sytest: Make 'Inbound federation can backfill events' pass (#1051)

* sytest: Make 'Inbound federation can backfill events' pass

This breaks 'Outbound federation can backfill events' because now
we are returning the right number of events, which the previous
test was relying on.

Previously, /messages was backfilling the membership event, causing
the test to pass. Now we are no longer backfilling the membership
event due to the change in this commit, causing the test to fail.

The test should instead be returning the membership event locally
from synacpis database, but it doesn't do it fast enough, resulting
in a no-op /sync response with a next_batch=s0_0 which will never
pick up the local membership event when it rolls in. The test
does attempt to retry, but doesn't take the new next_batch=s1_0
resulting in it missing from the /messages response.

* Linting

11 files changed, 86 insertions, 45 deletions

diff --git a/federationapi/routing/backfill.go b/federationapi/routing/backfill.go
index 651a4a2d..1f46b240 100644
--- a/federationapi/routing/backfill.go
+++ b/federationapi/routing/backfill.go
@@ -69,9 +69,14 @@ func Backfill(
 
 	// Populate the request.
 	req := api.QueryBackfillRequest{
-		RoomID:            roomID,
-		EarliestEventsIDs: eIDs,
-		ServerName:        request.Origin(),
+		RoomID: roomID,
+		// we don't know who the successors are for these events, which won't
+		// be a problem because we don't use that information when servicing /backfill requests,
+		// only when making them. TODO: Think of a better API shape
+		BackwardsExtremities: map[string][]string{
+			"": eIDs,
+		},
+		ServerName: request.Origin(),
 	}
 	if req.Limit, err = strconv.Atoi(limit); err != nil {
 		util.GetLogger(httpReq.Context()).WithError(err).Error("strconv.Atoi failed")
@@ -105,6 +110,12 @@ func Backfill(
 		eventJSONs = append(eventJSONs, e.JSON())
 	}
 
+	// sytest wants these in reversed order, similar to /messages, so reverse them now.
+	for i := len(eventJSONs)/2 - 1; i >= 0; i-- {
+		opp := len(eventJSONs) - 1 - i
+		eventJSONs[i], eventJSONs[opp] = eventJSONs[opp], eventJSONs[i]
+	}
+
 	txn := gomatrixserverlib.Transaction{
 		Origin:         cfg.Matrix.ServerName,
 		PDUs:           eventJSONs,
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index 9afc51f4..e92d2a99 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -21,6 +21,7 @@ import (
 
 	commonHTTP "github.com/matrix-org/dendrite/common/http"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
 	opentracing "github.com/opentracing/opentracing-go"
 )
 
@@ -228,14 +229,24 @@ type QueryStateAndAuthChainResponse struct {
 type QueryBackfillRequest struct {
 	// The room to backfill
 	RoomID string `json:"room_id"`
-	// Events to start paginating from.
-	EarliestEventsIDs []string `json:"earliest_event_ids"`
+	// A map of backwards extremity event ID to a list of its prev_event IDs.
+	BackwardsExtremities map[string][]string `json:"backwards_extremities"`
 	// The maximum number of events to retrieve.
 	Limit int `json:"limit"`
 	// The server interested in the events.
 	ServerName gomatrixserverlib.ServerName `json:"server_name"`
 }
 
+// PrevEventIDs returns the prev_event IDs of all backwards extremities, de-duplicated in a lexicographically sorted order.
+func (r *QueryBackfillRequest) PrevEventIDs() []string {
+	var prevEventIDs []string
+	for _, pes := range r.BackwardsExtremities {
+		prevEventIDs = append(prevEventIDs, pes...)
+	}
+	prevEventIDs = util.UniqueStrings(prevEventIDs)
+	return prevEventIDs
+}
+
 // QueryBackfillResponse is a response to QueryBackfill.
 type QueryBackfillResponse struct {
 	// Missing events, arbritrary order.
diff --git a/roomserver/internal/query.go b/roomserver/internal/query.go
index 8043746c..2d1c21c5 100644
--- a/roomserver/internal/query.go
+++ b/roomserver/internal/query.go
@@ -495,14 +495,8 @@ func (r *RoomserverInternalAPI) QueryBackfill(
 	// defines the highest number of elements in the map below.
 	visited := make(map[string]bool, request.Limit)
 
-	// The provided event IDs have already been seen by the request's emitter,
-	// and will be retrieved anyway, so there's no need to care about them if
-	// they appear in our exploration of the event tree.
-	for _, id := range request.EarliestEventsIDs {
-		visited[id] = true
-	}
-
-	front = request.EarliestEventsIDs
+	// this will include these events which is what we want
+	front = request.PrevEventIDs()
 
 	// Scan the event tree for events to send back.
 	resultNIDs, err := r.scanEventTree(ctx, front, visited, request.Limit, request.ServerName)
@@ -534,7 +528,7 @@ func (r *RoomserverInternalAPI) backfillViaFederation(ctx context.Context, req *
 	if err != nil {
 		return fmt.Errorf("backfillViaFederation: unknown room version for room %s : %w", req.RoomID, err)
 	}
-	requester := newBackfillRequester(r.DB, r.FedClient, r.ServerName)
+	requester := newBackfillRequester(r.DB, r.FedClient, r.ServerName, req.BackwardsExtremities)
 	// Request 100 items regardless of what the query asks for.
 	// We don't want to go much higher than this.
 	// We can't honour exactly the limit as some sytests rely on requesting more for tests to pass
@@ -542,7 +536,7 @@ func (r *RoomserverInternalAPI) backfillViaFederation(ctx context.Context, req *
 	// Specifically the test "Outbound federation can backfill events"
 	events, err := gomatrixserverlib.RequestBackfill(
 		ctx, requester,
-		r.KeyRing, req.RoomID, roomVer, req.EarliestEventsIDs, 100)
+		r.KeyRing, req.RoomID, roomVer, req.PrevEventIDs(), 100)
 	if err != nil {
 		return err
 	}
diff --git a/roomserver/internal/query_backfill.go b/roomserver/internal/query_backfill.go
index 82f7238d..49e0af34 100644
--- a/roomserver/internal/query_backfill.go
+++ b/roomserver/internal/query_backfill.go
@@ -16,6 +16,7 @@ type backfillRequester struct {
 	db         storage.Database
 	fedClient  *gomatrixserverlib.FederationClient
 	thisServer gomatrixserverlib.ServerName
+	bwExtrems  map[string][]string
 
 	// per-request state
 	servers                 []gomatrixserverlib.ServerName
@@ -23,13 +24,14 @@ type backfillRequester struct {
 	eventIDMap              map[string]gomatrixserverlib.Event
 }
 
-func newBackfillRequester(db storage.Database, fedClient *gomatrixserverlib.FederationClient, thisServer gomatrixserverlib.ServerName) *backfillRequester {
+func newBackfillRequester(db storage.Database, fedClient *gomatrixserverlib.FederationClient, thisServer gomatrixserverlib.ServerName, bwExtrems map[string][]string) *backfillRequester {
 	return &backfillRequester{
 		db:                      db,
 		fedClient:               fedClient,
 		thisServer:              thisServer,
 		eventIDToBeforeStateIDs: make(map[string][]string),
 		eventIDMap:              make(map[string]gomatrixserverlib.Event),
+		bwExtrems:               bwExtrems,
 	}
 }
 
@@ -160,26 +162,44 @@ func (b *backfillRequester) StateBeforeEvent(ctx context.Context, roomVer gomatr
 // It returns a list of servers which can be queried for backfill requests. These servers
 // will be servers that are in the room already. The entries at the beginning are preferred servers
 // and will be tried first. An empty list will fail the request.
-func (b *backfillRequester) ServersAtEvent(ctx context.Context, roomID, eventID string) (servers []gomatrixserverlib.ServerName) {
+func (b *backfillRequester) ServersAtEvent(ctx context.Context, roomID, eventID string) []gomatrixserverlib.ServerName {
+	// eventID will be a prev_event ID of a backwards extremity, meaning we will not have a database entry for it. Instead, use
+	// its successor, so look it up.
+	successor := ""
+FindSuccessor:
+	for sucID, prevEventIDs := range b.bwExtrems {
+		for _, pe := range prevEventIDs {
+			if pe == eventID {
+				successor = sucID
+				break FindSuccessor
+			}
+		}
+	}
+	if successor == "" {
+		logrus.WithField("event_id", eventID).Error("ServersAtEvent: failed to find successor of this event to determine room state")
+		return nil
+	}
+	eventID = successor
+
 	// getMembershipsBeforeEventNID requires a NID, so retrieving the NID for
 	// the event is necessary.
 	NIDs, err := b.db.EventNIDs(ctx, []string{eventID})
 	if err != nil {
 		logrus.WithField("event_id", eventID).WithError(err).Error("ServersAtEvent: failed to get event NID for event")
-		return
+		return nil
 	}
 
 	stateEntries, err := stateBeforeEvent(ctx, b.db, NIDs[eventID])
 	if err != nil {
 		logrus.WithField("event_id", eventID).WithError(err).Error("ServersAtEvent: failed to load state before event")
-		return
+		return nil
 	}
 
 	// possibly return all joined servers depending on history visiblity
 	memberEventsFromVis, err := joinEventsFromHistoryVisibility(ctx, b.db, roomID, stateEntries)
 	if err != nil {
 		logrus.WithError(err).Error("ServersAtEvent: failed calculate servers from history visibility rules")
-		return
+		return nil
 	}
 	logrus.Infof("ServersAtEvent including %d current events from history visibility", len(memberEventsFromVis))
 
@@ -189,7 +209,7 @@ func (b *backfillRequester) ServersAtEvent(ctx context.Context, roomID, eventID
 	memberEvents, err := getMembershipsAtState(ctx, b.db, stateEntries, true)
 	if err != nil {
 		logrus.WithField("event_id", eventID).WithError(err).Error("ServersAtEvent: failed to get memberships before event")
-		return
+		return nil
 	}
 	memberEvents = append(memberEvents, memberEventsFromVis...)
 
@@ -198,6 +218,7 @@ func (b *backfillRequester) ServersAtEvent(ctx context.Context, roomID, eventID
 	for _, event := range memberEvents {
 		serverSet[event.Origin()] = true
 	}
+	var servers []gomatrixserverlib.ServerName
 	for server := range serverSet {
 		if server == b.thisServer {
 			continue
@@ -205,7 +226,7 @@ func (b *backfillRequester) ServersAtEvent(ctx context.Context, roomID, eventID
 		servers = append(servers, server)
 	}
 	b.servers = servers
-	return
+	return servers
 }
 
 // Backfill performs a backfill request to the given server.
diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
index 811188cc..88e16fe5 100644
--- a/syncapi/routing/messages.go
+++ b/syncapi/routing/messages.go
@@ -205,6 +205,7 @@ func (r *messagesReq) retrieveEvents() (
 	}
 
 	var events []gomatrixserverlib.HeaderedEvent
+	util.GetLogger(r.ctx).WithField("start", start).WithField("end", end).Infof("Fetched %d events locally", len(streamEvents))
 
 	// There can be two reasons for streamEvents to be empty: either we've
 	// reached the oldest event in the room (or the most recent one, depending
@@ -373,13 +374,13 @@ func (e eventsByDepth) Less(i, j int) bool {
 // event, or if there is no remote homeserver to contact.
 // 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(roomID string, fromEventIDs []string, limit int) ([]gomatrixserverlib.HeaderedEvent, error) {
+func (r *messagesReq) backfill(roomID string, backwardsExtremities map[string][]string, limit int) ([]gomatrixserverlib.HeaderedEvent, error) {
 	var res api.QueryBackfillResponse
 	err := r.rsAPI.QueryBackfill(context.Background(), &api.QueryBackfillRequest{
-		RoomID:            roomID,
-		EarliestEventsIDs: fromEventIDs,
-		Limit:             limit,
-		ServerName:        r.cfg.Matrix.ServerName,
+		RoomID:               roomID,
+		BackwardsExtremities: backwardsExtremities,
+		Limit:                limit,
+		ServerName:           r.cfg.Matrix.ServerName,
 	}, &res)
 	if err != nil {
 		return nil, fmt.Errorf("QueryBackfill failed: %w", err)
diff --git a/syncapi/storage/interface.go b/syncapi/storage/interface.go
index eba008b3..121e94c7 100644
--- a/syncapi/storage/interface.go
+++ b/syncapi/storage/interface.go
@@ -94,9 +94,8 @@ type Database interface {
 	GetEventsInTopologicalRange(ctx context.Context, from, to *types.TopologyToken, roomID string, limit int, backwardOrdering bool) (events []types.StreamEvent, err error)
 	// EventPositionInTopology returns the depth and stream position of the given event.
 	EventPositionInTopology(ctx context.Context, eventID string) (types.TopologyToken, error)
-	// BackwardExtremitiesForRoom returns the event IDs of all of the backward
-	// extremities we know of for a given room.
-	BackwardExtremitiesForRoom(ctx context.Context, roomID string) (backwardExtremities []string, err error)
+	// BackwardExtremitiesForRoom returns a map of backwards extremity event ID to a list of its prev_events.
+	BackwardExtremitiesForRoom(ctx context.Context, roomID string) (backwardExtremities map[string][]string, err error)
 	// MaxTopologicalPosition returns the highest topological position for a given room.
 	MaxTopologicalPosition(ctx context.Context, roomID string) (types.TopologyToken, error)
 	// StreamEventsToEvents converts streamEvent to Event. If device is non-nil and
diff --git a/syncapi/storage/postgres/backwards_extremities_table.go b/syncapi/storage/postgres/backwards_extremities_table.go
index 3dd1b234..fa498a40 100644
--- a/syncapi/storage/postgres/backwards_extremities_table.go
+++ b/syncapi/storage/postgres/backwards_extremities_table.go
@@ -41,7 +41,7 @@ const insertBackwardExtremitySQL = "" +
 	" ON CONFLICT DO NOTHING"
 
 const selectBackwardExtremitiesForRoomSQL = "" +
-	"SELECT DISTINCT event_id FROM syncapi_backward_extremities WHERE room_id = $1"
+	"SELECT event_id, prev_event_id FROM syncapi_backward_extremities WHERE room_id = $1"
 
 const deleteBackwardExtremitySQL = "" +
 	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND prev_event_id = $2"
@@ -79,23 +79,24 @@ func (s *backwardExtremitiesStatements) InsertsBackwardExtremity(
 
 func (s *backwardExtremitiesStatements) SelectBackwardExtremitiesForRoom(
 	ctx context.Context, roomID string,
-) (eventIDs []string, err error) {
+) (bwExtrems map[string][]string, err error) {
 	rows, err := s.selectBackwardExtremitiesForRoomStmt.QueryContext(ctx, roomID)
 	if err != nil {
 		return
 	}
 	defer common.CloseAndLogIfError(ctx, rows, "selectBackwardExtremitiesForRoom: rows.close() failed")
 
+	bwExtrems = make(map[string][]string)
 	for rows.Next() {
 		var eID string
-		if err = rows.Scan(&eID); err != nil {
+		var prevEventID string
+		if err = rows.Scan(&eID, &prevEventID); err != nil {
 			return
 		}
-
-		eventIDs = append(eventIDs, eID)
+		bwExtrems[eID] = append(bwExtrems[eID], prevEventID)
 	}
 
-	return eventIDs, rows.Err()
+	return bwExtrems, rows.Err()
 }
 
 func (s *backwardExtremitiesStatements) DeleteBackwardExtremity(
diff --git a/syncapi/storage/shared/syncserver.go b/syncapi/storage/shared/syncserver.go
index be1f9c7a..543e5b4a 100644
--- a/syncapi/storage/shared/syncserver.go
+++ b/syncapi/storage/shared/syncserver.go
@@ -310,6 +310,7 @@ func (d *Database) updateRoomState(
 			}
 			membership = &value
 		}
+
 		if err := d.CurrentRoomState.UpsertRoomState(ctx, txn, event, membership, pduPosition); err != nil {
 			return err
 		}
@@ -367,7 +368,7 @@ func (d *Database) SyncPosition(ctx context.Context) (tok types.StreamingToken,
 
 func (d *Database) BackwardExtremitiesForRoom(
 	ctx context.Context, roomID string,
-) (backwardExtremities []string, err error) {
+) (backwardExtremities map[string][]string, err error) {
 	return d.BackwardExtremities.SelectBackwardExtremitiesForRoom(ctx, roomID)
 }
 
diff --git a/syncapi/storage/sqlite3/backwards_extremities_table.go b/syncapi/storage/sqlite3/backwards_extremities_table.go
index a172f6bf..c5d9cae5 100644
--- a/syncapi/storage/sqlite3/backwards_extremities_table.go
+++ b/syncapi/storage/sqlite3/backwards_extremities_table.go
@@ -41,7 +41,7 @@ const insertBackwardExtremitySQL = "" +
 	" ON CONFLICT (room_id, event_id, prev_event_id) DO NOTHING"
 
 const selectBackwardExtremitiesForRoomSQL = "" +
-	"SELECT DISTINCT event_id FROM syncapi_backward_extremities WHERE room_id = $1"
+	"SELECT event_id, prev_event_id FROM syncapi_backward_extremities WHERE room_id = $1"
 
 const deleteBackwardExtremitySQL = "" +
 	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND prev_event_id = $2"
@@ -79,23 +79,24 @@ func (s *backwardExtremitiesStatements) InsertsBackwardExtremity(
 
 func (s *backwardExtremitiesStatements) SelectBackwardExtremitiesForRoom(
 	ctx context.Context, roomID string,
-) (eventIDs []string, err error) {
+) (bwExtrems map[string][]string, err error) {
 	rows, err := s.selectBackwardExtremitiesForRoomStmt.QueryContext(ctx, roomID)
 	if err != nil {
 		return
 	}
 	defer common.CloseAndLogIfError(ctx, rows, "selectBackwardExtremitiesForRoom: rows.close() failed")
 
+	bwExtrems = make(map[string][]string)
 	for rows.Next() {
 		var eID string
-		if err = rows.Scan(&eID); err != nil {
+		var prevEventID string
+		if err = rows.Scan(&eID, &prevEventID); err != nil {
 			return
 		}
-
-		eventIDs = append(eventIDs, eID)
+		bwExtrems[eID] = append(bwExtrems[eID], prevEventID)
 	}
 
-	return eventIDs, rows.Err()
+	return bwExtrems, rows.Err()
 }
 
 func (s *backwardExtremitiesStatements) DeleteBackwardExtremity(
diff --git a/syncapi/storage/tables/interface.go b/syncapi/storage/tables/interface.go
index f31bdc2e..bc3b6941 100644
--- a/syncapi/storage/tables/interface.go
+++ b/syncapi/storage/tables/interface.go
@@ -89,8 +89,8 @@ type CurrentRoomState interface {
 type BackwardsExtremities interface {
 	// InsertsBackwardExtremity inserts a new backwards extremity.
 	InsertsBackwardExtremity(ctx context.Context, txn *sql.Tx, roomID, eventID string, prevEventID string) (err error)
-	// SelectBackwardExtremitiesForRoom retrieves all backwards extremities for the room.
-	SelectBackwardExtremitiesForRoom(ctx context.Context, roomID string) (eventIDs []string, err error)
+	// SelectBackwardExtremitiesForRoom retrieves all backwards extremities for the room, as a map of event_id to list of prev_event_ids.
+	SelectBackwardExtremitiesForRoom(ctx context.Context, roomID string) (bwExtrems map[string][]string, err error)
 	// DeleteBackwardExtremity removes a backwards extremity for a room, if one existed.
 	DeleteBackwardExtremity(ctx context.Context, txn *sql.Tx, roomID, knownEventID string) (err error)
 }
diff --git a/sytest-whitelist b/sytest-whitelist
index 4a8af13a..2e8205dc 100644
--- a/sytest-whitelist
+++ b/sytest-whitelist
@@ -280,3 +280,4 @@ Inbound federation can get public room list
 An event which redacts itself should be ignored
 A pair of events which redact each other should be ignored
 Outbound federation can backfill events
+Inbound federation can backfill events