Add event size checks similar to Synapse (#3140)

Companion to https://github.com/matrix-org/gomatrixserverlib/pull/400
This tries to mimic the logic found in Synapse, as dropping events can
break rooms (and we may end up in endless loops..)

2 files changed, 74 insertions, 12 deletions

diff --git a/roomserver/internal/input/input_events.go b/roomserver/internal/input/input_events.go
index db3c9550..93f6cc01 100644
--- a/roomserver/internal/input/input_events.go
+++ b/roomserver/internal/input/input_events.go
@@ -250,6 +250,21 @@ func (r *Inputer) processRoomEvent(
 				// really do anything with the event other than reject it at this point.
 				isRejected = true
 				rejectionErr = fmt.Errorf("missingState.processEventWithMissingState: %w", err)
+				switch e := err.(type) {
+				case gomatrixserverlib.EventValidationError:
+					if e.Persistable && stateSnapshot != nil {
+						// We retrieved some state and we ended up having to call /state_ids for
+						// the new event in question (probably because closing the gap by using
+						// /get_missing_events didn't do what we hoped) so we'll instead overwrite
+						// the state snapshot with the newly resolved state.
+						missingPrev = false
+						input.HasState = true
+						input.StateEventIDs = make([]string, 0, len(stateSnapshot.StateEvents))
+						for _, se := range stateSnapshot.StateEvents {
+							input.StateEventIDs = append(input.StateEventIDs, se.EventID())
+						}
+					}
+				}
 			} else if stateSnapshot != nil {
 				// We retrieved some state and we ended up having to call /state_ids for
 				// the new event in question (probably because closing the gap by using
diff --git a/roomserver/internal/input/input_missing.go b/roomserver/internal/input/input_missing.go
index 7ee84e4c..5b4c0727 100644
--- a/roomserver/internal/input/input_missing.go
+++ b/roomserver/internal/input/input_missing.go
@@ -259,12 +259,20 @@ func (t *missingStateReq) lookupResolvedStateBeforeEvent(ctx context.Context, e
 	// Therefore, we cannot just query /state_ids with this event to get the state before. Instead, we need to query
 	// the state AFTER all the prev_events for this event, then apply state resolution to that to get the state before the event.
 	var states []*respState
+	var validationError error
 	for _, prevEventID := range e.PrevEventIDs() {
 		// Look up what the state is after the backward extremity. This will either
 		// come from the roomserver, if we know all the required events, or it will
 		// come from a remote server via /state_ids if not.
 		prevState, trustworthy, err := t.lookupStateAfterEvent(ctx, roomVersion, e.RoomID(), prevEventID)
-		if err != nil {
+		switch err2 := err.(type) {
+		case gomatrixserverlib.EventValidationError:
+			if !err2.Persistable {
+				return nil, err2
+			}
+			validationError = err2
+		case nil:
+		default:
 			return nil, fmt.Errorf("t.lookupStateAfterEvent: %w", err)
 		}
 		// Append the state onto the collected state. We'll run this through the
@@ -311,12 +319,19 @@ func (t *missingStateReq) lookupResolvedStateBeforeEvent(ctx context.Context, e
 		t.roomsMu.Lock(e.RoomID())
 		resolvedState, err = t.resolveStatesAndCheck(ctx, roomVersion, respStates, e)
 		t.roomsMu.Unlock(e.RoomID())
-		if err != nil {
+		switch err2 := err.(type) {
+		case gomatrixserverlib.EventValidationError:
+			if !err2.Persistable {
+				return nil, err2
+			}
+			validationError = err2
+		case nil:
+		default:
 			return nil, fmt.Errorf("t.resolveStatesAndCheck: %w", err)
 		}
 	}
 
-	return resolvedState, nil
+	return resolvedState, validationError
 }
 
 // lookupStateAfterEvent returns the room state after `eventID`, which is the state before eventID with the state of `eventID` (if it's a state event)
@@ -339,8 +354,15 @@ func (t *missingStateReq) lookupStateAfterEvent(ctx context.Context, roomVersion
 	}
 
 	// fetch the event we're missing and add it to the pile
+	var validationError error
 	h, err := t.lookupEvent(ctx, roomVersion, roomID, eventID, false)
-	switch err.(type) {
+	switch e := err.(type) {
+	case gomatrixserverlib.EventValidationError:
+		if !e.Persistable {
+			logrus.WithContext(ctx).WithError(err).Errorf("Failed to look up event %s", eventID)
+			return nil, false, e
+		}
+		validationError = e
 	case verifySigError:
 		return respState, false, nil
 	case nil:
@@ -365,7 +387,7 @@ func (t *missingStateReq) lookupStateAfterEvent(ctx context.Context, roomVersion
 		}
 	}
 
-	return respState, false, nil
+	return respState, false, validationError
 }
 
 func (t *missingStateReq) cacheAndReturn(ev gomatrixserverlib.PDU) gomatrixserverlib.PDU {
@@ -481,6 +503,7 @@ func (t *missingStateReq) resolveStatesAndCheck(ctx context.Context, roomVersion
 		return nil, err
 	}
 	// apply the current event
+	var validationError error
 retryAllowedState:
 	if err = checkAllowedByState(backwardsExtremity, resolvedStateEvents, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 		return t.inputer.Queryer.QueryUserIDForSender(ctx, roomID, senderID)
@@ -488,7 +511,12 @@ retryAllowedState:
 		switch missing := err.(type) {
 		case gomatrixserverlib.MissingAuthEventError:
 			h, err2 := t.lookupEvent(ctx, roomVersion, backwardsExtremity.RoomID(), missing.AuthEventID, true)
-			switch err2.(type) {
+			switch e := err2.(type) {
+			case gomatrixserverlib.EventValidationError:
+				if !e.Persistable {
+					return nil, e
+				}
+				validationError = e
 			case verifySigError:
 				return &parsedRespState{
 					AuthEvents:  authEventList,
@@ -509,7 +537,7 @@ retryAllowedState:
 	return &parsedRespState{
 		AuthEvents:  authEventList,
 		StateEvents: resolvedStateEvents,
-	}, nil
+	}, validationError
 }
 
 // get missing events for `e`. If `isGapFilled`=true then `newEvents` contains all the events to inject,
@@ -779,7 +807,11 @@ func (t *missingStateReq) lookupMissingStateViaStateIDs(ctx context.Context, roo
 		// Define what we'll do in order to fetch the missing event ID.
 		fetch := func(missingEventID string) {
 			h, herr := t.lookupEvent(ctx, roomVersion, roomID, missingEventID, false)
-			switch herr.(type) {
+			switch e := herr.(type) {
+			case gomatrixserverlib.EventValidationError:
+				if !e.Persistable {
+					return
+				}
 			case verifySigError:
 				return
 			case nil:
@@ -869,6 +901,8 @@ func (t *missingStateReq) lookupEvent(ctx context.Context, roomVersion gomatrixs
 	}
 	var event gomatrixserverlib.PDU
 	found := false
+	var validationError error
+serverLoop:
 	for _, serverName := range t.servers {
 		reqctx, cancel := context.WithTimeout(ctx, time.Second*30)
 		defer cancel()
@@ -886,12 +920,25 @@ func (t *missingStateReq) lookupEvent(ctx context.Context, roomVersion gomatrixs
 			continue
 		}
 		event, err = verImpl.NewEventFromUntrustedJSON(txn.PDUs[0])
-		if err != nil {
+		switch e := err.(type) {
+		case gomatrixserverlib.EventValidationError:
+			// If the event is persistable, e.g. failed validation for exceeding
+			// byte sizes, we can "accept" the event.
+			if e.Persistable {
+				validationError = e
+				found = true
+				break serverLoop
+			}
+			// If we can't persist the event, we probably can't do so with results
+			// from other servers, so also break the loop.
+			break serverLoop
+		case nil:
+			found = true
+			break serverLoop
+		default:
 			t.log.WithError(err).WithField("missing_event_id", missingEventID).Warnf("Failed to parse event JSON of event returned from /event")
 			continue
 		}
-		found = true
-		break
 	}
 	if !found {
 		t.log.WithField("missing_event_id", missingEventID).Warnf("Failed to get missing /event for event ID from %d server(s)", len(t.servers))
@@ -903,7 +950,7 @@ func (t *missingStateReq) lookupEvent(ctx context.Context, roomVersion gomatrixs
 		t.log.WithError(err).Warnf("Couldn't validate signature of event %q from /event", event.EventID())
 		return nil, verifySigError{event.EventID(), err}
 	}
-	return t.cacheAndReturn(event), nil
+	return t.cacheAndReturn(event), validationError
 }
 
 func checkAllowedByState(e gomatrixserverlib.PDU, stateEvents []gomatrixserverlib.PDU, userIDForSender spec.UserIDForSender) error {