Optimize /sync and history visibility (#2961)

Should fix the following issues or make a lot less worse when using
Postgres:

The main issue behind #2911: The client gives up after a certain time,
causing a cascade of context errors, because the response couldn't be
built up fast enough. This mostly happens on accounts with many rooms,
due to the inefficient way we're getting recent events and current state

For #2777: The queries for getting the membership events for history
visibility were being executed for each room (I think 185?), resulting
in a whooping 2k queries for membership events. (Getting the
statesnapshot -> block nids -> actual wanted membership event)

Both should now be better by:
- Using a LATERAL join to get all recent events for all joined rooms in
one go (TODO: maybe do the same for room summary and current state etc)
- If we're lazy loading on initial syncs, we're now not getting the
whole current state, just to drop the majority of it because we're lazy
loading members - we add a filter to exclude membership events on the
first call to `CurrentState`.
- Using an optimized query to get the membership events needed to
calculate history visibility

---------

Co-authored-by: kegsay <kegan@matrix.org>

roomserver/internal/query/query.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 
+	"github.com/matrix-org/dendrite/roomserver/storage/tables"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 	"github.com/sirupsen/logrus"
@@ -216,7 +217,8 @@ func (r *Queryer) QueryMembershipAtEvent(
 	request *api.QueryMembershipAtEventRequest,
 	response *api.QueryMembershipAtEventResponse,
 ) error {
-	response.Memberships = make(map[string][]*gomatrixserverlib.HeaderedEvent)
+	response.Membership = make(map[string]*gomatrixserverlib.HeaderedEvent)
+
 	info, err := r.DB.RoomInfo(ctx, request.RoomID)
 	if err != nil {
 		return fmt.Errorf("unable to get roomInfo: %w", err)
@@ -234,7 +236,17 @@ func (r *Queryer) QueryMembershipAtEvent(
 		return fmt.Errorf("requested stateKeyNID for %s was not found", request.UserID)
 	}
 
-	stateEntries, err := helpers.MembershipAtEvent(ctx, r.DB, info, request.EventIDs, stateKeyNIDs[request.UserID])
+	response.Membership, err = r.DB.GetMembershipForHistoryVisibility(ctx, stateKeyNIDs[request.UserID], info, request.EventIDs...)
+	switch err {
+	case nil:
+		return nil
+	case tables.OptimisationNotSupportedError: // fallthrough, slow way of getting the membership events for each event
+	default:
+		return err
+	}
+
+	response.Membership = make(map[string]*gomatrixserverlib.HeaderedEvent)
+	stateEntries, err := helpers.MembershipAtEvent(ctx, r.DB, nil, request.EventIDs, stateKeyNIDs[request.UserID])
 	if err != nil {
 		return fmt.Errorf("unable to get state before event: %w", err)
 	}
@@ -258,7 +270,7 @@ func (r *Queryer) QueryMembershipAtEvent(
 	for _, eventID := range request.EventIDs {
 		stateEntry, ok := stateEntries[eventID]
 		if !ok || len(stateEntry) == 0 {
-			response.Memberships[eventID] = []*gomatrixserverlib.HeaderedEvent{}
+			response.Membership[eventID] = nil
 			continue
 		}
 
@@ -275,15 +287,15 @@ func (r *Queryer) QueryMembershipAtEvent(
 			return fmt.Errorf("unable to get memberships at state: %w", err)
 		}
 
-		res := make([]*gomatrixserverlib.HeaderedEvent, 0, len(memberships))
-
+		// Iterate over all membership events we got. Given we only query the membership for
+		// one user and assuming this user only ever has one membership event associated to
+		// a given event, overwrite any other existing membership events.
 		for i := range memberships {
 			ev := memberships[i]
 			if ev.Type() == gomatrixserverlib.MRoomMember && ev.StateKeyEquals(request.UserID) {
-				res = append(res, ev.Headered(info.RoomVersion))
+				response.Membership[eventID] = ev.Event.Headered(info.RoomVersion)
 			}
 		}
-		response.Memberships[eventID] = res
 	}
 
 	return nil