Sorta transplanted the code over

clientapi/routing/createroom.go

@@ -462,6 +462,7 @@ func createRoom(
 				AuthEvents:  accumulated,
 			},
 			ev.Headered(roomVersion),
+			cfg.Matrix.ServerName,
 			nil,
 		); err != nil {
 			util.GetLogger(req.Context()).WithError(err).Error("SendEventWithState failed")

clientapi/routing/membership.go

@@ -109,6 +109,7 @@ func sendMembership(ctx context.Context, accountDB accounts.Database, device *us
 		roomserverAPI.KindNew,
 		[]*gomatrixserverlib.HeaderedEvent{event.Event.Headered(roomVer)},
 		cfg.Matrix.ServerName,
+		cfg.Matrix.ServerName,
 		nil,
 	); err != nil {
 		util.GetLogger(ctx).WithError(err).Error("SendEvents failed")

clientapi/routing/profile.go

@@ -169,7 +169,7 @@ func SetAvatarURL(
 		return jsonerror.InternalServerError()
 	}
 
-	if err := api.SendEvents(req.Context(), rsAPI, api.KindNew, events, cfg.Matrix.ServerName, nil); err != nil {
+	if err := api.SendEvents(req.Context(), rsAPI, api.KindNew, events, cfg.Matrix.ServerName, cfg.Matrix.ServerName, nil); err != nil {
 		util.GetLogger(req.Context()).WithError(err).Error("SendEvents failed")
 		return jsonerror.InternalServerError()
 	}
@@ -286,7 +286,7 @@ func SetDisplayName(
 		return jsonerror.InternalServerError()
 	}
 
-	if err := api.SendEvents(req.Context(), rsAPI, api.KindNew, events, cfg.Matrix.ServerName, nil); err != nil {
+	if err := api.SendEvents(req.Context(), rsAPI, api.KindNew, events, cfg.Matrix.ServerName, cfg.Matrix.ServerName, nil); err != nil {
 		util.GetLogger(req.Context()).WithError(err).Error("SendEvents failed")
 		return jsonerror.InternalServerError()
 	}

clientapi/routing/redaction.go

@@ -120,7 +120,7 @@ func SendRedaction(
 			JSON: jsonerror.NotFound("Room does not exist"),
 		}
 	}
-	if err = roomserverAPI.SendEvents(context.Background(), rsAPI, roomserverAPI.KindNew, []*gomatrixserverlib.HeaderedEvent{e}, cfg.Matrix.ServerName, nil); err != nil {
+	if err = roomserverAPI.SendEvents(context.Background(), rsAPI, roomserverAPI.KindNew, []*gomatrixserverlib.HeaderedEvent{e}, cfg.Matrix.ServerName, cfg.Matrix.ServerName, nil); err != nil {
 		util.GetLogger(req.Context()).WithError(err).Errorf("failed to SendEvents")
 		return jsonerror.InternalServerError()
 	}

clientapi/routing/sendevent.go

@@ -121,6 +121,7 @@ func SendEvent(
 			e.Headered(verRes.RoomVersion),
 		},
 		cfg.Matrix.ServerName,
+		cfg.Matrix.ServerName,
 		txnAndSessionID,
 	); err != nil {
 		util.GetLogger(req.Context()).WithError(err).Error("SendEvents failed")

clientapi/threepid/invites.go

@@ -366,6 +366,7 @@ func emit3PIDInviteEvent(
 			event.Headered(queryRes.RoomVersion),
 		},
 		cfg.Matrix.ServerName,
+		cfg.Matrix.ServerName,
 		nil,
 	)
 }

federationapi/api/api.go

@@ -24,6 +24,7 @@ type FederationClient interface {
 	MSC2946Spaces(ctx context.Context, dst gomatrixserverlib.ServerName, roomID string, r gomatrixserverlib.MSC2946SpacesRequest) (res gomatrixserverlib.MSC2946SpacesResponse, err error)
 	LookupServerKeys(ctx context.Context, s gomatrixserverlib.ServerName, keyRequests map[gomatrixserverlib.PublicKeyLookupRequest]gomatrixserverlib.Timestamp) ([]gomatrixserverlib.ServerKeys, error)
 	GetEventAuth(ctx context.Context, s gomatrixserverlib.ServerName, roomID, eventID string) (res gomatrixserverlib.RespEventAuth, err error)
+	LookupMissingEvents(ctx context.Context, s gomatrixserverlib.ServerName, roomID string, missing gomatrixserverlib.MissingEvents, roomVersion gomatrixserverlib.RoomVersion) (res gomatrixserverlib.RespMissingEvents, err error)
 }
 
 // FederationClientError is returned from FederationClient methods in the event of a problem.

federationapi/internal/federationclient.go

@@ -106,6 +106,21 @@ func (a *FederationInternalAPI) LookupStateIDs(
 	return ires.(gomatrixserverlib.RespStateIDs), nil
 }
 
+func (a *FederationInternalAPI) LookupMissingEvents(
+	ctx context.Context, s gomatrixserverlib.ServerName, roomID string,
+	missing gomatrixserverlib.MissingEvents, roomVersion gomatrixserverlib.RoomVersion,
+) (res gomatrixserverlib.RespMissingEvents, err error) {
+	ctx, cancel := context.WithTimeout(ctx, time.Second*30)
+	defer cancel()
+	ires, err := a.doRequest(s, func() (interface{}, error) {
+		return a.federation.LookupMissingEvents(ctx, s, roomID, missing, roomVersion)
+	})
+	if err != nil {
+		return gomatrixserverlib.RespMissingEvents{}, err
+	}
+	return ires.(gomatrixserverlib.RespMissingEvents), nil
+}
+
 func (a *FederationInternalAPI) GetEvent(
 	ctx context.Context, s gomatrixserverlib.ServerName, eventID string,
 ) (res gomatrixserverlib.Transaction, err error) {

federationapi/internal/perform.go

@@ -249,6 +249,7 @@ func (r *FederationInternalAPI) performJoinUsingServer(
 			roomserverAPI.KindNew,
 			respState,
 			event.Headered(respMakeJoin.RoomVersion),
+			serverName,
 			nil,
 		); err != nil {
 			logrus.WithFields(logrus.Fields{
@@ -430,6 +431,7 @@ func (r *FederationInternalAPI) performOutboundPeekUsingServer(
 		roomserverAPI.KindNew,
 		&respState,
 		respPeek.LatestEvent.Headered(respPeek.RoomVersion),
+		serverName,
 		nil,
 	); err != nil {
 		return fmt.Errorf("r.producer.SendEventWithState: %w", err)

federationapi/inthttp/client.go

@@ -26,17 +26,18 @@ const (
 	FederationAPIPerformServersAlivePath           = "/federationapi/performServersAlive"
 	FederationAPIPerformBroadcastEDUPath           = "/federationapi/performBroadcastEDU"
 
-	FederationAPIGetUserDevicesPath     = "/federationapi/client/getUserDevices"
+	FederationAPIGetUserDevicesPath      = "/federationapi/client/getUserDevices"
-	FederationAPIClaimKeysPath          = "/federationapi/client/claimKeys"
+	FederationAPIClaimKeysPath           = "/federationapi/client/claimKeys"
-	FederationAPIQueryKeysPath          = "/federationapi/client/queryKeys"
+	FederationAPIQueryKeysPath           = "/federationapi/client/queryKeys"
-	FederationAPIBackfillPath           = "/federationapi/client/backfill"
+	FederationAPIBackfillPath            = "/federationapi/client/backfill"
-	FederationAPILookupStatePath        = "/federationapi/client/lookupState"
+	FederationAPILookupStatePath         = "/federationapi/client/lookupState"
-	FederationAPILookupStateIDsPath     = "/federationapi/client/lookupStateIDs"
+	FederationAPILookupStateIDsPath      = "/federationapi/client/lookupStateIDs"
-	FederationAPIGetEventPath           = "/federationapi/client/getEvent"
+	FederationAPILookupMissingEventsPath = "/federationapi/client/lookupMissingEvents"
-	FederationAPILookupServerKeysPath   = "/federationapi/client/lookupServerKeys"
+	FederationAPIGetEventPath            = "/federationapi/client/getEvent"
-	FederationAPIEventRelationshipsPath = "/federationapi/client/msc2836eventRelationships"
+	FederationAPILookupServerKeysPath    = "/federationapi/client/lookupServerKeys"
-	FederationAPISpacesSummaryPath      = "/federationapi/client/msc2946spacesSummary"
+	FederationAPIEventRelationshipsPath  = "/federationapi/client/msc2836eventRelationships"
-	FederationAPIGetEventAuthPath       = "/federationapi/client/getEventAuth"
+	FederationAPISpacesSummaryPath       = "/federationapi/client/msc2946spacesSummary"
+	FederationAPIGetEventAuthPath        = "/federationapi/client/getEventAuth"
 
 	FederationAPIInputPublicKeyPath = "/federationapi/inputPublicKey"
 	FederationAPIQueryPublicKeyPath = "/federationapi/queryPublicKey"
@@ -354,6 +355,40 @@ func (h *httpFederationInternalAPI) LookupStateIDs(
 	return *response.Res, nil
 }
 
+type lookupMissingEvents struct {
+	S           gomatrixserverlib.ServerName
+	RoomID      string
+	Missing     gomatrixserverlib.MissingEvents
+	RoomVersion gomatrixserverlib.RoomVersion
+	Res         *gomatrixserverlib.RespMissingEvents
+	Err         *api.FederationClientError
+}
+
+func (h *httpFederationInternalAPI) LookupMissingEvents(
+	ctx context.Context, s gomatrixserverlib.ServerName, roomID string,
+	missing gomatrixserverlib.MissingEvents, roomVersion gomatrixserverlib.RoomVersion,
+) (gomatrixserverlib.RespMissingEvents, error) {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "LookupMissingEvents")
+	defer span.Finish()
+
+	request := lookupMissingEvents{
+		S:           s,
+		RoomID:      roomID,
+		Missing:     missing,
+		RoomVersion: roomVersion,
+	}
+	var response lookupMissingEvents
+	apiURL := h.federationAPIURL + FederationAPILookupMissingEventsPath
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, &request, &response)
+	if err != nil {
+		return gomatrixserverlib.RespMissingEvents{}, err
+	}
+	if response.Err != nil {
+		return gomatrixserverlib.RespMissingEvents{}, response.Err
+	}
+	return *response.Res, nil
+}
+
 type getEvent struct {
 	S       gomatrixserverlib.ServerName
 	EventID string

federationapi/inthttp/server.go

@@ -241,6 +241,28 @@ func AddRoutes(intAPI api.FederationInternalAPI, internalAPIMux *mux.Router) {
 			return util.JSONResponse{Code: http.StatusOK, JSON: request}
 		}),
 	)
+	internalAPIMux.Handle(
+		FederationAPILookupMissingEventsPath,
+		httputil.MakeInternalAPI("LookupMissingEvents", func(req *http.Request) util.JSONResponse {
+			var request lookupMissingEvents
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.MessageResponse(http.StatusBadRequest, err.Error())
+			}
+			res, err := intAPI.LookupMissingEvents(req.Context(), request.S, request.RoomID, request.Missing, request.RoomVersion)
+			if err != nil {
+				ferr, ok := err.(*api.FederationClientError)
+				if ok {
+					request.Err = ferr
+				} else {
+					request.Err = &api.FederationClientError{
+						Err: err.Error(),
+					}
+				}
+			}
+			request.Res = &res
+			return util.JSONResponse{Code: http.StatusOK, JSON: request}
+		}),
+	)
 	internalAPIMux.Handle(
 		FederationAPIGetEventPath,
 		httputil.MakeInternalAPI("GetEvent", func(req *http.Request) util.JSONResponse {

federationapi/routing/send.go

@@ -17,7 +17,6 @@ package routing
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 	"sync"
@@ -201,8 +200,6 @@ func Send(
 		eduAPI:     eduAPI,
 		keys:       keys,
 		federation: federation,
-		hadEvents:  make(map[string]bool),
-		haveEvents: make(map[string]*gomatrixserverlib.HeaderedEvent),
 		servers:    servers,
 		keyAPI:     keyAPI,
 		roomsMu:    mu,
@@ -263,22 +260,8 @@ type txnReq struct {
 	keys       gomatrixserverlib.JSONVerifier
 	federation txnFederationClient
 	roomsMu    *internal.MutexByRoom
-	// something that can tell us about which servers are in a room right now
+	servers    federationAPI.ServersInRoomProvider
-	servers federationAPI.ServersInRoomProvider
+	work       string
-	// a list of events from the auth and prev events which we already had
-	hadEvents      map[string]bool
-	hadEventsMutex sync.Mutex
-	// local cache of events for auth checks, etc - this may include events
-	// which the roomserver is unaware of.
-	haveEvents      map[string]*gomatrixserverlib.HeaderedEvent
-	haveEventsMutex sync.Mutex
-	work            string // metrics
-}
-
-func (t *txnReq) hadEvent(eventID string, had bool) {
-	t.hadEventsMutex.Lock()
-	defer t.hadEventsMutex.Unlock()
-	t.hadEvents[eventID] = had
 }
 
 // A subset of FederationClient functionality that txn requires. Useful for testing.
@@ -440,22 +423,8 @@ func (t *inputWorker) run() {
 type roomNotFoundError struct {
 	roomID string
 }
-type verifySigError struct {
-	eventID string
-	err     error
-}
-type missingPrevEventsError struct {
-	eventID string
-	err     error
-}
 
 func (e roomNotFoundError) Error() string { return fmt.Sprintf("room %q not found", e.roomID) }
-func (e verifySigError) Error() string {
-	return fmt.Sprintf("unable to verify signature of event %q: %s", e.eventID, e.err)
-}
-func (e missingPrevEventsError) Error() string {
-	return fmt.Sprintf("unable to get prev_events for event %q: %s", e.eventID, e.err)
-}
 
 func (t *txnReq) processEDUs(ctx context.Context) {
 	for _, e := range t.EDUs {
@@ -599,28 +568,7 @@ func (t *txnReq) processDeviceListUpdate(ctx context.Context, e gomatrixserverli
 	}
 }
 
-func (t *txnReq) getServers(ctx context.Context, roomID string, event *gomatrixserverlib.Event) []gomatrixserverlib.ServerName {
-	// The server that sent us the event should be sufficient to tell us about missing
-	// prev and auth events.
-	servers := []gomatrixserverlib.ServerName{t.Origin}
-	// If the event origin is different to the transaction origin then we can use
-	// this as a last resort. The origin server that created the event would have
-	// had to know the auth and prev events.
-	if event != nil {
-		if origin := event.Origin(); origin != t.Origin {
-			servers = append(servers, origin)
-		}
-	}
-	// If a specific room-to-server provider exists then use that. This will primarily
-	// be used for the P2P demos.
-	if t.servers != nil {
-		servers = append(servers, t.servers.GetServersForRoom(ctx, roomID, event)...)
-	}
-	return servers
-}
-
 func (t *txnReq) processEvent(ctx context.Context, e *gomatrixserverlib.Event) error {
-	logger := util.GetLogger(ctx).WithField("event_id", e.EventID()).WithField("room_id", e.RoomID())
 	t.work = "" // reset from previous event
 
 	// Ask the roomserver if we know about the room and/or if we're joined
@@ -648,30 +596,14 @@ func (t *txnReq) processEvent(ctx context.Context, e *gomatrixserverlib.Event) e
 	// before the roomserver tries to work out
 	stateReq := api.QueryMissingAuthPrevEventsRequest{
 		RoomID:       e.RoomID(),
-		AuthEventIDs: e.AuthEventIDs(),
+		AuthEventIDs: nil, //e.AuthEventIDs(),
-		PrevEventIDs: e.PrevEventIDs(),
+		PrevEventIDs: nil, //e.PrevEventIDs(),
 	}
 	var stateResp api.QueryMissingAuthPrevEventsResponse
 	if err := t.rsAPI.QueryMissingAuthPrevEvents(ctx, &stateReq, &stateResp); err != nil {
 		return fmt.Errorf("t.rsAPI.QueryMissingAuthPrevEvents: %w", err)
 	}
 
-	// Prepare a map of all the events we already had before this point, so
-	// that we don't send them to the roomserver again.
-	for _, eventID := range append(e.AuthEventIDs(), e.PrevEventIDs()...) {
-		t.hadEvent(eventID, true)
-	}
-	for _, eventID := range append(stateResp.MissingAuthEventIDs, stateResp.MissingPrevEventIDs...) {
-		t.hadEvent(eventID, false)
-	}
-
-	if len(stateResp.MissingPrevEventIDs) > 0 {
-		t.work = MetricsWorkMissingPrevEvents
-		logger.Infof("Event refers to %d unknown prev_events", len(stateResp.MissingPrevEventIDs))
-		return t.processEventWithMissingState(ctx, e, stateResp.RoomVersion)
-	}
-	t.work = MetricsWorkDirect
-
 	// pass the event to the roomserver which will do auth checks
 	// If the event fail auth checks, gmsl.NotAllowed error will be returned which we be silently
 	// discarded by the caller of this function
@@ -682,656 +614,8 @@ func (t *txnReq) processEvent(ctx context.Context, e *gomatrixserverlib.Event) e
 		[]*gomatrixserverlib.HeaderedEvent{
 			e.Headered(stateResp.RoomVersion),
 		},
+		t.Origin,
 		api.DoNotSendToOtherServers,
 		nil,
 	)
 }
-
-func checkAllowedByState(e *gomatrixserverlib.Event, stateEvents []*gomatrixserverlib.Event) error {
-	authUsingState := gomatrixserverlib.NewAuthEvents(nil)
-	for i := range stateEvents {
-		err := authUsingState.AddEvent(stateEvents[i])
-		if err != nil {
-			return err
-		}
-	}
-	return gomatrixserverlib.Allowed(e, &authUsingState)
-}
-
-func (t *txnReq) processEventWithMissingState(
-	ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion,
-) error {
-	// We are missing the previous events for this events.
-	// This means that there is a gap in our view of the history of the
-	// room. There two ways that we can handle such a gap:
-	//   1) We can fill in the gap using /get_missing_events
-	//   2) We can leave the gap and request the state of the room at
-	//      this event from the remote server using either /state_ids
-	//      or /state.
-	// Synapse will attempt to do 1 and if that fails or if the gap is
-	// too large then it will attempt 2.
-	// Synapse will use /state_ids if possible since usually the state
-	// is largely unchanged and it is more efficient to fetch a list of
-	// event ids and then use /event to fetch the individual events.
-	// However not all version of synapse support /state_ids so you may
-	// need to fallback to /state.
-
-	// Attempt to fill in the gap using /get_missing_events
-	// This will either:
-	// - fill in the gap completely then process event `e` returning no backwards extremity
-	// - fail to fill in the gap and tell us to terminate the transaction err=not nil
-	// - fail to fill in the gap and tell us to fetch state at the new backwards extremity, and to not terminate the transaction
-	newEvents, err := t.getMissingEvents(ctx, e, roomVersion)
-	if err != nil {
-		return err
-	}
-	if len(newEvents) == 0 {
-		return nil
-	}
-
-	backwardsExtremity := newEvents[0]
-	newEvents = newEvents[1:]
-
-	type respState struct {
-		// A snapshot is considered trustworthy if it came from our own roomserver.
-		// That's because the state will have been through state resolution once
-		// already in QueryStateAfterEvent.
-		trustworthy bool
-		*gomatrixserverlib.RespState
-	}
-
-	// at this point we know we're going to have a gap: we need to work out the room state at the new backwards extremity.
-	// 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
-	for _, prevEventID := range backwardsExtremity.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, lerr := t.lookupStateAfterEvent(ctx, roomVersion, backwardsExtremity.RoomID(), prevEventID)
-		if lerr != nil {
-			util.GetLogger(ctx).WithError(lerr).Errorf("Failed to lookup state after prev_event: %s", prevEventID)
-			return lerr
-		}
-		// Append the state onto the collected state. We'll run this through the
-		// state resolution next.
-		states = append(states, &respState{trustworthy, prevState})
-	}
-
-	// Now that we have collected all of the state from the prev_events, we'll
-	// run the state through the appropriate state resolution algorithm for the
-	// room if needed. This does a couple of things:
-	// 1. Ensures that the state is deduplicated fully for each state-key tuple
-	// 2. Ensures that we pick the latest events from both sets, in the case that
-	//    one of the prev_events is quite a bit older than the others
-	resolvedState := &gomatrixserverlib.RespState{}
-	switch len(states) {
-	case 0:
-		extremityIsCreate := backwardsExtremity.Type() == gomatrixserverlib.MRoomCreate && backwardsExtremity.StateKeyEquals("")
-		if !extremityIsCreate {
-			// There are no previous states and this isn't the beginning of the
-			// room - this is an error condition!
-			util.GetLogger(ctx).Errorf("Failed to lookup any state after prev_events")
-			return fmt.Errorf("expected %d states but got %d", len(backwardsExtremity.PrevEventIDs()), len(states))
-		}
-	case 1:
-		// There's only one previous state - if it's trustworthy (came from a
-		// local state snapshot which will already have been through state res),
-		// use it as-is. There's no point in resolving it again.
-		if states[0].trustworthy {
-			resolvedState = states[0].RespState
-			break
-		}
-		// Otherwise, if it isn't trustworthy (came from federation), run it through
-		// state resolution anyway for safety, in case there are duplicates.
-		fallthrough
-	default:
-		respStates := make([]*gomatrixserverlib.RespState, len(states))
-		for i := range states {
-			respStates[i] = states[i].RespState
-		}
-		// There's more than one previous state - run them all through state res
-		t.roomsMu.Lock(e.RoomID())
-		resolvedState, err = t.resolveStatesAndCheck(ctx, roomVersion, respStates, backwardsExtremity)
-		t.roomsMu.Unlock(e.RoomID())
-		if err != nil {
-			util.GetLogger(ctx).WithError(err).Errorf("Failed to resolve state conflicts for event %s", backwardsExtremity.EventID())
-			return err
-		}
-	}
-
-	// First of all, send the backward extremity into the roomserver with the
-	// newly resolved state. This marks the "oldest" point in the backfill and
-	// sets the baseline state for any new events after this. We'll make a
-	// copy of the hadEvents map so that it can be taken downstream without
-	// worrying about concurrent map reads/writes, since t.hadEvents is meant
-	// to be protected by a mutex.
-	hadEvents := map[string]bool{}
-	t.hadEventsMutex.Lock()
-	for k, v := range t.hadEvents {
-		hadEvents[k] = v
-	}
-	t.hadEventsMutex.Unlock()
-	err = api.SendEventWithState(
-		context.Background(),
-		t.rsAPI,
-		api.KindOld,
-		resolvedState,
-		backwardsExtremity.Headered(roomVersion),
-		hadEvents,
-	)
-	if err != nil {
-		return fmt.Errorf("api.SendEventWithState: %w", err)
-	}
-
-	// Then send all of the newer backfilled events, of which will all be newer
-	// than the backward extremity, into the roomserver without state. This way
-	// they will automatically fast-forward based on the room state at the
-	// extremity in the last step.
-	headeredNewEvents := make([]*gomatrixserverlib.HeaderedEvent, len(newEvents))
-	for i, newEvent := range newEvents {
-		headeredNewEvents[i] = newEvent.Headered(roomVersion)
-	}
-	if err = api.SendEvents(
-		context.Background(),
-		t.rsAPI,
-		api.KindOld,
-		append(headeredNewEvents, e.Headered(roomVersion)),
-		api.DoNotSendToOtherServers,
-		nil,
-	); err != nil {
-		return fmt.Errorf("api.SendEvents: %w", err)
-	}
-
-	return nil
-}
-
-// lookupStateAfterEvent returns the room state after `eventID`, which is the state before eventID with the state of `eventID` (if it's a state event)
-// added into the mix.
-func (t *txnReq) lookupStateAfterEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, eventID string) (*gomatrixserverlib.RespState, bool, error) {
-	// try doing all this locally before we resort to querying federation
-	respState := t.lookupStateAfterEventLocally(ctx, roomID, eventID)
-	if respState != nil {
-		return respState, true, nil
-	}
-
-	respState, err := t.lookupStateBeforeEvent(ctx, roomVersion, roomID, eventID)
-	if err != nil {
-		return nil, false, fmt.Errorf("t.lookupStateBeforeEvent: %w", err)
-	}
-
-	// fetch the event we're missing and add it to the pile
-	h, err := t.lookupEvent(ctx, roomVersion, roomID, eventID, false)
-	switch err.(type) {
-	case verifySigError:
-		return respState, false, nil
-	case nil:
-		// do nothing
-	default:
-		return nil, false, fmt.Errorf("t.lookupEvent: %w", err)
-	}
-	h = t.cacheAndReturn(h)
-	if h.StateKey() != nil {
-		addedToState := false
-		for i := range respState.StateEvents {
-			se := respState.StateEvents[i]
-			if se.Type() == h.Type() && se.StateKeyEquals(*h.StateKey()) {
-				respState.StateEvents[i] = h.Unwrap()
-				addedToState = true
-				break
-			}
-		}
-		if !addedToState {
-			respState.StateEvents = append(respState.StateEvents, h.Unwrap())
-		}
-	}
-
-	return respState, false, nil
-}
-
-func (t *txnReq) cacheAndReturn(ev *gomatrixserverlib.HeaderedEvent) *gomatrixserverlib.HeaderedEvent {
-	t.haveEventsMutex.Lock()
-	defer t.haveEventsMutex.Unlock()
-	if cached, exists := t.haveEvents[ev.EventID()]; exists {
-		return cached
-	}
-	t.haveEvents[ev.EventID()] = ev
-	return ev
-}
-
-func (t *txnReq) lookupStateAfterEventLocally(ctx context.Context, roomID, eventID string) *gomatrixserverlib.RespState {
-	var res api.QueryStateAfterEventsResponse
-	err := t.rsAPI.QueryStateAfterEvents(ctx, &api.QueryStateAfterEventsRequest{
-		RoomID:       roomID,
-		PrevEventIDs: []string{eventID},
-	}, &res)
-	if err != nil || !res.PrevEventsExist {
-		util.GetLogger(ctx).WithField("room_id", roomID).WithError(err).Warnf("failed to query state after %s locally, prev exists=%v", eventID, res.PrevEventsExist)
-		return nil
-	}
-	stateEvents := make([]*gomatrixserverlib.HeaderedEvent, len(res.StateEvents))
-	for i, ev := range res.StateEvents {
-		// set the event from the haveEvents cache - this means we will share pointers with other prev_event branches for this
-		// processEvent request, which is better for memory.
-		stateEvents[i] = t.cacheAndReturn(ev)
-		t.hadEvent(ev.EventID(), true)
-	}
-	// we should never access res.StateEvents again so we delete it here to make GC faster
-	res.StateEvents = nil
-
-	var authEvents []*gomatrixserverlib.Event
-	missingAuthEvents := map[string]bool{}
-	for _, ev := range stateEvents {
-		t.haveEventsMutex.Lock()
-		for _, ae := range ev.AuthEventIDs() {
-			if aev, ok := t.haveEvents[ae]; ok {
-				authEvents = append(authEvents, aev.Unwrap())
-			} else {
-				missingAuthEvents[ae] = true
-			}
-		}
-		t.haveEventsMutex.Unlock()
-	}
-	// QueryStateAfterEvents does not return the auth events, so fetch them now. We know the roomserver has them else it wouldn't
-	// have stored the event.
-	if len(missingAuthEvents) > 0 {
-		var missingEventList []string
-		for evID := range missingAuthEvents {
-			missingEventList = append(missingEventList, evID)
-		}
-		queryReq := api.QueryEventsByIDRequest{
-			EventIDs: missingEventList,
-		}
-		util.GetLogger(ctx).WithField("count", len(missingEventList)).Infof("Fetching missing auth events")
-		var queryRes api.QueryEventsByIDResponse
-		if err = t.rsAPI.QueryEventsByID(ctx, &queryReq, &queryRes); err != nil {
-			return nil
-		}
-		for i, ev := range queryRes.Events {
-			authEvents = append(authEvents, t.cacheAndReturn(queryRes.Events[i]).Unwrap())
-			t.hadEvent(ev.EventID(), true)
-		}
-		queryRes.Events = nil
-	}
-
-	return &gomatrixserverlib.RespState{
-		StateEvents: gomatrixserverlib.UnwrapEventHeaders(stateEvents),
-		AuthEvents:  authEvents,
-	}
-}
-
-// lookuptStateBeforeEvent returns the room state before the event e, which is just /state_ids and/or /state depending on what
-// the server supports.
-func (t *txnReq) lookupStateBeforeEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, eventID string) (
-	*gomatrixserverlib.RespState, error) {
-
-	// Attempt to fetch the missing state using /state_ids and /events
-	return t.lookupMissingStateViaStateIDs(ctx, roomID, eventID, roomVersion)
-}
-
-func (t *txnReq) resolveStatesAndCheck(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, states []*gomatrixserverlib.RespState, backwardsExtremity *gomatrixserverlib.Event) (*gomatrixserverlib.RespState, error) {
-	var authEventList []*gomatrixserverlib.Event
-	var stateEventList []*gomatrixserverlib.Event
-	for _, state := range states {
-		authEventList = append(authEventList, state.AuthEvents...)
-		stateEventList = append(stateEventList, state.StateEvents...)
-	}
-	resolvedStateEvents, err := gomatrixserverlib.ResolveConflicts(roomVersion, stateEventList, authEventList)
-	if err != nil {
-		return nil, err
-	}
-	// apply the current event
-retryAllowedState:
-	if err = checkAllowedByState(backwardsExtremity, resolvedStateEvents); err != nil {
-		switch missing := err.(type) {
-		case gomatrixserverlib.MissingAuthEventError:
-			h, err2 := t.lookupEvent(ctx, roomVersion, backwardsExtremity.RoomID(), missing.AuthEventID, true)
-			switch err2.(type) {
-			case verifySigError:
-				return &gomatrixserverlib.RespState{
-					AuthEvents:  authEventList,
-					StateEvents: resolvedStateEvents,
-				}, nil
-			case nil:
-				// do nothing
-			default:
-				return nil, fmt.Errorf("missing auth event %s and failed to look it up: %w", missing.AuthEventID, err2)
-			}
-			util.GetLogger(ctx).Infof("fetched event %s", missing.AuthEventID)
-			resolvedStateEvents = append(resolvedStateEvents, h.Unwrap())
-			goto retryAllowedState
-		default:
-		}
-		return nil, err
-	}
-	return &gomatrixserverlib.RespState{
-		AuthEvents:  authEventList,
-		StateEvents: resolvedStateEvents,
-	}, nil
-}
-
-func (t *txnReq) getMissingEvents(ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) (newEvents []*gomatrixserverlib.Event, err error) {
-	logger := util.GetLogger(ctx).WithField("event_id", e.EventID()).WithField("room_id", e.RoomID())
-	needed := gomatrixserverlib.StateNeededForAuth([]*gomatrixserverlib.Event{e})
-	// query latest events (our trusted forward extremities)
-	req := api.QueryLatestEventsAndStateRequest{
-		RoomID:       e.RoomID(),
-		StateToFetch: needed.Tuples(),
-	}
-	var res api.QueryLatestEventsAndStateResponse
-	if err = t.rsAPI.QueryLatestEventsAndState(ctx, &req, &res); err != nil {
-		logger.WithError(err).Warn("Failed to query latest events")
-		return nil, err
-	}
-	latestEvents := make([]string, len(res.LatestEvents))
-	for i, ev := range res.LatestEvents {
-		latestEvents[i] = res.LatestEvents[i].EventID
-		t.hadEvent(ev.EventID, true)
-	}
-
-	var missingResp *gomatrixserverlib.RespMissingEvents
-	servers := t.getServers(ctx, e.RoomID(), e)
-	for _, server := range servers {
-		var m gomatrixserverlib.RespMissingEvents
-		if m, err = t.federation.LookupMissingEvents(ctx, server, e.RoomID(), gomatrixserverlib.MissingEvents{
-			Limit: 20,
-			// The latest event IDs that the sender already has. These are skipped when retrieving the previous events of latest_events.
-			EarliestEvents: latestEvents,
-			// The event IDs to retrieve the previous events for.
-			LatestEvents: []string{e.EventID()},
-		}, roomVersion); err == nil {
-			missingResp = &m
-			break
-		} else {
-			logger.WithError(err).Errorf("%s pushed us an event but %q did not respond to /get_missing_events", t.Origin, server)
-			if errors.Is(err, context.DeadlineExceeded) {
-				break
-			}
-		}
-	}
-
-	if missingResp == nil {
-		logger.WithError(err).Errorf(
-			"%s pushed us an event but %d server(s) couldn't give us details about prev_events via /get_missing_events - dropping this event until it can",
-			t.Origin, len(servers),
-		)
-		return nil, missingPrevEventsError{
-			eventID: e.EventID(),
-			err:     err,
-		}
-	}
-
-	// security: how we handle failures depends on whether or not this event will become the new forward extremity for the room.
-	// There's 2 scenarios to consider:
-	// - Case A: We got pushed an event and are now fetching missing prev_events. (isInboundTxn=true)
-	// - Case B: We are fetching missing prev_events already and now fetching some more  (isInboundTxn=false)
-	// In Case B, we know for sure that the event we are currently processing will not become the new forward extremity for the room,
-	// as it was called in response to an inbound txn which had it as a prev_event.
-	// In Case A, the event is a forward extremity, and could eventually become the _only_ forward extremity in the room. This is bad
-	// because it means we would trust the state at that event to be the state for the entire room, and allows rooms to be hijacked.
-	// https://github.com/matrix-org/synapse/pull/3456
-	// https://github.com/matrix-org/synapse/blob/229eb81498b0fe1da81e9b5b333a0285acde9446/synapse/handlers/federation.py#L335
-	// For now, we do not allow Case B, so reject the event.
-	logger.Infof("get_missing_events returned %d events", len(missingResp.Events))
-
-	// Make sure events from the missingResp are using the cache - missing events
-	// will be added and duplicates will be removed.
-	for i, ev := range missingResp.Events {
-		missingResp.Events[i] = t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap()
-	}
-
-	// topologically sort and sanity check that we are making forward progress
-	newEvents = gomatrixserverlib.ReverseTopologicalOrdering(missingResp.Events, gomatrixserverlib.TopologicalOrderByPrevEvents)
-	shouldHaveSomeEventIDs := e.PrevEventIDs()
-	hasPrevEvent := false
-Event:
-	for _, pe := range shouldHaveSomeEventIDs {
-		for _, ev := range newEvents {
-			if ev.EventID() == pe {
-				hasPrevEvent = true
-				break Event
-			}
-		}
-	}
-	if !hasPrevEvent {
-		err = fmt.Errorf("called /get_missing_events but server %s didn't return any prev_events with IDs %v", t.Origin, shouldHaveSomeEventIDs)
-		logger.WithError(err).Errorf(
-			"%s pushed us an event but couldn't give us details about prev_events via /get_missing_events - dropping this event until it can",
-			t.Origin,
-		)
-		return nil, missingPrevEventsError{
-			eventID: e.EventID(),
-			err:     err,
-		}
-	}
-
-	return newEvents, nil
-}
-
-func (t *txnReq) lookupMissingStateViaState(ctx context.Context, roomID, eventID string, roomVersion gomatrixserverlib.RoomVersion) (
-	respState *gomatrixserverlib.RespState, err error) {
-	state, err := t.federation.LookupState(ctx, t.Origin, roomID, eventID, roomVersion)
-	if err != nil {
-		return nil, err
-	}
-	// Check that the returned state is valid.
-	if err := state.Check(ctx, t.keys, nil); err != nil {
-		return nil, err
-	}
-	// Cache the results of this state lookup and deduplicate anything we already
-	// have in the cache, freeing up memory.
-	for i, ev := range state.AuthEvents {
-		state.AuthEvents[i] = t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap()
-	}
-	for i, ev := range state.StateEvents {
-		state.StateEvents[i] = t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap()
-	}
-	return &state, nil
-}
-
-func (t *txnReq) lookupMissingStateViaStateIDs(ctx context.Context, roomID, eventID string, roomVersion gomatrixserverlib.RoomVersion) (
-	*gomatrixserverlib.RespState, error) {
-	util.GetLogger(ctx).WithField("room_id", roomID).Infof("lookupMissingStateViaStateIDs %s", eventID)
-	// fetch the state event IDs at the time of the event
-	stateIDs, err := t.federation.LookupStateIDs(ctx, t.Origin, roomID, eventID)
-	if err != nil {
-		return nil, err
-	}
-	// work out which auth/state IDs are missing
-	wantIDs := append(stateIDs.StateEventIDs, stateIDs.AuthEventIDs...)
-	missing := make(map[string]bool)
-	var missingEventList []string
-	t.haveEventsMutex.Lock()
-	for _, sid := range wantIDs {
-		if _, ok := t.haveEvents[sid]; !ok {
-			if !missing[sid] {
-				missing[sid] = true
-				missingEventList = append(missingEventList, sid)
-			}
-		}
-	}
-	t.haveEventsMutex.Unlock()
-
-	// fetch as many as we can from the roomserver
-	queryReq := api.QueryEventsByIDRequest{
-		EventIDs: missingEventList,
-	}
-	var queryRes api.QueryEventsByIDResponse
-	if err = t.rsAPI.QueryEventsByID(ctx, &queryReq, &queryRes); err != nil {
-		return nil, err
-	}
-	for i, ev := range queryRes.Events {
-		queryRes.Events[i] = t.cacheAndReturn(queryRes.Events[i])
-		t.hadEvent(ev.EventID(), true)
-		evID := queryRes.Events[i].EventID()
-		if missing[evID] {
-			delete(missing, evID)
-		}
-	}
-	queryRes.Events = nil // allow it to be GCed
-
-	concurrentRequests := 8
-	missingCount := len(missing)
-	util.GetLogger(ctx).WithField("room_id", roomID).WithField("event_id", eventID).Infof("lookupMissingStateViaStateIDs missing %d/%d events", missingCount, len(wantIDs))
-
-	// If over 50% of the auth/state events from /state_ids are missing
-	// then we'll just call /state instead, otherwise we'll just end up
-	// hammering the remote side with /event requests unnecessarily.
-	if missingCount > concurrentRequests && missingCount > len(wantIDs)/2 {
-		util.GetLogger(ctx).WithFields(logrus.Fields{
-			"missing":           missingCount,
-			"event_id":          eventID,
-			"room_id":           roomID,
-			"total_state":       len(stateIDs.StateEventIDs),
-			"total_auth_events": len(stateIDs.AuthEventIDs),
-		}).Info("Fetching all state at event")
-		return t.lookupMissingStateViaState(ctx, roomID, eventID, roomVersion)
-	}
-
-	if missingCount > 0 {
-		util.GetLogger(ctx).WithFields(logrus.Fields{
-			"missing":             missingCount,
-			"event_id":            eventID,
-			"room_id":             roomID,
-			"total_state":         len(stateIDs.StateEventIDs),
-			"total_auth_events":   len(stateIDs.AuthEventIDs),
-			"concurrent_requests": concurrentRequests,
-		}).Info("Fetching missing state at event")
-
-		// Create a queue containing all of the missing event IDs that we want
-		// to retrieve.
-		pending := make(chan string, missingCount)
-		for missingEventID := range missing {
-			pending <- missingEventID
-		}
-		close(pending)
-
-		// Define how many workers we should start to do this.
-		if missingCount < concurrentRequests {
-			concurrentRequests = missingCount
-		}
-
-		// Create the wait group.
-		var fetchgroup sync.WaitGroup
-		fetchgroup.Add(concurrentRequests)
-
-		// This is the only place where we'll write to t.haveEvents from
-		// multiple goroutines, and everywhere else is blocked on this
-		// synchronous function anyway.
-		var haveEventsMutex sync.Mutex
-
-		// Define what we'll do in order to fetch the missing event ID.
-		fetch := func(missingEventID string) {
-			var h *gomatrixserverlib.HeaderedEvent
-			h, err = t.lookupEvent(ctx, roomVersion, roomID, missingEventID, false)
-			switch err.(type) {
-			case verifySigError:
-				return
-			case nil:
-				break
-			default:
-				util.GetLogger(ctx).WithFields(logrus.Fields{
-					"event_id": missingEventID,
-					"room_id":  roomID,
-				}).Info("Failed to fetch missing event")
-				return
-			}
-			haveEventsMutex.Lock()
-			t.cacheAndReturn(h)
-			haveEventsMutex.Unlock()
-		}
-
-		// Create the worker.
-		worker := func(ch <-chan string) {
-			defer fetchgroup.Done()
-			for missingEventID := range ch {
-				fetch(missingEventID)
-			}
-		}
-
-		// Start the workers.
-		for i := 0; i < concurrentRequests; i++ {
-			go worker(pending)
-		}
-
-		// Wait for the workers to finish.
-		fetchgroup.Wait()
-	}
-
-	resp, err := t.createRespStateFromStateIDs(stateIDs)
-	return resp, err
-}
-
-func (t *txnReq) createRespStateFromStateIDs(stateIDs gomatrixserverlib.RespStateIDs) (
-	*gomatrixserverlib.RespState, error) { // nolint:unparam
-	t.haveEventsMutex.Lock()
-	defer t.haveEventsMutex.Unlock()
-
-	// create a RespState response using the response to /state_ids as a guide
-	respState := gomatrixserverlib.RespState{}
-
-	for i := range stateIDs.StateEventIDs {
-		ev, ok := t.haveEvents[stateIDs.StateEventIDs[i]]
-		if !ok {
-			logrus.Warnf("Missing state event in createRespStateFromStateIDs: %s", stateIDs.StateEventIDs[i])
-			continue
-		}
-		respState.StateEvents = append(respState.StateEvents, ev.Unwrap())
-	}
-	for i := range stateIDs.AuthEventIDs {
-		ev, ok := t.haveEvents[stateIDs.AuthEventIDs[i]]
-		if !ok {
-			logrus.Warnf("Missing auth event in createRespStateFromStateIDs: %s", stateIDs.AuthEventIDs[i])
-			continue
-		}
-		respState.AuthEvents = append(respState.AuthEvents, ev.Unwrap())
-	}
-	// We purposefully do not do auth checks on the returned events, as they will still
-	// be processed in the exact same way, just as a 'rejected' event
-	// TODO: Add a field to HeaderedEvent to indicate if the event is rejected.
-	return &respState, nil
-}
-
-func (t *txnReq) lookupEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, missingEventID string, localFirst bool) (*gomatrixserverlib.HeaderedEvent, error) {
-	if localFirst {
-		// fetch from the roomserver
-		queryReq := api.QueryEventsByIDRequest{
-			EventIDs: []string{missingEventID},
-		}
-		var queryRes api.QueryEventsByIDResponse
-		if err := t.rsAPI.QueryEventsByID(ctx, &queryReq, &queryRes); err != nil {
-			util.GetLogger(ctx).Warnf("Failed to query roomserver for missing event %s: %s - falling back to remote", missingEventID, err)
-		} else if len(queryRes.Events) == 1 {
-			return queryRes.Events[0], nil
-		}
-	}
-	var event *gomatrixserverlib.Event
-	found := false
-	servers := t.getServers(ctx, roomID, nil)
-	for _, serverName := range servers {
-		txn, err := t.federation.GetEvent(ctx, serverName, missingEventID)
-		if err != nil || len(txn.PDUs) == 0 {
-			util.GetLogger(ctx).WithError(err).WithField("event_id", missingEventID).Warn("Failed to get missing /event for event ID")
-			if errors.Is(err, context.DeadlineExceeded) {
-				break
-			}
-			continue
-		}
-		event, err = gomatrixserverlib.NewEventFromUntrustedJSON(txn.PDUs[0], roomVersion)
-		if err != nil {
-			util.GetLogger(ctx).WithError(err).WithField("event_id", missingEventID).Warnf("Transaction: Failed to parse event JSON of event")
-			continue
-		}
-		found = true
-		break
-	}
-	if !found {
-		util.GetLogger(ctx).WithField("event_id", missingEventID).Warnf("Failed to get missing /event for event ID from %d server(s)", len(servers))
-		return nil, fmt.Errorf("wasn't able to find event via %d server(s)", len(servers))
-	}
-	if err := event.VerifyEventSignatures(ctx, t.keys); err != nil {
-		util.GetLogger(ctx).WithError(err).Warnf("Transaction: Couldn't validate signature of event %q", event.EventID())
-		return nil, verifySigError{event.EventID(), err}
-	}
-	return t.cacheAndReturn(event.Headered(roomVersion)), nil
-}

federationapi/routing/send_test.go

@@ -244,8 +244,6 @@ func mustCreateTransaction(rsAPI api.RoomserverInternalAPI, fedClient txnFederat
 		eduAPI:     &testEDUProducer{},
 		keys:       &test.NopJSONVerifier{},
 		federation: fedClient,
-		haveEvents: make(map[string]*gomatrixserverlib.HeaderedEvent),
-		hadEvents:  make(map[string]bool),
 		roomsMu:    internal.NewMutexByRoom(),
 	}
 	t.PDUs = pdus

federationapi/routing/threepid.go

@@ -89,7 +89,7 @@ func CreateInvitesFrom3PIDInvites(
 	}
 
 	// Send all the events
-	if err := api.SendEvents(req.Context(), rsAPI, api.KindNew, evs, cfg.Matrix.ServerName, nil); err != nil {
+	if err := api.SendEvents(req.Context(), rsAPI, api.KindNew, evs, "TODO", cfg.Matrix.ServerName, nil); err != nil {
 		util.GetLogger(req.Context()).WithError(err).Error("SendEvents failed")
 		return jsonerror.InternalServerError()
 	}
@@ -178,6 +178,7 @@ func ExchangeThirdPartyInvite(
 		[]*gomatrixserverlib.HeaderedEvent{
 			signedEvent.Event.Headered(verRes.RoomVersion),
 		},
+		request.Origin(),
 		cfg.Matrix.ServerName,
 		nil,
 	); err != nil {

roomserver/api/input.go

@@ -54,6 +54,8 @@ type InputRoomEvent struct {
 	Kind Kind `json:"kind"`
 	// The event JSON for the event to add.
 	Event *gomatrixserverlib.HeaderedEvent `json:"event"`
+	// Which server told us about this event.
+	Origin gomatrixserverlib.ServerName `json:"origin"`
 	// List of state event IDs that authenticate this event.
 	// These are likely derived from the "auth_events" JSON key of the event.
 	// But can be different because the "auth_events" key can be incomplete or wrong.

roomserver/api/wrapper.go

@@ -22,10 +22,15 @@ import (
 	"github.com/matrix-org/util"
 )
 
+type RoomEventInputter interface {
+	InputRoomEvents(context.Context, *InputRoomEventsRequest, *InputRoomEventsResponse)
+}
+
 // SendEvents to the roomserver The events are written with KindNew.
 func SendEvents(
-	ctx context.Context, rsAPI RoomserverInternalAPI,
+	ctx context.Context, rsAPI RoomEventInputter,
 	kind Kind, events []*gomatrixserverlib.HeaderedEvent,
+	origin gomatrixserverlib.ServerName,
 	sendAsServer gomatrixserverlib.ServerName, txnID *TransactionID,
 ) error {
 	ires := make([]InputRoomEvent, len(events))
@@ -33,6 +38,7 @@ func SendEvents(
 		ires[i] = InputRoomEvent{
 			Kind:          kind,
 			Event:         event,
+			Origin:        origin,
 			AuthEventIDs:  event.AuthEventIDs(),
 			SendAsServer:  string(sendAsServer),
 			TransactionID: txnID,
@@ -45,9 +51,9 @@ func SendEvents(
 // with the state at the event as KindOutlier before it. Will not send any event that is
 // marked as `true` in haveEventIDs.
 func SendEventWithState(
-	ctx context.Context, rsAPI RoomserverInternalAPI, kind Kind,
+	ctx context.Context, rsAPI RoomEventInputter, kind Kind,
 	state *gomatrixserverlib.RespState, event *gomatrixserverlib.HeaderedEvent,
-	haveEventIDs map[string]bool,
+	origin gomatrixserverlib.ServerName, haveEventIDs map[string]bool,
 ) error {
 	outliers, err := state.Events()
 	if err != nil {
@@ -62,6 +68,7 @@ func SendEventWithState(
 		ires = append(ires, InputRoomEvent{
 			Kind:         KindOutlier,
 			Event:        outlier.Headered(event.RoomVersion),
+			Origin:       origin,
 			AuthEventIDs: outlier.AuthEventIDs(),
 		})
 	}
@@ -74,6 +81,7 @@ func SendEventWithState(
 	ires = append(ires, InputRoomEvent{
 		Kind:          kind,
 		Event:         event,
+		Origin:        origin,
 		AuthEventIDs:  event.AuthEventIDs(),
 		HasState:      true,
 		StateEventIDs: stateEventIDs,
@@ -84,7 +92,7 @@ func SendEventWithState(
 
 // SendInputRoomEvents to the roomserver.
 func SendInputRoomEvents(
-	ctx context.Context, rsAPI RoomserverInternalAPI, ires []InputRoomEvent,
+	ctx context.Context, rsAPI RoomEventInputter, ires []InputRoomEvent,
 ) error {
 	request := InputRoomEventsRequest{InputRoomEvents: ires}
 	var response InputRoomEventsResponse

roomserver/internal/api.go

@@ -92,6 +92,7 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.FederationInternalA
 		FSAPI:                fsAPI,
 		KeyRing:              keyRing,
 		ACLs:                 r.ServerACLs,
+		Queryer:              r.Queryer,
 	}
 	r.Inviter = &perform.Inviter{
 		DB:      r.DB,

roomserver/internal/input/input.go

@@ -27,6 +27,7 @@ import (
 	"github.com/matrix-org/dendrite/internal/hooks"
 	"github.com/matrix-org/dendrite/roomserver/acls"
 	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/roomserver/internal/query"
 	"github.com/matrix-org/dendrite/roomserver/storage"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/prometheus/client_golang/prometheus"
@@ -50,6 +51,8 @@ type Inputer struct {
 	ACLs                 *acls.ServerACLs
 	OutputRoomEventTopic string
 	workers              sync.Map // room ID -> *inputWorker
+
+	Queryer *query.Queryer
 }
 
 type inputTask struct {

roomserver/internal/input/input_events.go

@@ -23,6 +23,7 @@ import (
 	"time"
 
 	fedapi "github.com/matrix-org/dendrite/federationapi/api"
+	"github.com/matrix-org/dendrite/internal"
 	"github.com/matrix-org/dendrite/internal/eventutil"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/internal/helpers"
@@ -76,6 +77,11 @@ func (r *Inputer) processRoomEvent(
 	// Parse and validate the event JSON
 	headered := input.Event
 	event := headered.Unwrap()
+	logger := util.GetLogger(ctx).WithFields(logrus.Fields{
+		"event_id": event.EventID(),
+		"room_id":  event.RoomID(),
+		"type":     event.Type(),
+	})
 
 	// if we have already got this event then do not process it again, if the input kind is an outlier.
 	// Outliers contain no extra information which may warrant a re-processing.
@@ -88,23 +94,35 @@ func (r *Inputer) processRoomEvent(
 				switch idFormat {
 				case gomatrixserverlib.EventIDFormatV1:
 					if bytes.Equal(event.EventReference().EventSHA256, evs[0].EventReference().EventSHA256) {
-						util.GetLogger(ctx).WithField("event_id", event.EventID()).Infof("Already processed event; ignoring")
+						logger.Debugf("Already processed event; ignoring")
 						return event.EventID(), nil
 					}
 				default:
-					util.GetLogger(ctx).WithField("event_id", event.EventID()).Infof("Already processed event; ignoring")
+					logger.Debugf("Already processed event; ignoring")
 					return event.EventID(), nil
 				}
 			}
 		}
 	}
 
+	missingReq := &api.QueryMissingAuthPrevEventsRequest{
+		RoomID:       event.RoomID(),
+		AuthEventIDs: event.AuthEventIDs(),
+		PrevEventIDs: event.PrevEventIDs(),
+	}
+	missingRes := &api.QueryMissingAuthPrevEventsResponse{}
+	if event.Type() != gomatrixserverlib.MRoomCreate {
+		if err = r.Queryer.QueryMissingAuthPrevEvents(ctx, missingReq, missingRes); err != nil {
+			return "", fmt.Errorf("r.Queryer.QueryMissingAuthPrevEvents: %w", err)
+		}
+	}
+
 	// First of all, check that the auth events of the event are known.
 	// If they aren't then we will ask the federation API for them.
 	isRejected := false
 	authEvents := gomatrixserverlib.NewAuthEvents(nil)
-	knownAuthEvents := map[string]types.Event{}
+	knownEvents := map[string]*types.Event{}
-	if err = r.checkForMissingAuthEvents(ctx, input.Event, &authEvents, knownAuthEvents); err != nil {
+	if err = r.checkForMissingAuthEvents(ctx, logger, input.Event, &authEvents, knownEvents); err != nil {
 		return "", fmt.Errorf("r.checkForMissingAuthEvents: %w", err)
 	}
 
@@ -113,14 +131,14 @@ func (r *Inputer) processRoomEvent(
 	var rejectionErr error
 	if rejectionErr = gomatrixserverlib.Allowed(event, &authEvents); rejectionErr != nil {
 		isRejected = true
-		logrus.WithError(rejectionErr).Warnf("Event %s rejected", event.EventID())
+		logger.WithError(rejectionErr).Warnf("Event %s rejected", event.EventID())
 	}
 
 	// Accumulate the auth event NIDs.
 	authEventIDs := event.AuthEventIDs()
 	authEventNIDs := make([]types.EventNID, 0, len(authEventIDs))
 	for _, authEventID := range authEventIDs {
-		authEventNIDs = append(authEventNIDs, knownAuthEvents[authEventID].EventNID)
+		authEventNIDs = append(authEventNIDs, knownEvents[authEventID].EventNID)
 	}
 
 	var softfail bool
@@ -129,11 +147,7 @@ func (r *Inputer) processRoomEvent(
 		// current room state.
 		softfail, err = helpers.CheckForSoftFail(ctx, r.DB, headered, input.StateEventIDs)
 		if err != nil {
-			logrus.WithFields(logrus.Fields{
+			logger.WithError(err).Info("Error authing soft-failed event")
-				"event_id": event.EventID(),
-				"type":     event.Type(),
-				"room":     event.RoomID(),
-			}).WithError(err).Info("Error authing soft-failed event")
 		}
 	}
 
@@ -156,12 +170,7 @@ func (r *Inputer) processRoomEvent(
 	// doesn't have any associated state to store and we don't need to
 	// notify anyone about it.
 	if input.Kind == api.KindOutlier {
-		logrus.WithFields(logrus.Fields{
+		logger.Debug("Stored outlier")
-			"event_id": event.EventID(),
-			"type":     event.Type(),
-			"room":     event.RoomID(),
-			"sender":   event.Sender(),
-		}).Debug("Stored outlier")
 		return event.EventID(), nil
 	}
 
@@ -173,6 +182,27 @@ func (r *Inputer) processRoomEvent(
 		return "", fmt.Errorf("r.DB.RoomInfo missing for room %s", event.RoomID())
 	}
 
+	if input.Origin == "" {
+		return "", fmt.Errorf("expected an origin")
+	}
+
+	if len(missingRes.MissingPrevEventIDs) > 0 {
+		missingState := missingStateReq{
+			origin:     input.Origin,
+			inputer:    r,
+			queryer:    r.Queryer,
+			db:         r.DB,
+			federation: r.FSAPI,
+			roomsMu:    internal.NewMutexByRoom(),
+			servers:    []gomatrixserverlib.ServerName{input.Origin},
+			hadEvents:  map[string]bool{},
+			haveEvents: map[string]*gomatrixserverlib.HeaderedEvent{},
+		}
+		if err = missingState.processEventWithMissingState(ctx, input.Event.Unwrap(), roomInfo.RoomVersion); err != nil {
+			return "", fmt.Errorf("r.checkForMissingPrevEvents: %w", err)
+		}
+	}
+
 	if stateAtEvent.BeforeStateSnapshotNID == 0 {
 		// We haven't calculated a state for this event yet.
 		// Lets calculate one.
@@ -184,13 +214,7 @@ func (r *Inputer) processRoomEvent(
 
 	// We stop here if the event is rejected: We've stored it but won't update forward extremities or notify anyone about it.
 	if isRejected || softfail {
-		logrus.WithFields(logrus.Fields{
+		logger.WithField("soft_fail", softfail).Debug("Stored rejected event")
-			"event_id":  event.EventID(),
-			"type":      event.Type(),
-			"room":      event.RoomID(),
-			"soft_fail": softfail,
-			"sender":    event.Sender(),
-		}).Debug("Stored rejected event")
 		return event.EventID(), rejectionErr
 	}
 
@@ -246,9 +270,10 @@ func (r *Inputer) processRoomEvent(
 
 func (r *Inputer) checkForMissingAuthEvents(
 	ctx context.Context,
+	logger *logrus.Entry,
 	event *gomatrixserverlib.HeaderedEvent,
 	auth *gomatrixserverlib.AuthEvents,
-	known map[string]types.Event,
+	known map[string]*types.Event,
 ) error {
 	authEventIDs := event.AuthEventIDs()
 	if len(authEventIDs) == 0 {
@@ -263,7 +288,8 @@ func (r *Inputer) checkForMissingAuthEvents(
 	}
 	for _, event := range authEvents {
 		if event.Event != nil {
-			known[event.EventID()] = event
+			ev := event // don't take the address of the iterated value
+			known[event.EventID()] = &ev
 			if err = auth.AddEvent(event.Event); err != nil {
 				return fmt.Errorf("auth.AddEvent: %w", err)
 			}
@@ -273,7 +299,7 @@ func (r *Inputer) checkForMissingAuthEvents(
 	}
 
 	if len(unknown) > 0 {
-		logrus.Printf("XXX: There are %d missing auth events", len(unknown))
+		logger.Printf("XXX: There are %d missing auth events", len(unknown))
 
 		serverReq := &fedapi.QueryJoinedHostServerNamesInRoomRequest{
 			RoomID: event.RoomID(),
@@ -283,22 +309,22 @@ func (r *Inputer) checkForMissingAuthEvents(
 			return fmt.Errorf("r.FSAPI.QueryJoinedHostServerNamesInRoom: %w", err)
 		}
 
-		logrus.Printf("XXX: Asking servers %+v", serverRes.ServerNames)
+		logger.Printf("XXX: Asking servers %+v", serverRes.ServerNames)
 
 		var res gomatrixserverlib.RespEventAuth
 		var found bool
 		for _, serverName := range serverRes.ServerNames {
 			res, err = r.FSAPI.GetEventAuth(ctx, serverName, event.RoomID(), event.EventID())
 			if err != nil {
-				logrus.WithError(err).Warnf("Failed to get event auth from federation for %q: %s", event.EventID(), err)
+				logger.WithError(err).Warnf("Failed to get event auth from federation for %q: %s", event.EventID(), err)
 				continue
 			}
-			logrus.Printf("XXX: Server %q provided us with %d auth events", serverName, len(res.AuthEvents))
+			logger.Printf("XXX: Server %q provided us with %d auth events", serverName, len(res.AuthEvents))
 			found = true
 			break
 		}
 		if !found {
-			logrus.Printf("XXX: None of the %d servers provided us with auth events", len(serverRes.ServerNames))
+			logger.Printf("XXX: None of the %d servers provided us with auth events", len(serverRes.ServerNames))
 			return fmt.Errorf("no servers provided event auth")
 		}
 
@@ -332,7 +358,7 @@ func (r *Inputer) checkForMissingAuthEvents(
 			}
 
 			// Let's take a note of the fact that we now know about this event.
-			known[event.EventID()] = types.Event{}
+			known[event.EventID()] = nil
 			if err := auth.AddEvent(event); err != nil {
 				return fmt.Errorf("auth.AddEvent: %w", err)
 			}
@@ -341,7 +367,7 @@ func (r *Inputer) checkForMissingAuthEvents(
 			isRejected := false
 			if err := gomatrixserverlib.Allowed(event, auth); err != nil {
 				isRejected = true
-				logrus.WithError(err).Warnf("Auth event %s rejected", event.EventID())
+				logger.WithError(err).Warnf("Auth event %s rejected", event.EventID())
 			}
 
 			// Finally, store the event in the database.
@@ -351,7 +377,7 @@ func (r *Inputer) checkForMissingAuthEvents(
 			}
 
 			// Now we know about this event, too.
-			known[event.EventID()] = types.Event{
+			known[event.EventID()] = &types.Event{
 				EventNID: eventNID,
 				Event:    event,
 			}
@@ -361,6 +387,228 @@ func (r *Inputer) checkForMissingAuthEvents(
 	return nil
 }
 
+/*
+func (r *Inputer) checkForMissingPrevEvents(
+	ctx context.Context,
+	logger *logrus.Entry,
+	event *gomatrixserverlib.HeaderedEvent,
+	roomInfo *types.RoomInfo,
+	known map[string]*types.Event,
+) error {
+	prevStates := map[string]*types.StateAtEvent{}
+	prevEventIDs := event.PrevEventIDs()
+	if len(prevEventIDs) == 0 && event.Type() != gomatrixserverlib.MRoomCreate {
+		return fmt.Errorf("expected to find some prev events for event type %q", event.Type())
+	}
+
+	for _, eventID := range prevEventIDs {
+		state, err := r.DB.StateAtEventIDs(ctx, []string{eventID})
+		if err != nil {
+			if _, ok := err.(types.MissingEventError); ok {
+				continue
+			}
+			return fmt.Errorf("r.DB.StateAtEventIDs: %w", err)
+		}
+		if len(state) == 1 {
+			prevStates[eventID] = &state[0]
+			continue
+		}
+	}
+
+	// If we know all of the states of the previous events then there is nothing more to
+	// do here, as the state across them will be resolved later.
+	if len(prevStates) == len(prevEventIDs) {
+		return nil
+	}
+	if r.FSAPI == nil {
+		return fmt.Errorf("cannot satisfy missing events without federation")
+	}
+
+	// Ask the federation API which servers we should ask. In theory the roomserver
+	// doesn't need the help of the federation API to do this because we already know
+	// all of the membership states, it's just that the federation API tracks this in
+	// a table for this purpose. TODO: Work out what makes most sense here.
+	serverReq := &fedapi.QueryJoinedHostServerNamesInRoomRequest{
+		RoomID: event.RoomID(),
+	}
+	serverRes := &fedapi.QueryJoinedHostServerNamesInRoomResponse{}
+	if err := r.FSAPI.QueryJoinedHostServerNamesInRoom(ctx, serverReq, serverRes); err != nil {
+		return fmt.Errorf("r.FSAPI.QueryJoinedHostServerNamesInRoom: %w", err)
+	}
+
+	// Attempt to fill in the gap using /get_missing_events
+	// This will either:
+	// - fill in the gap completely then process event `e` returning no backwards extremity
+	// - fail to fill in the gap and tell us to terminate the transaction err=not nil
+	// - fail to fill in the gap and tell us to fetch state at the new backwards extremity, and to not terminate the transaction
+	newEvents, err := r.getMissingEvents(ctx, logger, event, roomInfo, serverRes.ServerNames, known)
+	if err != nil {
+		return err
+	}
+	if len(newEvents) == 0 {
+		return fmt.Errorf("/get_missing_events returned no new events")
+	}
+
+	return nil
+}
+
+func (r *Inputer) getMissingEvents(
+	ctx context.Context,
+	logger *logrus.Entry,
+	event *gomatrixserverlib.HeaderedEvent,
+	roomInfo *types.RoomInfo,
+	servers []gomatrixserverlib.ServerName,
+	known map[string]*types.Event,
+) (newEvents []*gomatrixserverlib.Event, err error) {
+	logger.Printf("XXX: get_missing_events called")
+	needed := gomatrixserverlib.StateNeededForAuth([]*gomatrixserverlib.Event{event.Unwrap()})
+
+	// Ask the roomserver for our current forward extremities. These will form
+	// the "earliest" part of the `/get_missing_events` request.
+	req := &api.QueryLatestEventsAndStateRequest{
+		RoomID:       event.RoomID(),
+		StateToFetch: needed.Tuples(),
+	}
+	res := &api.QueryLatestEventsAndStateResponse{}
+	if err = r.Queryer.QueryLatestEventsAndState(ctx, req, res); err != nil {
+		logger.WithError(err).Warn("Failed to query latest events")
+		return nil, err
+	}
+
+	// Accumulate the event IDs of our forward extremities for use in the request.
+	latestEvents := make([]string, len(res.LatestEvents))
+	for i := range res.LatestEvents {
+		latestEvents[i] = res.LatestEvents[i].EventID
+	}
+
+	var missingResp *gomatrixserverlib.RespMissingEvents
+	for _, server := range servers {
+		logger.Printf("XXX: Calling /get_missing_events via %q", server)
+		var m gomatrixserverlib.RespMissingEvents
+		if m, err = r.FSAPI.LookupMissingEvents(ctx, server, event.RoomID(), gomatrixserverlib.MissingEvents{
+			Limit:          20,
+			EarliestEvents: latestEvents,
+			LatestEvents:   []string{event.EventID()},
+		}, event.RoomVersion); err == nil {
+			missingResp = &m
+			break
+		} else if errors.Is(err, context.DeadlineExceeded) {
+			break
+		}
+	}
+
+	if missingResp == nil {
+		return nil, fmt.Errorf("/get_missing_events failed via all candidate servers")
+	}
+	if len(missingResp.Events) == 0 {
+		return nil, fmt.Errorf("/get_missing_events returned no events")
+	}
+
+	// security: how we handle failures depends on whether or not this event will become the new forward extremity for the room.
+	// There's 2 scenarios to consider:
+	// - Case A: We got pushed an event and are now fetching missing prev_events. (isInboundTxn=true)
+	// - Case B: We are fetching missing prev_events already and now fetching some more  (isInboundTxn=false)
+	// In Case B, we know for sure that the event we are currently processing will not become the new forward extremity for the room,
+	// as it was called in response to an inbound txn which had it as a prev_event.
+	// In Case A, the event is a forward extremity, and could eventually become the _only_ forward extremity in the room. This is bad
+	// because it means we would trust the state at that event to be the state for the entire room, and allows rooms to be hijacked.
+	// https://github.com/matrix-org/synapse/pull/3456
+	// https://github.com/matrix-org/synapse/blob/229eb81498b0fe1da81e9b5b333a0285acde9446/synapse/handlers/federation.py#L335
+	// For now, we do not allow Case B, so reject the event.
+	logger.Printf("XXX: get_missing_events returned %d events", len(missingResp.Events))
+
+	newEvents = gomatrixserverlib.ReverseTopologicalOrdering(
+		missingResp.Events,
+		gomatrixserverlib.TopologicalOrderByPrevEvents,
+	)
+	for _, pe := range event.PrevEventIDs() {
+		hasPrevEvent := false
+		for _, ev := range newEvents {
+			if ev.EventID() == pe {
+				hasPrevEvent = true
+				break
+			}
+		}
+		if !hasPrevEvent {
+			logger.Errorf("Prev event %q is still missing after /get_missing_events", pe)
+		}
+	}
+
+	backwardExtremity := newEvents[0]
+	fastForwardEvents := newEvents[1:]
+
+	// Do we know about the state of the backward extremity already?
+	if _, err := r.DB.StateAtEventIDs(ctx, []string{backwardExtremity.EventID()}); err == nil {
+		// Yes, we do, so we don't need to store that event.
+	} else {
+		// No, we don't, so let's go find it.
+		// r.FSAPI.LookupStateIDs()
+	}
+
+	for _, ev := range fastForwardEvents {
+		if _, err := r.processRoomEvent(ctx, &api.InputRoomEvent{
+			Kind:         api.KindOld,
+			Event:        ev.Headered(event.RoomVersion),
+			AuthEventIDs: ev.AuthEventIDs(),
+		}); err != nil {
+			return nil, fmt.Errorf("r.processRoomEvent (prev event): %w", err)
+		}
+	}
+
+	return newEvents, nil
+}
+
+func (r *Inputer) lookupStateBeforeEvent(
+	ctx context.Context,
+	logger *logrus.Entry,
+	event *gomatrixserverlib.HeaderedEvent,
+	roomInfo *types.RoomInfo,
+	servers []gomatrixserverlib.ServerName,
+) error {
+	knownPrevStates := map[string]types.StateAtEvent{}
+	unknownPrevStates := map[string]struct{}{}
+	neededStateEvents := map[string]struct{}{}
+
+	for _, prevEventID := range event.PrevEventIDs() {
+		if state, err := r.DB.StateAtEventIDs(ctx, []string{prevEventID}); err == nil && len(state) == 1 {
+			knownPrevStates[prevEventID] = state[0]
+		} else {
+			unknownPrevStates[prevEventID] = struct{}{}
+		}
+	}
+
+	for prevEventID := range unknownPrevStates {
+		stateIDs, err := r.FSAPI.LookupStateIDs(ctx, "TODO: SERVER", event.RoomID(), prevEventID)
+		if err != nil {
+			return fmt.Errorf("r.FSAPI.LookupStateIDs: %w", err)
+		}
+		events, err := r.DB.EventsFromIDs(ctx, stateIDs.StateEventIDs)
+		if err != nil {
+			return fmt.Errorf("r.DB.EventsFromIDs: %w", err)
+		}
+		for i, eventID := range stateIDs.StateEventIDs {
+			if events[i].Event == nil || events[i].EventNID == 0 {
+				neededStateEvents[eventID] = struct{}{}
+			}
+		}
+
+		if len(neededStateEvents) > (len(stateIDs.StateEventIDs) / 2) {
+			// More than 50% of the state events are missing, so let's just
+			// call `/state` instead of fetching the events individually.
+			state, err := r.FSAPI.LookupState(ctx, "", event.RoomID(), prevEventID, roomInfo.RoomVersion)
+			if err != nil {
+				return fmt.Errorf("r.FSAPI.LookupState: %w", err)
+			}
+			knownPrevStates[prevEventID] = types.StateAtEvent{
+				StateEntry: types.StateEntry{},
+			}
+		}
+	}
+
+	return nil
+}
+*/
+
 func (r *Inputer) calculateAndSetState(
 	ctx context.Context,
 	input *api.InputRoomEvent,

roomserver/internal/input/input_missing.go

@@ -0,0 +1,708 @@
+package input
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"sync"
+
+	fedapi "github.com/matrix-org/dendrite/federationapi/api"
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/roomserver/internal/query"
+	"github.com/matrix-org/dendrite/roomserver/storage"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
+)
+
+type missingStateReq struct {
+	origin          gomatrixserverlib.ServerName
+	db              storage.Database
+	inputer         *Inputer
+	queryer         *query.Queryer
+	keys            gomatrixserverlib.JSONVerifier
+	federation      fedapi.FederationInternalAPI
+	roomsMu         *internal.MutexByRoom
+	servers         []gomatrixserverlib.ServerName
+	hadEvents       map[string]bool
+	hadEventsMutex  sync.Mutex
+	haveEvents      map[string]*gomatrixserverlib.HeaderedEvent
+	haveEventsMutex sync.Mutex
+}
+
+func (t *missingStateReq) processEventWithMissingState(
+	ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion,
+) error {
+	// We are missing the previous events for this events.
+	// This means that there is a gap in our view of the history of the
+	// room. There two ways that we can handle such a gap:
+	//   1) We can fill in the gap using /get_missing_events
+	//   2) We can leave the gap and request the state of the room at
+	//      this event from the remote server using either /state_ids
+	//      or /state.
+	// Synapse will attempt to do 1 and if that fails or if the gap is
+	// too large then it will attempt 2.
+	// Synapse will use /state_ids if possible since usually the state
+	// is largely unchanged and it is more efficient to fetch a list of
+	// event ids and then use /event to fetch the individual events.
+	// However not all version of synapse support /state_ids so you may
+	// need to fallback to /state.
+
+	// Attempt to fill in the gap using /get_missing_events
+	// This will either:
+	// - fill in the gap completely then process event `e` returning no backwards extremity
+	// - fail to fill in the gap and tell us to terminate the transaction err=not nil
+	// - fail to fill in the gap and tell us to fetch state at the new backwards extremity, and to not terminate the transaction
+	newEvents, err := t.getMissingEvents(ctx, e, roomVersion)
+	if err != nil {
+		return err
+	}
+	if len(newEvents) == 0 {
+		return nil
+	}
+
+	backwardsExtremity := newEvents[0]
+	newEvents = newEvents[1:]
+
+	type respState struct {
+		// A snapshot is considered trustworthy if it came from our own roomserver.
+		// That's because the state will have been through state resolution once
+		// already in QueryStateAfterEvent.
+		trustworthy bool
+		*gomatrixserverlib.RespState
+	}
+
+	// at this point we know we're going to have a gap: we need to work out the room state at the new backwards extremity.
+	// 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
+	for _, prevEventID := range backwardsExtremity.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, lerr := t.lookupStateAfterEvent(ctx, roomVersion, backwardsExtremity.RoomID(), prevEventID)
+		if lerr != nil {
+			util.GetLogger(ctx).WithError(lerr).Errorf("Failed to lookup state after prev_event: %s", prevEventID)
+			return lerr
+		}
+		// Append the state onto the collected state. We'll run this through the
+		// state resolution next.
+		states = append(states, &respState{trustworthy, prevState})
+	}
+
+	// Now that we have collected all of the state from the prev_events, we'll
+	// run the state through the appropriate state resolution algorithm for the
+	// room if needed. This does a couple of things:
+	// 1. Ensures that the state is deduplicated fully for each state-key tuple
+	// 2. Ensures that we pick the latest events from both sets, in the case that
+	//    one of the prev_events is quite a bit older than the others
+	resolvedState := &gomatrixserverlib.RespState{}
+	switch len(states) {
+	case 0:
+		extremityIsCreate := backwardsExtremity.Type() == gomatrixserverlib.MRoomCreate && backwardsExtremity.StateKeyEquals("")
+		if !extremityIsCreate {
+			// There are no previous states and this isn't the beginning of the
+			// room - this is an error condition!
+			util.GetLogger(ctx).Errorf("Failed to lookup any state after prev_events")
+			return fmt.Errorf("expected %d states but got %d", len(backwardsExtremity.PrevEventIDs()), len(states))
+		}
+	case 1:
+		// There's only one previous state - if it's trustworthy (came from a
+		// local state snapshot which will already have been through state res),
+		// use it as-is. There's no point in resolving it again.
+		if states[0].trustworthy {
+			resolvedState = states[0].RespState
+			break
+		}
+		// Otherwise, if it isn't trustworthy (came from federation), run it through
+		// state resolution anyway for safety, in case there are duplicates.
+		fallthrough
+	default:
+		respStates := make([]*gomatrixserverlib.RespState, len(states))
+		for i := range states {
+			respStates[i] = states[i].RespState
+		}
+		// There's more than one previous state - run them all through state res
+		t.roomsMu.Lock(e.RoomID())
+		resolvedState, err = t.resolveStatesAndCheck(ctx, roomVersion, respStates, backwardsExtremity)
+		t.roomsMu.Unlock(e.RoomID())
+		if err != nil {
+			util.GetLogger(ctx).WithError(err).Errorf("Failed to resolve state conflicts for event %s", backwardsExtremity.EventID())
+			return err
+		}
+	}
+
+	// First of all, send the backward extremity into the roomserver with the
+	// newly resolved state. This marks the "oldest" point in the backfill and
+	// sets the baseline state for any new events after this. We'll make a
+	// copy of the hadEvents map so that it can be taken downstream without
+	// worrying about concurrent map reads/writes, since t.hadEvents is meant
+	// to be protected by a mutex.
+	hadEvents := map[string]bool{}
+	t.hadEventsMutex.Lock()
+	for k, v := range t.hadEvents {
+		hadEvents[k] = v
+	}
+	t.hadEventsMutex.Unlock()
+
+	err = api.SendEventWithState(
+		context.Background(),
+		t.inputer,
+		api.KindOld,
+		resolvedState,
+		backwardsExtremity.Headered(roomVersion),
+		t.origin,
+		hadEvents,
+	)
+	if err != nil {
+		return fmt.Errorf("api.SendEventWithState: %w", err)
+	}
+
+	// Then send all of the newer backfilled events, of which will all be newer
+	// than the backward extremity, into the roomserver without state. This way
+	// they will automatically fast-forward based on the room state at the
+	// extremity in the last step.
+	headeredNewEvents := make([]*gomatrixserverlib.HeaderedEvent, len(newEvents))
+	for i, newEvent := range newEvents {
+		headeredNewEvents[i] = newEvent.Headered(roomVersion)
+	}
+	if err = api.SendEvents(
+		context.Background(),
+		t.inputer,
+		api.KindOld,
+		append(headeredNewEvents, e.Headered(roomVersion)),
+		t.origin,
+		api.DoNotSendToOtherServers,
+		nil,
+	); err != nil {
+		return fmt.Errorf("api.SendEvents: %w", err)
+	}
+
+	return nil
+}
+
+// lookupStateAfterEvent returns the room state after `eventID`, which is the state before eventID with the state of `eventID` (if it's a state event)
+// added into the mix.
+func (t *missingStateReq) lookupStateAfterEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, eventID string) (*gomatrixserverlib.RespState, bool, error) {
+	// try doing all this locally before we resort to querying federation
+	respState := t.lookupStateAfterEventLocally(ctx, roomID, eventID)
+	if respState != nil {
+		return respState, true, nil
+	}
+
+	respState, err := t.lookupStateBeforeEvent(ctx, roomVersion, roomID, eventID)
+	if err != nil {
+		return nil, false, fmt.Errorf("t.lookupStateBeforeEvent: %w", err)
+	}
+
+	// fetch the event we're missing and add it to the pile
+	h, err := t.lookupEvent(ctx, roomVersion, roomID, eventID, false)
+	switch err.(type) {
+	case verifySigError:
+		return respState, false, nil
+	case nil:
+		// do nothing
+	default:
+		return nil, false, fmt.Errorf("t.lookupEvent: %w", err)
+	}
+	h = t.cacheAndReturn(h)
+	if h.StateKey() != nil {
+		addedToState := false
+		for i := range respState.StateEvents {
+			se := respState.StateEvents[i]
+			if se.Type() == h.Type() && se.StateKeyEquals(*h.StateKey()) {
+				respState.StateEvents[i] = h.Unwrap()
+				addedToState = true
+				break
+			}
+		}
+		if !addedToState {
+			respState.StateEvents = append(respState.StateEvents, h.Unwrap())
+		}
+	}
+
+	return respState, false, nil
+}
+
+func (t *missingStateReq) cacheAndReturn(ev *gomatrixserverlib.HeaderedEvent) *gomatrixserverlib.HeaderedEvent {
+	t.haveEventsMutex.Lock()
+	defer t.haveEventsMutex.Unlock()
+	if cached, exists := t.haveEvents[ev.EventID()]; exists {
+		return cached
+	}
+	t.haveEvents[ev.EventID()] = ev
+	return ev
+}
+
+func (t *missingStateReq) lookupStateAfterEventLocally(ctx context.Context, roomID, eventID string) *gomatrixserverlib.RespState {
+	var res api.QueryStateAfterEventsResponse
+	err := t.queryer.QueryStateAfterEvents(ctx, &api.QueryStateAfterEventsRequest{
+		RoomID:       roomID,
+		PrevEventIDs: []string{eventID},
+	}, &res)
+	if err != nil || !res.PrevEventsExist {
+		util.GetLogger(ctx).WithField("room_id", roomID).WithError(err).Warnf("failed to query state after %s locally, prev exists=%v", eventID, res.PrevEventsExist)
+		return nil
+	}
+	stateEvents := make([]*gomatrixserverlib.HeaderedEvent, len(res.StateEvents))
+	for i, ev := range res.StateEvents {
+		// set the event from the haveEvents cache - this means we will share pointers with other prev_event branches for this
+		// processEvent request, which is better for memory.
+		stateEvents[i] = t.cacheAndReturn(ev)
+		t.hadEvent(ev.EventID(), true)
+	}
+	// we should never access res.StateEvents again so we delete it here to make GC faster
+	res.StateEvents = nil
+
+	var authEvents []*gomatrixserverlib.Event
+	missingAuthEvents := map[string]bool{}
+	for _, ev := range stateEvents {
+		t.haveEventsMutex.Lock()
+		for _, ae := range ev.AuthEventIDs() {
+			if aev, ok := t.haveEvents[ae]; ok {
+				authEvents = append(authEvents, aev.Unwrap())
+			} else {
+				missingAuthEvents[ae] = true
+			}
+		}
+		t.haveEventsMutex.Unlock()
+	}
+	// QueryStateAfterEvents does not return the auth events, so fetch them now. We know the roomserver has them else it wouldn't
+	// have stored the event.
+	if len(missingAuthEvents) > 0 {
+		var missingEventList []string
+		for evID := range missingAuthEvents {
+			missingEventList = append(missingEventList, evID)
+		}
+		queryReq := api.QueryEventsByIDRequest{
+			EventIDs: missingEventList,
+		}
+		util.GetLogger(ctx).WithField("count", len(missingEventList)).Infof("Fetching missing auth events")
+		var queryRes api.QueryEventsByIDResponse
+		if err = t.queryer.QueryEventsByID(ctx, &queryReq, &queryRes); err != nil {
+			return nil
+		}
+		for i, ev := range queryRes.Events {
+			authEvents = append(authEvents, t.cacheAndReturn(queryRes.Events[i]).Unwrap())
+			t.hadEvent(ev.EventID(), true)
+		}
+		queryRes.Events = nil
+	}
+
+	return &gomatrixserverlib.RespState{
+		StateEvents: gomatrixserverlib.UnwrapEventHeaders(stateEvents),
+		AuthEvents:  authEvents,
+	}
+}
+
+// lookuptStateBeforeEvent returns the room state before the event e, which is just /state_ids and/or /state depending on what
+// the server supports.
+func (t *missingStateReq) lookupStateBeforeEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, eventID string) (
+	*gomatrixserverlib.RespState, error) {
+
+	// Attempt to fetch the missing state using /state_ids and /events
+	return t.lookupMissingStateViaStateIDs(ctx, roomID, eventID, roomVersion)
+}
+
+func (t *missingStateReq) resolveStatesAndCheck(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, states []*gomatrixserverlib.RespState, backwardsExtremity *gomatrixserverlib.Event) (*gomatrixserverlib.RespState, error) {
+	var authEventList []*gomatrixserverlib.Event
+	var stateEventList []*gomatrixserverlib.Event
+	for _, state := range states {
+		authEventList = append(authEventList, state.AuthEvents...)
+		stateEventList = append(stateEventList, state.StateEvents...)
+	}
+	resolvedStateEvents, err := gomatrixserverlib.ResolveConflicts(roomVersion, stateEventList, authEventList)
+	if err != nil {
+		return nil, err
+	}
+	// apply the current event
+retryAllowedState:
+	if err = checkAllowedByState(backwardsExtremity, resolvedStateEvents); err != nil {
+		switch missing := err.(type) {
+		case gomatrixserverlib.MissingAuthEventError:
+			h, err2 := t.lookupEvent(ctx, roomVersion, backwardsExtremity.RoomID(), missing.AuthEventID, true)
+			switch err2.(type) {
+			case verifySigError:
+				return &gomatrixserverlib.RespState{
+					AuthEvents:  authEventList,
+					StateEvents: resolvedStateEvents,
+				}, nil
+			case nil:
+				// do nothing
+			default:
+				return nil, fmt.Errorf("missing auth event %s and failed to look it up: %w", missing.AuthEventID, err2)
+			}
+			util.GetLogger(ctx).Infof("fetched event %s", missing.AuthEventID)
+			resolvedStateEvents = append(resolvedStateEvents, h.Unwrap())
+			goto retryAllowedState
+		default:
+		}
+		return nil, err
+	}
+	return &gomatrixserverlib.RespState{
+		AuthEvents:  authEventList,
+		StateEvents: resolvedStateEvents,
+	}, nil
+}
+
+func (t *missingStateReq) getMissingEvents(ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) (newEvents []*gomatrixserverlib.Event, err error) {
+	logger := util.GetLogger(ctx).WithField("event_id", e.EventID()).WithField("room_id", e.RoomID())
+	needed := gomatrixserverlib.StateNeededForAuth([]*gomatrixserverlib.Event{e})
+	// query latest events (our trusted forward extremities)
+	req := api.QueryLatestEventsAndStateRequest{
+		RoomID:       e.RoomID(),
+		StateToFetch: needed.Tuples(),
+	}
+	var res api.QueryLatestEventsAndStateResponse
+	if err = t.queryer.QueryLatestEventsAndState(ctx, &req, &res); err != nil {
+		logger.WithError(err).Warn("Failed to query latest events")
+		return nil, err
+	}
+	latestEvents := make([]string, len(res.LatestEvents))
+	for i, ev := range res.LatestEvents {
+		latestEvents[i] = res.LatestEvents[i].EventID
+		t.hadEvent(ev.EventID, true)
+	}
+
+	var missingResp *gomatrixserverlib.RespMissingEvents
+	for _, server := range t.servers {
+		var m gomatrixserverlib.RespMissingEvents
+		if m, err = t.federation.LookupMissingEvents(ctx, server, e.RoomID(), gomatrixserverlib.MissingEvents{
+			Limit: 20,
+			// The latest event IDs that the sender already has. These are skipped when retrieving the previous events of latest_events.
+			EarliestEvents: latestEvents,
+			// The event IDs to retrieve the previous events for.
+			LatestEvents: []string{e.EventID()},
+		}, roomVersion); err == nil {
+			missingResp = &m
+			break
+		} else {
+			logger.WithError(err).Errorf("%s pushed us an event but %q did not respond to /get_missing_events", t.origin, server)
+			if errors.Is(err, context.DeadlineExceeded) {
+				break
+			}
+		}
+	}
+
+	if missingResp == nil {
+		logger.WithError(err).Errorf(
+			"%s pushed us an event but %d server(s) couldn't give us details about prev_events via /get_missing_events - dropping this event until it can",
+			t.origin, len(t.servers),
+		)
+		return nil, missingPrevEventsError{
+			eventID: e.EventID(),
+			err:     err,
+		}
+	}
+
+	// security: how we handle failures depends on whether or not this event will become the new forward extremity for the room.
+	// There's 2 scenarios to consider:
+	// - Case A: We got pushed an event and are now fetching missing prev_events. (isInboundTxn=true)
+	// - Case B: We are fetching missing prev_events already and now fetching some more  (isInboundTxn=false)
+	// In Case B, we know for sure that the event we are currently processing will not become the new forward extremity for the room,
+	// as it was called in response to an inbound txn which had it as a prev_event.
+	// In Case A, the event is a forward extremity, and could eventually become the _only_ forward extremity in the room. This is bad
+	// because it means we would trust the state at that event to be the state for the entire room, and allows rooms to be hijacked.
+	// https://github.com/matrix-org/synapse/pull/3456
+	// https://github.com/matrix-org/synapse/blob/229eb81498b0fe1da81e9b5b333a0285acde9446/synapse/handlers/federation.py#L335
+	// For now, we do not allow Case B, so reject the event.
+	logger.Infof("get_missing_events returned %d events", len(missingResp.Events))
+
+	// Make sure events from the missingResp are using the cache - missing events
+	// will be added and duplicates will be removed.
+	for i, ev := range missingResp.Events {
+		missingResp.Events[i] = t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap()
+	}
+
+	// topologically sort and sanity check that we are making forward progress
+	newEvents = gomatrixserverlib.ReverseTopologicalOrdering(missingResp.Events, gomatrixserverlib.TopologicalOrderByPrevEvents)
+	shouldHaveSomeEventIDs := e.PrevEventIDs()
+	hasPrevEvent := false
+Event:
+	for _, pe := range shouldHaveSomeEventIDs {
+		for _, ev := range newEvents {
+			if ev.EventID() == pe {
+				hasPrevEvent = true
+				break Event
+			}
+		}
+	}
+	if !hasPrevEvent {
+		err = fmt.Errorf("called /get_missing_events but server %s didn't return any prev_events with IDs %v", t.origin, shouldHaveSomeEventIDs)
+		logger.WithError(err).Errorf(
+			"%s pushed us an event but couldn't give us details about prev_events via /get_missing_events - dropping this event until it can",
+			t.origin,
+		)
+		return nil, missingPrevEventsError{
+			eventID: e.EventID(),
+			err:     err,
+		}
+	}
+
+	return newEvents, nil
+}
+
+func (t *missingStateReq) lookupMissingStateViaState(ctx context.Context, roomID, eventID string, roomVersion gomatrixserverlib.RoomVersion) (
+	respState *gomatrixserverlib.RespState, err error) {
+	state, err := t.federation.LookupState(ctx, t.origin, roomID, eventID, roomVersion)
+	if err != nil {
+		return nil, err
+	}
+	// Check that the returned state is valid.
+	if err := state.Check(ctx, t.keys, nil); err != nil {
+		return nil, err
+	}
+	// Cache the results of this state lookup and deduplicate anything we already
+	// have in the cache, freeing up memory.
+	for i, ev := range state.AuthEvents {
+		state.AuthEvents[i] = t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap()
+	}
+	for i, ev := range state.StateEvents {
+		state.StateEvents[i] = t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap()
+	}
+	return &state, nil
+}
+
+func (t *missingStateReq) lookupMissingStateViaStateIDs(ctx context.Context, roomID, eventID string, roomVersion gomatrixserverlib.RoomVersion) (
+	*gomatrixserverlib.RespState, error) {
+	util.GetLogger(ctx).WithField("room_id", roomID).Infof("lookupMissingStateViaStateIDs %s", eventID)
+	// fetch the state event IDs at the time of the event
+	stateIDs, err := t.federation.LookupStateIDs(ctx, t.origin, roomID, eventID)
+	if err != nil {
+		return nil, err
+	}
+	// work out which auth/state IDs are missing
+	wantIDs := append(stateIDs.StateEventIDs, stateIDs.AuthEventIDs...)
+	missing := make(map[string]bool)
+	var missingEventList []string
+	t.haveEventsMutex.Lock()
+	for _, sid := range wantIDs {
+		if _, ok := t.haveEvents[sid]; !ok {
+			if !missing[sid] {
+				missing[sid] = true
+				missingEventList = append(missingEventList, sid)
+			}
+		}
+	}
+	t.haveEventsMutex.Unlock()
+
+	// fetch as many as we can from the roomserver
+	queryReq := api.QueryEventsByIDRequest{
+		EventIDs: missingEventList,
+	}
+	var queryRes api.QueryEventsByIDResponse
+	if err = t.queryer.QueryEventsByID(ctx, &queryReq, &queryRes); err != nil {
+		return nil, err
+	}
+	for i, ev := range queryRes.Events {
+		queryRes.Events[i] = t.cacheAndReturn(queryRes.Events[i])
+		t.hadEvent(ev.EventID(), true)
+		evID := queryRes.Events[i].EventID()
+		if missing[evID] {
+			delete(missing, evID)
+		}
+	}
+	queryRes.Events = nil // allow it to be GCed
+
+	concurrentRequests := 8
+	missingCount := len(missing)
+	util.GetLogger(ctx).WithField("room_id", roomID).WithField("event_id", eventID).Infof("lookupMissingStateViaStateIDs missing %d/%d events", missingCount, len(wantIDs))
+
+	// If over 50% of the auth/state events from /state_ids are missing
+	// then we'll just call /state instead, otherwise we'll just end up
+	// hammering the remote side with /event requests unnecessarily.
+	if missingCount > concurrentRequests && missingCount > len(wantIDs)/2 {
+		util.GetLogger(ctx).WithFields(logrus.Fields{
+			"missing":           missingCount,
+			"event_id":          eventID,
+			"room_id":           roomID,
+			"total_state":       len(stateIDs.StateEventIDs),
+			"total_auth_events": len(stateIDs.AuthEventIDs),
+		}).Info("Fetching all state at event")
+		return t.lookupMissingStateViaState(ctx, roomID, eventID, roomVersion)
+	}
+
+	if missingCount > 0 {
+		util.GetLogger(ctx).WithFields(logrus.Fields{
+			"missing":             missingCount,
+			"event_id":            eventID,
+			"room_id":             roomID,
+			"total_state":         len(stateIDs.StateEventIDs),
+			"total_auth_events":   len(stateIDs.AuthEventIDs),
+			"concurrent_requests": concurrentRequests,
+		}).Info("Fetching missing state at event")
+
+		// Create a queue containing all of the missing event IDs that we want
+		// to retrieve.
+		pending := make(chan string, missingCount)
+		for missingEventID := range missing {
+			pending <- missingEventID
+		}
+		close(pending)
+
+		// Define how many workers we should start to do this.
+		if missingCount < concurrentRequests {
+			concurrentRequests = missingCount
+		}
+
+		// Create the wait group.
+		var fetchgroup sync.WaitGroup
+		fetchgroup.Add(concurrentRequests)
+
+		// This is the only place where we'll write to t.haveEvents from
+		// multiple goroutines, and everywhere else is blocked on this
+		// synchronous function anyway.
+		var haveEventsMutex sync.Mutex
+
+		// Define what we'll do in order to fetch the missing event ID.
+		fetch := func(missingEventID string) {
+			var h *gomatrixserverlib.HeaderedEvent
+			h, err = t.lookupEvent(ctx, roomVersion, roomID, missingEventID, false)
+			switch err.(type) {
+			case verifySigError:
+				return
+			case nil:
+				break
+			default:
+				util.GetLogger(ctx).WithFields(logrus.Fields{
+					"event_id": missingEventID,
+					"room_id":  roomID,
+				}).Info("Failed to fetch missing event")
+				return
+			}
+			haveEventsMutex.Lock()
+			t.cacheAndReturn(h)
+			haveEventsMutex.Unlock()
+		}
+
+		// Create the worker.
+		worker := func(ch <-chan string) {
+			defer fetchgroup.Done()
+			for missingEventID := range ch {
+				fetch(missingEventID)
+			}
+		}
+
+		// Start the workers.
+		for i := 0; i < concurrentRequests; i++ {
+			go worker(pending)
+		}
+
+		// Wait for the workers to finish.
+		fetchgroup.Wait()
+	}
+
+	resp, err := t.createRespStateFromStateIDs(stateIDs)
+	return resp, err
+}
+
+func (t *missingStateReq) createRespStateFromStateIDs(stateIDs gomatrixserverlib.RespStateIDs) (
+	*gomatrixserverlib.RespState, error) { // nolint:unparam
+	t.haveEventsMutex.Lock()
+	defer t.haveEventsMutex.Unlock()
+
+	// create a RespState response using the response to /state_ids as a guide
+	respState := gomatrixserverlib.RespState{}
+
+	for i := range stateIDs.StateEventIDs {
+		ev, ok := t.haveEvents[stateIDs.StateEventIDs[i]]
+		if !ok {
+			logrus.Warnf("Missing state event in createRespStateFromStateIDs: %s", stateIDs.StateEventIDs[i])
+			continue
+		}
+		respState.StateEvents = append(respState.StateEvents, ev.Unwrap())
+	}
+	for i := range stateIDs.AuthEventIDs {
+		ev, ok := t.haveEvents[stateIDs.AuthEventIDs[i]]
+		if !ok {
+			logrus.Warnf("Missing auth event in createRespStateFromStateIDs: %s", stateIDs.AuthEventIDs[i])
+			continue
+		}
+		respState.AuthEvents = append(respState.AuthEvents, ev.Unwrap())
+	}
+	// We purposefully do not do auth checks on the returned events, as they will still
+	// be processed in the exact same way, just as a 'rejected' event
+	// TODO: Add a field to HeaderedEvent to indicate if the event is rejected.
+	return &respState, nil
+}
+
+func (t *missingStateReq) lookupEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, missingEventID string, localFirst bool) (*gomatrixserverlib.HeaderedEvent, error) {
+	if localFirst {
+		// fetch from the roomserver
+		queryReq := api.QueryEventsByIDRequest{
+			EventIDs: []string{missingEventID},
+		}
+		var queryRes api.QueryEventsByIDResponse
+		if err := t.queryer.QueryEventsByID(ctx, &queryReq, &queryRes); err != nil {
+			util.GetLogger(ctx).Warnf("Failed to query roomserver for missing event %s: %s - falling back to remote", missingEventID, err)
+		} else if len(queryRes.Events) == 1 {
+			return queryRes.Events[0], nil
+		}
+	}
+	var event *gomatrixserverlib.Event
+	found := false
+	for _, serverName := range t.servers {
+		txn, err := t.federation.GetEvent(ctx, serverName, missingEventID)
+		if err != nil || len(txn.PDUs) == 0 {
+			util.GetLogger(ctx).WithError(err).WithField("event_id", missingEventID).Warn("Failed to get missing /event for event ID")
+			if errors.Is(err, context.DeadlineExceeded) {
+				break
+			}
+			continue
+		}
+		event, err = gomatrixserverlib.NewEventFromUntrustedJSON(txn.PDUs[0], roomVersion)
+		if err != nil {
+			util.GetLogger(ctx).WithError(err).WithField("event_id", missingEventID).Warnf("Transaction: Failed to parse event JSON of event")
+			continue
+		}
+		found = true
+		break
+	}
+	if !found {
+		util.GetLogger(ctx).WithField("event_id", missingEventID).Warnf("Failed to get missing /event for event ID from %d server(s)", len(t.servers))
+		return nil, fmt.Errorf("wasn't able to find event via %d server(s)", len(t.servers))
+	}
+	if err := event.VerifyEventSignatures(ctx, t.keys); err != nil {
+		util.GetLogger(ctx).WithError(err).Warnf("Transaction: Couldn't validate signature of event %q", event.EventID())
+		return nil, verifySigError{event.EventID(), err}
+	}
+	return t.cacheAndReturn(event.Headered(roomVersion)), nil
+}
+
+func checkAllowedByState(e *gomatrixserverlib.Event, stateEvents []*gomatrixserverlib.Event) error {
+	authUsingState := gomatrixserverlib.NewAuthEvents(nil)
+	for i := range stateEvents {
+		err := authUsingState.AddEvent(stateEvents[i])
+		if err != nil {
+			return err
+		}
+	}
+	return gomatrixserverlib.Allowed(e, &authUsingState)
+}
+
+func (t *missingStateReq) hadEvent(eventID string, had bool) {
+	t.hadEventsMutex.Lock()
+	defer t.hadEventsMutex.Unlock()
+	t.hadEvents[eventID] = had
+}
+
+type roomNotFoundError struct {
+	roomID string
+}
+type verifySigError struct {
+	eventID string
+	err     error
+}
+type missingPrevEventsError struct {
+	eventID string
+	err     error
+}
+
+func (e roomNotFoundError) Error() string { return fmt.Sprintf("room %q not found", e.roomID) }
+func (e verifySigError) Error() string {
+	return fmt.Sprintf("unable to verify signature of event %q: %s", e.eventID, e.err)
+}
+func (e missingPrevEventsError) Error() string {
+	return fmt.Sprintf("unable to get prev_events for event %q: %s", e.eventID, e.err)
+}

roomserver/internal/perform/perform_invite.go

@@ -172,6 +172,7 @@ func (r *Inviter) PerformInvite(
 				{
 					Kind:         api.KindNew,
 					Event:        event,
+					Origin:       event.Origin(),
 					AuthEventIDs: event.AuthEventIDs(),
 					SendAsServer: req.SendAsServer,
 				},

roomserver/roomserver_test.go

@@ -189,7 +189,7 @@ func mustSendEvents(t *testing.T, ver gomatrixserverlib.RoomVersion, events []js
 	t.Helper()
 	rsAPI, dp := mustCreateRoomserverAPI(t)
 	hevents := mustLoadRawEvents(t, ver, events)
-	if err := api.SendEvents(ctx, rsAPI, api.KindNew, hevents, testOrigin, nil); err != nil {
+	if err := api.SendEvents(ctx, rsAPI, api.KindNew, hevents, testOrigin, testOrigin, nil); err != nil {
 		t.Errorf("failed to SendEvents: %s", err)
 	}
 	return rsAPI, dp, hevents
@@ -335,7 +335,7 @@ func TestOutputRewritesState(t *testing.T) {
 	deleteDatabase()
 	rsAPI, producer := mustCreateRoomserverAPI(t)
 	defer deleteDatabase()
-	err := api.SendEvents(context.Background(), rsAPI, api.KindNew, originalEvents, testOrigin, nil)
+	err := api.SendEvents(context.Background(), rsAPI, api.KindNew, originalEvents, testOrigin, testOrigin, nil)
 	if err != nil {
 		t.Fatalf("failed to send original events: %s", err)
 	}