Implement backfill over federation (#938)

* Implement history visibility checks for /backfill Required for p2p to show history correctly. * Add sytest * Logging * Fix two backfill bugs which prevented backfill from working correctly - When receiving backfill requests, do not send the event that was in the original request. - When storing backfill results, correctly update the backwards extremity for the room. * hack: make backfill work multiple times * add sqlite impl and remove logging * Linting
2024-11-22 03:31:41 +00:00 · 2020-03-24 12:20:10 +00:00 · 2020-03-24 12:20:10 +00:00 · 6bac7e5efd
commit 6bac7e5efd
parent 5a1a1ded1b
11 changed files with 322 additions and 202 deletions
--- a/go.sum
+++ b/go.sum
@ -276,6 +276,8 @@ github.com/matrix-org/gomatrixserverlib v0.0.0-20200318145320-bc896516d72a h1:7+
 github.com/matrix-org/gomatrixserverlib v0.0.0-20200318145320-bc896516d72a/go.mod h1:FsKa2pWE/bpQql9H7U4boOPXFoJX/QcqaZZ6ijLkaZI=
 github.com/matrix-org/gomatrixserverlib v0.0.0-20200318150006-bc27294f9203 h1:7HkL6bF7/M2cYteNFVtvGW5qjD4wHIiR0HsdCm2Rqao=
 github.com/matrix-org/gomatrixserverlib v0.0.0-20200318150006-bc27294f9203/go.mod h1:FsKa2pWE/bpQql9H7U4boOPXFoJX/QcqaZZ6ijLkaZI=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200318180716-bc4ff56961e2 h1:y4DOMbhgPAnATHJ4lNxTWxIlJG0SlIPhvukx1sQkty4=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200318180716-bc4ff56961e2/go.mod h1:FsKa2pWE/bpQql9H7U4boOPXFoJX/QcqaZZ6ijLkaZI=
 github.com/matrix-org/naffka v0.0.0-20200127221512-0716baaabaf1 h1:osLoFdOy+ChQqVUn2PeTDETFftVkl4w9t/OW18g3lnk=
 github.com/matrix-org/naffka v0.0.0-20200127221512-0716baaabaf1/go.mod h1:cXoYQIENbdWIQHt1SyCo6Bl3C3raHwJ0wgVrXHSqf+A=
 github.com/matrix-org/util v0.0.0-20171127121716-2e2df66af2f5 h1:W7l5CP4V7wPyPb4tYE11dbmeAOwtFQBTW0rf4OonOS8=
@ -470,6 +472,8 @@ go.uber.org/atomic v1.3.0 h1:vs7fgriifsPbGdK3bNuMWapNn3qnZhCRXc19NRdq010=
 go.uber.org/atomic v1.3.0/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
 go.uber.org/atomic v1.4.0 h1:cxzIVoETapQEqDhQu3QfnvXAV4AlzcvUCxkVUFw3+EU=
 go.uber.org/atomic v1.4.0/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
+go.uber.org/atomic v1.6.0 h1:Ezj3JGmsOnG1MoRWQkPBsKLe9DwWD9QeXzTRzzldNVk=
+go.uber.org/atomic v1.6.0/go.mod h1:sABNBOSYdrvTF6hTgEIbc7YasKWGhgEQZyfxyTvoXHQ=
 go.uber.org/multierr v1.1.0/go.mod h1:wR5kodmAFQ0UK8QlbwjlSNy0Z68gJhDJUG5sjR94q/0=
 go.uber.org/zap v1.10.0/go.mod h1:vwi/ZaCAaUcBkycHslxD9B2zi4UTXhF60s6SWpuDF0Q=
 golang.org/x/crypto v0.0.0-20170930174604-9419663f5a44/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
@ -491,6 +495,7 @@ golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL
 golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=
 golang.org/x/lint v0.0.0-20190227174305-5b3e6a55c961/go.mod h1:wehouNa3lNwaWXcvxsM5YxQ5yQlVC4a0KAMCusXpPoU=
 golang.org/x/lint v0.0.0-20190313153728-d0100b6bd8b3/go.mod h1:6SW0HCj/g11FgYtHlgUYUwCkIfeOF89ocIRzGO/8vkc=
+golang.org/x/lint v0.0.0-20190930215403-16217165b5de/go.mod h1:6SW0HCj/g11FgYtHlgUYUwCkIfeOF89ocIRzGO/8vkc=
 golang.org/x/mod v0.0.0-20190513183733-4bf6d317e70e/go.mod h1:mXi4GBBbnImb6dmsKGUJ2LatrhH/nqhxcFungHvyanc=
 golang.org/x/net v0.0.0-20180724234803-3673e40ba225/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20180826012351-8a410e7b638d/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
@ -559,6 +564,7 @@ golang.org/x/tools v0.0.0-20190910044552-dd2b5c81c578/go.mod h1:b+2E5dAYhXwXZwtn
 golang.org/x/tools v0.0.0-20190911230505-6bfd74cf029c/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/tools v0.0.0-20190912215617-3720d1ec3678 h1:rM1Udd0CgtYI3KUIhu9ROz0QCqjW+n/ODp/hH7c60Xc=
 golang.org/x/tools v0.0.0-20190912215617-3720d1ec3678/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/tools v0.0.0-20191029041327-9cc4af7d6b2c/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
--- a/roomserver/auth/auth.go
+++ b/roomserver/auth/auth.go
@ -12,18 +12,76 @@

 package auth

-import "github.com/matrix-org/gomatrixserverlib"
+import (
+	"encoding/json"

-// IsServerAllowed returns true if there exists a event in authEvents
-// which allows server to view this event. That is true when a client on the server
-// can view the event. Otherwise returns false.
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// TODO: This logic should live in gomatrixserverlib
+
+// IsServerAllowed returns true if the server is allowed to see events in the room
+// at this particular state. This function implements https://matrix.org/docs/spec/client_server/r0.6.0#id87
 func IsServerAllowed(
 	serverName gomatrixserverlib.ServerName,
+	serverCurrentlyInRoom bool,
 	authEvents []gomatrixserverlib.Event,
 ) bool {
+	historyVisibility := historyVisibilityForRoom(authEvents)
+
+	// 1. If the history_visibility was set to world_readable, allow.
+	if historyVisibility == "world_readable" {
+		return true
+	}
+	// 2. If the user's membership was join, allow.
+	joinedUserExists := IsAnyUserOnServerWithMembership(serverName, authEvents, gomatrixserverlib.Join)
+	if joinedUserExists {
+		return true
+	}
+	// 3. If history_visibility was set to shared, and the user joined the room at any point after the event was sent, allow.
+	if historyVisibility == "shared" && serverCurrentlyInRoom {
+		return true
+	}
+	// 4. If the user's membership was invite, and the history_visibility was set to invited, allow.
+	invitedUserExists := IsAnyUserOnServerWithMembership(serverName, authEvents, gomatrixserverlib.Invite)
+	if invitedUserExists && historyVisibility == "invited" {
+		return true
+	}
+
+	// 5. Otherwise, deny.
+	return false
+}
+
+func historyVisibilityForRoom(authEvents []gomatrixserverlib.Event) string {
+	// https://matrix.org/docs/spec/client_server/r0.6.0#id87
+	// By default if no history_visibility is set, or if the value is not understood, the visibility is assumed to be shared.
+	visibility := "shared"
+	knownStates := []string{"invited", "joined", "shared", "world_readable"}
+	for _, ev := range authEvents {
+		if ev.Type() != gomatrixserverlib.MRoomHistoryVisibility {
+			continue
+		}
+		// TODO: This should be HistoryVisibilityContent to match things like 'MemberContent'. Do this when moving to GMSL
+		content := struct {
+			HistoryVisibility string `json:"history_visibility"`
+		}{}
+		if err := json.Unmarshal(ev.Content(), &content); err != nil {
+			break // value is not understood
+		}
+		for _, s := range knownStates {
+			if s == content.HistoryVisibility {
+				visibility = s
+				break
+			}
+		}
+	}
+	return visibility
+}
+
+func IsAnyUserOnServerWithMembership(serverName gomatrixserverlib.ServerName, authEvents []gomatrixserverlib.Event, wantMembership string) bool {
 	for _, ev := range authEvents {
 		membership, err := ev.Membership()
-		if err != nil || membership != gomatrixserverlib.Join {
+		if err != nil || membership != wantMembership {
 			continue
 		}

@ -41,7 +99,5 @@ func IsServerAllowed(
 			return true
 		}
 	}
-
-	// TODO: Check if history visibility is shared and if the server is currently in the room
 	return false
 }
--- a/roomserver/query/query.go
+++ b/roomserver/query/query.go
@ -447,14 +447,26 @@ func (r *RoomserverQueryAPI) QueryServerAllowedToSeeEvent(
 	request *api.QueryServerAllowedToSeeEventRequest,
 	response *api.QueryServerAllowedToSeeEventResponse,
 ) (err error) {
+	events, err := r.DB.EventsFromIDs(ctx, []string{request.EventID})
+	if err != nil {
+		return
+	}
+	if len(events) == 0 {
+		response.AllowedToSeeEvent = false // event doesn't exist so not allowed to see
+		return
+	}
+	isServerInRoom, err := r.isServerCurrentlyInRoom(ctx, request.ServerName, events[0].RoomID())
+	if err != nil {
+		return
+	}
 	response.AllowedToSeeEvent, err = r.checkServerAllowedToSeeEvent(
-		ctx, request.EventID, request.ServerName,
+		ctx, request.EventID, request.ServerName, isServerInRoom,
 	)
 	return
 }

 func (r *RoomserverQueryAPI) checkServerAllowedToSeeEvent(
-	ctx context.Context, eventID string, serverName gomatrixserverlib.ServerName,
+	ctx context.Context, eventID string, serverName gomatrixserverlib.ServerName, isServerInRoom bool,
 ) (bool, error) {
 	roomState := state.NewStateResolution(r.DB)
 	stateEntries, err := roomState.LoadStateAtEvent(ctx, eventID)
@ -469,7 +481,7 @@ func (r *RoomserverQueryAPI) checkServerAllowedToSeeEvent(
 		return false, err
 	}

-	return auth.IsServerAllowed(serverName, stateAtEvent), nil
+	return auth.IsServerAllowed(serverName, isServerInRoom, stateAtEvent), nil
 }

 // QueryMissingEvents implements api.RoomserverQueryAPI
@ -564,17 +576,55 @@ func (r *RoomserverQueryAPI) QueryBackfill(
 	return err
 }

+func (r *RoomserverQueryAPI) isServerCurrentlyInRoom(ctx context.Context, serverName gomatrixserverlib.ServerName, roomID string) (bool, error) {
+	roomNID, err := r.DB.RoomNID(ctx, roomID)
+	if err != nil {
+		return false, err
+	}
+
+	eventNIDs, err := r.DB.GetMembershipEventNIDsForRoom(ctx, roomNID, true)
+	if err != nil {
+		return false, err
+	}
+
+	events, err := r.DB.Events(ctx, eventNIDs)
+	if err != nil {
+		return false, err
+	}
+	gmslEvents := make([]gomatrixserverlib.Event, len(events))
+	for i := range events {
+		gmslEvents[i] = events[i].Event
+	}
+	return auth.IsAnyUserOnServerWithMembership(serverName, gmslEvents, gomatrixserverlib.Join), nil
+}
+
+// TODO: Remove this when we have tests to assert correctness of this function
+// nolint:gocyclo
 func (r *RoomserverQueryAPI) scanEventTree(
 	ctx context.Context, front []string, visited map[string]bool, limit int,
 	serverName gomatrixserverlib.ServerName,
-) (resultNIDs []types.EventNID, err error) {
+) ([]types.EventNID, error) {
+	var resultNIDs []types.EventNID
+	var err error
 	var allowed bool
 	var events []types.Event
 	var next []string
 	var pre string

+	// TODO: add tests for this function to ensure it meets the contract that callers expect (and doc what that is supposed to be)
+	// Currently, callers like QueryBackfill will call scanEventTree with a pre-populated `visited` map, assuming that by doing
+	// so means that the events in that map will NOT be returned from this function. That is not currently true, resulting in
+	// duplicate events being sent in response to /backfill requests.
+	initialIgnoreList := make(map[string]bool, len(visited))
+	for k, v := range visited {
+		initialIgnoreList[k] = v
+	}
+
 	resultNIDs = make([]types.EventNID, 0, limit)

+	var checkedServerInRoom bool
+	var isServerInRoom bool
+
 	// Loop through the event IDs to retrieve the requested events and go
 	// through the whole tree (up to the provided limit) using the events'
 	// "prev_event" key.
@ -587,7 +637,18 @@ BFSLoop:
 		// Retrieve the events to process from the database.
 		events, err = r.DB.EventsFromIDs(ctx, front)
 		if err != nil {
-			return
+			return resultNIDs, err
+		}
+
+		if !checkedServerInRoom && len(events) > 0 {
+			// It's nasty that we have to extract the room ID from an event, but many federation requests
+			// only talk in event IDs, no room IDs at all (!!!)
+			ev := events[0]
+			isServerInRoom, err = r.isServerCurrentlyInRoom(ctx, serverName, ev.RoomID())
+			if err != nil {
+				util.GetLogger(ctx).WithError(err).Error("Failed to check if server is currently in room, assuming not.")
+			}
+			checkedServerInRoom = true
 		}

 		for _, ev := range events {
@ -595,17 +656,23 @@ BFSLoop:
 			if len(resultNIDs) == limit {
 				break BFSLoop
 			}
+
+			if !initialIgnoreList[ev.EventID()] {
 				// Update the list of events to retrieve.
 				resultNIDs = append(resultNIDs, ev.EventNID)
+			}
 			// Loop through the event's parents.
 			for _, pre = range ev.PrevEventIDs() {
 				// Only add an event to the list of next events to process if it
 				// hasn't been seen before.
 				if !visited[pre] {
 					visited[pre] = true
-					allowed, err = r.checkServerAllowedToSeeEvent(ctx, pre, serverName)
+					allowed, err = r.checkServerAllowedToSeeEvent(ctx, pre, serverName, isServerInRoom)
 					if err != nil {
-						return
+						util.GetLogger(ctx).WithField("server", serverName).WithField("event_id", pre).WithError(err).Error(
+							"Error checking if allowed to see event",
+						)
+						return resultNIDs, err
 					}

 					// If the event hasn't been seen before and the HS
@ -613,6 +680,8 @@ BFSLoop:
 					// the list of events to retrieve.
 					if allowed {
 						next = append(next, pre)
+					} else {
+						util.GetLogger(ctx).WithField("server", serverName).WithField("event_id", pre).Info("Not allowed to see event")
 					}
 				}
 			}
@ -621,7 +690,7 @@ BFSLoop:
 		front = next
 	}

-	return
+	return resultNIDs, err
 }

 // QueryStateAndAuthChain implements api.RoomserverQueryAPI
--- a/syncapi/routing/messages.go
+++ b/syncapi/routing/messages.go
@ -28,7 +28,7 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
-	log "github.com/sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )

 type messagesReq struct {
@ -151,6 +151,14 @@ func OnIncomingMessagesRequest(
 		util.GetLogger(req.Context()).WithError(err).Error("mreq.retrieveEvents failed")
 		return jsonerror.InternalServerError()
 	}
+	util.GetLogger(req.Context()).WithFields(logrus.Fields{
+		"from":         from.String(),
+		"to":           to.String(),
+		"limit":        limit,
+		"backwards":    backwardOrdering,
+		"return_start": start.String(),
+		"return_end":   end.String(),
+	}).Info("Responding")

 	// Respond with the events.
 	return util.JSONResponse{
@ -302,8 +310,9 @@ func (r *messagesReq) handleNonEmptyEventsSlice(streamEvents []types.StreamEvent
 	events []gomatrixserverlib.HeaderedEvent, err error,
 ) {
 	// Check if we have enough events.
-	isSetLargeEnough := true
-	if len(streamEvents) < r.limit {
+	isSetLargeEnough := len(streamEvents) >= r.limit
+	if !isSetLargeEnough {
+		// it might be fine we don't have up to 'limit' events, let's find out
 		if r.backwardOrdering {
 			if r.wasToProvided {
 				// The condition in the SQL query is a strict "greater than" so
@ -343,54 +352,6 @@ func (r *messagesReq) handleNonEmptyEventsSlice(streamEvents []types.StreamEvent
 	return
 }

-// containsBackwardExtremity checks if a slice of StreamEvent contains a
-// backward extremity. It does so by selecting the earliest event in the slice
-// and by checking the presence in the database of all of its parent events, and
-// considers the event itself a backward extremity if at least one of the parent
-// events doesn't exist in the database.
-// Returns an error if there was an issue with talking to the database.
-//
-// This function is unused but currently set to nolint for now until we are
-// absolutely sure that the changes in matrix-org/dendrite#847 are behaving
-// properly.
-// nolint:unused
-func (r *messagesReq) containsBackwardExtremity(events []types.StreamEvent) (bool, error) {
-	// Select the earliest retrieved event.
-	var ev *types.StreamEvent
-	if r.backwardOrdering {
-		ev = &(events[len(events)-1])
-	} else {
-		ev = &(events[0])
-	}
-	// Get the earliest retrieved event's parents.
-	prevIDs := ev.PrevEventIDs()
-	prevs, err := r.db.Events(r.ctx, prevIDs)
-	if err != nil {
-		return false, nil
-	}
-	// Check if we have all of the events we requested. If not, it means we've
-	// reached a backward extremity.
-	var eventInDB bool
-	var id string
-	// Iterate over the IDs we used in the request.
-	for _, id = range prevIDs {
-		eventInDB = false
-		// Iterate over the events we got in response.
-		for _, ev := range prevs {
-			if ev.EventID() == id {
-				eventInDB = true
-			}
-		}
-		// One occurrence of one the event's parents not being present in the
-		// database is enough to say that the event is a backward extremity.
-		if !eventInDB {
-			return true, nil
-		}
-	}
-
-	return false, nil
-}
-
 // backfill performs a backfill request over the federation on another
 // homeserver in the room.
 // See: https://matrix.org/docs/spec/server_server/latest#get-matrix-federation-v1-backfill-roomid
@ -401,38 +362,16 @@ func (r *messagesReq) containsBackwardExtremity(events []types.StreamEvent) (boo
 // Returns an error if there was an issue with retrieving the list of servers in
 // the room or sending the request.
 func (r *messagesReq) backfill(fromEventIDs []string, limit int) ([]gomatrixserverlib.HeaderedEvent, error) {
-	// Query the list of servers in the room when one of the backward extremities
-	// was sent.
-	var serversResponse api.QueryServersInRoomAtEventResponse
-	serversRequest := api.QueryServersInRoomAtEventRequest{
-		RoomID:  r.roomID,
-		EventID: fromEventIDs[0],
-	}
-	if err := r.queryAPI.QueryServersInRoomAtEvent(r.ctx, &serversRequest, &serversResponse); err != nil {
-		return nil, err
-	}
-
-	// Use the first server from the response, except if that server is us.
-	// In that case, use the second one if the roomserver responded with
-	// enough servers. If not, use an empty string to prevent the backfill
-	// from happening as there's no server to direct the request towards.
-	// TODO: Be smarter at selecting the server to direct the request
-	// towards.
-	srvToBackfillFrom := serversResponse.Servers[0]
-	if srvToBackfillFrom == r.cfg.Matrix.ServerName {
-		if len(serversResponse.Servers) > 1 {
-			srvToBackfillFrom = serversResponse.Servers[1]
-		} else {
-			srvToBackfillFrom = gomatrixserverlib.ServerName("")
-			log.Warn("Not enough servers to backfill from")
-		}
+	srvToBackfillFrom, err := r.serverToBackfillFrom(fromEventIDs)
+	if err != nil {
+		return nil, fmt.Errorf("Cannot find server to backfill from: %w", err)
 	}

 	pdus := make([]gomatrixserverlib.HeaderedEvent, 0)

 	// If the roomserver responded with at least one server that isn't us,
 	// send it a request for backfill.
-	if len(srvToBackfillFrom) > 0 {
+	util.GetLogger(r.ctx).WithField("server", srvToBackfillFrom).WithField("limit", limit).Info("Backfilling from server")
 	txn, err := r.federation.Backfill(
 		r.ctx, srvToBackfillFrom, r.roomID, limit, fromEventIDs,
 	)
@ -443,6 +382,7 @@ func (r *messagesReq) backfill(fromEventIDs []string, limit int) ([]gomatrixserv
 	for _, p := range txn.PDUs {
 		pdus = append(pdus, p.Headered(gomatrixserverlib.RoomVersionV1))
 	}
+	util.GetLogger(r.ctx).WithField("server", srvToBackfillFrom).WithField("new_events", len(pdus)).Info("Storing new events from backfill")

 	// Store the events in the database, while marking them as unfit to show
 	// up in responses to sync requests.
@ -459,11 +399,66 @@ func (r *messagesReq) backfill(fromEventIDs []string, limit int) ([]gomatrixserv
 			return nil, err
 		}
 	}
-	}

 	return pdus, nil
 }

+func (r *messagesReq) serverToBackfillFrom(fromEventIDs []string) (gomatrixserverlib.ServerName, error) {
+	// Query the list of servers in the room when one of the backward extremities
+	// was sent.
+	var serversResponse api.QueryServersInRoomAtEventResponse
+	serversRequest := api.QueryServersInRoomAtEventRequest{
+		RoomID:  r.roomID,
+		EventID: fromEventIDs[0],
+	}
+	if err := r.queryAPI.QueryServersInRoomAtEvent(r.ctx, &serversRequest, &serversResponse); err != nil {
+		util.GetLogger(r.ctx).WithError(err).Warn("Failed to query servers in room at event, falling back to event sender")
+		// FIXME: We shouldn't be doing this but in situations where we have already backfilled once
+		// the query API doesn't work as backfilled events do not make it to the room server.
+		// This means QueryServersInRoomAtEvent returns an error as it doesn't have the event ID in question.
+		// We need to inject backfilled events into the room server and store them appropriately.
+		events, err := r.db.Events(r.ctx, fromEventIDs)
+		if err != nil {
+			return "", err
+		}
+		if len(events) == 0 {
+			// should be impossible as these event IDs are backwards extremities
+			return "", fmt.Errorf("backfill: missing backwards extremities, event IDs: %s", fromEventIDs)
+		}
+		// The rationale here is that the last event was unlikely to be sent by us, so poke the server who sent it.
+		// We shouldn't be doing this really, but as a heuristic it should work pretty well for now.
+		for _, e := range events {
+			_, srv, srverr := gomatrixserverlib.SplitID('@', e.Sender())
+			if srverr != nil {
+				util.GetLogger(r.ctx).WithError(srverr).Warn("Failed to extract domain from event sender")
+				continue
+			}
+			if srv != r.cfg.Matrix.ServerName {
+				return srv, nil
+			}
+		}
+		// no valid events which have a remote server, fail.
+		return "", err
+	}
+
+	// Use the first server from the response, except if that server is us.
+	// In that case, use the second one if the roomserver responded with
+	// enough servers. If not, use an empty string to prevent the backfill
+	// from happening as there's no server to direct the request towards.
+	// TODO: Be smarter at selecting the server to direct the request
+	// towards.
+	srvToBackfillFrom := serversResponse.Servers[0]
+	if srvToBackfillFrom == r.cfg.Matrix.ServerName {
+		if len(serversResponse.Servers) > 1 {
+			srvToBackfillFrom = serversResponse.Servers[1]
+		} else {
+			util.GetLogger(r.ctx).Info("Not enough servers to backfill from")
+			return "", nil
+		}
+	}
+	return srvToBackfillFrom, nil
+}
+
 // setToDefault returns the default value for the "to" query parameter of a
 // request to /messages if not provided. It defaults to either the earliest
 // topological position (if we're going backward) or to the latest one (if we're
@ -475,7 +470,8 @@ func setToDefault(
 	roomID string,
 ) (to *types.PaginationToken, err error) {
 	if backwardOrdering {
-		to = types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, 1, 0)
+		// go 1 earlier than the first event so we correctly fetch the earliest event
+		to = types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, 0, 0)
 	} else {
 		var pos types.StreamPosition
 		pos, err = db.MaxTopologicalPosition(ctx, roomID)
--- a/syncapi/storage/postgres/backward_extremities_table.go
+++ b/syncapi/storage/postgres/backward_extremities_table.go
@ -21,39 +21,53 @@ import (
 	"github.com/matrix-org/dendrite/common"
 )

+// The purpose of this table is to keep track of backwards extremities for a room.
+// Backwards extremities are the earliest (DAG-wise) known events which we have
+// the entire event JSON. These event IDs are used in federation requests to fetch
+// even earlier events.
+//
+// We persist the previous event IDs as well, one per row, so when we do fetch even
+// earlier events we can simply delete rows which referenced it. Consider the graph:
+//        A
+//        |   Event C has 1 prev_event ID: A.
+//    B   C
+//    |___|   Event D has 2 prev_event IDs: B and C.
+//      |
+//      D
+// The earliest known event we have is D, so this table has 2 rows.
+// A backfill request gives us C but not B. We delete rows where prev_event=C. This
+// still means that D is a backwards extremity as we do not have event B. However, event
+// C is *also* a backwards extremity at this point as we do not have event A. Later,
+// when we fetch event B, we delete rows where prev_event=B. This then removes D as
+// a backwards extremity because there are no more rows with event_id=B.
 const backwardExtremitiesSchema = `
 -- Stores output room events received from the roomserver.
 CREATE TABLE IF NOT EXISTS syncapi_backward_extremities (
 	-- The 'room_id' key for the event.
 	room_id TEXT NOT NULL,
-	-- The event ID for the event.
+	-- The event ID for the last known event. This is the backwards extremity.
 	event_id TEXT NOT NULL,
+	-- The prev_events for the last known event. This is used to update extremities.
+	prev_event_id TEXT NOT NULL,

-	PRIMARY KEY(room_id, event_id)
+	PRIMARY KEY(room_id, event_id, prev_event_id)
 );
 `

 const insertBackwardExtremitySQL = "" +
-	"INSERT INTO syncapi_backward_extremities (room_id, event_id)" +
-	" VALUES ($1, $2)" +
+	"INSERT INTO syncapi_backward_extremities (room_id, event_id, prev_event_id)" +
+	" VALUES ($1, $2, $3)" +
 	" ON CONFLICT DO NOTHING"

 const selectBackwardExtremitiesForRoomSQL = "" +
 	"SELECT event_id FROM syncapi_backward_extremities WHERE room_id = $1"

-const isBackwardExtremitySQL = "" +
-	"SELECT EXISTS (" +
-	" SELECT TRUE FROM syncapi_backward_extremities" +
-	" WHERE room_id = $1 AND event_id = $2" +
-	")"
-
 const deleteBackwardExtremitySQL = "" +
-	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND event_id = $2"
+	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND prev_event_id = $2"

 type backwardExtremitiesStatements struct {
 	insertBackwardExtremityStmt          *sql.Stmt
 	selectBackwardExtremitiesForRoomStmt *sql.Stmt
-	isBackwardExtremityStmt              *sql.Stmt
 	deleteBackwardExtremityStmt          *sql.Stmt
 }

@ -68,9 +82,6 @@ func (s *backwardExtremitiesStatements) prepare(db *sql.DB) (err error) {
 	if s.selectBackwardExtremitiesForRoomStmt, err = db.Prepare(selectBackwardExtremitiesForRoomSQL); err != nil {
 		return
 	}
-	if s.isBackwardExtremityStmt, err = db.Prepare(isBackwardExtremitySQL); err != nil {
-		return
-	}
 	if s.deleteBackwardExtremityStmt, err = db.Prepare(deleteBackwardExtremitySQL); err != nil {
 		return
 	}
@ -78,17 +89,15 @@ func (s *backwardExtremitiesStatements) prepare(db *sql.DB) (err error) {
 }

 func (s *backwardExtremitiesStatements) insertsBackwardExtremity(
-	ctx context.Context, roomID, eventID string,
+	ctx context.Context, txn *sql.Tx, roomID, eventID string, prevEventID string,
 ) (err error) {
-	_, err = s.insertBackwardExtremityStmt.ExecContext(ctx, roomID, eventID)
+	_, err = txn.Stmt(s.insertBackwardExtremityStmt).ExecContext(ctx, roomID, eventID, prevEventID)
 	return
 }

 func (s *backwardExtremitiesStatements) selectBackwardExtremitiesForRoom(
 	ctx context.Context, roomID string,
 ) (eventIDs []string, err error) {
-	eventIDs = make([]string, 0)
-
 	rows, err := s.selectBackwardExtremitiesForRoomStmt.QueryContext(ctx, roomID)
 	if err != nil {
 		return
@ -107,16 +116,9 @@ func (s *backwardExtremitiesStatements) selectBackwardExtremitiesForRoom(
 	return eventIDs, rows.Err()
 }

-func (s *backwardExtremitiesStatements) isBackwardExtremity(
-	ctx context.Context, roomID, eventID string,
-) (isBE bool, err error) {
-	err = s.isBackwardExtremityStmt.QueryRowContext(ctx, roomID, eventID).Scan(&isBE)
-	return
-}
-
 func (s *backwardExtremitiesStatements) deleteBackwardExtremity(
-	ctx context.Context, roomID, eventID string,
+	ctx context.Context, txn *sql.Tx, roomID, knownEventID string,
 ) (err error) {
-	_, err = s.insertBackwardExtremityStmt.ExecContext(ctx, roomID, eventID)
+	_, err = txn.Stmt(s.deleteBackwardExtremityStmt).ExecContext(ctx, roomID, knownEventID)
 	return
 }
--- a/syncapi/storage/postgres/output_room_events_table.go
+++ b/syncapi/storage/postgres/output_room_events_table.go
@ -305,7 +305,6 @@ func (s *outputRoomEventsStatements) selectRecentEvents(
 	} else {
 		stmt = common.TxStmt(txn, s.selectRecentEventsStmt)
 	}
-
 	rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
 	if err != nil {
 		return nil, err
--- a/syncapi/storage/postgres/syncserver.go
+++ b/syncapi/storage/postgres/syncserver.go
@ -111,22 +111,17 @@ func (d *SyncServerDatasource) Events(ctx context.Context, eventIDs []string) ([
 	return d.StreamEventsToEvents(nil, streamEvents), nil
 }

-func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, ev *gomatrixserverlib.HeaderedEvent) error {
-	// If the event is already known as a backward extremity, don't consider
-	// it as such anymore now that we have it.
-	isBackwardExtremity, err := d.backwardExtremities.isBackwardExtremity(ctx, ev.RoomID(), ev.EventID())
-	if err != nil {
+// handleBackwardExtremities adds this event as a backwards extremity if and only if we do not have all of
+// the events listed in the event's 'prev_events'. This function also updates the backwards extremities table
+// to account for the fact that the given event is no longer a backwards extremity, but may be marked as such.
+func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, txn *sql.Tx, ev *gomatrixserverlib.HeaderedEvent) error {
+	if err := d.backwardExtremities.deleteBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID()); err != nil {
 		return err
 	}
-	if isBackwardExtremity {
-		if err = d.backwardExtremities.deleteBackwardExtremity(ctx, ev.RoomID(), ev.EventID()); err != nil {
-			return err
-		}
-	}

 	// Check if we have all of the event's previous events. If an event is
 	// missing, add it to the room's backward extremities.
-	prevEvents, err := d.events.selectEvents(ctx, nil, ev.PrevEventIDs())
+	prevEvents, err := d.events.selectEvents(ctx, txn, ev.PrevEventIDs())
 	if err != nil {
 		return err
 	}
@ -141,7 +136,7 @@ func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, ev

 		// If the event is missing, consider it a backward extremity.
 		if !found {
-			if err = d.backwardExtremities.insertsBackwardExtremity(ctx, ev.RoomID(), ev.EventID()); err != nil {
+			if err = d.backwardExtremities.insertsBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID(), eID); err != nil {
 				return err
 			}
 		}
@ -174,7 +169,7 @@ func (d *SyncServerDatasource) WriteEvent(
 			return err
 		}

-		if err = d.handleBackwardExtremities(ctx, ev); err != nil {
+		if err = d.handleBackwardExtremities(ctx, txn, ev); err != nil {
 			return err
 		}

--- a/syncapi/storage/sqlite3/backward_extremities_table.go
+++ b/syncapi/storage/sqlite3/backward_extremities_table.go
@ -21,38 +21,53 @@ import (
 	"github.com/matrix-org/dendrite/common"
 )

+// The purpose of this table is to keep track of backwards extremities for a room.
+// Backwards extremities are the earliest (DAG-wise) known events which we have
+// the entire event JSON. These event IDs are used in federation requests to fetch
+// even earlier events.
+//
+// We persist the previous event IDs as well, one per row, so when we do fetch even
+// earlier events we can simply delete rows which referenced it. Consider the graph:
+//        A
+//        |   Event C has 1 prev_event ID: A.
+//    B   C
+//    |___|   Event D has 2 prev_event IDs: B and C.
+//      |
+//      D
+// The earliest known event we have is D, so this table has 2 rows.
+// A backfill request gives us C but not B. We delete rows where prev_event=C. This
+// still means that D is a backwards extremity as we do not have event B. However, event
+// C is *also* a backwards extremity at this point as we do not have event A. Later,
+// when we fetch event B, we delete rows where prev_event=B. This then removes D as
+// a backwards extremity because there are no more rows with event_id=B.
 const backwardExtremitiesSchema = `
 -- Stores output room events received from the roomserver.
 CREATE TABLE IF NOT EXISTS syncapi_backward_extremities (
+	-- The 'room_id' key for the event.
 	room_id TEXT NOT NULL,
+	-- The event ID for the last known event. This is the backwards extremity.
 	event_id TEXT NOT NULL,
+	-- The prev_events for the last known event. This is used to update extremities.
+	prev_event_id TEXT NOT NULL,

-	PRIMARY KEY(room_id, event_id)
+	PRIMARY KEY(room_id, event_id, prev_event_id)
 );
 `

 const insertBackwardExtremitySQL = "" +
-	"INSERT INTO syncapi_backward_extremities (room_id, event_id)" +
-	" VALUES ($1, $2)" +
-	" ON CONFLICT (room_id, event_id) DO NOTHING"
+	"INSERT INTO syncapi_backward_extremities (room_id, event_id, prev_event_id)" +
+	" VALUES ($1, $2, $3)" +
+	" ON CONFLICT (room_id, event_id, prev_event_id) DO NOTHING"

 const selectBackwardExtremitiesForRoomSQL = "" +
 	"SELECT event_id FROM syncapi_backward_extremities WHERE room_id = $1"

-const isBackwardExtremitySQL = "" +
-	"SELECT EXISTS (" +
-	" SELECT TRUE FROM syncapi_backward_extremities" +
-	" WHERE room_id = $1 AND event_id = $2" +
-	")"
-
 const deleteBackwardExtremitySQL = "" +
-	"DELETE FROM syncapi_backward_extremities" +
-	" WHERE room_id = $1 AND event_id = $2"
+	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND prev_event_id = $2"

 type backwardExtremitiesStatements struct {
 	insertBackwardExtremityStmt          *sql.Stmt
 	selectBackwardExtremitiesForRoomStmt *sql.Stmt
-	isBackwardExtremityStmt              *sql.Stmt
 	deleteBackwardExtremityStmt          *sql.Stmt
 }

@ -67,9 +82,6 @@ func (s *backwardExtremitiesStatements) prepare(db *sql.DB) (err error) {
 	if s.selectBackwardExtremitiesForRoomStmt, err = db.Prepare(selectBackwardExtremitiesForRoomSQL); err != nil {
 		return
 	}
-	if s.isBackwardExtremityStmt, err = db.Prepare(isBackwardExtremitySQL); err != nil {
-		return
-	}
 	if s.deleteBackwardExtremityStmt, err = db.Prepare(deleteBackwardExtremitySQL); err != nil {
 		return
 	}
@ -77,23 +89,20 @@ func (s *backwardExtremitiesStatements) prepare(db *sql.DB) (err error) {
 }

 func (s *backwardExtremitiesStatements) insertsBackwardExtremity(
-	ctx context.Context, txn *sql.Tx, roomID, eventID string,
+	ctx context.Context, txn *sql.Tx, roomID, eventID string, prevEventID string,
 ) (err error) {
-	stmt := common.TxStmt(txn, s.insertBackwardExtremityStmt)
-	_, err = stmt.ExecContext(ctx, roomID, eventID)
+	_, err = txn.Stmt(s.insertBackwardExtremityStmt).ExecContext(ctx, roomID, eventID, prevEventID)
 	return
 }

 func (s *backwardExtremitiesStatements) selectBackwardExtremitiesForRoom(
-	ctx context.Context, txn *sql.Tx, roomID string,
+	ctx context.Context, roomID string,
 ) (eventIDs []string, err error) {
-	eventIDs = make([]string, 0)
-
-	stmt := common.TxStmt(txn, s.selectBackwardExtremitiesForRoomStmt)
-	rows, err := stmt.QueryContext(ctx, roomID)
+	rows, err := s.selectBackwardExtremitiesForRoomStmt.QueryContext(ctx, roomID)
 	if err != nil {
 		return
 	}
+	defer common.CloseAndLogIfError(ctx, rows, "selectBackwardExtremitiesForRoom: rows.close() failed")

 	for rows.Next() {
 		var eID string
@ -104,21 +113,12 @@ func (s *backwardExtremitiesStatements) selectBackwardExtremitiesForRoom(
 		eventIDs = append(eventIDs, eID)
 	}

-	return
-}
-
-func (s *backwardExtremitiesStatements) isBackwardExtremity(
-	ctx context.Context, txn *sql.Tx, roomID, eventID string,
-) (isBE bool, err error) {
-	stmt := common.TxStmt(txn, s.isBackwardExtremityStmt)
-	err = stmt.QueryRowContext(ctx, roomID, eventID).Scan(&isBE)
-	return
+	return eventIDs, rows.Err()
 }

 func (s *backwardExtremitiesStatements) deleteBackwardExtremity(
-	ctx context.Context, txn *sql.Tx, roomID, eventID string,
+	ctx context.Context, txn *sql.Tx, roomID, knownEventID string,
 ) (err error) {
-	stmt := common.TxStmt(txn, s.deleteBackwardExtremityStmt)
-	_, err = stmt.ExecContext(ctx, roomID, eventID)
+	_, err = txn.Stmt(s.deleteBackwardExtremityStmt).ExecContext(ctx, roomID, knownEventID)
 	return
 }
--- a/syncapi/storage/sqlite3/syncserver.go
+++ b/syncapi/storage/sqlite3/syncserver.go
@ -137,18 +137,13 @@ func (d *SyncServerDatasource) Events(ctx context.Context, eventIDs []string) ([
 	return d.StreamEventsToEvents(nil, streamEvents), nil
 }

+// handleBackwardExtremities adds this event as a backwards extremity if and only if we do not have all of
+// the events listed in the event's 'prev_events'. This function also updates the backwards extremities table
+// to account for the fact that the given event is no longer a backwards extremity, but may be marked as such.
 func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, txn *sql.Tx, ev *gomatrixserverlib.HeaderedEvent) error {
-	// If the event is already known as a backward extremity, don't consider
-	// it as such anymore now that we have it.
-	isBackwardExtremity, err := d.backwardExtremities.isBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID())
-	if err != nil {
+	if err := d.backwardExtremities.deleteBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID()); err != nil {
 		return err
 	}
-	if isBackwardExtremity {
-		if err = d.backwardExtremities.deleteBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID()); err != nil {
-			return err
-		}
-	}

 	// Check if we have all of the event's previous events. If an event is
 	// missing, add it to the room's backward extremities.
@ -167,7 +162,7 @@ func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, tx

 		// If the event is missing, consider it a backward extremity.
 		if !found {
-			if err = d.backwardExtremities.insertsBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID()); err != nil {
+			if err = d.backwardExtremities.insertsBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID(), eID); err != nil {
 				return err
 			}
 		}
@ -348,7 +343,7 @@ func (d *SyncServerDatasource) SyncPosition(ctx context.Context) (tok types.Pagi
 func (d *SyncServerDatasource) BackwardExtremitiesForRoom(
 	ctx context.Context, roomID string,
 ) (backwardExtremities []string, err error) {
-	return d.backwardExtremities.selectBackwardExtremitiesForRoom(ctx, nil, roomID)
+	return d.backwardExtremities.selectBackwardExtremitiesForRoom(ctx, roomID)
 }

 // MaxTopologicalPosition returns the highest topological position for a given
--- a/syncapi/sync/requestpool.go
+++ b/syncapi/sync/requestpool.go
@ -47,7 +47,6 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 	var syncData *types.Response

 	// Extract values from request
-	logger := util.GetLogger(req.Context())
 	userID := device.UserID
 	syncReq, err := newSyncRequest(req, *device)
 	if err != nil {
@ -56,20 +55,21 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 			JSON: jsonerror.Unknown(err.Error()),
 		}
 	}
-	logger.WithFields(log.Fields{
+	logger := util.GetLogger(req.Context()).WithFields(log.Fields{
 		"userID":  userID,
 		"since":   syncReq.since,
 		"timeout": syncReq.timeout,
-	}).Info("Incoming /sync request")
+	})

 	currPos := rp.notifier.CurrentPosition()

 	if shouldReturnImmediately(syncReq) {
 		syncData, err = rp.currentSyncForUser(*syncReq, currPos)
 		if err != nil {
-			util.GetLogger(req.Context()).WithError(err).Error("rp.currentSyncForUser failed")
+			logger.WithError(err).Error("rp.currentSyncForUser failed")
 			return jsonerror.InternalServerError()
 		}
+		logger.WithField("next", syncData.NextBatch).Info("Responding immediately")
 		return util.JSONResponse{
 			Code: http.StatusOK,
 			JSON: syncData,
@ -107,7 +107,7 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 			hasTimedOut = true
 		// Or for the request to be cancelled
 		case <-req.Context().Done():
-			util.GetLogger(req.Context()).WithError(err).Error("request cancelled")
+			logger.WithError(err).Error("request cancelled")
 			return jsonerror.InternalServerError()
 		}

@ -118,11 +118,12 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype

 		syncData, err = rp.currentSyncForUser(*syncReq, currPos)
 		if err != nil {
-			util.GetLogger(req.Context()).WithError(err).Error("rp.currentSyncForUser failed")
+			logger.WithError(err).Error("rp.currentSyncForUser failed")
 			return jsonerror.InternalServerError()
 		}

 		if !syncData.IsEmpty() || hasTimedOut {
+			logger.WithField("next", syncData.NextBatch).WithField("timed_out", hasTimedOut).Info("Responding")
 			return util.JSONResponse{
 				Code: http.StatusOK,
 				JSON: syncData,
--- a/1
+++ b/1
@ -220,3 +220,4 @@ Regular users can add and delete aliases when m.room.aliases is restricted
 GET /r0/capabilities is not public
 GET /joined_rooms lists newly-created room
 /joined_rooms returns only joined rooms
+Message history can be paginated over federation