::ready};
-use ruma::{
-	CanonicalJsonObject, RoomId, ServerName, api::error::ErrorKind, events::StateEventType,
 };
+use ruma::{CanonicalJsonObject, RoomId, ServerName, events::StateEventType};
 use tokio::join;
 
 use super::get_room_version_rules;
-use crate::rooms::{
-	state_compressor::{CompressedState, HashSetCompressStateEvent},
-	timeline::RawPduId,
-};
+use crate::rooms::timeline::RawPduId;
 
 impl super::Service {
 	#[tracing::instrument(name="upgrade_outlier", skip_all, fields(event_id=%incoming_pdu.event_id()))]
@@ -46,16 +34,24 @@ pub(super) async fn upgrade_outlier_to_timeline_pdu(
 			trace!(event_id=%incoming_pdu.event_id(), "Skipping upgrade of already upgraded PDU");
 			return Ok(Some(id));
 		} else if rejected {
-			return Err!(Request(Forbidden("Event has been rejected")));
+			return Err!(Request(Forbidden(debug_info!("Event has been rejected"))));
 		} else if soft_failed {
-			return Err!(Request(Forbidden("Event has been soft-failed")));
+			// Soft-failed events cannot be promoted.
+			return Err!(Request(Forbidden(debug_info!("Event has been soft-failed"))));
 		}
 
+		// These should never happen, but they're good last-minute sanity checks to
+		// ensure we never promote totally illegal events.
 		assert_eq!(
 			*create_event.kind(),
 			StateEventType::RoomCreate.into(),
 			"tried to upgrade a PDU with a create_event that is not a room create event"
 		);
+		assert_eq!(
+			incoming_pdu.room_id_or_hash(),
+			*room_id,
+			"room ID mismatch: PDU room ID differs from parameter"
+		);
 
 		debug!(
 			event_id = %incoming_pdu.event_id,
@@ -65,326 +61,118 @@ pub(super) async fn upgrade_outlier_to_timeline_pdu(
 		let min_depth = self.services.metadata.get_mindepth(room_id).await;
 		let room_version_rules = get_room_version_rules(create_event)?;
 
-		// 10. Fetch missing state and auth chain events by calling /state_ids at
-		//     backwards extremities doing all the checks in this list starting at 1.
-		//     These are not timeline events.
-
-		debug!(
-			event_id = %incoming_pdu.event_id,
-			"Resolving state at event"
-		);
-		let state_at_incoming_event = if incoming_pdu.prev_events().count() == 1 {
-			self.state_at_incoming_degree_one(&incoming_pdu).await?
-		} else {
-			self.state_at_incoming_resolved(&incoming_pdu, room_id, &room_version_rules)
-				.await?
-		};
-		let state_at_incoming_event = match state_at_incoming_event {
-			| Some(s) => s,
-			| None => {
-				trace!("Could not calculate incoming state, asking remote {origin} for it");
-				self.fetch_state(origin, create_event, room_id, incoming_pdu.event_id())
-					.await
-					.debug_inspect_err(|e| {
-						debug_error!("Could not fetch state from {origin}: {e}");
-					})?
-			},
-		};
-
-		if state_at_incoming_event.is_empty()
-			&& *incoming_pdu.event_type() != StateEventType::RoomCreate.into()
-		{
-			// This can happen if the remote sends an event but cannot be reached to fetch
-			// the state at it, and all other servers in the room (which might just be the
-			// unreachable server) are unable to provide required info.
-			// returning an error here allows the upgrade to be attempted at another time.
-			return Err!(Request(Forbidden("Could not resolve incoming state at event")));
-		}
-		trace!(state_events = state_at_incoming_event.len(), "Calculated incoming state");
-
-		debug!(
-			event_id = %incoming_pdu.event_id,
-			"Performing auth check to upgrade"
-		);
-		// 11. Check the auth of the event passes based on the state of the event
-		let state_fetch_state = &state_at_incoming_event;
-		let state_fetch = |k: StateEventType, s: StateKey| async move {
-			let shortstatekey = self.services.short.get_shortstatekey(&k, &s).await.ok()?;
-
-			let event_id = state_fetch_state.get(&shortstatekey)?;
-			self.services.timeline.get_pdu(event_id).await.ok()
-		};
-
-		debug!(
-			event_id = %incoming_pdu.event_id,
-			"Running initial auth check"
-		);
-		// PDU check: 5
-		let auth_check = state_res::event_auth::auth_check(
-			&room_version_rules,
-			&incoming_pdu,
-			None, // TODO: third party invite
-			|ty, sk| state_fetch(ty.clone(), sk.into()),
-			create_event.as_pdu(),
-		)
-		.await
-		.map_err(|e| err!(Request(Forbidden("Auth check failed: {e:?}"))))?;
-
-		if !auth_check {
-			self.services
-				.pdu_metadata
-				.mark_event_rejected(incoming_pdu.event_id());
-			return Err!(Request(Forbidden(
-				"Event authorisation fails based on the state before the event"
-			)));
-		}
-
-		debug!(
-			event_id = %incoming_pdu.event_id,
-			"Gathering auth events"
-		);
-		let auth_events = self
-			.services
-			.state
-			.get_auth_events(
-				room_id,
-				incoming_pdu.kind(),
-				incoming_pdu.sender(),
-				incoming_pdu.state_key(),
-				incoming_pdu.content(),
-				&room_version_rules,
-			)
+		// We now need to resolve the state before the event so that we can perform PDU
+		// check 5 (event auth passes based on state before the event). To do this, we
+		// either need to have all the prev events locally, or ask a remote server
+		// for the state at the event.
+		let (passes_state_before, state_before) = self
+			.state_before_check_5(&incoming_pdu, &room_version_rules, create_event, origin)
 			.await?;
 
-		let state_fetch = |k: &StateEventType, s: &str| {
-			let key = k.with_state_key(s);
-			ready(auth_events.get(&key).map(ToOwned::to_owned))
-		};
-
-		debug!(
-			event_id = %incoming_pdu.event_id,
-			"Running auth check with claimed state auth"
-		);
-		// PDU check: 6
-		let auth_check = state_res::event_auth::auth_check(
-			&room_version_rules,
-			&incoming_pdu,
-			None, // third-party invite
-			state_fetch,
-			create_event.as_pdu(),
-		)
-		.await
-		.map_err(|e| err!(Request(Forbidden("Auth check failed: {e:?}"))))?;
-		if !auth_check {
-			warn!(
-				event_id = %incoming_pdu.event_id,
-				"Event authorization fails based on the current state of the room"
-			);
+		if !passes_state_before {
+			self.reject_and_persist(incoming_pdu.event_id(), &val);
+			return Err!(Request(Forbidden(debug_warn!(
+				"Event authorisation fails based on the state before the event"
+			))));
 		}
 
-		// Soft fail check before doing state res
-		debug!(
-			event_id = %incoming_pdu.event_id,
-			"Performing soft-fail check"
-		);
-		let mut soft_fail = match (auth_check, incoming_pdu.redacts_id(&room_version_rules)) {
-			| (false, _) => true,
-			| (true, None) => false,
-			| (true, Some(redact_id)) => {
-				if !self
-					.services
-					.state_accessor
-					.user_can_redact(&redact_id, incoming_pdu.sender(), room_id, true)
-					.await?
-				{
-					warn!(redacts = %redact_id, "User is not allowed to redact event");
-					true
-				} else {
-					false
-				}
-			},
-		};
+		// Now that we know the event passes both self-authentication, and
+		// authentication based on the state before the event, we need to check that it
+		// passes based on the *current* room state (state across all forward
+		// extremities). If it doesn't, we accept it, but soft-fail it, and this
+		// prevents it being promoted.
 
-		// 13. Use state resolution to find new room state
-		// We start looking at current room state now, so lets lock the room
+		// We lock the room here to prevent the current state from changing beneath us
+		// mid-check.
 		trace!(
 			room_id = %room_id,
 			"Locking the room"
 		);
 		let state_lock = self.services.state.mutex.lock(room_id).await;
-
-		let state_ids_compressed: Arc<CompressedState> = self
-			.services
-			.state_compressor
-			.compress_state_events(
-				state_at_incoming_event
-					.iter()
-					.map(|(ssk, eid)| (ssk, eid.borrow())),
-			)
-			.collect()
-			.map(Arc::new)
-			.await;
-
-		if incoming_pdu.state_key().is_some() {
-			debug!("Event is a state-event. Deriving new room state");
-
-			// We also add state after incoming event to the fork states
-			let mut state_after = state_at_incoming_event.clone();
-			if let Some(state_key) = incoming_pdu.state_key() {
-				let shortstatekey = self
-					.services
-					.short
-					.get_or_create_shortstatekey(
-						&incoming_pdu.kind().to_string().into(),
-						state_key,
-					)
-					.await;
-
-				let event_id = incoming_pdu.event_id();
-				state_after.insert(shortstatekey, event_id.to_owned());
-			}
-
-			let new_room_state = self
-				.resolve_state(room_id, &room_version_rules, state_after)
-				.await?;
-
-			// Set the new room state to the resolved state
-			debug!("Forcing new room state");
-			let HashSetCompressStateEvent { shortstatehash, added, removed } = self
-				.services
-				.state_compressor
-				.save_state(room_id, new_room_state)
-				.await?;
-
-			self.services
-				.state
-				.force_state(room_id, shortstatehash, added, removed, &state_lock)
-				.await?;
-		}
-
-		if !soft_fail {
-			// Don't call the below checks on events that have already soft-failed, there's
-			// no reason to re-calculate that.
-			// 14-pre. ask the policy server to sign the event, if possible
-			debug!(event_id = %incoming_pdu.event_id, "Checking policy server for event");
-			let tmp_evt_id = val.remove("event_id");
-			if let Err(e) = self
-				.policy_server_allows_event(
-					&incoming_pdu,
-					&mut val,
-					room_id,
-					&room_version_rules,
-					true,
-				)
-				.await
-			{
-				if matches!(e.kind(), ErrorKind::Forbidden) {
-					info!(
-						event_id = %incoming_pdu.event_id,
-						error = %e,
-						"Event has been marked as spam by policy server: {}",
-						e.message(),
+		let passes_current_state = self
+			.current_state_check_6(&incoming_pdu, &room_version_rules, create_event)
+			.await
+			.inspect(|passes| {
+				if !*passes {
+					debug_warn!(
+						"Event authorisation fails based on the current room state - will be \
+						 soft-failed"
 					);
-					soft_fail = true;
-				} else {
-					return Err(e);
 				}
-			} else {
-				debug!(
-					event_id = %incoming_pdu.event_id,
-					"Event has passed policy server check."
-				);
-			}
-			if let Some(id) = tmp_evt_id {
-				val.insert("event_id".to_owned(), id);
-			}
+			})?;
 
-			// Additionally, if this is a redaction for a soft-failed event, we soft-fail it
-			// also.
+		// Determine whether this PDU should be soft-failed.
+		// If the auth check failed, invariably yes. Otherwise, only if the user isn't
+		// allowed to redact the target event (if any).
+		let mut should_soft_fail =
+			match (passes_current_state, incoming_pdu.redacts_id(&room_version_rules)) {
+				| (false, _) => true,
+				| (true, None) => false,
+				| (true, Some(redact_id)) => self
+					.services
+					.state_accessor
+					.user_can_redact(&redact_id, incoming_pdu.sender(), room_id, true)
+					.await
+					.is_ok_and(is_true!()),
+			};
 
-			// TODO: this is supposed to hide redactions from policy servers, however, for
-			// full efficacy it also needs to hide redactions for unknown events. This
-			// needs to be investigated at a later time.
+		if !should_soft_fail {
+			// Now we can perform check 7, which is ensuring the event passes policy server
+			// checks.
+			// We explicitly only do this if we aren't already going to soft-fail the event,
+			// since the policy server refusing this event also soft-fails it.
+			debug!(event_id = %incoming_pdu.event_id, "Checking policy server for event");
+			should_soft_fail = !self
+				.policy_server_check_7(&incoming_pdu, &mut val, &room_version_rules)
+				.await
+				.inspect(|passes| {
+					if !*passes {
+						debug_warn!(
+							"Event did not pass the policy server check and will be soft-failed"
+						);
+					}
+				})?;
+
+			// TODO: this is supposed to hide redactions from policy servers and janitorial
+			// bots, however, for full efficacy it also needs to hide redactions for
+			// unknown events. This needs to be investigated at a later time.
 			if let Some(redact_id) = incoming_pdu.redacts_id(&room_version_rules) {
 				debug!(
 					redact_id = %redact_id,
-					"Checking if redaction is for a soft-failed event"
+					"Checking if redaction is for a soft-failed/rejected event"
 				);
-				if self
+				if !self
 					.services
 					.pdu_metadata
-					.is_event_soft_failed(&redact_id)
+					.is_event_accepted(&redact_id)
 					.await
 				{
-					info!(
-						redact_id = %redact_id,
-						"Redaction is for a soft-failed event"
+					debug_info!(
+						"Soft-failing valid redaction because it targets a non-accepted event"
 					);
-					soft_fail = true;
+					should_soft_fail = true;
 				}
 			}
 		}
 
+		// The PDU has now passed all checks! We can now promote it (or soft-fail it if
+		// the verdict is such).
 		trace!("Appending pdu to timeline");
-		let mut extremities: Vec<_> = self
-			.services
-			.state
-			.get_forward_extremities(room_id)
-			.collect()
-			.await;
-		if !soft_fail {
-			// Per https://spec.matrix.org/unstable/server-server-api/#soft-failure, soft-failed events
-			// are not added as forward extremities.
-
-			// Now we calculate the set of extremities this room has after the incoming
-			// event has been applied. We start with the previous extremities (aka leaves)
-			trace!("Calculating extremities");
-			extremities = extremities
-				.into_iter()
-				.stream()
-				.ready_filter(|event_id| {
-					// Remove any that are referenced by this incoming event's prev_events
-					!incoming_pdu.prev_events().any(is_equal_to!(event_id))
-				})
-				.broad_filter_map(|event_id| async move {
-					// Only keep those extremities were not referenced yet
-					self.services
-						.pdu_metadata
-						.is_event_referenced(room_id, &event_id)
-						.await
-						.eq(&false)
-						.then_some(event_id)
-				})
-				.collect::<Vec<_>>()
-				.await;
-			extremities.push(incoming_pdu.event_id().to_owned());
-			debug!(
-				"Retained {} extremities checked against {} prev_events",
-				extremities.len(),
-				incoming_pdu.prev_events().count()
-			);
-			assert!(!extremities.is_empty(), "extremities must not empty");
-		}
-
 		let pdu_id = self
 			.services
 			.timeline
 			.append_incoming_pdu(
 				&incoming_pdu,
 				val,
-				extremities.iter().map(Borrow::borrow),
-				state_ids_compressed,
-				soft_fail,
+				&room_version_rules,
+				state_before,
+				should_soft_fail,
 				&state_lock,
-				room_id,
 			)
 			.await?;
-		if soft_fail {
-			self.services
-				.pdu_metadata
-				.mark_event_soft_failed(incoming_pdu.event_id());
 
-			info!(
+		if should_soft_fail {
+			debug_info!(
 				elapsed = ?timer.elapsed(),
 				event_id = %incoming_pdu.event_id,
 				"Event was soft failed"

src/service/rooms/membership/mod.rs

@@ -520,7 +520,7 @@ pub async fn join_remote_room(
 						return state;
 					},
 				};
-				if !pdu_fits(&mut value.clone()) {
+				if !pdu_fits(&value.clone()) {
 					warn!(
 						"dropping incoming PDU {event_id} in room {room_id} from room join \
 						 because it exceeds 65535 bytes or is otherwise too large."

src/service/rooms/timeline/append.rs

@@ -1,69 +1,113 @@
 use std::{
-	collections::{BTreeMap, HashSet},
+	borrow::Borrow,
+	collections::{BTreeMap, HashMap, HashSet},
 	sync::Arc,
 };
 
 use conduwuit::{
-	debug_warn,
+	Result, debug, debug_warn, is_equal_to,
+	matrix::{Event, PduEvent},
 	pdu::{Count, ShortRoomId},
 	trace,
-	utils::{IterStream, TryFutureExtExt, stream::BroadbandExt},
+	utils::{
+		IterStream, TryFutureExtExt,
+		mutex_map::Guard,
+		stream::{BroadbandExt, ReadyExt},
+	},
 	warn,
 };
 use conduwuit_core::{
-	Result, err, error,
-	matrix::{
-		event::Event,
-		pdu::{PduCount, PduEvent, PduId, RawPduId},
-	},
-	utils::{self, ReadyExt},
+	err, error,
+	matrix::pdu::{PduCount, PduId, RawPduId},
+	utils::{self},
 };
-use futures::{StreamExt, TryFutureExt};
+use futures::{FutureExt, StreamExt, TryFutureExt};
 use ruma::{
-	CanonicalJsonObject, CanonicalJsonValue, EventId, RoomId, RoomVersionId, UserId,
+	CanonicalJsonObject, CanonicalJsonValue, EventId, OwnedEventId, OwnedRoomId, RoomId,
+	RoomVersionId, UserId,
 	events::{
 		GlobalAccountDataEventType, TimelineEventType,
 		push_rules::PushRulesEvent,
 		room::{encrypted::Relation, redaction::RoomRedactionEventContent},
 	},
 	push::{Action, Ruleset, Tweak},
+	room_version_rules::RoomVersionRules,
 };
 
 use super::{ExtractBody, ExtractRelatesTo, ExtractRelatesToEventId, RoomMutexGuard};
-use crate::{appservice::RegistrationInfo, rooms::state_compressor::CompressedState};
+use crate::{
+	appservice::RegistrationInfo,
+	rooms::state_compressor::{CompressedState, HashSetCompressStateEvent},
+};
 
 impl super::Service {
 	/// Append the incoming event setting the state snapshot to the state from
 	/// the server that sent the event.
-	#[allow(clippy::too_many_arguments)]
-	pub async fn append_incoming_pdu<'a, Leaves>(
+	pub async fn append_incoming_pdu<'a>(
 		&'a self,
 		pdu: &'a PduEvent,
 		pdu_json: CanonicalJsonObject,
-		new_room_leaves: Leaves,
-		state_ids_compressed: Arc<CompressedState>,
-		soft_fail: bool,
+		room_version_rules: &RoomVersionRules,
+		state_before: HashMap<u64, OwnedEventId>,
+		should_soft_fail: bool,
 		state_lock: &'a RoomMutexGuard,
-		room_id: &'a RoomId,
-	) -> Result<Option<RawPduId>>
-	where
-		Leaves: Iterator<Item = &'a EventId> + Send + 'a,
-	{
+	) -> Result<Option<RawPduId>> {
+		let room_id = pdu.room_id_or_hash();
+
+		let sb = state_before.iter().map(|(ssk, eid)| (ssk, eid.as_ref()));
+		let state_ids_compressed: Arc<CompressedState> = self
+			.services
+			.state_compressor
+			.compress_state_events(sb)
+			.collect()
+			.map(Arc::new)
+			.await;
+
 		// We append to state before appending the pdu, so we don't have a moment in
 		// time with the pdu without it's state. This is okay because append_pdu can't
 		// fail.
 		self.services
 			.state
-			.set_event_state(&pdu.event_id, room_id, state_ids_compressed)
+			.set_event_state(&pdu.event_id, &room_id, state_ids_compressed)
 			.await?;
 
-		if soft_fail {
+		if should_soft_fail {
 			// Nothing else to do with a soft-failed event.
+			self.services
+				.pdu_metadata
+				.mark_event_soft_failed(pdu.event_id());
 			return Ok(None);
 		}
 
+		// Per https://spec.matrix.org/unstable/server-server-api/#soft-failure, soft-failed events
+		// are not added as forward extremities.
+		// This also means we set the state here.
+		// We do this BEFORE setting the extremities so that there's never a point in
+		// time where we have fresh extremities referencing stale state.
+		let mut extremities: Vec<_> = self
+			.services
+			.state
+			.get_forward_extremities(&room_id)
+			.collect()
+			.await;
+		extremities = self
+			.progress_state_and_extremities(
+				pdu,
+				room_version_rules,
+				state_before.clone(),
+				extremities,
+				state_lock,
+			)
+			.await?;
+
 		let pdu_id = self
-			.append_pdu(pdu, pdu_json, new_room_leaves, state_lock, room_id)
+			.append_pdu(
+				pdu,
+				pdu_json,
+				extremities.iter().map(Borrow::borrow),
+				state_lock,
+				&room_id,
+			)
 			.await?;
 
 		// Process admin commands for federation events
@@ -86,11 +130,110 @@ pub async fn append_incoming_pdu<'a, Leaves>(
 			}
 		}
 
-		self.services.sync.wake_all_joined(room_id).await;
+		self.services.sync.wake_all_joined(&room_id).await;
 
 		Ok(Some(pdu_id))
 	}
 
+	/// Derives new room state from the incoming event and filters forward
+	/// extremities accordingly. Does not set forward extremities.
+	///
+	/// Only call this function if the incoming PDU is not soft-failed or
+	/// rejected.
+	async fn progress_state_and_extremities(
+		&self,
+		incoming_pdu: &PduEvent,
+		room_version_rules: &RoomVersionRules,
+		state_before: HashMap<u64, OwnedEventId>,
+		forward_extremities: Vec<OwnedEventId>,
+		state_lock: &Guard<OwnedRoomId, ()>,
+	) -> Result<Vec<OwnedEventId>> {
+		if incoming_pdu.state_key().is_some() {
+			debug!("Event is a state-event. Deriving new room state");
+			self.derive_new_state(incoming_pdu, room_version_rules, state_before, state_lock)
+				.await?;
+		}
+
+		// Now we calculate the set of extremities this room has after the incoming
+		// event has been applied. We start with the previous extremities
+		trace!("Calculating extremities");
+		let mut forward_extremities = forward_extremities
+			.into_iter()
+			.stream()
+			.ready_filter(|event_id| {
+				// Remove any that are referenced by this incoming event's prev_events
+				!incoming_pdu.prev_events().any(is_equal_to!(event_id))
+			})
+			.broad_filter_map(|event_id| async move {
+				// Only keep those extremities were not referenced yet
+				self.services
+					.pdu_metadata
+					.is_event_referenced(&incoming_pdu.room_id_or_hash(), &event_id)
+					.await
+					.eq(&false)
+					.then_some(event_id)
+			})
+			.collect::<Vec<_>>()
+			.await;
+		forward_extremities.push(incoming_pdu.event_id().to_owned());
+		debug!(
+			"Retained {} extremities checked against {} prev_events",
+			forward_extremities.len(),
+			incoming_pdu.prev_events().count()
+		);
+		assert!(!forward_extremities.is_empty(), "resolved extremities cannot be empty");
+		Ok(forward_extremities)
+	}
+
+	/// Derives a new room state by adding the incoming PDU to the state before
+	/// it to create the state at, which then becomes the current room state.
+	///
+	/// The caller MUST ensure forward extremities are set appropriately,
+	/// including this incoming pdu, either before or after calling this
+	/// function. Failing to do so will result in an inconsistent current state
+	/// cache, which may affect event authentication.
+	async fn derive_new_state(
+		&self,
+		incoming_pdu: &PduEvent,
+		room_version_rules: &RoomVersionRules,
+		state_before: HashMap<u64, OwnedEventId>,
+		state_lock: &Guard<OwnedRoomId, ()>,
+	) -> Result {
+		let room_id = incoming_pdu.room_id_or_hash();
+		// We also add state after incoming event to the fork states
+		let mut state_at_incoming_event = state_before;
+		let shortstatekey = self
+			.services
+			.short
+			.get_or_create_shortstatekey(
+				&incoming_pdu.kind().to_string().into(),
+				incoming_pdu.state_key().unwrap(),
+			)
+			.await;
+
+		let event_id = incoming_pdu.event_id();
+		state_at_incoming_event.insert(shortstatekey, event_id.to_owned());
+
+		debug!("Resolving new room state");
+		let new_room_state = self
+			.services
+			.event_handler
+			.resolve_state(&room_id, room_version_rules, state_at_incoming_event)
+			.await?;
+
+		debug!("Forcing new room state");
+		let HashSetCompressStateEvent { shortstatehash, added, removed } = self
+			.services
+			.state_compressor
+			.save_state(&room_id, new_room_state)
+			.await?;
+
+		self.services
+			.state
+			.force_state(&room_id, shortstatehash, added, removed, state_lock)
+			.await
+	}
+
 	/// Populates the unsigned data of a PDU
 	async fn populate_unsigned(&self, pdu: &PduEvent, pdu_json: &mut CanonicalJsonObject) {
 		let Some(state_key) = pdu.state_key() else {

src/service/rooms/timeline/backfill.rs

@@ -143,7 +143,7 @@ pub async fn get_remote_pdu(&self, room_id: &RoomId, event_id: &EventId) -> Resu
 				| Ok(value) => {
 					self.services
 						.event_handler
-						.handle_incoming_pdu(&backfill_server, room_id, event_id, value, false)
+						.handle_incoming_pdu(&backfill_server, room_id, event_id, value, true)
 						.boxed()
 						.await?;
 					debug!("Successfully backfilled {event_id} from {backfill_server}");
@@ -185,7 +185,7 @@ pub async fn backfill_pdu(&self, origin: &ServerName, pdu: Box<RawJsonValue>) ->
 
 		self.services
 			.event_handler
-			.handle_incoming_pdu(origin, &room_id, &event_id, value, false)
+			.handle_incoming_pdu(origin, &room_id, &event_id, value, true)
 			.boxed()
 			.await?;
 

src/service/rooms/timeline/create.rs

@@ -22,7 +22,9 @@
 
 use super::RoomMutexGuard;
 
-pub fn pdu_fits(owned_obj: &mut CanonicalJsonObject) -> bool {
+/// Ensures the given PDU fits inside the size limits for a PDU.
+#[must_use]
+pub fn pdu_fits(owned_obj: &CanonicalJsonObject) -> bool {
 	// room IDs, event IDs, senders, types, and state keys must all be <= 255 bytes
 	if let Some(CanonicalJsonValue::String(room_id)) = owned_obj.get("room_id") {
 		if room_id.len() > 255 {
@@ -311,7 +313,7 @@ pub async fn create_hash_and_sign_event(
 		pdu_json
 			.insert("event_id".into(), CanonicalJsonValue::String(pdu.event_id.clone().into()));
 		// Verify that the *full* PDU isn't over 64KiB.
-		if !pdu_fits(&mut pdu_json.clone()) {
+		if !pdu_fits(&pdu_json.clone()) {
 			// feckin huge PDU mate
 			return Err!(Request(TooLarge("Message/PDU is too long (exceeds 65535 bytes)")));
 		}

src/service/rooms/timeline/mod.rs

@@ -77,6 +77,7 @@ struct Services {
 	short: Dep<rooms::short::Service>,
 	state: Dep<rooms::state::Service>,
 	state_accessor: Dep<rooms::state_accessor::Service>,
+	state_compressor: Dep<rooms::state_compressor::Service>,
 	state_cache: Dep<rooms::state_cache::Service>,
 	sync: Dep<sync::Service>,
 	threads: Dep<rooms::threads::Service>,
@@ -110,6 +111,8 @@ fn build(args: crate::Args<'_>) -> Result<Arc<Self>> {
 				state: args.depend::<rooms::state::Service>("rooms::state"),
 				state_accessor: args
 					.depend::<rooms::state_accessor::Service>("rooms::state_accessor"),
+				state_compressor: args
+					.depend::<rooms::state_compressor::Service>("rooms::state_compressor"),
 				state_cache: args.depend::<rooms::state_cache::Service>("rooms::state_cache"),
 				sync: args.depend::<sync::Service>("sync"),
 				threads: args.depend::<rooms::threads::Service>("rooms::threads"),