Warp/server/internal/syncapi/sliding.go

package syncapi

import (
	"cmp"
	"context"
	"encoding/json"
	"errors"

	"slices"
	"strconv"
	"strings"
	"time"

	"go.opentelemetry.io/otel/attribute"

	"github.com/dariuszjaniak/warp/server/internal/matrix/event"
	"github.com/dariuszjaniak/warp/server/internal/roomserver"
	"github.com/dariuszjaniak/warp/server/internal/storage"
	"github.com/dariuszjaniak/warp/server/internal/tracing"
)

// Simplified Sliding Sync (MSC4186): /sync for clients with a thousand
// rooms that want to show the top twenty.
//
// A classic initial sync sends every room a user is in, with its state and
// a timeline, before the client can draw anything; a client with a large
// account waits minutes. Sliding sync turns that round. The client asks for
// lists — "my rooms, most recently active first, positions 0 to 19" — and
// for the few pieces of state it needs to draw each one, and the server
// sends those rooms and a count of the rest. As the user scrolls, the
// client widens the window; as rooms become active, they move into it and
// are sent.
//
// What the server keeps for this is per connection (slidingconn.go): which
// rooms it has sent and as of which position, so that a room is sent once
// in full and then only as it changes, and a room that leaves the window
// and comes back is not sent again unless something happened to it. The
// streams that are not about one room — to-device messages, device lists,
// account data, receipts, typing — are extensions a client turns on, and
// are read with the same helpers classic /sync uses.

// SlidingRequest is one sliding sync request.
type SlidingRequest struct {
	User      string
	Localpart string
	Device    string

	// Pos is the ?pos= of the previous response, empty for a new
	// connection.
	Pos string

	// Timeout is how long to wait for something to happen.
	Timeout time.Duration

	// SetPresence is ?set_presence=, as for /sync.
	SetPresence string

	Body SlidingBody
}

// SlidingBody is the request body.
type SlidingBody struct {
	// ConnID tells apart the connections one device keeps: a client with a
	// room list and a notification process runs two.
	ConnID string `json:"conn_id,omitempty"`

	Lists             map[string]SlidingList      `json:"lists,omitempty"`
	RoomSubscriptions map[string]RoomSubscription `json:"room_subscriptions,omitempty"`
	Extensions        SlidingExtensionsRequest    `json:"extensions,omitempty"`
}

// RoomSubscription is what a client wants of each room it is sent.
type RoomSubscription struct {
	// RequiredState lists [type, state_key] pairs. "*" is a wildcard in
	// either place, "$ME" is the user, and ["m.room.member", "$LAZY"] is the
	// members who sent something in the timeline.
	RequiredState [][]string `json:"required_state,omitempty"`

	TimelineLimit int `json:"timeline_limit,omitempty"`
}

// SlidingList is one room list.
type SlidingList struct {
	RoomSubscription

	// Ranges are inclusive windows into the sorted list.
	Ranges [][2]int `json:"ranges,omitempty"`

	Filters *SlidingFilters `json:"filters,omitempty"`
}

// SlidingFilters narrows which rooms a list holds.
type SlidingFilters struct {
	IsDM        *bool    `json:"is_dm,omitempty"`
	Spaces      []string `json:"spaces,omitempty"`
	IsEncrypted *bool    `json:"is_encrypted,omitempty"`
	IsInvite    *bool    `json:"is_invite,omitempty"`

	// RoomTypes and NotRoomTypes match m.room.create's type; a null entry
	// is a room with no type, which is to say an ordinary room.
	RoomTypes    []*string `json:"room_types,omitempty"`
	NotRoomTypes []*string `json:"not_room_types,omitempty"`

	Tags    []string `json:"tags,omitempty"`
	NotTags []string `json:"not_tags,omitempty"`
}

// extensionToggle is the shape every extension's request shares.
type extensionToggle struct {
	Enabled *bool `json:"enabled,omitempty"`

	// Lists and Rooms narrow which rooms a per-room extension reports on;
	// absent means every room in the response's lists and subscriptions.
	Lists []string `json:"lists,omitempty"`
	Rooms []string `json:"rooms,omitempty"`
}

func (t *extensionToggle) on() bool { return t != nil && t.Enabled != nil && *t.Enabled }

// SlidingExtensionsRequest is which extensions the client wants.
type SlidingExtensionsRequest struct {
	ToDevice *struct {
		extensionToggle
		Since string `json:"since,omitempty"`
		Limit int    `json:"limit,omitempty"`
	} `json:"to_device,omitempty"`
	E2EE        *extensionToggle `json:"e2ee,omitempty"`
	AccountData *extensionToggle `json:"account_data,omitempty"`
	Receipts    *extensionToggle `json:"receipts,omitempty"`
	Typing      *extensionToggle `json:"typing,omitempty"`
}

// SlidingResponse is one response.
type SlidingResponse struct {
	Pos        string                       `json:"pos"`
	Lists      map[string]SlidingListResult `json:"lists"`
	Rooms      map[string]*SlidingRoom      `json:"rooms"`
	Extensions SlidingExtensions            `json:"extensions"`
}

// SlidingListResult is how many rooms a list holds.
type SlidingListResult struct {
	Count int `json:"count"`
}

// SlidingHero is a member a room without a name is named after.
type SlidingHero struct {
	UserID      string `json:"user_id"`
	DisplayName string `json:"displayname,omitempty"`
	AvatarURL   string `json:"avatar_url,omitempty"`
}

// SlidingRoom is one room of a response.
type SlidingRoom struct {
	Name   string        `json:"name,omitempty"`
	Avatar string        `json:"avatar,omitempty"`
	Heroes []SlidingHero `json:"heroes,omitempty"`
	IsDM   bool          `json:"is_dm,omitempty"`

	// Initial says this is the room in full: the client replaces what it
	// had rather than adding to it.
	Initial bool `json:"initial,omitempty"`

	RequiredState []roomserver.ClientEvent `json:"required_state,omitempty"`
	Timeline      []roomserver.ClientEvent `json:"timeline,omitempty"`
	PrevBatch     string                   `json:"prev_batch,omitempty"`
	Limited       bool                     `json:"limited,omitempty"`

	// NumLive is how many of the timeline's events are new since the
	// previous response, rather than history sent to fill the timeline.
	NumLive *int `json:"num_live,omitempty"`

	// BumpStamp orders rooms by activity: the position of the last event
	// that moved the room up a list.
	BumpStamp int64 `json:"bump_stamp,omitempty"`

	JoinedCount  *int `json:"joined_count,omitempty"`
	InvitedCount *int `json:"invited_count,omitempty"`

	NotificationCount int `json:"notification_count"`
	HighlightCount    int `json:"highlight_count"`

	InviteState []roomserver.StrippedEvent `json:"invite_state,omitempty"`
	KnockState  []roomserver.StrippedEvent `json:"knock_state,omitempty"`
}

// SlidingExtensions is the extensions' half of a response.
type SlidingExtensions struct {
	ToDevice    *ToDeviceExtension    `json:"to_device,omitempty"`
	E2EE        *E2EEExtension        `json:"e2ee,omitempty"`
	AccountData *AccountDataExtension `json:"account_data,omitempty"`
	Receipts    *EphemeralExtension   `json:"receipts,omitempty"`
	Typing      *EphemeralExtension   `json:"typing,omitempty"`
}

// ToDeviceExtension is the to-device messages for the syncing device.
type ToDeviceExtension struct {
	NextBatch string          `json:"next_batch"`
	Events    []toDeviceEvent `json:"events"`
}

// E2EEExtension is the end-to-end encryption bookkeeping a classic sync
// carries at its top level.
type E2EEExtension struct {
	DeviceLists                  DeviceLists    `json:"device_lists"`
	DeviceOneTimeKeysCount       map[string]int `json:"device_one_time_keys_count"`
	DeviceUnusedFallbackKeyTypes []string       `json:"device_unused_fallback_key_types"`
}

// AccountDataExtension is global and per-room account data.
type AccountDataExtension struct {
	Global []AccountDataEvent            `json:"global"`
	Rooms  map[string][]AccountDataEvent `json:"rooms"`
}

// EphemeralExtension is receipts or typing, by room.
type EphemeralExtension struct {
	Rooms map[string]EphemeralEvent `json:"rooms"`
}

// defaultToDeviceLimit is the to_device extension's batch size when the
// client names none. A device that has been away drains its queue over
// several requests rather than in one response of unbounded size.
const defaultToDeviceLimit = 100

// directType is the account data a client records its direct chats in.
const directType = "m.direct"

// tagType is the room account data a client records a room's tags in.
const tagType = "m.tag"

// SlidingSync answers one sliding sync request, blocking for up to
// req.Timeout when there is nothing to report.
func (s *API) SlidingSync(ctx context.Context, req SlidingRequest) (SlidingResponse, error) {
	key := connKey{user: req.User, device: req.Device, connID: req.Body.ConnID}
	snap, err := s.conns.load(key, req.Pos)
	if err != nil {
		return SlidingResponse{}, err
	}
	if err := s.applySetPresence(ctx, SyncRequest{User: req.User, SetPresence: req.SetPresence}); err != nil {
		return SlidingResponse{}, err
	}

	timeout := min(req.Timeout, s.longPollTimeout)
	deadline := s.now().Add(timeout)
	for {
		rooms, err := s.rooms.UserMemberships(ctx, req.User, nil)
		if err != nil {
			return SlidingResponse{}, err
		}
		listener := s.notifier.Listen(req.User, roomIDsOf(rooms))

		resp, next, interesting, err := s.slidingAttempt(ctx, req, snap, rooms)
		if err != nil {
			listener.Close()
			return SlidingResponse{}, err
		}
		wait := time.Until(deadline)
		if interesting || timeout <= 0 || wait <= 0 {
			listener.Close()
			resp.Pos = s.conns.save(key, next)
			return resp, nil
		}
		woken, err := waitFor(ctx, listener, wait)
		listener.Close()
		if err != nil {
			return SlidingResponse{}, err
		}
		if !woken {
			resp.Pos = s.conns.save(key, next)
			return resp, nil
		}
	}
}

// slidingRoomConfig is what a room is to be sent with, merged over every
// list and subscription that selects it.
type slidingRoomConfig struct {
	timelineLimit int
	requiredState [][]string
}

func (c slidingRoomConfig) merge(sub RoomSubscription) slidingRoomConfig {
	c.timelineLimit = max(c.timelineLimit, min(sub.TimelineLimit, MaxTimelineLimit))
	for _, pair := range sub.RequiredState {
		if len(pair) == 2 && !slices.ContainsFunc(c.requiredState, func(p []string) bool { return slices.Equal(p, pair) }) {
			c.requiredState = append(c.requiredState, pair)
		}
	}
	return c
}

// key renders a config for comparison with the one a room was last sent
// with.
func (c slidingRoomConfig) key() string {
	pairs := make([]string, 0, len(c.requiredState))
	for _, p := range c.requiredState {
		pairs = append(pairs, p[0]+"\x00"+p[1])
	}
	slices.Sort(pairs)
	return strconv.Itoa(c.timelineLimit) + "\x01" + strings.Join(pairs, "\x01")
}

// slidingAttempt builds one candidate response and the connection state it
// leaves behind.
func (s *API) slidingAttempt(ctx context.Context, req SlidingRequest, prev *connSnapshot, memberships []roomserver.MembershipAt) (_ SlidingResponse, _ *connSnapshot, _ bool, err error) {
	ctx, span := tracing.Start(ctx, "syncapi.sliding_sync", attribute.Bool("matrix.initial", prev == nil))
	defer func() { tracing.End(span, err) }()

	positions, err := s.store.StreamPositions(ctx)
	if err != nil {
		return SlidingResponse{}, nil, false, err
	}
	to := tokenAt(positions)

	var since Token
	next := &connSnapshot{token: to, startPDU: to.PDU, rooms: map[string]sentRoom{}, lists: map[string]int{}}
	if prev != nil {
		since = prev.token
		next = prev.clone()
		next.token = to
	}

	resp := SlidingResponse{
		Lists: map[string]SlidingListResult{},
		Rooms: map[string]*SlidingRoom{},
	}
	byRoom := make(map[string]roomserver.MembershipAt, len(memberships))
	for _, m := range memberships {
		byRoom[m.RoomID] = m
	}

	// The rooms a list may hold: those the user is in, invited to or
	// knocking on, and those they left while this connection was open —
	// which is how a client hears that a room it is showing is gone.
	var candidates []roomserver.MembershipAt
	for _, m := range memberships {
		switch m.Membership {
		case event.MembershipJoin, event.MembershipInvite, event.MembershipKnock:
			candidates = append(candidates, m)
		case event.MembershipLeave, event.MembershipBan:
			if prev != nil && m.Position > prev.startPDU {
				candidates = append(candidates, m)
			}
		}
	}

	facts, err := s.roomFacts(ctx, req, candidates, to, req.Body.Lists)
	if err != nil {
		return SlidingResponse{}, nil, false, err
	}

	selected := map[string]slidingRoomConfig{}
	listsChanged := prev == nil
	for name, list := range req.Body.Lists {
		var rooms []roomserver.MembershipAt
		for _, m := range candidates {
			ok, err := s.listAdmits(ctx, req, list.Filters, m, facts)
			if err != nil {
				return SlidingResponse{}, nil, false, err
			}
			if ok {
				rooms = append(rooms, m)
			}
		}
		slices.SortFunc(rooms, func(a, b roomserver.MembershipAt) int {
			return cmp.Or(cmp.Compare(facts.bump[b.RoomID], facts.bump[a.RoomID]), strings.Compare(a.RoomID, b.RoomID))
		})
		resp.Lists[name] = SlidingListResult{Count: len(rooms)}
		if count, ok := next.lists[name]; !ok || count != len(rooms) {
			listsChanged = true
		}
		next.lists[name] = len(rooms)

		for _, r := range list.Ranges {
			lo, hi := max(r[0], 0), min(r[1], len(rooms)-1)
			for i := lo; i <= hi; i++ {
				selected[rooms[i].RoomID] = selected[rooms[i].RoomID].merge(list.RoomSubscription)
			}
		}
	}

	// Subscriptions name a room directly: a room the client has open,
	// whatever list it is or is not in. The user has to be able to read
	// it, which for a room they are not in means a world-readable one.
	for roomID, sub := range req.Body.RoomSubscriptions {
		if _, ok := byRoom[roomID]; !ok {
			visible, err := s.rooms.VisibleRange(ctx, roomID, req.User)
			if err != nil {
				if errors.Is(err, ErrNoSuchRoom) {
					continue
				}
				return SlidingResponse{}, nil, false, err
			}
			if !visible.Allowed {
				continue
			}
			byRoom[roomID] = roomserver.MembershipAt{RoomID: roomID}
		}
		selected[roomID] = selected[roomID].merge(sub)
	}

	changed, err := s.slidingChangedRooms(ctx, req, prev, selected, byRoom, since, to)
	if err != nil {
		return SlidingResponse{}, nil, false, err
	}

	selectedIDs := make([]string, 0, len(selected))
	for roomID := range selected {
		selectedIDs = append(selectedIDs, roomID)
	}
	slices.Sort(selectedIDs)

	var counts map[string]storage.NotificationCounts
	if s.push != nil && len(changed) > 0 {
		if counts, err = s.push.NotificationCounts(ctx, req.User, selectedIDs); err != nil {
			return SlidingResponse{}, nil, false, err
		}
	}

	initialRooms := map[string]bool{}
	for _, roomID := range selectedIDs {
		initial, ok := changed[roomID]
		if !ok {
			continue
		}
		config := selected[roomID]
		m := byRoom[roomID]
		sent := next.rooms[roomID]
		from := sent.pos
		if initial {
			from = 0
		}
		room, err := s.slidingRoom(ctx, req, m, config, from, to, facts)
		if err != nil {
			if errors.Is(err, ErrForbidden) {
				// Lost access between the membership read and this one.
				continue
			}
			return SlidingResponse{}, nil, false, err
		}
		room.Initial = initial
		room.BumpStamp = facts.bump[roomID]
		c := counts[roomID]
		room.NotificationCount, room.HighlightCount = c.Notifications, c.Highlights
		resp.Rooms[roomID] = room
		if initial {
			initialRooms[roomID] = true
		}
		next.rooms[roomID] = sentRoom{pos: to.PDU, config: config.key(), membership: m.Membership, typing: sent.typing}
	}

	extensionsInteresting, err := s.slidingExtensions(ctx, req, prev, next, since, to, memberships, selectedIDs, byRoom, initialRooms, &resp)
	if err != nil {
		return SlidingResponse{}, nil, false, err
	}

	interesting := prev == nil || listsChanged || len(resp.Rooms) > 0 || extensionsInteresting
	return resp, next, interesting, nil
}

// slidingFacts are what a room list is filtered and sorted by.
type slidingFacts struct {
	bump  map[string]int64
	dms   map[string]bool
	entry map[string]roomserver.RoomListEntry
}

// roomFacts reads what the lists need to know about the candidate rooms.
//
// The bump stamp of a joined room is its last activity; a room the user is
// invited to, knocking on or has left is bumped by their own membership,
// because they cannot see what happens in it.
func (s *API) roomFacts(ctx context.Context, req SlidingRequest, candidates []roomserver.MembershipAt, to Token, lists map[string]SlidingList) (slidingFacts, error) {
	facts := slidingFacts{bump: map[string]int64{}, dms: map[string]bool{}, entry: map[string]roomserver.RoomListEntry{}}

	var joined []string
	for _, m := range candidates {
		if m.Membership == event.MembershipJoin {
			joined = append(joined, m.RoomID)
		} else {
			facts.bump[m.RoomID] = m.Position
		}
	}
	bumps, err := s.rooms.BumpStamps(ctx, joined, to.PDU)
	if err != nil {
		return slidingFacts{}, err
	}
	for _, roomID := range joined {
		facts.bump[roomID] = bumps[roomID]
	}

	raw, err := s.accounts.AccountData(ctx, req.Localpart, "", directType)
	switch {
	case errors.Is(err, storage.ErrNoSuchAccountData):
	case err != nil:
		return slidingFacts{}, err
	default:
		var direct map[string][]string
		if json.Unmarshal(raw, &direct) == nil {
			for _, rooms := range direct {
				for _, roomID := range rooms {
					facts.dms[roomID] = true
				}
			}
		}
	}
	return facts, nil
}

// entryOf reads a room's list entry once per attempt.
func (s *API) entryOf(ctx context.Context, roomID string, facts slidingFacts) (roomserver.RoomListEntry, error) {
	if e, ok := facts.entry[roomID]; ok {
		return e, nil
	}
	e, err := s.rooms.RoomListEntry(ctx, roomID)
	if err != nil {
		return roomserver.RoomListEntry{}, err
	}
	facts.entry[roomID] = e
	return e, nil
}

// listAdmits applies a list's filters to one room.
func (s *API) listAdmits(ctx context.Context, req SlidingRequest, f *SlidingFilters, m roomserver.MembershipAt, facts slidingFacts) (bool, error) {
	if f == nil {
		return true, nil
	}
	if f.IsDM != nil && facts.dms[m.RoomID] != *f.IsDM {
		return false, nil
	}
	if f.IsInvite != nil && (m.Membership == event.MembershipInvite) != *f.IsInvite {
		return false, nil
	}
	if f.IsEncrypted != nil || len(f.RoomTypes) > 0 || len(f.NotRoomTypes) > 0 {
		entry, err := s.entryOf(ctx, m.RoomID, facts)
		if err != nil {
			return false, err
		}
		if f.IsEncrypted != nil && entry.Encrypted != *f.IsEncrypted {
			return false, nil
		}
		typeMatches := func(types []*string) bool {
			return slices.ContainsFunc(types, func(t *string) bool {
				if t == nil {
					return entry.RoomType == ""
				}
				return *t == entry.RoomType
			})
		}
		if len(f.RoomTypes) > 0 && !typeMatches(f.RoomTypes) {
			return false, nil
		}
		if typeMatches(f.NotRoomTypes) {
			return false, nil
		}
	}
	if len(f.Spaces) > 0 {
		in := false
		for _, space := range f.Spaces {
			children, err := s.rooms.SpaceChildRooms(ctx, space)
			if err != nil {
				return false, err
			}
			if slices.Contains(children, m.RoomID) {
				in = true
				break
			}
		}
		if !in {
			return false, nil
		}
	}
	if len(f.Tags) > 0 || len(f.NotTags) > 0 {
		tags, err := s.roomTags(ctx, req.Localpart, m.RoomID)
		if err != nil {
			return false, err
		}
		if len(f.Tags) > 0 && !slices.ContainsFunc(f.Tags, func(t string) bool { return tags[t] }) {
			return false, nil
		}
		if slices.ContainsFunc(f.NotTags, func(t string) bool { return tags[t] }) {
			return false, nil
		}
	}
	return true, nil
}

// roomTags reads the tags a user has put on a room.
func (s *API) roomTags(ctx context.Context, localpart, roomID string) (map[string]bool, error) {
	raw, err := s.accounts.AccountData(ctx, localpart, roomID, tagType)
	if errors.Is(err, storage.ErrNoSuchAccountData) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	var content struct {
		Tags map[string]json.RawMessage `json:"tags"`
	}
	// Tags a client wrote in a shape that is not the spec's are no tags:
	// a list filter is not the place to refuse somebody's account data.
	if json.Unmarshal(raw, &content) != nil {
		content.Tags = nil
	}
	out := make(map[string]bool, len(content.Tags))
	for t := range content.Tags {
		out[t] = true
	}
	return out, nil
}

// slidingChangedRooms decides which selected rooms go into the response,
// and whether each goes in full. The value is true for a room sent in full.
func (s *API) slidingChangedRooms(ctx context.Context, req SlidingRequest, prev *connSnapshot, selected map[string]slidingRoomConfig, byRoom map[string]roomserver.MembershipAt, since, to Token) (map[string]bool, error) {
	out := map[string]bool{}
	// The rooms already sent, grouped by the position they were sent up
	// to, so that "has anything happened since" is one query per group.
	byPos := map[int64][]string{}
	for roomID, config := range selected {
		var sent sentRoom
		var ok bool
		if prev != nil {
			sent, ok = prev.rooms[roomID]
		}
		m := byRoom[roomID]
		switch {
		case !ok, sent.config != config.key():
			out[roomID] = true
		case m.Membership != sent.membership:
			// A membership the room was last sent under no longer
			// applies: an invite accepted is a room the client has only
			// seen stripped state of.
			out[roomID] = sent.membership == event.MembershipInvite ||
				sent.membership == event.MembershipKnock || sent.membership == ""
		case m.Position > sent.pos:
			out[roomID] = false
		case m.Membership == event.MembershipJoin || m.Membership == "":
			byPos[sent.pos] = append(byPos[sent.pos], roomID)
		}
	}
	for pos, roomIDs := range byPos {
		slices.Sort(roomIDs)
		withEvents, err := s.rooms.RoomsWithEvents(ctx, roomIDs, pos, to.PDU)
		if err != nil {
			return nil, err
		}
		for _, roomID := range withEvents {
			out[roomID] = false
		}
	}

	// Rooms whose unread counts moved without an event: read on another
	// device.
	if s.push != nil && prev != nil && to.Notification > since.Notification {
		moved, err := s.push.NotificationCountChanges(ctx, req.User, since.Notification, to.Notification)
		if err != nil {
			return nil, err
		}
		for _, roomID := range moved {
			if _, ok := selected[roomID]; ok {
				if _, already := out[roomID]; !already {
					out[roomID] = false
				}
			}
		}
	}
	return out, nil
}

// slidingRoom reads one room for a response. from is where the client's
// copy of the room ends, zero for a room sent in full.
func (s *API) slidingRoom(ctx context.Context, req SlidingRequest, m roomserver.MembershipAt, config slidingRoomConfig, from int64, to Token, facts slidingFacts) (*SlidingRoom, error) {
	room := &SlidingRoom{IsDM: facts.dms[m.RoomID]}
	entry, err := s.entryOf(ctx, m.RoomID, facts)
	if err != nil {
		return nil, err
	}
	room.Name, room.Avatar = entry.Name, entry.AvatarURL

	switch m.Membership {
	case event.MembershipInvite, event.MembershipKnock:
		state, err := s.rooms.StrippedState(ctx, m.RoomID, []string{req.User})
		if err != nil {
			return nil, err
		}
		if m.Membership == event.MembershipInvite {
			room.InviteState = state
		} else {
			room.KnockState = state
		}
		return room, nil
	}

	if config.timelineLimit > 0 {
		timeline, err := s.rooms.Timeline(ctx, roomserver.TimelineRequest{
			RoomID:    m.RoomID,
			User:      req.User,
			Low:       from,
			High:      to.PDU,
			Direction: storage.DirectionBackward,
			Limit:     config.timelineLimit,
		})
		if err != nil {
			return nil, err
		}
		room.Timeline = withoutRoomID(timeline.Events)
		room.Limited = timeline.Limited
		if len(timeline.Events) > 0 {
			room.PrevBatch = pduToken(timeline.Start).String()
		}
		live := 0
		if from != 0 {
			live = len(timeline.Events)
		}
		room.NumLive = &live
	}

	match := newStateMatcher(config.requiredState, req.User, room.Timeline)
	if !match.empty() {
		state, err := s.rooms.RequiredState(ctx, roomserver.RequiredStateRequest{
			RoomID: m.RoomID,
			User:   req.User,
			Since:  from,
			To:     to.PDU,
			Match:  match.matches,
		})
		if err != nil {
			return nil, err
		}
		room.RequiredState = withoutRoomID(state)
	}

	summary, err := s.rooms.RoomSummaryFor(ctx, m.RoomID, req.User)
	if err != nil {
		return nil, err
	}
	joined, invited := summary.JoinedMemberCount, summary.InvitedMemberCount
	room.JoinedCount, room.InvitedCount = &joined, &invited
	if room.Name == "" && len(summary.Heroes) > 0 {
		members, err := s.rooms.MemberEvents(ctx, m.RoomID, summary.Heroes)
		if err != nil {
			return nil, err
		}
		profiles := map[string]SlidingHero{}
		for _, e := range members {
			if e.StateKey == nil {
				continue
			}
			var content struct {
				DisplayName string `json:"displayname"`
				AvatarURL   string `json:"avatar_url"`
			}
			_ = json.Unmarshal(e.Content, &content)
			profiles[*e.StateKey] = SlidingHero{UserID: *e.StateKey, DisplayName: content.DisplayName, AvatarURL: content.AvatarURL}
		}
		for _, userID := range summary.Heroes {
			hero, ok := profiles[userID]
			if !ok {
				hero = SlidingHero{UserID: userID}
			}
			room.Heroes = append(room.Heroes, hero)
		}
	}
	return room, nil
}

// stateMatcher is a room's required_state, compiled.
type stateMatcher struct {
	all       bool
	wholeType map[string]bool
	anyType   map[string]bool
	tuples    map[[2]string]bool
}

func newStateMatcher(required [][]string, userID string, timeline []roomserver.ClientEvent) stateMatcher {
	m := stateMatcher{wholeType: map[string]bool{}, anyType: map[string]bool{}, tuples: map[[2]string]bool{}}
	for _, pair := range required {
		eventType, stateKey := pair[0], pair[1]
		switch stateKey {
		case "$ME":
			stateKey = userID
		case "$LAZY":
			if eventType == event.TypeMember {
				// The members whose names the timeline needs, and the user
				// themselves, whose own membership a client always shows.
				m.tuples[[2]string{event.TypeMember, userID}] = true
				for _, e := range timeline {
					m.tuples[[2]string{event.TypeMember, e.Sender}] = true
				}
			}
			continue
		}
		switch {
		case eventType == "*" && stateKey == "*":
			m.all = true
		case stateKey == "*":
			m.wholeType[eventType] = true
		case eventType == "*":
			m.anyType[stateKey] = true
		default:
			m.tuples[[2]string{eventType, stateKey}] = true
		}
	}
	return m
}

func (m stateMatcher) empty() bool {
	return !m.all && len(m.wholeType) == 0 && len(m.anyType) == 0 && len(m.tuples) == 0
}

func (m stateMatcher) matches(eventType, stateKey string) bool {
	return m.all || m.wholeType[eventType] || m.anyType[stateKey] || m.tuples[[2]string{eventType, stateKey}]
}

// extensionRooms narrows the rooms a per-room extension reports on to the
// ones its request names, by list or by room.
//
// Naming any list the request has, or "*", is every selected room: the
// windows of the lists are merged before the extensions run, and a client
// naming its main list means the rooms it is showing, which is a superset
// at worst. Naming rooms narrows to those of them that are selected.
func extensionRooms(t *extensionToggle, body SlidingBody, selected []string) []string {
	if len(t.Lists) == 0 && len(t.Rooms) == 0 {
		return selected
	}
	for _, name := range t.Lists {
		if _, ok := body.Lists[name]; ok || name == "*" {
			return selected
		}
	}
	if slices.Contains(t.Rooms, "*") {
		return selected
	}
	var out []string
	for _, roomID := range t.Rooms {
		if slices.Contains(selected, roomID) {
			out = append(out, roomID)
		}
	}
	return out
}

// slidingExtensions fills in the extensions a request turned on, and
// reports whether any of them has something to say.
func (s *API) slidingExtensions(
	ctx context.Context,
	req SlidingRequest,
	prev, next *connSnapshot,
	since, to Token,
	memberships []roomserver.MembershipAt,
	selected []string,
	byRoom map[string]roomserver.MembershipAt,
	initialRooms map[string]bool,
	resp *SlidingResponse,
) (bool, error) {
	ext := req.Body.Extensions
	interesting := false

	if ext.ToDevice != nil && ext.ToDevice.on() {
		from := next.toDevice
		if ext.ToDevice.Since != "" {
			parsed, err := strconv.ParseInt(ext.ToDevice.Since, 10, 64)
			if err != nil || parsed < 0 {
				return false, errInvalidTokenf("%q is not a to_device token", ext.ToDevice.Since)
			}
			from = parsed
			// Coming back with a since acknowledges everything up to it.
			s.ackToDevice(ctx, req.Localpart, req.Device, from)
		}
		limit := ext.ToDevice.Limit
		if limit <= 0 {
			limit = defaultToDeviceLimit
		}
		msgs, pos, err := s.toDeviceMessages(ctx, req.Localpart, req.Device, from, to.ToDevice, limit)
		if err != nil {
			return false, err
		}
		if msgs == nil {
			msgs = []toDeviceEvent{}
		}
		next.toDevice = pos
		resp.Extensions.ToDevice = &ToDeviceExtension{NextBatch: strconv.FormatInt(pos, 10), Events: msgs}
		interesting = interesting || len(msgs) > 0
	}

	if ext.E2EE.on() {
		e2ee := &E2EEExtension{DeviceLists: DeviceLists{Changed: []string{}, Left: []string{}}}
		if prev != nil {
			lists, err := s.deviceLists(ctx, req.User, since, to, memberships)
			if err != nil {
				return false, err
			}
			e2ee.DeviceLists = lists
		}
		var keys Response
		if err := s.deviceKeyCounts(ctx, SyncRequest{User: req.User, Device: req.Device}, &keys); err != nil {
			return false, err
		}
		e2ee.DeviceOneTimeKeysCount = keys.DeviceOneTimeKeysCount
		if e2ee.DeviceOneTimeKeysCount == nil {
			e2ee.DeviceOneTimeKeysCount = map[string]int{}
		}
		e2ee.DeviceUnusedFallbackKeyTypes = []string{}
		if keys.DeviceUnusedFallbackKeyTypes != nil {
			e2ee.DeviceUnusedFallbackKeyTypes = *keys.DeviceUnusedFallbackKeyTypes
		}
		resp.Extensions.E2EE = e2ee
		interesting = interesting || len(e2ee.DeviceLists.Changed) > 0 || len(e2ee.DeviceLists.Left) > 0
	}

	if ext.AccountData.on() {
		from := since.AccountData
		global, err := s.globalAccountData(ctx, SyncRequest{User: req.User, Localpart: req.Localpart}, from, to.AccountData, prev == nil, Filter{})
		if err != nil {
			return false, err
		}
		out := &AccountDataExtension{Global: global, Rooms: map[string][]AccountDataEvent{}}
		for _, roomID := range extensionRooms(ext.AccountData, req.Body, selected) {
			window := roomWindow{since: since, to: to, full: initialRooms[roomID] || prev == nil}
			events, err := s.roomAccountData(ctx, req.Localpart, roomID, window, Filter{})
			if err != nil {
				return false, err
			}
			if len(events) > 0 {
				out.Rooms[roomID] = events
			}
		}
		resp.Extensions.AccountData = out
		interesting = interesting || len(global) > 0 || len(out.Rooms) > 0
	}

	joinedOnly := func(roomIDs []string) []string {
		var out []string
		for _, roomID := range roomIDs {
			if byRoom[roomID].Membership == event.MembershipJoin {
				out = append(out, roomID)
			}
		}
		return out
	}

	if ext.Receipts.on() {
		out := &EphemeralExtension{Rooms: map[string]EphemeralEvent{}}
		rooms := joinedOnly(extensionRooms(ext.Receipts, req.Body, selected))
		var fresh, ongoing []string
		for _, roomID := range rooms {
			if initialRooms[roomID] || prev == nil {
				fresh = append(fresh, roomID)
			} else {
				ongoing = append(ongoing, roomID)
			}
		}
		for _, set := range []struct {
			rooms []string
			from  int64
		}{{fresh, 0}, {ongoing, since.Receipt}} {
			if len(set.rooms) == 0 || (set.from != 0 && to.Receipt <= set.from) {
				continue
			}
			byRoomReceipts, err := s.roomReceipts(ctx, req.User, set.rooms, set.from, to.Receipt)
			if err != nil {
				return false, err
			}
			for roomID, receipts := range byRoomReceipts {
				if e := receiptsEvent(receipts); e != nil {
					out.Rooms[roomID] = *e
				}
			}
		}
		resp.Extensions.Receipts = out
		interesting = interesting || len(out.Rooms) > 0
	}

	if ext.Typing.on() {
		out := &EphemeralExtension{Rooms: map[string]EphemeralEvent{}}
		for _, roomID := range joinedOnly(extensionRooms(ext.Typing, req.Body, selected)) {
			e := s.typingEvent(roomID)
			current := ""
			if e != nil {
				current = string(e.Content)
			}
			sent := next.rooms[roomID]
			if current == sent.typing {
				continue
			}
			if e == nil {
				// Somebody stopped: the client has to be told the list is
				// now empty, or it would show them typing until it
				// restarted.
				e = &EphemeralEvent{Type: "m.typing", Content: json.RawMessage(`{"user_ids":[]}`)}
			}
			out.Rooms[roomID] = *e
			sent.typing = current
			next.rooms[roomID] = sent
		}
		resp.Extensions.Typing = out
		interesting = interesting || len(out.Rooms) > 0
	}
	return interesting, nil
}