Fix ordering when backfilling (#1000)

* Fix ordering when backfilling

The problem was that we weren't sorting the returned events
by depth when sending them back to the caller, instead we
were sorting by prev_events which is not the same thing.

* Fixup tests

syncapi/routing/messages.go

@@ -210,15 +210,16 @@ func (r *messagesReq) retrieveEvents() (
 	}
 
 	// Sort the events to ensure we send them in the right order.
-	events = gomatrixserverlib.HeaderedReverseTopologicalOrdering(
-		events,
-		gomatrixserverlib.TopologicalOrderByPrevEvents,
-	)
 	if r.backwardOrdering {
 		// This reverses the array from old->new to new->old
-		sort.SliceStable(events, func(i, j int) bool {
-			return true
-		})
+		reversed := func(in []gomatrixserverlib.HeaderedEvent) []gomatrixserverlib.HeaderedEvent {
+			out := make([]gomatrixserverlib.HeaderedEvent, len(in))
+			for i := 0; i < len(in); i++ {
+				out[i] = in[len(in)-i-1]
+			}
+			return out
+		}
+		events = reversed(events)
 	}
 
 	// Convert all of the events into client events.
@@ -259,6 +260,7 @@ func (r *messagesReq) retrieveEvents() (
 		// to them by the event on their left, therefore we need to decrement the
 		// end position we send in the response if we're going backward.
 		end.PDUPosition--
+		end.EDUTypingPosition += 1000
 	}
 
 	// The lowest token value is 1, therefore we need to manually set it to that
@@ -345,10 +347,23 @@ func (r *messagesReq) handleNonEmptyEventsSlice(streamEvents []types.StreamEvent
 
 	// Append the events ve previously retrieved locally.
 	events = append(events, r.db.StreamEventsToEvents(nil, streamEvents)...)
+	sort.Sort(eventsByDepth(events))
 
 	return
 }
 
+type eventsByDepth []gomatrixserverlib.HeaderedEvent
+
+func (e eventsByDepth) Len() int {
+	return len(e)
+}
+func (e eventsByDepth) Swap(i, j int) {
+	e[i], e[j] = e[j], e[i]
+}
+func (e eventsByDepth) Less(i, j int) bool {
+	return e[i].Depth() < e[j].Depth()
+}
+
 // 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
@@ -375,17 +390,24 @@ func (r *messagesReq) backfill(roomID string, fromEventIDs []string, limit int)
 	// Currently, this can race with live events for the room and cause problems. It's also just a bit unclear
 	// when you have multiple entry points to write events.
 
+	// we have to order these by depth, starting with the lowest because otherwise the topology tokens
+	// will skip over events that have the same depth but different stream positions due to the query which is:
+	//  - anything less than the depth OR
+	//  - anything with the same depth and a lower stream position.
+	sort.Sort(eventsByDepth(res.Events))
+
 	// Store the events in the database, while marking them as unfit to show
 	// up in responses to sync requests.
 	for i := range res.Events {
-		if _, err = r.db.WriteEvent(
+		_, err = r.db.WriteEvent(
 			r.ctx,
 			&res.Events[i],
 			[]gomatrixserverlib.HeaderedEvent{},
 			[]string{},
 			[]string{},
 			nil, true,
-		); err != nil {
+		)
+		if err != nil {
 			return nil, err
 		}
 	}