fetch missing state on backfill to remember snapshots correctly

federationapi/routing/events.go

@@ -16,7 +16,9 @@ package routing
 
 import (
 	"context"
+	"encoding/json"
 	"net/http"
+	"time"
 
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
@@ -29,13 +31,20 @@ func GetEvent(
 	request *gomatrixserverlib.FederationRequest,
 	query api.RoomserverQueryAPI,
 	eventID string,
+	origin gomatrixserverlib.ServerName,
 ) util.JSONResponse {
 	event, err := getEvent(ctx, request, query, eventID)
 	if err != nil {
 		return *err
 	}
 
-	return util.JSONResponse{Code: http.StatusOK, JSON: event}
+	return util.JSONResponse{Code: http.StatusOK, JSON: gomatrixserverlib.Transaction{
+		Origin:         origin,
+		OriginServerTS: gomatrixserverlib.AsTimestamp(time.Now()),
+		PDUs: []json.RawMessage{
+			event.JSON(),
+		},
+	}}
 }
 
 // getEvent returns the requested event,

federationapi/routing/routing.go

@@ -126,7 +126,7 @@ func Setup(
 				return util.ErrorResponse(err)
 			}
 			return GetEvent(
-				httpReq.Context(), request, query, vars["eventID"],
+				httpReq.Context(), request, query, vars["eventID"], cfg.Matrix.ServerName,
 			)
 		},
 	)).Methods(http.MethodGet)

roomserver/query/query.go

@@ -572,8 +572,14 @@ func (r *RoomserverQueryAPI) backfillViaFederation(ctx context.Context, req *api
 		}
 		var entries []types.StateEntry
 		if entries, err = r.DB.StateEntriesForEventIDs(ctx, stateIDs); err != nil {
-			logrus.WithError(err).WithField("event_id", ev.EventID()).Error("backfillViaFederation: failed to get state entries for event")
-			return err
+			// attempt to fetch the missing events
+			r.fetchAndStoreMissingEvents(ctx, roomVer, requester, stateIDs)
+			// try again
+			entries, err = r.DB.StateEntriesForEventIDs(ctx, stateIDs)
+			if err != nil {
+				logrus.WithError(err).WithField("event_id", ev.EventID()).Error("backfillViaFederation: failed to get state entries for event")
+				return err
+			}
 		}
 
 		var beforeStateSnapshotNID types.StateSnapshotNID
@@ -615,6 +621,66 @@ func (r *RoomserverQueryAPI) isServerCurrentlyInRoom(ctx context.Context, server
 	return auth.IsAnyUserOnServerWithMembership(serverName, gmslEvents, gomatrixserverlib.Join), nil
 }
 
+// fetchAndStoreMissingEvents does a best-effort fetch and store of missing events specified in stateIDs. Returns no error as it is just
+// best effort.
+func (r *RoomserverQueryAPI) fetchAndStoreMissingEvents(ctx context.Context, roomVer gomatrixserverlib.RoomVersion,
+	backfillRequester *backfillRequester, stateIDs []string) {
+
+	servers := backfillRequester.servers
+
+	// work out which are missing
+	nidMap, err := r.DB.EventNIDs(ctx, stateIDs)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Warn("cannot query missing events")
+		return
+	}
+	missingMap := make(map[string]*gomatrixserverlib.HeaderedEvent) // id -> event
+	for _, id := range stateIDs {
+		if _, ok := nidMap[id]; !ok {
+			missingMap[id] = nil
+		}
+	}
+	util.GetLogger(ctx).Infof("Fetching %d missing state events (from %d possible servers)", len(missingMap), len(servers))
+
+	// fetch the events from federation. Loop the servers first so if we find one that works we stick with them
+	for _, srv := range servers {
+		for id, ev := range missingMap {
+			if ev != nil {
+				continue // already found
+			}
+			logger := util.GetLogger(ctx).WithField("server", srv).WithField("event_id", id)
+			res, err := r.FedClient.GetEvent(ctx, srv, id)
+			if err != nil {
+				logger.WithError(err).Warn("failed to get event from server")
+				continue
+			}
+			loader := gomatrixserverlib.NewEventsLoader(roomVer, r.KeyRing, backfillRequester, backfillRequester.ProvideEvents, false)
+			result, err := loader.LoadAndVerify(ctx, res.PDUs, gomatrixserverlib.TopologicalOrderByPrevEvents)
+			if err != nil {
+				logger.WithError(err).Warn("failed to load and verify event")
+				continue
+			}
+			logger.Infof("returned %d PDUs which made events %+v", len(res.PDUs), result)
+			for _, res := range result {
+				if res.Error != nil {
+					logger.WithError(err).Warn("event failed PDU checks")
+					continue
+				}
+				missingMap[id] = res.Event
+			}
+		}
+	}
+
+	var newEvents []gomatrixserverlib.HeaderedEvent
+	for _, ev := range missingMap {
+		if ev != nil {
+			newEvents = append(newEvents, *ev)
+		}
+	}
+	util.GetLogger(ctx).Infof("Persisting %d new events", len(newEvents))
+	persistEvents(ctx, r.DB, newEvents)
+}
+
 // TODO: Remove this when we have tests to assert correctness of this function
 // nolint:gocyclo
 func (r *RoomserverQueryAPI) scanEventTree(