MSC2836: Threading - part one (#1589)

* Add mscs/hooks package, begin work for msc2836

* Flesh out hooks and add SQL schema

* Begin implementing core msc2836 logic

* Add test harness

* Linting

* Implement visibility checks; stub out APIs for tests

* Flesh out testing

* Flesh out walkThread a bit

* Persist the origin_server_ts as well

* Edges table instead of relationships

* Add nodes table for event metadata

* LEFT JOIN to extract origin_server_ts for children

* Add graph walking structs

* Implement walking algorithm

* Add more graph walking tests

* Add auto_join for local rooms

* Fix create table syntax on postgres

* Add relationship_room_id|servers to the unsigned section of events

* Persist the parent room_id/servers in edge metadata

Other events cannot assert the true room_id/servers for the
parent event, only make claims to them, hence why this is
edge metadata.

* guts to pass through room_id/servers

* Refactor msc2836 to allow handling from federation

* Add JoinedVia to PerformJoin responses

* Fix tests; review comments

14 files changed, 1517 insertions, 35 deletions

diff --git a/cmd/dendrite-monolith-server/main.go b/cmd/dendrite-monolith-server/main.go
index e935805f..70b81bbc 100644
--- a/cmd/dendrite-monolith-server/main.go
+++ b/cmd/dendrite-monolith-server/main.go
@@ -23,6 +23,7 @@ import (
 	"github.com/matrix-org/dendrite/eduserver/cache"
 	"github.com/matrix-org/dendrite/federationsender"
 	"github.com/matrix-org/dendrite/internal/config"
+	"github.com/matrix-org/dendrite/internal/mscs"
 	"github.com/matrix-org/dendrite/internal/setup"
 	"github.com/matrix-org/dendrite/keyserver"
 	"github.com/matrix-org/dendrite/roomserver"
@@ -148,6 +149,12 @@ func main() {
 		base.PublicMediaAPIMux,
 	)
 
+	if len(base.Cfg.MSCs.MSCs) > 0 {
+		if err := mscs.Enable(base, &monolith); err != nil {
+			logrus.WithError(err).Fatalf("Failed to enable MSCs")
+		}
+	}
+
 	// Expose the matrix APIs directly rather than putting them under a /api path.
 	go func() {
 		base.SetupAndServeHTTP(
diff --git a/federationsender/api/api.go b/federationsender/api/api.go
index b0522516..a4d15f1f 100644
--- a/federationsender/api/api.go
+++ b/federationsender/api/api.go
@@ -48,6 +48,7 @@ type FederationSenderInternalAPI interface {
 	// Query the server names of the joined hosts in a room.
 	// Unlike QueryJoinedHostsInRoom, this function returns a de-duplicated slice
 	// containing only the server names (without information for membership events).
+	// The response will include this server if they are joined to the room.
 	QueryJoinedHostServerNamesInRoom(
 		ctx context.Context,
 		request *QueryJoinedHostServerNamesInRoomRequest,
@@ -104,6 +105,7 @@ type PerformJoinRequest struct {
 }
 
 type PerformJoinResponse struct {
+	JoinedVia gomatrixserverlib.ServerName
 	LastError *gomatrix.HTTPError
 }
 
diff --git a/federationsender/internal/perform.go b/federationsender/internal/perform.go
index a7484476..45f33ff7 100644
--- a/federationsender/internal/perform.go
+++ b/federationsender/internal/perform.go
@@ -105,6 +105,7 @@ func (r *FederationSenderInternalAPI) PerformJoin(
 		}
 
 		// We're all good.
+		response.JoinedVia = serverName
 		return
 	}
 
diff --git a/federationsender/internal/query.go b/federationsender/internal/query.go
index 253400a2..8ba228d1 100644
--- a/federationsender/internal/query.go
+++ b/federationsender/internal/query.go
@@ -4,7 +4,6 @@ import (
 	"context"
 
 	"github.com/matrix-org/dendrite/federationsender/api"
-	"github.com/matrix-org/gomatrixserverlib"
 )
 
 // QueryJoinedHostServerNamesInRoom implements api.FederationSenderInternalAPI
@@ -13,17 +12,11 @@ func (f *FederationSenderInternalAPI) QueryJoinedHostServerNamesInRoom(
 	request *api.QueryJoinedHostServerNamesInRoomRequest,
 	response *api.QueryJoinedHostServerNamesInRoomResponse,
 ) (err error) {
-	joinedHosts, err := f.db.GetJoinedHosts(ctx, request.RoomID)
+	joinedHosts, err := f.db.GetJoinedHostsForRooms(ctx, []string{request.RoomID})
 	if err != nil {
 		return
 	}
-
-	response.ServerNames = make([]gomatrixserverlib.ServerName, 0, len(joinedHosts))
-	for _, host := range joinedHosts {
-		response.ServerNames = append(response.ServerNames, host.ServerName)
-	}
-
-	// TODO: remove duplicates?
+	response.ServerNames = joinedHosts
 
 	return
 }
diff --git a/internal/config/config.go b/internal/config/config.go
index 9d9e2414..b8b12d0c 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -66,6 +66,8 @@ type Dendrite struct {
 	SyncAPI          SyncAPI          `yaml:"sync_api"`
 	UserAPI          UserAPI          `yaml:"user_api"`
 
+	MSCs MSCs `yaml:"mscs"`
+
 	// The config for tracing the dendrite servers.
 	Tracing struct {
 		// Set to true to enable tracer hooks. If false, no tracing is set up.
@@ -306,6 +308,7 @@ func (c *Dendrite) Defaults() {
 	c.SyncAPI.Defaults()
 	c.UserAPI.Defaults()
 	c.AppServiceAPI.Defaults()
+	c.MSCs.Defaults()
 
 	c.Wiring()
 }
@@ -319,7 +322,7 @@ func (c *Dendrite) Verify(configErrs *ConfigErrors, isMonolith bool) {
 		&c.EDUServer, &c.FederationAPI, &c.FederationSender,
 		&c.KeyServer, &c.MediaAPI, &c.RoomServer,
 		&c.SigningKeyServer, &c.SyncAPI, &c.UserAPI,
-		&c.AppServiceAPI,
+		&c.AppServiceAPI, &c.MSCs,
 	} {
 		c.Verify(configErrs, isMonolith)
 	}
@@ -337,6 +340,7 @@ func (c *Dendrite) Wiring() {
 	c.SyncAPI.Matrix = &c.Global
 	c.UserAPI.Matrix = &c.Global
 	c.AppServiceAPI.Matrix = &c.Global
+	c.MSCs.Matrix = &c.Global
 
 	c.ClientAPI.Derived = &c.Derived
 	c.AppServiceAPI.Derived = &c.Derived
diff --git a/internal/config/config_mscs.go b/internal/config/config_mscs.go
new file mode 100644
index 00000000..776d0b64
--- /dev/null
+++ b/internal/config/config_mscs.go
@@ -0,0 +1,19 @@
+package config
+
+type MSCs struct {
+	Matrix *Global `yaml:"-"`
+
+	// The MSCs to enable, currently only `msc2836` is supported.
+	MSCs []string `yaml:"mscs"`
+
+	Database DatabaseOptions `yaml:"database"`
+}
+
+func (c *MSCs) Defaults() {
+	c.Database.Defaults()
+	c.Database.ConnectionString = "file:mscs.db"
+}
+
+func (c *MSCs) Verify(configErrs *ConfigErrors, isMonolith bool) {
+	checkNotEmpty(configErrs, "mscs.database.connection_string", string(c.Database.ConnectionString))
+}
diff --git a/internal/hooks/hooks.go b/internal/hooks/hooks.go
new file mode 100644
index 00000000..223282a2
--- /dev/null
+++ b/internal/hooks/hooks.go
@@ -0,0 +1,74 @@
+// Copyright 2020 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 hooks exposes places in Dendrite where custom code can be executed, useful for MSCs.
+// Hooks can only be run in monolith mode.
+package hooks
+
+import "sync"
+
+const (
+	// KindNewEventPersisted is a hook which is called with *gomatrixserverlib.HeaderedEvent
+	// It is run when a new event is persisted in the roomserver.
+	// Usage:
+	//   hooks.Attach(hooks.KindNewEventPersisted, func(headeredEvent interface{}) { ... })
+	KindNewEventPersisted = "new_event_persisted"
+	// KindNewEventReceived is a hook which is called with *gomatrixserverlib.HeaderedEvent
+	// It is run before a new event is processed by the roomserver. This hook can be used
+	// to modify the event before it is persisted by adding data to `unsigned`.
+	// Usage:
+	//   hooks.Attach(hooks.KindNewEventReceived, func(headeredEvent interface{}) {
+	//     ev := headeredEvent.(*gomatrixserverlib.HeaderedEvent)
+	//     _ = ev.SetUnsignedField("key", "val")
+	//   })
+	KindNewEventReceived = "new_event_received"
+)
+
+var (
+	hookMap = make(map[string][]func(interface{}))
+	hookMu  = sync.Mutex{}
+	enabled = false
+)
+
+// Enable all hooks. This may slow down the server slightly. Required for MSCs to work.
+func Enable() {
+	enabled = true
+}
+
+// Run any hooks
+func Run(kind string, data interface{}) {
+	if !enabled {
+		return
+	}
+	cbs := callbacks(kind)
+	for _, cb := range cbs {
+		cb(data)
+	}
+}
+
+// Attach a hook
+func Attach(kind string, callback func(interface{})) {
+	if !enabled {
+		return
+	}
+	hookMu.Lock()
+	defer hookMu.Unlock()
+	hookMap[kind] = append(hookMap[kind], callback)
+}
+
+func callbacks(kind string) []func(interface{}) {
+	hookMu.Lock()
+	defer hookMu.Unlock()
+	return hookMap[kind]
+}
diff --git a/internal/mscs/msc2836/msc2836.go b/internal/mscs/msc2836/msc2836.go
new file mode 100644
index 00000000..865bc311
--- /dev/null
+++ b/internal/mscs/msc2836/msc2836.go
@@ -0,0 +1,530 @@
+// Copyright 2020 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 msc2836 'Threading' implements https://github.com/matrix-org/matrix-doc/pull/2836
+package msc2836
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	fs "github.com/matrix-org/dendrite/federationsender/api"
+	"github.com/matrix-org/dendrite/internal/hooks"
+	"github.com/matrix-org/dendrite/internal/httputil"
+	"github.com/matrix-org/dendrite/internal/setup"
+	roomserver "github.com/matrix-org/dendrite/roomserver/api"
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+)
+
+const (
+	constRelType     = "m.reference"
+	constRoomIDKey   = "relationship_room_id"
+	constRoomServers = "relationship_servers"
+)
+
+type EventRelationshipRequest struct {
+	EventID         string `json:"event_id"`
+	MaxDepth        int    `json:"max_depth"`
+	MaxBreadth      int    `json:"max_breadth"`
+	Limit           int    `json:"limit"`
+	DepthFirst      bool   `json:"depth_first"`
+	RecentFirst     bool   `json:"recent_first"`
+	IncludeParent   bool   `json:"include_parent"`
+	IncludeChildren bool   `json:"include_children"`
+	Direction       string `json:"direction"`
+	Batch           string `json:"batch"`
+	AutoJoin        bool   `json:"auto_join"`
+}
+
+func NewEventRelationshipRequest(body io.Reader) (*EventRelationshipRequest, error) {
+	var relation EventRelationshipRequest
+	relation.Defaults()
+	if err := json.NewDecoder(body).Decode(&relation); err != nil {
+		return nil, err
+	}
+	return &relation, nil
+}
+
+func (r *EventRelationshipRequest) Defaults() {
+	r.Limit = 100
+	r.MaxBreadth = 10
+	r.MaxDepth = 3
+	r.DepthFirst = false
+	r.RecentFirst = true
+	r.IncludeParent = false
+	r.IncludeChildren = false
+	r.Direction = "down"
+}
+
+type EventRelationshipResponse struct {
+	Events    []gomatrixserverlib.ClientEvent `json:"events"`
+	NextBatch string                          `json:"next_batch"`
+	Limited   bool                            `json:"limited"`
+}
+
+// Enable this MSC
+// nolint:gocyclo
+func Enable(
+	base *setup.BaseDendrite, rsAPI roomserver.RoomserverInternalAPI, fsAPI fs.FederationSenderInternalAPI,
+	userAPI userapi.UserInternalAPI, keyRing gomatrixserverlib.JSONVerifier,
+) error {
+	db, err := NewDatabase(&base.Cfg.MSCs.Database)
+	if err != nil {
+		return fmt.Errorf("Cannot enable MSC2836: %w", err)
+	}
+	hooks.Enable()
+	hooks.Attach(hooks.KindNewEventPersisted, func(headeredEvent interface{}) {
+		he := headeredEvent.(*gomatrixserverlib.HeaderedEvent)
+		hookErr := db.StoreRelation(context.Background(), he)
+		if hookErr != nil {
+			util.GetLogger(context.Background()).WithError(hookErr).Error(
+				"failed to StoreRelation",
+			)
+		}
+	})
+	hooks.Attach(hooks.KindNewEventReceived, func(headeredEvent interface{}) {
+		he := headeredEvent.(*gomatrixserverlib.HeaderedEvent)
+		ctx := context.Background()
+		// we only inject metadata for events our server sends
+		userID := he.Sender()
+		_, domain, err := gomatrixserverlib.SplitID('@', userID)
+		if err != nil {
+			return
+		}
+		if domain != base.Cfg.Global.ServerName {
+			return
+		}
+		// if this event has an m.relationship, add on the room_id and servers to unsigned
+		parent, child, relType := parentChildEventIDs(he)
+		if parent == "" || child == "" || relType == "" {
+			return
+		}
+		event, joinedToRoom := getEventIfVisible(ctx, rsAPI, parent, userID)
+		if !joinedToRoom {
+			return
+		}
+		err = he.SetUnsignedField(constRoomIDKey, event.RoomID())
+		if err != nil {
+			util.GetLogger(context.Background()).WithError(err).Warn("Failed to SetUnsignedField")
+			return
+		}
+
+		var servers []gomatrixserverlib.ServerName
+		if fsAPI != nil {
+			var res fs.QueryJoinedHostServerNamesInRoomResponse
+			err = fsAPI.QueryJoinedHostServerNamesInRoom(ctx, &fs.QueryJoinedHostServerNamesInRoomRequest{
+				RoomID: event.RoomID(),
+			}, &res)
+			if err != nil {
+				util.GetLogger(context.Background()).WithError(err).Warn("Failed to QueryJoinedHostServerNamesInRoom")
+				return
+			}
+			servers = res.ServerNames
+		} else {
+			servers = []gomatrixserverlib.ServerName{
+				base.Cfg.Global.ServerName,
+			}
+		}
+		err = he.SetUnsignedField(constRoomServers, servers)
+		if err != nil {
+			util.GetLogger(context.Background()).WithError(err).Warn("Failed to SetUnsignedField")
+			return
+		}
+	})
+
+	base.PublicClientAPIMux.Handle("/unstable/event_relationships",
+		httputil.MakeAuthAPI("eventRelationships", userAPI, eventRelationshipHandler(db, rsAPI)),
+	).Methods(http.MethodPost, http.MethodOptions)
+
+	base.PublicFederationAPIMux.Handle("/unstable/event_relationships", httputil.MakeExternalAPI(
+		"msc2836_event_relationships", func(req *http.Request) util.JSONResponse {
+			fedReq, errResp := gomatrixserverlib.VerifyHTTPRequest(
+				req, time.Now(), base.Cfg.Global.ServerName, keyRing,
+			)
+			if fedReq == nil {
+				return errResp
+			}
+			return federatedEventRelationship(req.Context(), fedReq, db, rsAPI)
+		},
+	)).Methods(http.MethodPost, http.MethodOptions)
+	return nil
+}
+
+type reqCtx struct {
+	ctx                context.Context
+	rsAPI              roomserver.RoomserverInternalAPI
+	db                 Database
+	req                *EventRelationshipRequest
+	userID             string
+	isFederatedRequest bool
+}
+
+func eventRelationshipHandler(db Database, rsAPI roomserver.RoomserverInternalAPI) func(*http.Request, *userapi.Device) util.JSONResponse {
+	return func(req *http.Request, device *userapi.Device) util.JSONResponse {
+		relation, err := NewEventRelationshipRequest(req.Body)
+		if err != nil {
+			util.GetLogger(req.Context()).WithError(err).Error("failed to decode HTTP request as JSON")
+			return util.JSONResponse{
+				Code: 400,
+				JSON: jsonerror.BadJSON(fmt.Sprintf("invalid json: %s", err)),
+			}
+		}
+		rc := reqCtx{
+			ctx:                req.Context(),
+			req:                relation,
+			userID:             device.UserID,
+			rsAPI:              rsAPI,
+			isFederatedRequest: false,
+			db:                 db,
+		}
+		res, resErr := rc.process()
+		if resErr != nil {
+			return *resErr
+		}
+
+		return util.JSONResponse{
+			Code: 200,
+			JSON: res,
+		}
+	}
+}
+
+func federatedEventRelationship(ctx context.Context, fedReq *gomatrixserverlib.FederationRequest, db Database, rsAPI roomserver.RoomserverInternalAPI) util.JSONResponse {
+	relation, err := NewEventRelationshipRequest(bytes.NewBuffer(fedReq.Content()))
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Error("failed to decode HTTP request as JSON")
+		return util.JSONResponse{
+			Code: 400,
+			JSON: jsonerror.BadJSON(fmt.Sprintf("invalid json: %s", err)),
+		}
+	}
+	rc := reqCtx{
+		ctx:                ctx,
+		req:                relation,
+		userID:             "",
+		rsAPI:              rsAPI,
+		isFederatedRequest: true,
+		db:                 db,
+	}
+	res, resErr := rc.process()
+	if resErr != nil {
+		return *resErr
+	}
+
+	return util.JSONResponse{
+		Code: 200,
+		JSON: res,
+	}
+}
+
+func (rc *reqCtx) process() (*EventRelationshipResponse, *util.JSONResponse) {
+	var res EventRelationshipResponse
+	var returnEvents []*gomatrixserverlib.HeaderedEvent
+	// Can the user see (according to history visibility) event_id? If no, reject the request, else continue.
+	// We should have the event being referenced so don't give any claimed room ID / servers
+	event := rc.getEventIfVisible(rc.req.EventID, "", nil)
+	if event == nil {
+		return nil, &util.JSONResponse{
+			Code: 403,
+			JSON: jsonerror.Forbidden("Event does not exist or you are not authorised to see it"),
+		}
+	}
+
+	// Retrieve the event. Add it to response array.
+	returnEvents = append(returnEvents, event)
+
+	if rc.req.IncludeParent {
+		if parentEvent := rc.includeParent(event); parentEvent != nil {
+			returnEvents = append(returnEvents, parentEvent)
+		}
+	}
+
+	if rc.req.IncludeChildren {
+		remaining := rc.req.Limit - len(returnEvents)
+		if remaining > 0 {
+			children, resErr := rc.includeChildren(rc.db, event.EventID(), remaining, rc.req.RecentFirst)
+			if resErr != nil {
+				return nil, resErr
+			}
+			returnEvents = append(returnEvents, children...)
+		}
+	}
+
+	remaining := rc.req.Limit - len(returnEvents)
+	var walkLimited bool
+	if remaining > 0 {
+		included := make(map[string]bool, len(returnEvents))
+		for _, ev := range returnEvents {
+			included[ev.EventID()] = true
+		}
+		var events []*gomatrixserverlib.HeaderedEvent
+		events, walkLimited = walkThread(
+			rc.ctx, rc.db, rc, included, remaining,
+		)
+		returnEvents = append(returnEvents, events...)
+	}
+	res.Events = make([]gomatrixserverlib.ClientEvent, len(returnEvents))
+	for i, ev := range returnEvents {
+		res.Events[i] = gomatrixserverlib.HeaderedToClientEvent(ev, gomatrixserverlib.FormatAll)
+	}
+	res.Limited = remaining == 0 || walkLimited
+	return &res, nil
+}
+
+// If include_parent: true and there is a valid m.relationship field in the event,
+// retrieve the referenced event. Apply history visibility check to that event and if it passes, add it to the response array.
+func (rc *reqCtx) includeParent(event *gomatrixserverlib.HeaderedEvent) (parent *gomatrixserverlib.HeaderedEvent) {
+	parentID, _, _ := parentChildEventIDs(event)
+	if parentID == "" {
+		return nil
+	}
+	claimedRoomID, claimedServers := roomIDAndServers(event)
+	return rc.getEventIfVisible(parentID, claimedRoomID, claimedServers)
+}
+
+// If include_children: true, lookup all events which have event_id as an m.relationship
+// Apply history visibility checks to all these events and add the ones which pass into the response array,
+// honouring the recent_first flag and the limit.
+func (rc *reqCtx) includeChildren(db Database, parentID string, limit int, recentFirst bool) ([]*gomatrixserverlib.HeaderedEvent, *util.JSONResponse) {
+	children, err := db.ChildrenForParent(rc.ctx, parentID, constRelType, recentFirst)
+	if err != nil {
+		util.GetLogger(rc.ctx).WithError(err).Error("failed to get ChildrenForParent")
+		resErr := jsonerror.InternalServerError()
+		return nil, &resErr
+	}
+	var childEvents []*gomatrixserverlib.HeaderedEvent
+	for _, child := range children {
+		// in order for us to even know about the children the server must be joined to those rooms, hence pass no claimed room ID or servers.
+		childEvent := rc.getEventIfVisible(child.EventID, "", nil)
+		if childEvent != nil {
+			childEvents = append(childEvents, childEvent)
+		}
+	}
+	if len(childEvents) > limit {
+		return childEvents[:limit], nil
+	}
+	return childEvents, nil
+}
+
+// Begin to walk the thread DAG in the direction specified, either depth or breadth first according to the depth_first flag,
+// honouring the limit, max_depth and max_breadth values according to the following rules
+// nolint: unparam
+func walkThread(
+	ctx context.Context, db Database, rc *reqCtx, included map[string]bool, limit int,
+) ([]*gomatrixserverlib.HeaderedEvent, bool) {
+	if rc.req.Direction != "down" {
+		util.GetLogger(ctx).Error("not implemented: direction=up")
+		return nil, false
+	}
+	var result []*gomatrixserverlib.HeaderedEvent
+	eventWalker := walker{
+		ctx: ctx,
+		req: rc.req,
+		db:  db,
+		fn: func(wi *walkInfo) bool {
+			// If already processed event, skip.
+			if included[wi.EventID] {
+				return false
+			}
+
+			// If the response array is >= limit, stop.
+			if len(result) >= limit {
+				return true
+			}
+
+			// Process the event.
+			// TODO: Include edge information: room ID and servers
+			event := rc.getEventIfVisible(wi.EventID, "", nil)
+			if event != nil {
+				result = append(result, event)
+			}
+			included[wi.EventID] = true
+			return false
+		},
+	}
+	limited, err := eventWalker.WalkFrom(rc.req.EventID)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Errorf("Failed to WalkFrom %s", rc.req.EventID)
+	}
+	return result, limited
+}
+
+func (rc *reqCtx) getEventIfVisible(eventID string, claimedRoomID string, claimedServers []string) *gomatrixserverlib.HeaderedEvent {
+	event, joinedToRoom := getEventIfVisible(rc.ctx, rc.rsAPI, eventID, rc.userID)
+	if event != nil && joinedToRoom {
+		return event
+	}
+	// either we don't have the event or we aren't joined to the room, regardless we should try joining if auto join is enabled
+	if !rc.req.AutoJoin {
+		return nil
+	}
+	// if we're doing this on behalf of a random server don't auto-join rooms regardless of what the request says
+	if rc.isFederatedRequest {
+		return nil
+	}
+	roomID := claimedRoomID
+	var servers []gomatrixserverlib.ServerName
+	if event != nil {
+		roomID = event.RoomID()
+	}
+	for _, s := range claimedServers {
+		servers = append(servers, gomatrixserverlib.ServerName(s))
+	}
+	var joinRes roomserver.PerformJoinResponse
+	rc.rsAPI.PerformJoin(rc.ctx, &roomserver.PerformJoinRequest{
+		UserID:        rc.userID,
+		Content:       map[string]interface{}{},
+		RoomIDOrAlias: roomID,
+		ServerNames:   servers,
+	}, &joinRes)
+	if joinRes.Error != nil {
+		util.GetLogger(rc.ctx).WithError(joinRes.Error).WithField("room_id", roomID).Error("Failed to auto-join room")
+		return nil
+	}
+	if event != nil {
+		return event
+	}
+	// TODO: hit /event_relationships on the server we joined via
+	util.GetLogger(rc.ctx).Infof("joined room but need to fetch event TODO")
+	return nil
+}
+
+func getEventIfVisible(ctx context.Context, rsAPI roomserver.RoomserverInternalAPI, eventID, userID string) (*gomatrixserverlib.HeaderedEvent, bool) {
+	var queryEventsRes roomserver.QueryEventsByIDResponse
+	err := rsAPI.QueryEventsByID(ctx, &roomserver.QueryEventsByIDRequest{
+		EventIDs: []string{eventID},
+	}, &queryEventsRes)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Error("getEventIfVisible: failed to QueryEventsByID")
+		return nil, false
+	}
+	if len(queryEventsRes.Events) == 0 {
+		util.GetLogger(ctx).Infof("event does not exist")
+		return nil, false // event does not exist
+	}
+	event := queryEventsRes.Events[0]
+
+	// Allow events if the member is in the room
+	// TODO: This does not honour history_visibility
+	// TODO: This does not honour m.room.create content
+	var queryMembershipRes roomserver.QueryMembershipForUserResponse
+	err = rsAPI.QueryMembershipForUser(ctx, &roomserver.QueryMembershipForUserRequest{
+		RoomID: event.RoomID(),
+		UserID: userID,
+	}, &queryMembershipRes)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Error("getEventIfVisible: failed to QueryMembershipForUser")
+		return nil, false
+	}
+	return event, queryMembershipRes.IsInRoom
+}
+
+type walkInfo struct {
+	eventInfo
+	SiblingNumber int
+	Depth         int
+}
+
+type walker struct {
+	ctx context.Context
+	req *EventRelationshipRequest
+	db  Database
+	fn  func(wi *walkInfo) bool // callback invoked for each event walked, return true to terminate the walk
+}
+
+// WalkFrom the event ID given
+func (w *walker) WalkFrom(eventID string) (limited bool, err error) {
+	children, err := w.db.ChildrenForParent(w.ctx, eventID, constRelType, w.req.RecentFirst)
+	if err != nil {
+		util.GetLogger(w.ctx).WithError(err).Error("WalkFrom() ChildrenForParent failed, cannot walk")
+		return false, err
+	}
+	var next *walkInfo
+	toWalk := w.addChildren(nil, children, 1)
+	next, toWalk = w.nextChild(toWalk)
+	for next != nil {
+		stop := w.fn(next)
+		if stop {
+			return true, nil
+		}
+		// find the children's children
+		children, err = w.db.ChildrenForParent(w.ctx, next.EventID, constRelType, w.req.RecentFirst)
+		if err != nil {
+			util.GetLogger(w.ctx).WithError(err).Error("WalkFrom() ChildrenForParent failed, cannot walk")
+			return false, err
+		}
+		toWalk = w.addChildren(toWalk, children, next.Depth+1)
+		next, toWalk = w.nextChild(toWalk)
+	}
+
+	return false, nil
+}
+
+// addChildren adds an event's children to the to walk data structure
+func (w *walker) addChildren(toWalk []walkInfo, children []eventInfo, depthOfChildren int) []walkInfo {
+	// Check what number child this event is (ordered by recent_first) compared to its parent, does it exceed (greater than) max_breadth? If yes, skip.
+	if len(children) > w.req.MaxBreadth {
+		children = children[:w.req.MaxBreadth]
+	}
+	// Check how deep the event is compared to event_id, does it exceed (greater than) max_depth? If yes, skip.
+	if depthOfChildren > w.req.MaxDepth {
+		return toWalk
+	}
+
+	if w.req.DepthFirst {
+		// the slice is a stack so push them in reverse order so we pop them in the correct order
+		// e.g [3,2,1] => [3,2] , 1 => [3] , 2 => [] , 3
+		for i := len(children) - 1; i >= 0; i-- {
+			toWalk = append(toWalk, walkInfo{
+				eventInfo:     children[i],
+				SiblingNumber: i + 1, // index from 1
+				Depth:         depthOfChildren,
+			})
+		}
+	} else {
+		// the slice is a queue so push them in normal order to we dequeue them in the correct order
+		// e.g [1,2,3] => 1, [2, 3] => 2 , [3] => 3, []
+		for i := range children {
+			toWalk = append(toWalk, walkInfo{
+				eventInfo:     children[i],
+				SiblingNumber: i + 1, // index from 1
+				Depth:         depthOfChildren,
+			})
+		}
+	}
+	return toWalk
+}
+
+func (w *walker) nextChild(toWalk []walkInfo) (*walkInfo, []walkInfo) {
+	if len(toWalk) == 0 {
+		return nil, nil
+	}
+	var child walkInfo
+	if w.req.DepthFirst {
+		// toWalk is a stack so pop the child off
+		child, toWalk = toWalk[len(toWalk)-1], toWalk[:len(toWalk)-1]
+		return &child, toWalk
+	}
+	// toWalk is a queue so shift the child off
+	child, toWalk = toWalk[0], toWalk[1:]
+	return &child, toWalk
+}
diff --git a/internal/mscs/msc2836/msc2836_test.go b/internal/mscs/msc2836/msc2836_test.go
new file mode 100644
index 00000000..cbf8b726
--- /dev/null
+++ b/internal/mscs/msc2836/msc2836_test.go
@@ -0,0 +1,574 @@
+package msc2836_test
+
+import (
+	"bytes"
+	"context"
+	"crypto/ed25519"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/gorilla/mux"
+	"github.com/matrix-org/dendrite/internal/config"
+	"github.com/matrix-org/dendrite/internal/hooks"
+	"github.com/matrix-org/dendrite/internal/httputil"
+	"github.com/matrix-org/dendrite/internal/mscs/msc2836"
+	"github.com/matrix-org/dendrite/internal/setup"
+	roomserver "github.com/matrix-org/dendrite/roomserver/api"
+	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 MSC2836 logic. Injects a thread that looks like:
+//   A
+//   |
+//   B
+//  / \
+// C   D
+//    /|\
+//   E F G
+//   |
+//   H
+// And makes sure POST /event_relationships works with various parameters
+func TestMSC2836(t *testing.T) {
+	alice := "@alice:localhost"
+	bob := "@bob:localhost"
+	charlie := "@charlie:localhost"
+	roomIDA := "!alice:localhost"
+	roomIDB := "!bob:localhost"
+	roomIDC := "!charlie:localhost"
+	// give access tokens to all three users
+	nopUserAPI := &testUserAPI{
+		accessTokens: make(map[string]userapi.Device),
+	}
+	nopUserAPI.accessTokens["alice"] = userapi.Device{
+		AccessToken: "alice",
+		DisplayName: "Alice",
+		UserID:      alice,
+	}
+	nopUserAPI.accessTokens["bob"] = userapi.Device{
+		AccessToken: "bob",
+		DisplayName: "Bob",
+		UserID:      bob,
+	}
+	nopUserAPI.accessTokens["charlie"] = userapi.Device{
+		AccessToken: "charlie",
+		DisplayName: "Charles",
+		UserID:      charlie,
+	}
+	eventA := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDA,
+		Sender: alice,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[A] Do you know shelties?",
+		},
+	})
+	eventB := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDB,
+		Sender: bob,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[B] I <3 shelties",
+			"m.relationship": map[string]string{
+				"rel_type": "m.reference",
+				"event_id": eventA.EventID(),
+			},
+		},
+	})
+	eventC := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDB,
+		Sender: bob,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[C] like so much",
+			"m.relationship": map[string]string{
+				"rel_type": "m.reference",
+				"event_id": eventB.EventID(),
+			},
+		},
+	})
+	eventD := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDA,
+		Sender: alice,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[D] but what are shelties???",
+			"m.relationship": map[string]string{
+				"rel_type": "m.reference",
+				"event_id": eventB.EventID(),
+			},
+		},
+	})
+	eventE := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDB,
+		Sender: bob,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[E] seriously???",
+			"m.relationship": map[string]string{
+				"rel_type": "m.reference",
+				"event_id": eventD.EventID(),
+			},
+		},
+	})
+	eventF := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDC,
+		Sender: charlie,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[F] omg how do you not know what shelties are",
+			"m.relationship": map[string]string{
+				"rel_type": "m.reference",
+				"event_id": eventD.EventID(),
+			},
+		},
+	})
+	eventG := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDA,
+		Sender: alice,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[G] looked it up, it's a sheltered person?",
+			"m.relationship": map[string]string{
+				"rel_type": "m.reference",
+				"event_id": eventD.EventID(),
+			},
+		},
+	})
+	eventH := mustCreateEvent(t, fledglingEvent{
+		RoomID: roomIDB,
+		Sender: bob,
+		Type:   "m.room.message",
+		Content: map[string]interface{}{
+			"body": "[H] it's a dog!!!!!",
+			"m.relationship": map[string]string{
+				"rel_type": "m.reference",
+				"event_id": eventE.EventID(),
+			},
+		},
+	})
+	// make everyone joined to each other's rooms
+	nopRsAPI := &testRoomserverAPI{
+		userToJoinedRooms: map[string][]string{
+			alice:   []string{roomIDA, roomIDB, roomIDC},
+			bob:     []string{roomIDA, roomIDB, roomIDC},
+			charlie: []string{roomIDA, roomIDB, roomIDC},
+		},
+		events: map[string]*gomatrixserverlib.HeaderedEvent{
+			eventA.EventID(): eventA,
+			eventB.EventID(): eventB,
+			eventC.EventID(): eventC,
+			eventD.EventID(): eventD,
+			eventE.EventID(): eventE,
+			eventF.EventID(): eventF,
+			eventG.EventID(): eventG,
+			eventH.EventID(): eventH,
+		},
+	}
+	router := injectEvents(t, nopUserAPI, nopRsAPI, []*gomatrixserverlib.HeaderedEvent{
+		eventA, eventB, eventC, eventD, eventE, eventF, eventG, eventH,
+	})
+	cancel := runServer(t, router)
+	defer cancel()
+
+	t.Run("returns 403 on invalid event IDs", func(t *testing.T) {
+		_ = postRelationships(t, 403, "alice", newReq(t, map[string]interface{}{
+			"event_id": "$invalid",
+		}))
+	})
+	t.Run("returns 403 if not joined to the room of specified event in request", func(t *testing.T) {
+		nopUserAPI.accessTokens["frank"] = userapi.Device{
+			AccessToken: "frank",
+			DisplayName: "Frank Not In Room",
+			UserID:      "@frank:localhost",
+		}
+		_ = postRelationships(t, 403, "frank", newReq(t, map[string]interface{}{
+			"event_id":       eventB.EventID(),
+			"limit":          1,
+			"include_parent": true,
+		}))
+	})
+	t.Run("omits parent if not joined to the room of parent of event", func(t *testing.T) {
+		nopUserAPI.accessTokens["frank2"] = userapi.Device{
+			AccessToken: "frank2",
+			DisplayName: "Frank2 Not In Room",
+			UserID:      "@frank2:localhost",
+		}
+		// Event B is in roomB, Event A is in roomA, so make frank2 joined to roomB
+		nopRsAPI.userToJoinedRooms["@frank2:localhost"] = []string{roomIDB}
+		body := postRelationships(t, 200, "frank2", newReq(t, map[string]interface{}{
+			"event_id":       eventB.EventID(),
+			"limit":          1,
+			"include_parent": true,
+		}))
+		assertContains(t, body, []string{eventB.EventID()})
+	})
+	t.Run("returns the parent if include_parent is true", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":       eventB.EventID(),
+			"include_parent": true,
+			"limit":          2,
+		}))
+		assertContains(t, body, []string{eventB.EventID(), eventA.EventID()})
+	})
+	t.Run("returns the children in the right order if include_children is true", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":         eventD.EventID(),
+			"include_children": true,
+			"recent_first":     true,
+			"limit":            4,
+		}))
+		assertContains(t, body, []string{eventD.EventID(), eventG.EventID(), eventF.EventID(), eventE.EventID()})
+		body = postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":         eventD.EventID(),
+			"include_children": true,
+			"recent_first":     false,
+			"limit":            4,
+		}))
+		assertContains(t, body, []string{eventD.EventID(), eventE.EventID(), eventF.EventID(), eventG.EventID()})
+	})
+	t.Run("walks the graph depth first", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": false,
+			"depth_first":  true,
+			"limit":        6,
+		}))
+		// Oldest first so:
+		//   A
+		//   |
+		//   B1
+		//  / \
+		// C2  D3
+		//    /| \
+		//  4E 6F G
+		//   |
+		//  5H
+		assertContains(t, body, []string{eventB.EventID(), eventC.EventID(), eventD.EventID(), eventE.EventID(), eventH.EventID(), eventF.EventID()})
+		body = postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": true,
+			"depth_first":  true,
+			"limit":        6,
+		}))
+		// Recent first so:
+		//   A
+		//   |
+		//   B1
+		//  / \
+		// C   D2
+		//    /| \
+		//  E5 F4 G3
+		//   |
+		//  H6
+		assertContains(t, body, []string{eventB.EventID(), eventD.EventID(), eventG.EventID(), eventF.EventID(), eventE.EventID(), eventH.EventID()})
+	})
+	t.Run("walks the graph breadth first", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": false,
+			"depth_first":  false,
+			"limit":        6,
+		}))
+		// Oldest first so:
+		//   A
+		//   |
+		//   B1
+		//  / \
+		// C2  D3
+		//    /| \
+		//  E4 F5 G6
+		//   |
+		//   H
+		assertContains(t, body, []string{eventB.EventID(), eventC.EventID(), eventD.EventID(), eventE.EventID(), eventF.EventID(), eventG.EventID()})
+		body = postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": true,
+			"depth_first":  false,
+			"limit":        6,
+		}))
+		// Recent first so:
+		//   A
+		//   |
+		//   B1
+		//  / \
+		// C3  D2
+		//    /| \
+		//  E6 F5 G4
+		//   |
+		//   H
+		assertContains(t, body, []string{eventB.EventID(), eventD.EventID(), eventC.EventID(), eventG.EventID(), eventF.EventID(), eventE.EventID()})
+	})
+	t.Run("caps via max_breadth", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": false,
+			"depth_first":  false,
+			"max_breadth":  2,
+			"limit":        10,
+		}))
+		// Event G gets omitted because of max_breadth
+		assertContains(t, body, []string{eventB.EventID(), eventC.EventID(), eventD.EventID(), eventE.EventID(), eventF.EventID(), eventH.EventID()})
+	})
+	t.Run("caps via max_depth", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": false,
+			"depth_first":  false,
+			"max_depth":    2,
+			"limit":        10,
+		}))
+		// Event H gets omitted because of max_depth
+		assertContains(t, body, []string{eventB.EventID(), eventC.EventID(), eventD.EventID(), eventE.EventID(), eventF.EventID(), eventG.EventID()})
+	})
+	t.Run("terminates when reaching the limit", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": false,
+			"depth_first":  false,
+			"limit":        4,
+		}))
+		assertContains(t, body, []string{eventB.EventID(), eventC.EventID(), eventD.EventID(), eventE.EventID()})
+	})
+	t.Run("returns all events with a high enough limit", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": false,
+			"depth_first":  false,
+			"limit":        400,
+		}))
+		assertContains(t, body, []string{eventB.EventID(), eventC.EventID(), eventD.EventID(), eventE.EventID(), eventF.EventID(), eventG.EventID(), eventH.EventID()})
+	})
+}
+
+// TODO: TestMSC2836TerminatesLoops (short and long)
+// TODO: TestMSC2836UnknownEventsSkipped
+// TODO: TestMSC2836SkipEventIfNotInRoom
+
+func newReq(t *testing.T, jsonBody map[string]interface{}) *msc2836.EventRelationshipRequest {
+	t.Helper()
+	b, err := json.Marshal(jsonBody)
+	if err != nil {
+		t.Fatalf("Failed to marshal request: %s", err)
+	}
+	r, err := msc2836.NewEventRelationshipRequest(bytes.NewBuffer(b))
+	if err != nil {
+		t.Fatalf("Failed to NewEventRelationshipRequest: %s", err)
+	}
+	return r
+}
+
+func runServer(t *testing.T, router *mux.Router) func() {
+	t.Helper()
+	externalServ := &http.Server{
+		Addr:         string(":8009"),
+		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 postRelationships(t *testing.T, expectCode int, accessToken string, req *msc2836.EventRelationshipRequest) *msc2836.EventRelationshipResponse {
+	t.Helper()
+	var r msc2836.EventRelationshipRequest
+	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:8009/_matrix/client/unstable/event_relationships",
+		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 msc2836.EventRelationshipResponse
+		if err := json.NewDecoder(res.Body).Decode(&result); err != nil {
+			t.Fatalf("response 200 OK but failed to deserialise JSON : %s", err)
+		}
+		return &result
+	}
+	return nil
+}
+
+func assertContains(t *testing.T, result *msc2836.EventRelationshipResponse, wantEventIDs []string) {
+	t.Helper()
+	gotEventIDs := make([]string, len(result.Events))
+	for i, ev := range result.Events {
+		gotEventIDs[i] = ev.EventID
+	}
+	if len(gotEventIDs) != len(wantEventIDs) {
+		t.Fatalf("length mismatch: got %v want %v", gotEventIDs, wantEventIDs)
+	}
+	for i := range gotEventIDs {
+		if gotEventIDs[i] != wantEventIDs[i] {
+			t.Errorf("wrong item in position %d - got %s want %s", i, gotEventIDs[i], wantEventIDs[i])
+		}
+	}
+}
+
+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) 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
+	userToJoinedRooms map[string][]string
+	events            map[string]*gomatrixserverlib.HeaderedEvent
+}
+
+func (r *testRoomserverAPI) QueryEventsByID(ctx context.Context, req *roomserver.QueryEventsByIDRequest, res *roomserver.QueryEventsByIDResponse) error {
+	for _, eventID := range req.EventIDs {
+		ev := r.events[eventID]
+		if ev != nil {
+			res.Events = append(res.Events, ev)
+		}
+	}
+	return nil
+}
+
+func (r *testRoomserverAPI) QueryMembershipForUser(ctx context.Context, req *roomserver.QueryMembershipForUserRequest, res *roomserver.QueryMembershipForUserResponse) error {
+	rooms := r.userToJoinedRooms[req.UserID]
+	for _, roomID := range rooms {
+		if roomID == req.RoomID {
+			res.IsInRoom = true
+			res.HasBeenInRoom = true
+			res.Membership = "join"
+			break
+		}
+	}
+	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:msc2836_test.db"
+	cfg.MSCs.MSCs = []string{"msc2836"}
+	base := &setup.BaseDendrite{
+		Cfg:                    cfg,
+		PublicClientAPIMux:     mux.NewRouter().PathPrefix(httputil.PublicClientPathPrefix).Subrouter(),
+		PublicFederationAPIMux: mux.NewRouter().PathPrefix(httputil.PublicFederationPathPrefix).Subrouter(),
+	}
+
+	err := msc2836.Enable(base, rsAPI, nil, userAPI, nil)
+	if err != nil {
+		t.Fatalf("failed to enable MSC2836: %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/internal/mscs/msc2836/storage.go b/internal/mscs/msc2836/storage.go
new file mode 100644
index 00000000..f524165f
--- /dev/null
+++ b/internal/mscs/msc2836/storage.go
@@ -0,0 +1,226 @@
+package msc2836
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+
+	"github.com/matrix-org/dendrite/internal/config"
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+type eventInfo struct {
+	EventID        string
+	OriginServerTS gomatrixserverlib.Timestamp
+	RoomID         string
+	Servers        []string
+}
+
+type Database interface {
+	// StoreRelation stores the parent->child and child->parent relationship for later querying.
+	// Also stores the event metadata e.g timestamp
+	StoreRelation(ctx context.Context, ev *gomatrixserverlib.HeaderedEvent) error
+	// ChildrenForParent returns the events who have the given `eventID` as an m.relationship with the
+	// provided `relType`. The returned slice is sorted by origin_server_ts according to whether
+	// `recentFirst` is true or false.
+	ChildrenForParent(ctx context.Context, eventID, relType string, recentFirst bool) ([]eventInfo, error)
+}
+
+type DB struct {
+	db                                     *sql.DB
+	writer                                 sqlutil.Writer
+	insertEdgeStmt                         *sql.Stmt
+	insertNodeStmt                         *sql.Stmt
+	selectChildrenForParentOldestFirstStmt *sql.Stmt
+	selectChildrenForParentRecentFirstStmt *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 msc2836_edges (
+		parent_event_id TEXT NOT NULL,
+		child_event_id TEXT NOT NULL,
+		rel_type TEXT NOT NULL,
+		parent_room_id TEXT NOT NULL,
+		parent_servers TEXT NOT NULL,
+		CONSTRAINT msc2836_edges_uniq UNIQUE (parent_event_id, child_event_id, rel_type)
+	);
+
+	CREATE TABLE IF NOT EXISTS msc2836_nodes (
+		event_id TEXT PRIMARY KEY NOT NULL,
+		origin_server_ts BIGINT NOT NULL,
+		room_id TEXT NOT NULL
+	);
+	`)
+	if err != nil {
+		return nil, err
+	}
+	if d.insertEdgeStmt, err = d.db.Prepare(`
+		INSERT INTO msc2836_edges(parent_event_id, child_event_id, rel_type, parent_room_id, parent_servers) VALUES($1, $2, $3, $4, $5) ON CONFLICT DO NOTHING
+	`); err != nil {
+		return nil, err
+	}
+	if d.insertNodeStmt, err = d.db.Prepare(`
+		INSERT INTO msc2836_nodes(event_id, origin_server_ts, room_id) VALUES($1, $2, $3) ON CONFLICT DO NOTHING
+	`); err != nil {
+		return nil, err
+	}
+	selectChildrenQuery := `
+	SELECT child_event_id, origin_server_ts, room_id FROM msc2836_edges
+	LEFT JOIN msc2836_nodes ON msc2836_edges.child_event_id = msc2836_nodes.event_id
+	WHERE parent_event_id = $1 AND rel_type = $2
+	ORDER BY origin_server_ts
+	`
+	if d.selectChildrenForParentOldestFirstStmt, err = d.db.Prepare(selectChildrenQuery + "ASC"); err != nil {
+		return nil, err
+	}
+	if d.selectChildrenForParentRecentFirstStmt, err = d.db.Prepare(selectChildrenQuery + "DESC"); 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 msc2836_edges (
+		parent_event_id TEXT NOT NULL,
+		child_event_id TEXT NOT NULL,
+		rel_type TEXT NOT NULL,
+		parent_room_id TEXT NOT NULL,
+		parent_servers TEXT NOT NULL,
+		UNIQUE (parent_event_id, child_event_id, rel_type)
+	);
+
+	CREATE TABLE IF NOT EXISTS msc2836_nodes (
+		event_id TEXT PRIMARY KEY NOT NULL,
+		origin_server_ts BIGINT NOT NULL,
+		room_id TEXT NOT NULL
+	);
+	`)
+	if err != nil {
+		return nil, err
+	}
+	if d.insertEdgeStmt, err = d.db.Prepare(`
+		INSERT INTO msc2836_edges(parent_event_id, child_event_id, rel_type, parent_room_id, parent_servers) VALUES($1, $2, $3, $4, $5) ON CONFLICT (parent_event_id, child_event_id, rel_type) DO NOTHING
+	`); err != nil {
+		return nil, err
+	}
+	if d.insertNodeStmt, err = d.db.Prepare(`
+		INSERT INTO msc2836_nodes(event_id, origin_server_ts, room_id) VALUES($1, $2, $3) ON CONFLICT DO NOTHING
+	`); err != nil {
+		return nil, err
+	}
+	selectChildrenQuery := `
+	SELECT child_event_id, origin_server_ts, room_id FROM msc2836_edges
+	LEFT JOIN msc2836_nodes ON msc2836_edges.child_event_id = msc2836_nodes.event_id
+	WHERE parent_event_id = $1 AND rel_type = $2
+	ORDER BY origin_server_ts
+	`
+	if d.selectChildrenForParentOldestFirstStmt, err = d.db.Prepare(selectChildrenQuery + "ASC"); err != nil {
+		return nil, err
+	}
+	if d.selectChildrenForParentRecentFirstStmt, err = d.db.Prepare(selectChildrenQuery + "DESC"); err != nil {
+		return nil, err
+	}
+	return &d, nil
+}
+
+func (p *DB) StoreRelation(ctx context.Context, ev *gomatrixserverlib.HeaderedEvent) error {
+	parent, child, relType := parentChildEventIDs(ev)
+	if parent == "" || child == "" {
+		return nil
+	}
+	relationRoomID, relationServers := roomIDAndServers(ev)
+	relationServersJSON, err := json.Marshal(relationServers)
+	if err != nil {
+		return err
+	}
+	return p.writer.Do(p.db, nil, func(txn *sql.Tx) error {
+		_, err := txn.Stmt(p.insertEdgeStmt).ExecContext(ctx, parent, child, relType, relationRoomID, string(relationServersJSON))
+		if err != nil {
+			return err
+		}
+		_, err = txn.Stmt(p.insertNodeStmt).ExecContext(ctx, ev.EventID(), ev.OriginServerTS(), ev.RoomID())
+		return err
+	})
+}
+
+func (p *DB) ChildrenForParent(ctx context.Context, eventID, relType string, recentFirst bool) ([]eventInfo, error) {
+	var rows *sql.Rows
+	var err error
+	if recentFirst {
+		rows, err = p.selectChildrenForParentRecentFirstStmt.QueryContext(ctx, eventID, relType)
+	} else {
+		rows, err = p.selectChildrenForParentOldestFirstStmt.QueryContext(ctx, eventID, relType)
+	}
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close() // nolint: errcheck
+	var children []eventInfo
+	for rows.Next() {
+		var evInfo eventInfo
+		if err := rows.Scan(&evInfo.EventID, &evInfo.OriginServerTS, &evInfo.RoomID); err != nil {
+			return nil, err
+		}
+		children = append(children, evInfo)
+	}
+	return children, nil
+}
+
+func parentChildEventIDs(ev *gomatrixserverlib.HeaderedEvent) (parent, child, relType string) {
+	if ev == nil {
+		return
+	}
+	body := struct {
+		Relationship struct {
+			RelType string `json:"rel_type"`
+			EventID string `json:"event_id"`
+		} `json:"m.relationship"`
+	}{}
+	if err := json.Unmarshal(ev.Content(), &body); err != nil {
+		return
+	}
+	if body.Relationship.EventID == "" || body.Relationship.RelType == "" {
+		return
+	}
+	return body.Relationship.EventID, ev.EventID(), body.Relationship.RelType
+}
+
+func roomIDAndServers(ev *gomatrixserverlib.HeaderedEvent) (roomID string, servers []string) {
+	servers = []string{}
+	if ev == nil {
+		return
+	}
+	body := struct {
+		RoomID  string   `json:"relationship_room_id"`
+		Servers []string `json:"relationship_servers"`
+	}{}
+	if err := json.Unmarshal(ev.Unsigned(), &body); err != nil {
+		return
+	}
+	return body.RoomID, body.Servers
+}
diff --git a/internal/mscs/mscs.go b/internal/mscs/mscs.go
new file mode 100644
index 00000000..0a896ab0
--- /dev/null
+++ b/internal/mscs/mscs.go
@@ -0,0 +1,42 @@
+// Copyright 2020 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 mscs implements Matrix Spec Changes from https://github.com/matrix-org/matrix-doc
+package mscs
+
+import (
+	"fmt"
+
+	"github.com/matrix-org/dendrite/internal/mscs/msc2836"
+	"github.com/matrix-org/dendrite/internal/setup"
+)
+
+// Enable MSCs - returns an error on unknown MSCs
+func Enable(base *setup.BaseDendrite, monolith *setup.Monolith) error {
+	for _, msc := range base.Cfg.MSCs.MSCs {
+		if err := EnableMSC(base, monolith, msc); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func EnableMSC(base *setup.BaseDendrite, monolith *setup.Monolith, msc string) error {
+	switch msc {
+	case "msc2836":
+		return msc2836.Enable(base, monolith.RoomserverAPI, monolith.FederationSenderAPI, monolith.UserAPI, monolith.KeyRing)
+	default:
+		return fmt.Errorf("EnableMSC: unknown msc '%s'", msc)
+	}
+}
diff --git a/roomserver/api/perform.go b/roomserver/api/perform.go
index 29dbd25c..ec561f11 100644
--- a/roomserver/api/perform.go
+++ b/roomserver/api/perform.go
@@ -83,7 +83,8 @@ type PerformJoinRequest struct {
 
 type PerformJoinResponse struct {
 	// The room ID, populated on success.
-	RoomID string `json:"room_id"`
+	RoomID    string `json:"room_id"`
+	JoinedVia gomatrixserverlib.ServerName
 	// If non-nil, the join request failed. Contains more information why it failed.
 	Error *PerformError
 }
diff --git a/roomserver/internal/input/input.go b/roomserver/internal/input/input.go
index d7257539..79dc2fe1 100644
--- a/roomserver/internal/input/input.go
+++ b/roomserver/internal/input/input.go
@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"github.com/Shopify/sarama"
+	"github.com/matrix-org/dendrite/internal/hooks"
 	"github.com/matrix-org/dendrite/roomserver/acls"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/storage"
@@ -61,7 +62,11 @@ func (w *inputWorker) start() {
 	for {
 		select {
 		case task := <-w.input:
+			hooks.Run(hooks.KindNewEventReceived, &task.event.Event)
 			_, task.err = w.r.processRoomEvent(task.ctx, task.event)
+			if task.err == nil {
+				hooks.Run(hooks.KindNewEventPersisted, &task.event.Event)
+			}
 			task.wg.Done()
 		case <-time.After(time.Second * 5):
 			return
diff --git a/roomserver/internal/perform/perform_join.go b/roomserver/internal/perform/perform_join.go
index 56ae6d0b..f3745a7f 100644
--- a/roomserver/internal/perform/perform_join.go
+++ b/roomserver/internal/perform/perform_join.go
@@ -47,7 +47,7 @@ func (r *Joiner) PerformJoin(
 	req *api.PerformJoinRequest,
 	res *api.PerformJoinResponse,
 ) {
-	roomID, err := r.performJoin(ctx, req)
+	roomID, joinedVia, err := r.performJoin(ctx, req)
 	if err != nil {
 		perr, ok := err.(*api.PerformError)
 		if ok {
@@ -59,21 +59,22 @@ func (r *Joiner) PerformJoin(
 		}
 	}
 	res.RoomID = roomID
+	res.JoinedVia = joinedVia
 }
 
 func (r *Joiner) performJoin(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
-) (string, error) {
+) (string, gomatrixserverlib.ServerName, error) {
 	_, domain, err := gomatrixserverlib.SplitID('@', req.UserID)
 	if err != nil {
-		return "", &api.PerformError{
+		return "", "", &api.PerformError{
 			Code: api.PerformErrorBadRequest,
 			Msg:  fmt.Sprintf("Supplied user ID %q in incorrect format", req.UserID),
 		}
 	}
 	if domain != r.Cfg.Matrix.ServerName {
-		return "", &api.PerformError{
+		return "", "", &api.PerformError{
 			Code: api.PerformErrorBadRequest,
 			Msg:  fmt.Sprintf("User %q does not belong to this homeserver", req.UserID),
 		}
@@ -84,7 +85,7 @@ func (r *Joiner) performJoin(
 	if strings.HasPrefix(req.RoomIDOrAlias, "#") {
 		return r.performJoinRoomByAlias(ctx, req)
 	}
-	return "", &api.PerformError{
+	return "", "", &api.PerformError{
 		Code: api.PerformErrorBadRequest,
 		Msg:  fmt.Sprintf("Room ID or alias %q is invalid", req.RoomIDOrAlias),
 	}
@@ -93,11 +94,11 @@ func (r *Joiner) performJoin(
 func (r *Joiner) performJoinRoomByAlias(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
-) (string, error) {
+) (string, gomatrixserverlib.ServerName, error) {
 	// Get the domain part of the room alias.
 	_, domain, err := gomatrixserverlib.SplitID('#', req.RoomIDOrAlias)
 	if err != nil {
-		return "", fmt.Errorf("Alias %q is not in the correct format", req.RoomIDOrAlias)
+		return "", "", fmt.Errorf("Alias %q is not in the correct format", req.RoomIDOrAlias)
 	}
 	req.ServerNames = append(req.ServerNames, domain)
 
@@ -115,7 +116,7 @@ func (r *Joiner) performJoinRoomByAlias(
 		err = r.FSAPI.PerformDirectoryLookup(ctx, &dirReq, &dirRes)
 		if err != nil {
 			logrus.WithError(err).Errorf("error looking up alias %q", req.RoomIDOrAlias)
-			return "", fmt.Errorf("Looking up alias %q over federation failed: %w", req.RoomIDOrAlias, err)
+			return "", "", fmt.Errorf("Looking up alias %q over federation failed: %w", req.RoomIDOrAlias, err)
 		}
 		roomID = dirRes.RoomID
 		req.ServerNames = append(req.ServerNames, dirRes.ServerNames...)
@@ -123,13 +124,13 @@ func (r *Joiner) performJoinRoomByAlias(
 		// Otherwise, look up if we know this room alias locally.
 		roomID, err = r.DB.GetRoomIDForAlias(ctx, req.RoomIDOrAlias)
 		if err != nil {
-			return "", fmt.Errorf("Lookup room alias %q failed: %w", req.RoomIDOrAlias, err)
+			return "", "", fmt.Errorf("Lookup room alias %q failed: %w", req.RoomIDOrAlias, err)
 		}
 	}
 
 	// If the room ID is empty then we failed to look up the alias.
 	if roomID == "" {
-		return "", fmt.Errorf("Alias %q not found", req.RoomIDOrAlias)
+		return "", "", fmt.Errorf("Alias %q not found", req.RoomIDOrAlias)
 	}
 
 	// If we do, then pluck out the room ID and continue the join.
@@ -142,11 +143,11 @@ func (r *Joiner) performJoinRoomByAlias(
 func (r *Joiner) performJoinRoomByID(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
-) (string, error) {
+) (string, gomatrixserverlib.ServerName, error) {
 	// Get the domain part of the room ID.
 	_, domain, err := gomatrixserverlib.SplitID('!', req.RoomIDOrAlias)
 	if err != nil {
-		return "", &api.PerformError{
+		return "", "", &api.PerformError{
 			Code: api.PerformErrorBadRequest,
 			Msg:  fmt.Sprintf("Room ID %q is invalid: %s", req.RoomIDOrAlias, err),
 		}
@@ -169,7 +170,7 @@ func (r *Joiner) performJoinRoomByID(
 		Redacts:  "",
 	}
 	if err = eb.SetUnsigned(struct{}{}); err != nil {
-		return "", fmt.Errorf("eb.SetUnsigned: %w", err)
+		return "", "", fmt.Errorf("eb.SetUnsigned: %w", err)
 	}
 
 	// It is possible for the request to include some "content" for the
@@ -180,7 +181,7 @@ func (r *Joiner) performJoinRoomByID(
 	}
 	req.Content["membership"] = gomatrixserverlib.Join
 	if err = eb.SetContent(req.Content); err != nil {
-		return "", fmt.Errorf("eb.SetContent: %w", err)
+		return "", "", fmt.Errorf("eb.SetContent: %w", err)
 	}
 
 	// Force a federated join if we aren't in the room and we've been
@@ -194,7 +195,7 @@ func (r *Joiner) performJoinRoomByID(
 	if err == nil && isInvitePending {
 		_, inviterDomain, ierr := gomatrixserverlib.SplitID('@', inviteSender)
 		if ierr != nil {
-			return "", fmt.Errorf("gomatrixserverlib.SplitID: %w", err)
+			return "", "", fmt.Errorf("gomatrixserverlib.SplitID: %w", err)
 		}
 
 		// If we were invited by someone from another server then we can
@@ -206,8 +207,10 @@ func (r *Joiner) performJoinRoomByID(
 	}
 
 	// If we should do a forced federated join then do that.
+	var joinedVia gomatrixserverlib.ServerName
 	if forceFederatedJoin {
-		return req.RoomIDOrAlias, r.performFederatedJoinRoomByID(ctx, req)
+		joinedVia, err = r.performFederatedJoinRoomByID(ctx, req)
+		return req.RoomIDOrAlias, joinedVia, err
 	}
 
 	// Try to construct an actual join event from the template.
@@ -249,7 +252,7 @@ func (r *Joiner) performJoinRoomByID(
 			inputRes := api.InputRoomEventsResponse{}
 			r.Inputer.InputRoomEvents(ctx, &inputReq, &inputRes)
 			if err = inputRes.Err(); err != nil {
-				return "", &api.PerformError{
+				return "", "", &api.PerformError{
 					Code: api.PerformErrorNotAllowed,
 					Msg:  fmt.Sprintf("InputRoomEvents auth failed: %s", err),
 				}
@@ -265,7 +268,7 @@ func (r *Joiner) performJoinRoomByID(
 			// Otherwise we'll try a federated join as normal, since it's quite
 			// possible that the room still exists on other servers.
 			if len(req.ServerNames) == 0 {
-				return "", &api.PerformError{
+				return "", "", &api.PerformError{
 					Code: api.PerformErrorNoRoom,
 					Msg:  fmt.Sprintf("Room ID %q does not exist", req.RoomIDOrAlias),
 				}
@@ -273,24 +276,25 @@ func (r *Joiner) performJoinRoomByID(
 		}
 
 		// Perform a federated room join.
-		return req.RoomIDOrAlias, r.performFederatedJoinRoomByID(ctx, req)
+		joinedVia, err = r.performFederatedJoinRoomByID(ctx, req)
+		return req.RoomIDOrAlias, joinedVia, err
 
 	default:
 		// Something else went wrong.
-		return "", fmt.Errorf("Error joining local room: %q", err)
+		return "", "", fmt.Errorf("Error joining local room: %q", err)
 	}
 
 	// By this point, if req.RoomIDOrAlias contained an alias, then
 	// it will have been overwritten with a room ID by performJoinRoomByAlias.
 	// We should now include this in the response so that the CS API can
 	// return the right room ID.
-	return req.RoomIDOrAlias, nil
+	return req.RoomIDOrAlias, r.Cfg.Matrix.ServerName, nil
 }
 
 func (r *Joiner) performFederatedJoinRoomByID(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
-) error {
+) (gomatrixserverlib.ServerName, error) {
 	// Try joining by all of the supplied server names.
 	fedReq := fsAPI.PerformJoinRequest{
 		RoomID:      req.RoomIDOrAlias, // the room ID to try and join
@@ -301,13 +305,13 @@ func (r *Joiner) performFederatedJoinRoomByID(
 	fedRes := fsAPI.PerformJoinResponse{}
 	r.FSAPI.PerformJoin(ctx, &fedReq, &fedRes)
 	if fedRes.LastError != nil {
-		return &api.PerformError{
+		return "", &api.PerformError{
 			Code:       api.PerformErrRemote,
 			Msg:        fedRes.LastError.Message,
 			RemoteCode: fedRes.LastError.Code,
 		}
 	}
-	return nil
+	return fedRes.JoinedVia, nil
 }
 
 func buildEvent(