MSC2946: Spaces Summary (#1700)

* Add stub functions for MSC2946

* Implement core space walking algorithm

* Flesh out stub functions; add test stubs

* Implement storage bits and add sanity check test

* Implement world_readable auth with test

* Linting

5 files changed, 1039 insertions, 1 deletions

diff --git a/setup/config/config_mscs.go b/setup/config/config_mscs.go
index 776d0b64..4b53495f 100644
--- a/setup/config/config_mscs.go
+++ b/setup/config/config_mscs.go
@@ -3,7 +3,7 @@ package config
 type MSCs struct {
 	Matrix *Global `yaml:"-"`
 
-	// The MSCs to enable, currently only `msc2836` is supported.
+	// The MSCs to enable
 	MSCs []string `yaml:"mscs"`
 
 	Database DatabaseOptions `yaml:"database"`
diff --git a/setup/mscs/msc2946/msc2946.go b/setup/mscs/msc2946/msc2946.go
new file mode 100644
index 00000000..244a54bc
--- /dev/null
+++ b/setup/mscs/msc2946/msc2946.go
@@ -0,0 +1,366 @@
+// Copyright 2021 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package msc2946 'Spaces Summary' implements https://github.com/matrix-org/matrix-doc/pull/2946
+package msc2946
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"sync"
+
+	"github.com/gorilla/mux"
+	chttputil "github.com/matrix-org/dendrite/clientapi/httputil"
+	"github.com/matrix-org/dendrite/internal/hooks"
+	"github.com/matrix-org/dendrite/internal/httputil"
+	roomserver "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/setup"
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/tidwall/gjson"
+)
+
+const (
+	ConstCreateEventContentKey = "org.matrix.msc1772.type"
+	ConstSpaceChildEventType   = "org.matrix.msc1772.space.child"
+	ConstSpaceParentEventType  = "org.matrix.msc1772.room.parent"
+)
+
+// SpacesRequest is the request body to POST /_matrix/client/r0/rooms/{roomID}/spaces
+type SpacesRequest struct {
+	MaxRoomsPerSpace int    `json:"max_rooms_per_space"`
+	Limit            int    `json:"limit"`
+	Batch            string `json:"batch"`
+}
+
+// Defaults sets the request defaults
+func (r *SpacesRequest) Defaults() {
+	r.Limit = 100
+	r.MaxRoomsPerSpace = -1
+}
+
+// SpacesResponse is the response body to POST /_matrix/client/r0/rooms/{roomID}/spaces
+type SpacesResponse struct {
+	NextBatch string `json:"next_batch"`
+	// Rooms are nodes on the space graph.
+	Rooms []Room `json:"rooms"`
+	// Events are edges on the space graph, exclusively m.space.child or m.room.parent events
+	Events []gomatrixserverlib.ClientEvent `json:"events"`
+}
+
+// Room is a node on the space graph
+type Room struct {
+	gomatrixserverlib.PublicRoom
+	NumRefs  int    `json:"num_refs"`
+	RoomType string `json:"room_type"`
+}
+
+// Enable this MSC
+func Enable(
+	base *setup.BaseDendrite, rsAPI roomserver.RoomserverInternalAPI, userAPI userapi.UserInternalAPI,
+) error {
+	db, err := NewDatabase(&base.Cfg.MSCs.Database)
+	if err != nil {
+		return fmt.Errorf("Cannot enable MSC2946: %w", err)
+	}
+	hooks.Enable()
+	hooks.Attach(hooks.KindNewEventPersisted, func(headeredEvent interface{}) {
+		he := headeredEvent.(*gomatrixserverlib.HeaderedEvent)
+		hookErr := db.StoreReference(context.Background(), he)
+		if hookErr != nil {
+			util.GetLogger(context.Background()).WithError(hookErr).WithField("event_id", he.EventID()).Error(
+				"failed to StoreReference",
+			)
+		}
+	})
+
+	base.PublicClientAPIMux.Handle("/unstable/rooms/{roomID}/spaces",
+		httputil.MakeAuthAPI("spaces", userAPI, spacesHandler(db, rsAPI)),
+	).Methods(http.MethodPost, http.MethodOptions)
+	return nil
+}
+
+func spacesHandler(db Database, rsAPI roomserver.RoomserverInternalAPI) func(*http.Request, *userapi.Device) util.JSONResponse {
+	inMemoryBatchCache := make(map[string]set)
+	return func(req *http.Request, device *userapi.Device) util.JSONResponse {
+		// Extract the room ID from the request. Sanity check request data.
+		params := mux.Vars(req)
+		roomID := params["roomID"]
+		var r SpacesRequest
+		r.Defaults()
+		if resErr := chttputil.UnmarshalJSONRequest(req, &r); resErr != nil {
+			return *resErr
+		}
+		if r.Limit > 100 {
+			r.Limit = 100
+		}
+		w := walker{
+			req:        &r,
+			rootRoomID: roomID,
+			caller:     device,
+			ctx:        req.Context(),
+
+			db:                 db,
+			rsAPI:              rsAPI,
+			inMemoryBatchCache: inMemoryBatchCache,
+		}
+		res := w.walk()
+		return util.JSONResponse{
+			Code: 200,
+			JSON: res,
+		}
+	}
+}
+
+type walker struct {
+	req        *SpacesRequest
+	rootRoomID string
+	caller     *userapi.Device
+	db         Database
+	rsAPI      roomserver.RoomserverInternalAPI
+	ctx        context.Context
+
+	// user ID|device ID|batch_num => event/room IDs sent to client
+	inMemoryBatchCache map[string]set
+	mu                 sync.Mutex
+}
+
+func (w *walker) alreadySent(id string) bool {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	m, ok := w.inMemoryBatchCache[w.caller.UserID+"|"+w.caller.ID]
+	if !ok {
+		return false
+	}
+	return m[id]
+}
+
+func (w *walker) markSent(id string) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	m := w.inMemoryBatchCache[w.caller.UserID+"|"+w.caller.ID]
+	if m == nil {
+		m = make(set)
+	}
+	m[id] = true
+	w.inMemoryBatchCache[w.caller.UserID+"|"+w.caller.ID] = m
+}
+
+// nolint:gocyclo
+func (w *walker) walk() *SpacesResponse {
+	var res SpacesResponse
+	// Begin walking the graph starting with the room ID in the request in a queue of unvisited rooms
+	unvisited := []string{w.rootRoomID}
+	processed := make(set)
+	for len(unvisited) > 0 {
+		roomID := unvisited[0]
+		unvisited = unvisited[1:]
+		// If this room has already been processed, skip. NB: do not remember this between calls
+		if processed[roomID] || roomID == "" {
+			continue
+		}
+		// Mark this room as processed.
+		processed[roomID] = true
+		// Is the caller currently joined to the room or is the room `world_readable`
+		// If no, skip this room. If yes, continue.
+		if !w.authorised(roomID) {
+			continue
+		}
+		// Get all `m.space.child` and `m.room.parent` state events for the room. *In addition*, get
+		// all `m.space.child` and `m.room.parent` state events which *point to* (via `state_key` or `content.room_id`)
+		// this room. This requires servers to store reverse lookups.
+		refs, err := w.references(roomID)
+		if err != nil {
+			util.GetLogger(w.ctx).WithError(err).WithField("room_id", roomID).Error("failed to extract references for room")
+			continue
+		}
+
+		// If this room has not ever been in `rooms` (across multiple requests), extract the
+		// `PublicRoomsChunk` for this room.
+		if !w.alreadySent(roomID) {
+			pubRoom := w.publicRoomsChunk(roomID)
+			roomType := ""
+			create := w.stateEvent(roomID, "m.room.create", "")
+			if create != nil {
+				roomType = gjson.GetBytes(create.Content(), ConstCreateEventContentKey).Str
+			}
+
+			// Add the total number of events to `PublicRoomsChunk` under `num_refs`. Add `PublicRoomsChunk` to `rooms`.
+			res.Rooms = append(res.Rooms, Room{
+				PublicRoom: *pubRoom,
+				NumRefs:    refs.len(),
+				RoomType:   roomType,
+			})
+		}
+
+		uniqueRooms := make(set)
+
+		// If this is the root room from the original request, insert all these events into `events` if
+		// they haven't been added before (across multiple requests).
+		if w.rootRoomID == roomID {
+			for _, ev := range refs.events() {
+				if !w.alreadySent(ev.EventID()) {
+					res.Events = append(res.Events, gomatrixserverlib.HeaderedToClientEvent(
+						ev, gomatrixserverlib.FormatAll,
+					))
+					uniqueRooms[ev.RoomID()] = true
+					uniqueRooms[SpaceTarget(ev)] = true
+					w.markSent(ev.EventID())
+				}
+			}
+		} else {
+			// Else add them to `events` honouring the `limit` and `max_rooms_per_space` values. If either
+			// are exceeded, stop adding events. If the event has already been added, do not add it again.
+			numAdded := 0
+			for _, ev := range refs.events() {
+				if w.req.Limit > 0 && len(res.Events) >= w.req.Limit {
+					break
+				}
+				if w.req.MaxRoomsPerSpace > 0 && numAdded >= w.req.MaxRoomsPerSpace {
+					break
+				}
+				if w.alreadySent(ev.EventID()) {
+					continue
+				}
+				res.Events = append(res.Events, gomatrixserverlib.HeaderedToClientEvent(
+					ev, gomatrixserverlib.FormatAll,
+				))
+				uniqueRooms[ev.RoomID()] = true
+				uniqueRooms[SpaceTarget(ev)] = true
+				w.markSent(ev.EventID())
+				// we don't distinguish between child state events and parent state events for the purposes of
+				// max_rooms_per_space, maybe we should?
+				numAdded++
+			}
+		}
+
+		// For each referenced room ID in the events being returned to the caller (both parent and child)
+		// add the room ID to the queue of unvisited rooms. Loop from the beginning.
+		for roomID := range uniqueRooms {
+			unvisited = append(unvisited, roomID)
+		}
+	}
+	return &res
+}
+
+func (w *walker) stateEvent(roomID, evType, stateKey string) *gomatrixserverlib.HeaderedEvent {
+	var queryRes roomserver.QueryCurrentStateResponse
+	tuple := gomatrixserverlib.StateKeyTuple{
+		EventType: evType,
+		StateKey:  stateKey,
+	}
+	err := w.rsAPI.QueryCurrentState(w.ctx, &roomserver.QueryCurrentStateRequest{
+		RoomID:      roomID,
+		StateTuples: []gomatrixserverlib.StateKeyTuple{tuple},
+	}, &queryRes)
+	if err != nil {
+		return nil
+	}
+	return queryRes.StateEvents[tuple]
+}
+
+func (w *walker) publicRoomsChunk(roomID string) *gomatrixserverlib.PublicRoom {
+	pubRooms, err := roomserver.PopulatePublicRooms(w.ctx, []string{roomID}, w.rsAPI)
+	if err != nil {
+		util.GetLogger(w.ctx).WithError(err).Error("failed to PopulatePublicRooms")
+		return nil
+	}
+	if len(pubRooms) == 0 {
+		return nil
+	}
+	return &pubRooms[0]
+}
+
+// authorised returns true iff the user is joined this room or the room is world_readable
+func (w *walker) authorised(roomID string) bool {
+	hisVisTuple := gomatrixserverlib.StateKeyTuple{
+		EventType: gomatrixserverlib.MRoomHistoryVisibility,
+		StateKey:  "",
+	}
+	roomMemberTuple := gomatrixserverlib.StateKeyTuple{
+		EventType: gomatrixserverlib.MRoomMember,
+		StateKey:  w.caller.UserID,
+	}
+	var queryRes roomserver.QueryCurrentStateResponse
+	err := w.rsAPI.QueryCurrentState(w.ctx, &roomserver.QueryCurrentStateRequest{
+		RoomID: roomID,
+		StateTuples: []gomatrixserverlib.StateKeyTuple{
+			hisVisTuple, roomMemberTuple,
+		},
+	}, &queryRes)
+	if err != nil {
+		util.GetLogger(w.ctx).WithError(err).Error("failed to QueryCurrentState")
+		return false
+	}
+	memberEv := queryRes.StateEvents[roomMemberTuple]
+	hisVisEv := queryRes.StateEvents[hisVisTuple]
+	if memberEv != nil {
+		membership, _ := memberEv.Membership()
+		if membership == gomatrixserverlib.Join {
+			return true
+		}
+	}
+	if hisVisEv != nil {
+		hisVis, _ := hisVisEv.HistoryVisibility()
+		if hisVis == "world_readable" {
+			return true
+		}
+	}
+	return false
+}
+
+// references returns all references pointing to or from this room.
+func (w *walker) references(roomID string) (eventLookup, error) {
+	events, err := w.db.References(w.ctx, roomID)
+	if err != nil {
+		return nil, err
+	}
+	el := make(eventLookup)
+	for _, ev := range events {
+		el.set(ev)
+	}
+	return el, nil
+}
+
+// state event lookup across multiple rooms keyed on event type
+// NOT THREAD SAFE
+type eventLookup map[string][]*gomatrixserverlib.HeaderedEvent
+
+func (el eventLookup) set(ev *gomatrixserverlib.HeaderedEvent) {
+	evs := el[ev.Type()]
+	if evs == nil {
+		evs = make([]*gomatrixserverlib.HeaderedEvent, 0)
+	}
+	evs = append(evs, ev)
+	el[ev.Type()] = evs
+}
+
+func (el eventLookup) len() int {
+	sum := 0
+	for _, evs := range el {
+		sum += len(evs)
+	}
+	return sum
+}
+
+func (el eventLookup) events() (events []*gomatrixserverlib.HeaderedEvent) {
+	for _, evs := range el {
+		events = append(events, evs...)
+	}
+	return
+}
+
+type set map[string]bool
diff --git a/setup/mscs/msc2946/msc2946_test.go b/setup/mscs/msc2946/msc2946_test.go
new file mode 100644
index 00000000..017319dc
--- /dev/null
+++ b/setup/mscs/msc2946/msc2946_test.go
@@ -0,0 +1,486 @@
+// Copyright 2021 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package msc2946_test
+
+import (
+	"bytes"
+	"context"
+	"crypto/ed25519"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"testing"
+	"time"
+
+	"github.com/gorilla/mux"
+	"github.com/matrix-org/dendrite/internal/hooks"
+	"github.com/matrix-org/dendrite/internal/httputil"
+	roomserver "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/setup"
+	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/dendrite/setup/mscs/msc2946"
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+var (
+	client = &http.Client{
+		Timeout: 10 * time.Second,
+	}
+)
+
+// Basic sanity check of MSC2946 logic. Tests a single room with a few state events
+// and a bit of recursion to subspaces. Makes a graph like:
+//      Root
+//    ____|_____
+//   |    |    |
+//  R1   R2    S1
+//             |_________
+//             |    |   |
+//             R3   R4  S2
+//                      |  <-- this link is just a parent, not a child
+//                      R5
+//
+// Alice is not joined to R4, but R4 is "world_readable".
+func TestMSC2946(t *testing.T) {
+	alice := "@alice:localhost"
+	// give access token to alice
+	nopUserAPI := &testUserAPI{
+		accessTokens: make(map[string]userapi.Device),
+	}
+	nopUserAPI.accessTokens["alice"] = userapi.Device{
+		AccessToken: "alice",
+		DisplayName: "Alice",
+		UserID:      alice,
+	}
+	rootSpace := "!rootspace:localhost"
+	subSpaceS1 := "!subspaceS1:localhost"
+	subSpaceS2 := "!subspaceS2:localhost"
+	room1 := "!room1:localhost"
+	room2 := "!room2:localhost"
+	room3 := "!room3:localhost"
+	room4 := "!room4:localhost"
+	empty := ""
+	room5 := "!room5:localhost"
+	allRooms := []string{
+		rootSpace, subSpaceS1, subSpaceS2,
+		room1, room2, room3, room4, room5,
+	}
+	rootToR1 := mustCreateEvent(t, fledglingEvent{
+		RoomID:   rootSpace,
+		Sender:   alice,
+		Type:     msc2946.ConstSpaceChildEventType,
+		StateKey: &room1,
+		Content: map[string]interface{}{
+			"via":     []string{"localhost"},
+			"present": true,
+		},
+	})
+	rootToR2 := mustCreateEvent(t, fledglingEvent{
+		RoomID:   rootSpace,
+		Sender:   alice,
+		Type:     msc2946.ConstSpaceChildEventType,
+		StateKey: &room2,
+		Content: map[string]interface{}{
+			"via":     []string{"localhost"},
+			"present": true,
+		},
+	})
+	rootToS1 := mustCreateEvent(t, fledglingEvent{
+		RoomID:   rootSpace,
+		Sender:   alice,
+		Type:     msc2946.ConstSpaceChildEventType,
+		StateKey: &subSpaceS1,
+		Content: map[string]interface{}{
+			"via":     []string{"localhost"},
+			"present": true,
+		},
+	})
+	s1ToR3 := mustCreateEvent(t, fledglingEvent{
+		RoomID:   subSpaceS1,
+		Sender:   alice,
+		Type:     msc2946.ConstSpaceChildEventType,
+		StateKey: &room3,
+		Content: map[string]interface{}{
+			"via":     []string{"localhost"},
+			"present": true,
+		},
+	})
+	s1ToR4 := mustCreateEvent(t, fledglingEvent{
+		RoomID:   subSpaceS1,
+		Sender:   alice,
+		Type:     msc2946.ConstSpaceChildEventType,
+		StateKey: &room4,
+		Content: map[string]interface{}{
+			"via":     []string{"localhost"},
+			"present": true,
+		},
+	})
+	s1ToS2 := mustCreateEvent(t, fledglingEvent{
+		RoomID:   subSpaceS1,
+		Sender:   alice,
+		Type:     msc2946.ConstSpaceChildEventType,
+		StateKey: &subSpaceS2,
+		Content: map[string]interface{}{
+			"via":     []string{"localhost"},
+			"present": true,
+		},
+	})
+	// This is a parent link only
+	s2ToR5 := mustCreateEvent(t, fledglingEvent{
+		RoomID:   room5,
+		Sender:   alice,
+		Type:     msc2946.ConstSpaceParentEventType,
+		StateKey: &empty,
+		Content: map[string]interface{}{
+			"room_id": subSpaceS2,
+			"via":     []string{"localhost"},
+			"present": true,
+		},
+	})
+	// history visibility for R4
+	r4HisVis := mustCreateEvent(t, fledglingEvent{
+		RoomID:   room4,
+		Sender:   "@someone:localhost",
+		Type:     gomatrixserverlib.MRoomHistoryVisibility,
+		StateKey: &empty,
+		Content: map[string]interface{}{
+			"history_visibility": "world_readable",
+		},
+	})
+	var joinEvents []*gomatrixserverlib.HeaderedEvent
+	for _, roomID := range allRooms {
+		if roomID == room4 {
+			continue // not joined to that room
+		}
+		joinEvents = append(joinEvents, mustCreateEvent(t, fledglingEvent{
+			RoomID:   roomID,
+			Sender:   alice,
+			StateKey: &alice,
+			Type:     gomatrixserverlib.MRoomMember,
+			Content: map[string]interface{}{
+				"membership": "join",
+			},
+		}))
+	}
+	roomNameTuple := gomatrixserverlib.StateKeyTuple{
+		EventType: "m.room.name",
+		StateKey:  "",
+	}
+	hisVisTuple := gomatrixserverlib.StateKeyTuple{
+		EventType: "m.room.history_visibility",
+		StateKey:  "",
+	}
+	nopRsAPI := &testRoomserverAPI{
+		joinEvents: joinEvents,
+		events: map[string]*gomatrixserverlib.HeaderedEvent{
+			rootToR1.EventID(): rootToR1,
+			rootToR2.EventID(): rootToR2,
+			rootToS1.EventID(): rootToS1,
+			s1ToR3.EventID():   s1ToR3,
+			s1ToR4.EventID():   s1ToR4,
+			s1ToS2.EventID():   s1ToS2,
+			s2ToR5.EventID():   s2ToR5,
+			r4HisVis.EventID(): r4HisVis,
+		},
+		pubRoomState: map[string]map[gomatrixserverlib.StateKeyTuple]string{
+			rootSpace: {
+				roomNameTuple: "Root",
+				hisVisTuple:   "shared",
+			},
+			subSpaceS1: {
+				roomNameTuple: "Sub-Space 1",
+				hisVisTuple:   "joined",
+			},
+			subSpaceS2: {
+				roomNameTuple: "Sub-Space 2",
+				hisVisTuple:   "shared",
+			},
+			room1: {
+				hisVisTuple: "joined",
+			},
+			room2: {
+				hisVisTuple: "joined",
+			},
+			room3: {
+				hisVisTuple: "joined",
+			},
+			room4: {
+				hisVisTuple: "world_readable",
+			},
+			room5: {
+				hisVisTuple: "joined",
+			},
+		},
+	}
+	allEvents := []*gomatrixserverlib.HeaderedEvent{
+		rootToR1, rootToR2, rootToS1,
+		s1ToR3, s1ToR4, s1ToS2,
+		s2ToR5, r4HisVis,
+	}
+	allEvents = append(allEvents, joinEvents...)
+	router := injectEvents(t, nopUserAPI, nopRsAPI, allEvents)
+	cancel := runServer(t, router)
+	defer cancel()
+
+	t.Run("returns no events for unknown rooms", func(t *testing.T) {
+		res := postSpaces(t, 200, "alice", "!unknown:localhost", newReq(t, map[string]interface{}{}))
+		if len(res.Events) > 0 {
+			t.Errorf("got %d events, want 0", len(res.Events))
+		}
+		if len(res.Rooms) > 0 {
+			t.Errorf("got %d rooms, want 0", len(res.Rooms))
+		}
+	})
+	t.Run("returns the entire graph", func(t *testing.T) {
+		res := postSpaces(t, 200, "alice", rootSpace, newReq(t, map[string]interface{}{}))
+		if len(res.Events) != 7 {
+			t.Errorf("got %d events, want 7", len(res.Events))
+		}
+		if len(res.Rooms) != len(allRooms) {
+			t.Errorf("got %d rooms, want %d", len(res.Rooms), len(allRooms))
+		}
+
+	})
+}
+
+func newReq(t *testing.T, jsonBody map[string]interface{}) *msc2946.SpacesRequest {
+	t.Helper()
+	b, err := json.Marshal(jsonBody)
+	if err != nil {
+		t.Fatalf("Failed to marshal request: %s", err)
+	}
+	var r msc2946.SpacesRequest
+	if err := json.Unmarshal(b, &r); err != nil {
+		t.Fatalf("Failed to unmarshal request: %s", err)
+	}
+	return &r
+}
+
+func runServer(t *testing.T, router *mux.Router) func() {
+	t.Helper()
+	externalServ := &http.Server{
+		Addr:         string(":8010"),
+		WriteTimeout: 60 * time.Second,
+		Handler:      router,
+	}
+	go func() {
+		externalServ.ListenAndServe()
+	}()
+	// wait to listen on the port
+	time.Sleep(500 * time.Millisecond)
+	return func() {
+		externalServ.Shutdown(context.TODO())
+	}
+}
+
+func postSpaces(t *testing.T, expectCode int, accessToken, roomID string, req *msc2946.SpacesRequest) *msc2946.SpacesResponse {
+	t.Helper()
+	var r msc2946.SpacesRequest
+	r.Defaults()
+	data, err := json.Marshal(req)
+	if err != nil {
+		t.Fatalf("failed to marshal request: %s", err)
+	}
+	httpReq, err := http.NewRequest(
+		"POST", "http://localhost:8010/_matrix/client/unstable/rooms/"+url.PathEscape(roomID)+"/spaces",
+		bytes.NewBuffer(data),
+	)
+	httpReq.Header.Set("Authorization", "Bearer "+accessToken)
+	if err != nil {
+		t.Fatalf("failed to prepare request: %s", err)
+	}
+	res, err := client.Do(httpReq)
+	if err != nil {
+		t.Fatalf("failed to do request: %s", err)
+	}
+	if res.StatusCode != expectCode {
+		body, _ := ioutil.ReadAll(res.Body)
+		t.Fatalf("wrong response code, got %d want %d - body: %s", res.StatusCode, expectCode, string(body))
+	}
+	if res.StatusCode == 200 {
+		var result msc2946.SpacesResponse
+		body, err := ioutil.ReadAll(res.Body)
+		if err != nil {
+			t.Fatalf("response 200 OK but failed to read response body: %s", err)
+		}
+		t.Logf("Body: %s", string(body))
+		if err := json.Unmarshal(body, &result); err != nil {
+			t.Fatalf("response 200 OK but failed to deserialise JSON : %s\nbody: %s", err, string(body))
+		}
+		return &result
+	}
+	return nil
+}
+
+type testUserAPI struct {
+	accessTokens map[string]userapi.Device
+}
+
+func (u *testUserAPI) InputAccountData(ctx context.Context, req *userapi.InputAccountDataRequest, res *userapi.InputAccountDataResponse) error {
+	return nil
+}
+func (u *testUserAPI) PerformAccountCreation(ctx context.Context, req *userapi.PerformAccountCreationRequest, res *userapi.PerformAccountCreationResponse) error {
+	return nil
+}
+func (u *testUserAPI) PerformPasswordUpdate(ctx context.Context, req *userapi.PerformPasswordUpdateRequest, res *userapi.PerformPasswordUpdateResponse) error {
+	return nil
+}
+func (u *testUserAPI) PerformDeviceCreation(ctx context.Context, req *userapi.PerformDeviceCreationRequest, res *userapi.PerformDeviceCreationResponse) error {
+	return nil
+}
+func (u *testUserAPI) PerformDeviceDeletion(ctx context.Context, req *userapi.PerformDeviceDeletionRequest, res *userapi.PerformDeviceDeletionResponse) error {
+	return nil
+}
+func (u *testUserAPI) PerformDeviceUpdate(ctx context.Context, req *userapi.PerformDeviceUpdateRequest, res *userapi.PerformDeviceUpdateResponse) error {
+	return nil
+}
+func (u *testUserAPI) PerformLastSeenUpdate(ctx context.Context, req *userapi.PerformLastSeenUpdateRequest, res *userapi.PerformLastSeenUpdateResponse) error {
+	return nil
+}
+func (u *testUserAPI) PerformAccountDeactivation(ctx context.Context, req *userapi.PerformAccountDeactivationRequest, res *userapi.PerformAccountDeactivationResponse) error {
+	return nil
+}
+func (u *testUserAPI) QueryProfile(ctx context.Context, req *userapi.QueryProfileRequest, res *userapi.QueryProfileResponse) error {
+	return nil
+}
+func (u *testUserAPI) QueryAccessToken(ctx context.Context, req *userapi.QueryAccessTokenRequest, res *userapi.QueryAccessTokenResponse) error {
+	dev, ok := u.accessTokens[req.AccessToken]
+	if !ok {
+		res.Err = fmt.Errorf("unknown token")
+		return nil
+	}
+	res.Device = &dev
+	return nil
+}
+func (u *testUserAPI) QueryDevices(ctx context.Context, req *userapi.QueryDevicesRequest, res *userapi.QueryDevicesResponse) error {
+	return nil
+}
+func (u *testUserAPI) QueryAccountData(ctx context.Context, req *userapi.QueryAccountDataRequest, res *userapi.QueryAccountDataResponse) error {
+	return nil
+}
+func (u *testUserAPI) QueryDeviceInfos(ctx context.Context, req *userapi.QueryDeviceInfosRequest, res *userapi.QueryDeviceInfosResponse) error {
+	return nil
+}
+func (u *testUserAPI) QuerySearchProfiles(ctx context.Context, req *userapi.QuerySearchProfilesRequest, res *userapi.QuerySearchProfilesResponse) error {
+	return nil
+}
+
+type testRoomserverAPI struct {
+	// use a trace API as it implements method stubs so we don't need to have them here.
+	// We'll override the functions we care about.
+	roomserver.RoomserverInternalAPITrace
+	joinEvents   []*gomatrixserverlib.HeaderedEvent
+	events       map[string]*gomatrixserverlib.HeaderedEvent
+	pubRoomState map[string]map[gomatrixserverlib.StateKeyTuple]string
+}
+
+func (r *testRoomserverAPI) QueryBulkStateContent(ctx context.Context, req *roomserver.QueryBulkStateContentRequest, res *roomserver.QueryBulkStateContentResponse) error {
+	res.Rooms = make(map[string]map[gomatrixserverlib.StateKeyTuple]string)
+	for _, roomID := range req.RoomIDs {
+		pubRoomData, ok := r.pubRoomState[roomID]
+		if ok {
+			res.Rooms[roomID] = pubRoomData
+		}
+	}
+	return nil
+}
+
+func (r *testRoomserverAPI) QueryCurrentState(ctx context.Context, req *roomserver.QueryCurrentStateRequest, res *roomserver.QueryCurrentStateResponse) error {
+	res.StateEvents = make(map[gomatrixserverlib.StateKeyTuple]*gomatrixserverlib.HeaderedEvent)
+	checkEvent := func(he *gomatrixserverlib.HeaderedEvent) {
+		if he.RoomID() != req.RoomID {
+			return
+		}
+		if he.StateKey() == nil {
+			return
+		}
+		tuple := gomatrixserverlib.StateKeyTuple{
+			EventType: he.Type(),
+			StateKey:  *he.StateKey(),
+		}
+		for _, t := range req.StateTuples {
+			if t == tuple {
+				res.StateEvents[t] = he
+			}
+		}
+	}
+	for _, he := range r.joinEvents {
+		checkEvent(he)
+	}
+	for _, he := range r.events {
+		checkEvent(he)
+	}
+	return nil
+}
+
+func injectEvents(t *testing.T, userAPI userapi.UserInternalAPI, rsAPI roomserver.RoomserverInternalAPI, events []*gomatrixserverlib.HeaderedEvent) *mux.Router {
+	t.Helper()
+	cfg := &config.Dendrite{}
+	cfg.Defaults()
+	cfg.Global.ServerName = "localhost"
+	cfg.MSCs.Database.ConnectionString = "file:msc2946_test.db"
+	cfg.MSCs.MSCs = []string{"msc2946"}
+	base := &setup.BaseDendrite{
+		Cfg:                    cfg,
+		PublicClientAPIMux:     mux.NewRouter().PathPrefix(httputil.PublicClientPathPrefix).Subrouter(),
+		PublicFederationAPIMux: mux.NewRouter().PathPrefix(httputil.PublicFederationPathPrefix).Subrouter(),
+	}
+
+	err := msc2946.Enable(base, rsAPI, userAPI)
+	if err != nil {
+		t.Fatalf("failed to enable MSC2946: %s", err)
+	}
+	for _, ev := range events {
+		hooks.Run(hooks.KindNewEventPersisted, ev)
+	}
+	return base.PublicClientAPIMux
+}
+
+type fledglingEvent struct {
+	Type     string
+	StateKey *string
+	Content  interface{}
+	Sender   string
+	RoomID   string
+}
+
+func mustCreateEvent(t *testing.T, ev fledglingEvent) (result *gomatrixserverlib.HeaderedEvent) {
+	t.Helper()
+	roomVer := gomatrixserverlib.RoomVersionV6
+	seed := make([]byte, ed25519.SeedSize) // zero seed
+	key := ed25519.NewKeyFromSeed(seed)
+	eb := gomatrixserverlib.EventBuilder{
+		Sender:   ev.Sender,
+		Depth:    999,
+		Type:     ev.Type,
+		StateKey: ev.StateKey,
+		RoomID:   ev.RoomID,
+	}
+	err := eb.SetContent(ev.Content)
+	if err != nil {
+		t.Fatalf("mustCreateEvent: failed to marshal event content %+v", ev.Content)
+	}
+	// make sure the origin_server_ts changes so we can test recency
+	time.Sleep(1 * time.Millisecond)
+	signedEvent, err := eb.Build(time.Now(), gomatrixserverlib.ServerName("localhost"), "ed25519:test", key, roomVer)
+	if err != nil {
+		t.Fatalf("mustCreateEvent: failed to sign event: %s", err)
+	}
+	h := signedEvent.Headered(roomVer)
+	return h
+}
diff --git a/setup/mscs/msc2946/storage.go b/setup/mscs/msc2946/storage.go
new file mode 100644
index 00000000..eb4a5efb
--- /dev/null
+++ b/setup/mscs/msc2946/storage.go
@@ -0,0 +1,183 @@
+// Copyright 2021 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package msc2946
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/tidwall/gjson"
+)
+
+var (
+	relTypes = map[string]int{
+		ConstSpaceChildEventType:  1,
+		ConstSpaceParentEventType: 2,
+	}
+)
+
+type Database interface {
+	// StoreReference persists a child or parent space mapping.
+	StoreReference(ctx context.Context, he *gomatrixserverlib.HeaderedEvent) error
+	// References returns all events which have the given roomID as a parent or child space.
+	References(ctx context.Context, roomID string) ([]*gomatrixserverlib.HeaderedEvent, error)
+}
+
+type DB struct {
+	db              *sql.DB
+	writer          sqlutil.Writer
+	insertEdgeStmt  *sql.Stmt
+	selectEdgesStmt *sql.Stmt
+}
+
+// NewDatabase loads the database for msc2836
+func NewDatabase(dbOpts *config.DatabaseOptions) (Database, error) {
+	if dbOpts.ConnectionString.IsPostgres() {
+		return newPostgresDatabase(dbOpts)
+	}
+	return newSQLiteDatabase(dbOpts)
+}
+
+func newPostgresDatabase(dbOpts *config.DatabaseOptions) (Database, error) {
+	d := DB{
+		writer: sqlutil.NewDummyWriter(),
+	}
+	var err error
+	if d.db, err = sqlutil.Open(dbOpts); err != nil {
+		return nil, err
+	}
+	_, err = d.db.Exec(`
+	CREATE TABLE IF NOT EXISTS msc2946_edges (
+		room_version TEXT NOT NULL,
+		-- the room ID of the event, the source of the arrow
+		source_room_id TEXT NOT NULL,
+		-- the target room ID, the arrow destination
+		dest_room_id TEXT NOT NULL,
+		-- the kind of relation, either child or parent (1,2)
+		rel_type SMALLINT NOT NULL,
+		event_json TEXT NOT NULL,
+		CONSTRAINT msc2946_edges_uniq UNIQUE (source_room_id, dest_room_id, rel_type)
+	);
+	`)
+	if err != nil {
+		return nil, err
+	}
+	if d.insertEdgeStmt, err = d.db.Prepare(`
+		INSERT INTO msc2946_edges(room_version, source_room_id, dest_room_id, rel_type, event_json)
+		VALUES($1, $2, $3, $4, $5)
+		ON CONFLICT DO NOTHING
+	`); err != nil {
+		return nil, err
+	}
+	if d.selectEdgesStmt, err = d.db.Prepare(`
+		SELECT room_version, event_json FROM msc2946_edges
+		WHERE source_room_id = $1 OR dest_room_id = $2
+	`); err != nil {
+		return nil, err
+	}
+	return &d, err
+}
+
+func newSQLiteDatabase(dbOpts *config.DatabaseOptions) (Database, error) {
+	d := DB{
+		writer: sqlutil.NewExclusiveWriter(),
+	}
+	var err error
+	if d.db, err = sqlutil.Open(dbOpts); err != nil {
+		return nil, err
+	}
+	_, err = d.db.Exec(`
+	CREATE TABLE IF NOT EXISTS msc2946_edges (
+		room_version TEXT NOT NULL,
+		-- the room ID of the event, the source of the arrow
+		source_room_id TEXT NOT NULL,
+		-- the target room ID, the arrow destination
+		dest_room_id TEXT NOT NULL,
+		-- the kind of relation, either child or parent (1,2)
+		rel_type SMALLINT NOT NULL,
+		event_json TEXT NOT NULL,
+		UNIQUE (source_room_id, dest_room_id, rel_type)
+	);
+	`)
+	if err != nil {
+		return nil, err
+	}
+	if d.insertEdgeStmt, err = d.db.Prepare(`
+		INSERT INTO msc2946_edges(room_version, source_room_id, dest_room_id, rel_type, event_json)
+		VALUES($1, $2, $3, $4, $5)
+		ON CONFLICT DO NOTHING
+	`); err != nil {
+		return nil, err
+	}
+	if d.selectEdgesStmt, err = d.db.Prepare(`
+		SELECT room_version, event_json FROM msc2946_edges
+		WHERE source_room_id = $1 OR dest_room_id = $2
+	`); err != nil {
+		return nil, err
+	}
+	return &d, err
+}
+
+func (d *DB) StoreReference(ctx context.Context, he *gomatrixserverlib.HeaderedEvent) error {
+	target := SpaceTarget(he)
+	if target == "" {
+		return nil // malformed event
+	}
+	relType := relTypes[he.Type()]
+	_, err := d.insertEdgeStmt.ExecContext(ctx, he.RoomVersion, he.RoomID(), target, relType, he.JSON())
+	return err
+}
+
+func (d *DB) References(ctx context.Context, roomID string) ([]*gomatrixserverlib.HeaderedEvent, error) {
+	rows, err := d.selectEdgesStmt.QueryContext(ctx, roomID, roomID)
+	if err != nil {
+		return nil, err
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "failed to close References")
+	refs := make([]*gomatrixserverlib.HeaderedEvent, 0)
+	for rows.Next() {
+		var roomVer string
+		var jsonBytes []byte
+		if err := rows.Scan(&roomVer, &jsonBytes); err != nil {
+			return nil, err
+		}
+		ev, err := gomatrixserverlib.NewEventFromTrustedJSON(jsonBytes, false, gomatrixserverlib.RoomVersion(roomVer))
+		if err != nil {
+			return nil, err
+		}
+		he := ev.Headered(gomatrixserverlib.RoomVersion(roomVer))
+		refs = append(refs, he)
+	}
+	return refs, nil
+}
+
+// SpaceTarget returns the destination room ID for the space event. This is either a child or a parent
+// depending on the event type.
+func SpaceTarget(he *gomatrixserverlib.HeaderedEvent) string {
+	if he.StateKey() == nil {
+		return "" // no-op
+	}
+	switch he.Type() {
+	case ConstSpaceParentEventType:
+		return gjson.GetBytes(he.Content(), "room_id").Str
+	case ConstSpaceChildEventType:
+		return *he.StateKey()
+	}
+	return ""
+}
diff --git a/setup/mscs/mscs.go b/setup/mscs/mscs.go
index a8e5668e..bf210362 100644
--- a/setup/mscs/mscs.go
+++ b/setup/mscs/mscs.go
@@ -21,6 +21,7 @@ import (
 
 	"github.com/matrix-org/dendrite/setup"
 	"github.com/matrix-org/dendrite/setup/mscs/msc2836"
+	"github.com/matrix-org/dendrite/setup/mscs/msc2946"
 	"github.com/matrix-org/util"
 )
 
@@ -39,6 +40,8 @@ func EnableMSC(base *setup.BaseDendrite, monolith *setup.Monolith, msc string) e
 	switch msc {
 	case "msc2836":
 		return msc2836.Enable(base, monolith.RoomserverAPI, monolith.FederationSenderAPI, monolith.UserAPI, monolith.KeyRing)
+	case "msc2946":
+		return msc2946.Enable(base, monolith.RoomserverAPI, monolith.UserAPI)
 	default:
 		return fmt.Errorf("EnableMSC: unknown msc '%s'", msc)
 	}