Phase 4: sync

Implements PLAN.md Phase 4: the read model. /sync, /messages and /context,
the filters that narrow them, and the streams a response carries that are
not room events — typing, receipts, presence and to-device messages. With
this a client can be used against Warp, which is milestone M1.

The one architectural choice is that the syncapi has no copy of the rooms:
it reads them through the roomserver. Dendrite gives its syncapi its own
tables and feeds them from the output stream, which is right for a design
where the two can be separate processes, and Dendrite's could be. Warp is
one process by decision, and there the copy buys nothing and costs the
hardest class of bug a homeserver has — a read model that disagrees with
the room, where a client is shown a message that was rejected and the only
way to find out is to compare two tables. So the roomserver grew a read
model (stream.go, roomsync.go, visibility.go) rather than exporting its
store: reading an event means parsing it under its room's version, and a
second copy of the version registry, the parser and the client projection
would be a second place for them to disagree.

syncapi: stream tokens naming a position in each of five independent
streams, so a client that has seen receipt 7 but only event 4 can say so in
one opaque string. A token with fewer positions than this build writes is
accepted and the rest read as zero, which is what lets a token stored
before a stream existed go on working. The PDU position comes from
MAX(stream_ordering) and not from the counter row the other four use: the
counter is advanced inside the writing transaction, so a token taken from
it would name a position whose event no reader can see yet, and the event
would be skipped for good once that transaction committed.

Long polling goes through a notifier that wakes by room and by user rather
than globally — an event wakes the listeners of its room, a membership
event also wakes the user it names, so an invite reaches a client that is
not in the room yet. A wake-up is only ever a hint: the listener re-reads
the streams and decides for itself what changed, which is what makes it
safe for the notifier to hold no state about what each client has seen. The
listener registers before the read, so an event landing in between leaves
the channel signalled and the sync reads again rather than blocking on a
change it just missed. EventPersisted must not block, because it is called
with the room's write lock still held.

"Something happened" is not "a token moved". A stream can move without this
user being concerned by it, and returning on every such move would turn a
long poll back into a busy loop. The same trap caught presence twice over:
a client says it is still online by syncing, so ?set_presence= makes every
sync a presence write, and a stream that moved each time would wake every
peer, which would sync, which would move it again. SetPresence therefore
reports whether anything a peer would notice actually changed, and a write
that only refreshes the last-active timestamp leaves the stream alone.

Timeline windows are named by their two ends and a direction, Low exclusive
and High inclusive in both directions, rather than by "from" and "to" as
the wire format spells them. Each of those would otherwise be the low bound
in one direction and the high bound in the other, which is exactly the
off-by-one every pagination bug is made of; the handlers translate once and
nothing below them thinks about it again. A sync's timeline is read
backwards and reported forwards, because a client wants the newest events
of the window it missed — a forward read with a limit hands it the oldest
and then calls the rest a gap.

visibility: history visibility is now a span rather than a yes or no, which
is what Phase 3 deferred here. A user who has left a room is served the
room as it was when they left — their state reads, their members and their
timeline — instead of being refused, and is bounded above at their own
membership event so a kicked client cannot go on streaming. Their span is
found from their own member events, so a user who joined, left and rejoined
sees their latest span. ?at= on /members is honoured for the same reason.
What is left is that the span is computed from the room's current
history_visibility where the spec judges each event by the one in force
when it was sent; that is a state read per event and belongs with open
question 1.

Typing is held in memory and nowhere else: notifications expire in seconds,
are worthless across a restart, and arrive one per keystroke-burst per
room, so persisting them would be the busiest table in the database holding
the least valuable data in it. Receipts only ever advance, compared on the
position of the event acknowledged rather than on arrival, because two of a
user's clients read at different speeds and the slower one must not drag
the marker backwards. To-device queues are per device and drained by the
next sync rather than by the one that delivered them, so a client that
crashes in between gets the batch again — the right way round, since a lost
key share leaves a message permanently undecryptable and a duplicate is
ignored. A wildcard addressee is expanded when sent, not stored, because
"every device" means the ones that existed then.

Presence is off by default, which answers PLAN.md open question 3. It is
the noisiest thing in Matrix and the one genuinely fan-out notification
here; a private homeserver gains little from it. With it off the endpoints
refuse rather than disappearing, because a client that gets a 404 from
/presence concludes the server is too old to have it and stops asking.

storage: one migration per driver adding the receipt, to-device, presence
and filter tables with a counter row each, and a stream position on the
existing account data table rather than a parallel one — there is exactly
one current value per (user, room, type), and a second table would be a
second thing to keep in step. Account data is reported in an event list but
is not an event, so it gets its own response type: a room event with an
empty sender and no timestamp would make a client decide what that meant.

Deliberately missing, each named in docs/sync.md: notification counts (they
need push rules, Phase 8), device lists (the keyserver, Phase 7), anything
remote (Phase 5), threads (Phase 10). device_lists is sent as two empty
lists because a client reads a missing one as malformed; the notification
counts are left out entirely, because a zero there is a claim and an absent
field is not.

Tested through HTTP rather than against the builder, because what is worth
pinning down is the response a client sees — which section a room lands in,
whether a token handed back can be handed in again, whether paging
terminates and loses nothing. The storage tests run against both SQL
drivers; the notifier has its own tests for the concurrent case. A live
server was driven end to end, which is what caught the two bugs the unit
tests had missed: a backwards /messages page must be in reverse
chronological order, and `start` must name where the page began rather than
position zero when ?from= was omitted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/internal/clientapi/sync_test.go

@@ -0,0 +1,664 @@
+package clientapi_test
+
+import (
+	"fmt"
+	"net/http"
+	"net/url"
+	"testing"
+	"time"
+)
+
+// The /sync tests. They go through HTTP rather than calling the syncapi
+// directly, because what is worth pinning down is the response a client
+// sees: the shape of the body, which section a room lands in, and whether a
+// token handed back can be handed in again.
+
+// syncResult is the handful of things these tests read out of a response.
+// It is a typed struct rather than a map because a sync response is nested
+// four deep, and `body["rooms"].(map[string]any)["join"]` repeated thirty
+// times is unreadable.
+type syncResult struct {
+	NextBatch string `json:"next_batch"`
+	Rooms     struct {
+		Join map[string]struct {
+			Timeline struct {
+				Events    []syncEvent `json:"events"`
+				Limited   bool        `json:"limited"`
+				PrevBatch string      `json:"prev_batch"`
+			} `json:"timeline"`
+			State struct {
+				Events []syncEvent `json:"events"`
+			} `json:"state"`
+			Ephemeral struct {
+				Events []syncEvent `json:"events"`
+			} `json:"ephemeral"`
+			AccountData struct {
+				Events []syncEvent `json:"events"`
+			} `json:"account_data"`
+			Summary *struct {
+				Heroes             []string `json:"m.heroes"`
+				JoinedMemberCount  int      `json:"m.joined_member_count"`
+				InvitedMemberCount int      `json:"m.invited_member_count"`
+			} `json:"summary"`
+		} `json:"join"`
+		Invite map[string]struct {
+			InviteState struct {
+				Events []syncEvent `json:"events"`
+			} `json:"invite_state"`
+		} `json:"invite"`
+		Leave map[string]struct {
+			Timeline struct {
+				Events []syncEvent `json:"events"`
+			} `json:"timeline"`
+			State struct {
+				Events []syncEvent `json:"events"`
+			} `json:"state"`
+		} `json:"leave"`
+	} `json:"rooms"`
+	Presence struct {
+		Events []syncEvent `json:"events"`
+	} `json:"presence"`
+	AccountData struct {
+		Events []syncEvent `json:"events"`
+	} `json:"account_data"`
+	ToDevice struct {
+		Events []syncEvent `json:"events"`
+	} `json:"to_device"`
+	DeviceLists struct {
+		Changed []string `json:"changed"`
+		Left    []string `json:"left"`
+	} `json:"device_lists"`
+}
+
+type syncEvent struct {
+	EventID  string         `json:"event_id"`
+	RoomID   string         `json:"room_id"`
+	Type     string         `json:"type"`
+	StateKey *string        `json:"state_key"`
+	Sender   string         `json:"sender"`
+	Content  map[string]any `json:"content"`
+}
+
+// sync performs one /sync, with no timeout: every test but the long-poll
+// ones wants the answer as it stands.
+func (ts *testServer) sync(acc account, since string) syncResult {
+	ts.t.Helper()
+	return ts.syncWith(acc, url.Values{"since": {since}})
+}
+
+// syncWith performs a /sync with arbitrary query parameters.
+func (ts *testServer) syncWith(acc account, query url.Values) syncResult {
+	ts.t.Helper()
+
+	path := "/_matrix/client/v3/sync"
+	if encoded := query.Encode(); encoded != "" {
+		path += "?" + encoded
+	}
+	rec := ts.get(path, acc.AccessToken)
+	expectStatus(ts.t, rec, http.StatusOK)
+
+	result := decode[syncResult](ts.t, rec)
+	if result.NextBatch == "" {
+		ts.t.Fatalf("/sync returned no next_batch: %s", rec.Body.String())
+	}
+	return result
+}
+
+// sendMessage sends an m.room.message and returns its event ID.
+func (ts *testServer) sendMessage(acc account, roomID, body string) string {
+	ts.t.Helper()
+
+	path := fmt.Sprintf("/_matrix/client/v3/rooms/%s/send/m.room.message/%s",
+		url.PathEscape(roomID), url.PathEscape(fmt.Sprintf("txn-%s-%d", body, time.Now().UnixNano())))
+	rec := ts.put(path, acc.AccessToken, map[string]any{"msgtype": "m.text", "body": body})
+	expectStatus(ts.t, rec, http.StatusOK)
+
+	eventID, _ := jsonBody(ts.t, rec)["event_id"].(string)
+	if eventID == "" {
+		ts.t.Fatalf("send returned no event ID: %s", rec.Body.String())
+	}
+	return eventID
+}
+
+// bodiesOf projects timeline events onto their message bodies, which is
+// what most of these tests actually assert on.
+func bodiesOf(events []syncEvent) []string {
+	out := make([]string, 0, len(events))
+	for _, e := range events {
+		if body, ok := e.Content["body"].(string); ok {
+			out = append(out, body)
+		}
+	}
+	return out
+}
+
+// typesOf projects events onto their types.
+func typesOf(events []syncEvent) []string {
+	out := make([]string, 0, len(events))
+	for _, e := range events {
+		out = append(out, e.Type)
+	}
+	return out
+}
+
+func contains(haystack []string, needle string) bool {
+	for _, s := range haystack {
+		if s == needle {
+			return true
+		}
+	}
+	return false
+}
+
+func TestSyncInitialReportsTheRoomAndItsHistory(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, map[string]any{"name": "Rome"})
+	ts.sendMessage(alice, roomID, "hello")
+
+	result := ts.sync(alice, "")
+
+	room, ok := result.Rooms.Join[roomID]
+	if !ok {
+		t.Fatalf("the room is not in the join section: %+v", result.Rooms.Join)
+	}
+	if got := bodiesOf(room.Timeline.Events); !contains(got, "hello") {
+		t.Errorf("timeline bodies = %v, want it to contain \"hello\"", got)
+	}
+
+	// The room was created and filled in fewer events than the default
+	// timeline limit, so the whole of it is in the timeline and there is
+	// nothing before it to report as state.
+	if room.Timeline.Limited {
+		t.Error("a room shorter than the timeline limit must not be reported as limited")
+	}
+
+	// A sync's events carry no room_id: they are already nested under the
+	// room they belong to.
+	for _, e := range room.Timeline.Events {
+		if e.RoomID != "" {
+			t.Errorf("%s carries a room_id in a sync response", e.Type)
+			break
+		}
+	}
+
+	// device_lists is empty until Phase 7 and must still be present: a
+	// client reads a missing one as a malformed response.
+	if result.DeviceLists.Changed == nil || result.DeviceLists.Left == nil {
+		t.Error("device_lists must carry both lists even when empty")
+	}
+}
+
+func TestSyncIncrementalReturnsOnlyWhatIsNew(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+	ts.sendMessage(alice, roomID, "first")
+
+	first := ts.sync(alice, "")
+	ts.sendMessage(alice, roomID, "second")
+	second := ts.sync(alice, first.NextBatch)
+
+	room, ok := second.Rooms.Join[roomID]
+	if !ok {
+		t.Fatalf("the room is not in the join section: %+v", second.Rooms.Join)
+	}
+	got := bodiesOf(room.Timeline.Events)
+	if len(got) != 1 || got[0] != "second" {
+		t.Errorf("timeline bodies = %v, want exactly [second]", got)
+	}
+
+	// An incremental sync with no gap reports no state: every state change
+	// in the window is a timeline event the client already has.
+	if len(room.State.Events) != 0 {
+		t.Errorf("state = %v, want none on a gapless incremental sync", typesOf(room.State.Events))
+	}
+}
+
+func TestSyncIncrementalIsEmptyWhenNothingHappened(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+
+	first := ts.sync(alice, "")
+	second := ts.sync(alice, first.NextBatch)
+
+	if len(second.Rooms.Join) != 0 {
+		t.Errorf("join = %v, want nothing when nothing happened", second.Rooms.Join)
+	}
+	_ = roomID
+}
+
+func TestSyncTokenRoundTripsThroughTheClient(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+
+	first := ts.sync(alice, "")
+	// Syncing twice from the same token must give the same answer: a token
+	// is a position, not a cursor the server advances for itself.
+	a := ts.sync(alice, first.NextBatch)
+	ts.sendMessage(alice, roomID, "once")
+	b := ts.sync(alice, first.NextBatch)
+	c := ts.sync(alice, first.NextBatch)
+
+	if len(a.Rooms.Join) != 0 {
+		t.Error("the first re-sync should have been empty")
+	}
+	if got := bodiesOf(b.Rooms.Join[roomID].Timeline.Events); len(got) != 1 {
+		t.Errorf("second re-sync timeline = %v, want one event", got)
+	}
+	if got := bodiesOf(c.Rooms.Join[roomID].Timeline.Events); len(got) != 1 {
+		t.Errorf("third re-sync timeline = %v, want the same one event", got)
+	}
+}
+
+func TestSyncRejectsAnUnreadableToken(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+
+	for _, since := range []string{"nonsense", "s-1", "sx", "s1_2_3_4_5_6"} {
+		rec := ts.get("/_matrix/client/v3/sync?since="+url.QueryEscape(since), alice.AccessToken)
+		expectError(t, rec, http.StatusBadRequest, "M_INVALID_PARAM")
+	}
+}
+
+func TestSyncAcceptsATokenFromAnOlderServer(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+	ts.sendMessage(alice, roomID, "hello")
+
+	// A token with only the event position in it, as a client stored
+	// before the other streams existed. It must be read as "seen nothing"
+	// of those streams rather than refused.
+	result := ts.syncWith(alice, url.Values{"since": {"s1"}})
+	if len(result.Rooms.Join) == 0 {
+		t.Error("a short token must still sync")
+	}
+}
+
+func TestSyncLongPollWakesOnAnEvent(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+	since := ts.sync(alice, "").NextBatch
+
+	done := make(chan syncResult, 1)
+	go func() {
+		done <- ts.syncWith(alice, url.Values{
+			"since":   {since},
+			"timeout": {"10000"},
+		})
+	}()
+
+	// The sync has to be blocked before the event is sent, or it would
+	// find the event without ever having waited and the test would pass
+	// without testing anything.
+	waitForListener(t, ts)
+	ts.sendMessage(alice, roomID, "woken")
+
+	select {
+	case result := <-done:
+		got := bodiesOf(result.Rooms.Join[roomID].Timeline.Events)
+		if !contains(got, "woken") {
+			t.Errorf("timeline = %v, want it to contain \"woken\"", got)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("a blocked /sync was not woken by an event in its room")
+	}
+}
+
+func TestSyncLongPollReturnsEmptyOnTimeout(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	ts.createRoom(alice, nil)
+	since := ts.sync(alice, "").NextBatch
+
+	start := time.Now()
+	result := ts.syncWith(alice, url.Values{"since": {since}, "timeout": {"150"}})
+	elapsed := time.Since(start)
+
+	if len(result.Rooms.Join) != 0 {
+		t.Errorf("join = %v, want nothing", result.Rooms.Join)
+	}
+	if elapsed < 100*time.Millisecond {
+		t.Errorf("a 150ms long poll returned after %v; it did not wait", elapsed)
+	}
+}
+
+// waitForListener blocks until a /sync has registered with the notifier, so
+// that a test can be sure the sync it started is actually waiting.
+func waitForListener(t *testing.T, ts *testServer) {
+	t.Helper()
+
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		if ts.syncAPI.Notifier().Listeners() > 0 {
+			return
+		}
+		time.Sleep(time.Millisecond)
+	}
+	t.Fatal("no /sync ever blocked on the notifier")
+}
+
+func TestSyncReportsAnInviteAsStrippedState(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	bob := ts.register("bob", "hunter2-hunter2-hunter2")
+	roomID := ts.createRoom(alice, map[string]any{"name": "Carthage"})
+
+	rec := ts.post("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/invite",
+		alice.AccessToken, map[string]any{"user_id": bob.UserID})
+	expectStatus(t, rec, http.StatusOK)
+
+	result := ts.sync(bob, "")
+	invite, ok := result.Rooms.Invite[roomID]
+	if !ok {
+		t.Fatalf("the room is not in the invite section: %+v", result.Rooms.Invite)
+	}
+	if !contains(typesOf(invite.InviteState.Events), "m.room.name") {
+		t.Errorf("invite_state = %v, want it to name the room", typesOf(invite.InviteState.Events))
+	}
+	if !contains(typesOf(invite.InviteState.Events), "m.room.member") {
+		t.Error("invite_state must carry the member events that make the invite legible")
+	}
+	// An invitee gets no timeline at all: they are not a member.
+	if _, joined := result.Rooms.Join[roomID]; joined {
+		t.Error("an invited room must not be in the join section")
+	}
+}
+
+func TestSyncReportsALeaveAndThenStops(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	bob := ts.register("bob", "hunter2-hunter2-hunter2")
+	roomID := ts.createRoom(alice, map[string]any{"preset": "public_chat"})
+
+	expectStatus(t, ts.post("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/join",
+		bob.AccessToken, map[string]any{}), http.StatusOK)
+	ts.sendMessage(alice, roomID, "while bob is here")
+
+	since := ts.sync(bob, "").NextBatch
+	expectStatus(t, ts.post("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/leave",
+		bob.AccessToken, map[string]any{}), http.StatusOK)
+
+	afterLeave := ts.sync(bob, since)
+	if _, ok := afterLeave.Rooms.Leave[roomID]; !ok {
+		t.Fatalf("the leave was not reported: %+v", afterLeave.Rooms)
+	}
+
+	// Everything after the leave is somebody else's business.
+	ts.sendMessage(alice, roomID, "after bob left")
+	quiet := ts.sync(bob, afterLeave.NextBatch)
+	for _, section := range []any{quiet.Rooms.Join[roomID], quiet.Rooms.Leave[roomID]} {
+		_ = section
+	}
+	if left, ok := quiet.Rooms.Leave[roomID]; ok {
+		if got := bodiesOf(left.Timeline.Events); contains(got, "after bob left") {
+			t.Errorf("a departed member was sent %v", got)
+		}
+	}
+}
+
+func TestSyncSummaryNamesARoomWithNoName(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	bob := ts.register("bob", "hunter2-hunter2-hunter2")
+	roomID := ts.createRoom(alice, map[string]any{
+		"preset": "trusted_private_chat",
+		"invite": []string{bob.UserID},
+	})
+
+	result := ts.sync(alice, "")
+	room := result.Rooms.Join[roomID]
+	if room.Summary == nil {
+		t.Fatal("a room with no name must carry a summary to name it by")
+	}
+	if !contains(room.Summary.Heroes, bob.UserID) {
+		t.Errorf("heroes = %v, want it to name %s", room.Summary.Heroes, bob.UserID)
+	}
+	if room.Summary.JoinedMemberCount != 1 {
+		t.Errorf("joined_member_count = %d, want 1", room.Summary.JoinedMemberCount)
+	}
+	if room.Summary.InvitedMemberCount != 1 {
+		t.Errorf("invited_member_count = %d, want 1", room.Summary.InvitedMemberCount)
+	}
+}
+
+func TestSyncLimitedTimelineCarriesTheStateBeforeIt(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+
+	since := ts.sync(alice, "").NextBatch
+
+	// A state change and then more messages than the timeline limit, so
+	// the state change falls into the gap and has to be reported
+	// separately.
+	expectStatus(t, ts.put("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/state/m.room.topic",
+		alice.AccessToken, map[string]any{"topic": "in the gap"}), http.StatusOK)
+	for i := range 5 {
+		ts.sendMessage(alice, roomID, fmt.Sprintf("message %d", i))
+	}
+
+	result := ts.syncWith(alice, url.Values{
+		"since":  {since},
+		"filter": {`{"room":{"timeline":{"limit":2}}}`},
+	})
+	room := result.Rooms.Join[roomID]
+	if !room.Timeline.Limited {
+		t.Fatal("a timeline cut short by the limit must say so")
+	}
+	if room.Timeline.PrevBatch == "" {
+		t.Error("a limited timeline must carry a prev_batch to close the gap with")
+	}
+	if !contains(typesOf(room.State.Events), "m.room.topic") {
+		t.Errorf("state = %v, want the topic change that fell into the gap",
+			typesOf(room.State.Events))
+	}
+}
+
+func TestSyncFullStateReturnsEverythingAgain(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, map[string]any{"name": "Thebes"})
+
+	since := ts.sync(alice, "").NextBatch
+	result := ts.syncWith(alice, url.Values{
+		"since":      {since},
+		"full_state": {"true"},
+		"filter":     {`{"room":{"timeline":{"limit":1}}}`},
+	})
+
+	room, ok := result.Rooms.Join[roomID]
+	if !ok {
+		t.Fatalf("full_state must report every room: %+v", result.Rooms.Join)
+	}
+	if !contains(typesOf(room.State.Events), "m.room.create") {
+		t.Errorf("state = %v, want the whole state including m.room.create",
+			typesOf(room.State.Events))
+	}
+}
+
+func TestSyncLazyLoadingLeavesOutUninvolvedMembers(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	bob := ts.register("bob", "hunter2-hunter2-hunter2")
+	roomID := ts.createRoom(alice, map[string]any{"preset": "public_chat"})
+	expectStatus(t, ts.post("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/join",
+		bob.AccessToken, map[string]any{}), http.StatusOK)
+
+	// Only Alice speaks, so only Alice's member event is needed to render
+	// the timeline.
+	for i := range 4 {
+		ts.sendMessage(alice, roomID, fmt.Sprintf("line %d", i))
+	}
+
+	result := ts.syncWith(alice, url.Values{
+		"filter": {`{"room":{"timeline":{"limit":2,"lazy_load_members":true}}}`},
+	})
+	room := result.Rooms.Join[roomID]
+
+	members := map[string]bool{}
+	for _, e := range room.State.Events {
+		if e.Type == "m.room.member" && e.StateKey != nil {
+			members[*e.StateKey] = true
+		}
+	}
+	if !members[alice.UserID] {
+		t.Error("lazy loading must still send the member event of a timeline sender")
+	}
+	if members[bob.UserID] {
+		t.Error("lazy loading must leave out a member who does not appear in the timeline")
+	}
+}
+
+func TestSyncAppliesATypeFilter(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+	since := ts.sync(alice, "").NextBatch
+
+	ts.sendMessage(alice, roomID, "a message")
+	expectStatus(t, ts.put("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/state/m.room.topic",
+		alice.AccessToken, map[string]any{"topic": "a topic"}), http.StatusOK)
+
+	result := ts.syncWith(alice, url.Values{
+		"since":  {since},
+		"filter": {`{"room":{"timeline":{"not_types":["m.room.message"]}}}`},
+	})
+	got := typesOf(result.Rooms.Join[roomID].Timeline.Events)
+	if contains(got, "m.room.message") {
+		t.Errorf("timeline = %v, want the message excluded", got)
+	}
+	if !contains(got, "m.room.topic") {
+		t.Errorf("timeline = %v, want the topic kept", got)
+	}
+}
+
+func TestSyncUsesAnUploadedFilter(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+
+	rec := ts.post("/_matrix/client/v3/user/"+url.PathEscape(alice.UserID)+"/filter",
+		alice.AccessToken, map[string]any{
+			"room": map[string]any{
+				"timeline": map[string]any{"types": []string{"m.room.message"}},
+			},
+		})
+	expectStatus(t, rec, http.StatusOK)
+	filterID, _ := jsonBody(t, rec)["filter_id"].(string)
+	if filterID == "" {
+		t.Fatalf("no filter ID was returned: %s", rec.Body.String())
+	}
+
+	// It comes back exactly as it was uploaded.
+	rec = ts.get("/_matrix/client/v3/user/"+url.PathEscape(alice.UserID)+"/filter/"+filterID,
+		alice.AccessToken)
+	expectStatus(t, rec, http.StatusOK)
+
+	ts.sendMessage(alice, roomID, "kept")
+	result := ts.syncWith(alice, url.Values{"filter": {filterID}})
+
+	got := typesOf(result.Rooms.Join[roomID].Timeline.Events)
+	if !contains(got, "m.room.message") {
+		t.Errorf("timeline = %v, want the message", got)
+	}
+	if contains(got, "m.room.create") {
+		t.Errorf("timeline = %v, want only messages", got)
+	}
+}
+
+func TestGetUnknownFilterIsNotFound(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+
+	rec := ts.get("/_matrix/client/v3/user/"+url.PathEscape(alice.UserID)+"/filter/nope",
+		alice.AccessToken)
+	expectError(t, rec, http.StatusNotFound, "M_NOT_FOUND")
+}
+
+func TestFilterBelongsToItsOwner(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	bob := ts.register("bob", "hunter2-hunter2-hunter2")
+
+	rec := ts.post("/_matrix/client/v3/user/"+url.PathEscape(bob.UserID)+"/filter",
+		alice.AccessToken, map[string]any{})
+	expectError(t, rec, http.StatusForbidden, "M_FORBIDDEN")
+}
+
+func TestSyncReportsAccountData(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	since := ts.sync(alice, "").NextBatch
+
+	expectStatus(t, ts.put("/_matrix/client/v3/user/"+url.PathEscape(alice.UserID)+
+		"/account_data/m.direct", alice.AccessToken, map[string]any{"@bob:warp.example": []string{}}),
+		http.StatusOK)
+
+	result := ts.sync(alice, since)
+	if !contains(typesOf(result.AccountData.Events), "m.direct") {
+		t.Errorf("account_data = %v, want m.direct", typesOf(result.AccountData.Events))
+	}
+	// Account data is not an event: it has a type and a content and
+	// nothing else.
+	for _, e := range result.AccountData.Events {
+		if e.EventID != "" || e.Sender != "" {
+			t.Errorf("account data was reported as an event: %+v", e)
+		}
+	}
+}
+
+func TestMembersAtAnEarlierPosition(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	bob := ts.register("bob", "hunter2-hunter2-hunter2")
+	roomID := ts.createRoom(alice, map[string]any{"preset": "public_chat"})
+
+	// A token from before Bob joined.
+	before := ts.sync(alice, "").NextBatch
+	expectStatus(t, ts.post("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/join",
+		bob.AccessToken, map[string]any{}), http.StatusOK)
+
+	membersAt := func(at string) []string {
+		t.Helper()
+		path := "/_matrix/client/v3/rooms/" + url.PathEscape(roomID) + "/members"
+		if at != "" {
+			path += "?at=" + url.QueryEscape(at)
+		}
+		rec := ts.get(path, alice.AccessToken)
+		expectStatus(t, rec, http.StatusOK)
+
+		body := decode[struct {
+			Chunk []syncEvent `json:"chunk"`
+		}](t, rec)
+
+		out := make([]string, 0, len(body.Chunk))
+		for _, e := range body.Chunk {
+			if e.StateKey != nil {
+				out = append(out, *e.StateKey)
+			}
+		}
+		return out
+	}
+
+	if got := membersAt(""); !contains(got, bob.UserID) {
+		t.Errorf("current members = %v, want it to include %s", got, bob.UserID)
+	}
+	if got := membersAt(before); contains(got, bob.UserID) {
+		t.Errorf("members at an earlier position = %v, want it to leave out %s", got, bob.UserID)
+	}
+}
+
+func TestMembersRejectsABadAtToken(t *testing.T) {
+	ts := newTestServer(t, nil)
+	alice := ts.register("alice", "correct-horse-battery-staple")
+	roomID := ts.createRoom(alice, nil)
+
+	rec := ts.get("/_matrix/client/v3/rooms/"+url.PathEscape(roomID)+"/members?at=nonsense",
+		alice.AccessToken)
+	expectError(t, rec, http.StatusBadRequest, "M_INVALID_PARAM")
+}