Handle case where we get no events from the database and prevent end token < 1

src/github.com/matrix-org/dendrite/syncapi/routing/messages.go

@@ -182,6 +182,40 @@ func retrieveEvents(
 		return
 	}
 
+	var backwardExtremity bool
+	var events []gomatrixserverlib.Event
+
+	// 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
+	// on the ordering), or we've reached a backward extremity.
+	if len(streamEvents) == 0 {
+		var evs []storage.StreamEvent
+		var laterPosition types.StreamPosition
+		if backwardOrdering {
+			laterPosition = from.Position + 1
+		} else {
+			laterPosition = to.Position + 1
+		}
+
+		evs, err = db.EventsAtTopologicalPosition(ctx, roomID, laterPosition)
+		if err != nil {
+			return
+		}
+
+		backwardExtremity, err = containsBackwardExtremity(ctx, db, evs, backwardOrdering)
+		if err != nil {
+			return
+		}
+
+		if backwardExtremity {
+			events, err = backfill(ctx, db, roomID, evs[0].EventID(), limit, queryAPI, cfg, federation)
+			if err != nil {
+				return
+			}
+		} else {
+			return []gomatrixserverlib.ClientEvent{}, from, to, nil
+		}
+	} else {
 		// Check if we have enough events.
 		isSetLargeEnough := true
 		if len(streamEvents) < limit {
@@ -197,15 +231,13 @@ func retrieveEvents(
 		}
 
 		// Check if the slice contains a backward extremity.
-	backwardExtremity, err := containsBackwardExtremity(
+		backwardExtremity, err = containsBackwardExtremity(
 			ctx, db, streamEvents, backwardOrdering,
 		)
 		if err != nil {
 			return
 		}
 
-	var events []gomatrixserverlib.Event
-
 		// Backfill is needed if we've reached a backward extremity and need more
 		// events. It's only needed if the direction is backward.
 		if backwardExtremity && !isSetLargeEnough && backwardOrdering {
@@ -225,6 +257,8 @@ func retrieveEvents(
 
 		// Append the events ve previously retrieved locally.
 		events = append(events, storage.StreamEventsToEvents(nil, streamEvents)...)
+	}
+
 	// Sort the events to ensure we send them in the right order. We currently
 	// do that based on the event's timestamp.
 	if backwardOrdering {
@@ -250,6 +284,10 @@ func retrieveEvents(
 		types.PaginationTokenTypeTopology, streamEvents[len(streamEvents)-1].StreamPosition,
 	)
 
+	if end.Position == types.StreamPosition(0) {
+		end.Position = types.StreamPosition(1)
+	}
+
 	return
 }
 
@@ -286,7 +324,7 @@ func containsBackwardExtremity(
 		return false, nil
 	}
 	// Check if we have all of the events we requested. If not, it means we've
-	// reached a backard extremity.
+	// reached a backward extremity.
 	var eventInDB bool
 	var id string
 	// Iterate over the IDs we used in the request.

src/github.com/matrix-org/dendrite/syncapi/storage/output_room_events_topology_table.go

@@ -59,12 +59,17 @@ const selectMaxPositionInTopologySQL = "" +
 	"SELECT MAX(topological_position) FROM syncapi_output_room_events_topology" +
 	" WHERE room_id = $1"
 
+const selectEventIDsFromPositionSQL = "" +
+	"SELECT event_id FROM syncapi_output_room_events_topology" +
+	" WHERE room_id = $1 AND topological_position = $2"
+
 type outputRoomEventsTopologyStatements struct {
 	insertEventInTopologyStmt       *sql.Stmt
 	selectEventIDsInRangeASCStmt    *sql.Stmt
 	selectEventIDsInRangeDESCStmt   *sql.Stmt
 	selectPositionInTopologyStmt    *sql.Stmt
 	selectMaxPositionInTopologyStmt *sql.Stmt
+	selectEventIDsFromPositionStmt  *sql.Stmt
 }
 
 func (s *outputRoomEventsTopologyStatements) prepare(db *sql.DB) (err error) {
@@ -87,6 +92,9 @@ func (s *outputRoomEventsTopologyStatements) prepare(db *sql.DB) (err error) {
 	if s.selectMaxPositionInTopologyStmt, err = db.Prepare(selectMaxPositionInTopologySQL); err != nil {
 		return
 	}
+	if s.selectEventIDsFromPositionStmt, err = db.Prepare(selectEventIDsFromPositionSQL); err != nil {
+		return
+	}
 	return
 }
 
@@ -153,3 +161,27 @@ func (s *outputRoomEventsTopologyStatements) selectMaxPositionInTopology(
 	err = s.selectMaxPositionInTopologyStmt.QueryRowContext(ctx, roomID).Scan(&pos)
 	return
 }
+
+// selectEventIDsFromPosition returns the IDs of all events that have a given
+// position in the topology of a given room.
+func (s *outputRoomEventsTopologyStatements) selectEventIDsFromPosition(
+	ctx context.Context, roomID string, pos types.StreamPosition,
+) (eventIDs []string, err error) {
+	// Query the event IDs.
+	rows, err := s.selectEventIDsFromPositionStmt.QueryContext(ctx, roomID, pos)
+	if err == sql.ErrNoRows {
+		// If no event matched the request, return an empty slice.
+		return []string{}, nil
+	} else if err != nil {
+		return
+	}
+	// Return the IDs.
+	var eventID string
+	for rows.Next() {
+		if err = rows.Scan(&eventID); err != nil {
+			return
+		}
+		eventIDs = append(eventIDs, eventID)
+	}
+	return
+}

src/github.com/matrix-org/dendrite/syncapi/storage/syncserver.go

@@ -269,6 +269,19 @@ func (d *SyncServerDatabase) MaxTopologicalPosition(
 	return d.topology.selectMaxPositionInTopology(ctx, roomID)
 }
 
+// EventsAtTopologicalPosition returns all of the events matching a given
+// position in the topology of a given room.
+func (d *SyncServerDatabase) EventsAtTopologicalPosition(
+	ctx context.Context, roomID string, pos types.StreamPosition,
+) ([]StreamEvent, error) {
+	eIDs, err := d.topology.selectEventIDsFromPosition(ctx, roomID, pos)
+	if err != nil {
+		return nil, err
+	}
+
+	return d.events.selectEvents(ctx, nil, eIDs)
+}
+
 // SyncStreamPosition returns the latest position in the sync stream. Returns 0 if there are no events yet.
 func (d *SyncServerDatabase) SyncStreamPosition(ctx context.Context) (types.StreamPosition, error) {
 	return d.syncStreamPositionTx(ctx, nil)
@@ -397,6 +410,8 @@ func (d *SyncServerDatabase) CompleteSync(
 		}
 		if backwardTopologyPos-1 <= 0 {
 			backwardTopologyPos = types.StreamPosition(1)
+		} else {
+			backwardTopologyPos = backwardTopologyPos - 1
 		}
 
 		// We don't include a device here as we don't need to send down
@@ -405,7 +420,7 @@ func (d *SyncServerDatabase) CompleteSync(
 		stateEvents = removeDuplicates(stateEvents, recentEvents)
 		jr := types.NewJoinResponse()
 		jr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
-			types.PaginationTokenTypeTopology, backwardTopologyPos-1,
+			types.PaginationTokenTypeTopology, backwardTopologyPos,
 		).String()
 		jr.Timeline.Events = gomatrixserverlib.ToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
 		jr.Timeline.Limited = true
@@ -542,6 +557,8 @@ func (d *SyncServerDatabase) addRoomDeltaToResponse(
 	}
 	if backwardTopologyPos-1 <= 0 {
 		backwardTopologyPos = types.StreamPosition(1)
+	} else {
+		backwardTopologyPos = backwardTopologyPos - 1
 	}
 
 	switch delta.membership {
@@ -549,7 +566,7 @@ func (d *SyncServerDatabase) addRoomDeltaToResponse(
 		jr := types.NewJoinResponse()
 
 		jr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
-			types.PaginationTokenTypeTopology, backwardTopologyPos-1,
+			types.PaginationTokenTypeTopology, backwardTopologyPos,
 		).String()
 		jr.Timeline.Events = gomatrixserverlib.ToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
 		jr.Timeline.Limited = false // TODO: if len(events) >= numRecents + 1 and then set limited:true
@@ -562,7 +579,7 @@ func (d *SyncServerDatabase) addRoomDeltaToResponse(
 		//       no longer in the room.
 		lr := types.NewLeaveResponse()
 		lr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
-			types.PaginationTokenTypeStream, backwardTopologyPos-1,
+			types.PaginationTokenTypeStream, backwardTopologyPos,
 		).String()
 		lr.Timeline.Events = gomatrixserverlib.ToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
 		lr.Timeline.Limited = false // TODO: if len(events) >= numRecents + 1 and then set limited:true