Join room support in federation sender (#989)

* Implement PerformJoinRequest

* Rename perform functions

* Check send join response

* Temporary wiring to test federation sender room joins

* Actually pass through the config

* Make sure membership content shows join

14 files changed, 264 insertions, 134 deletions

diff --git a/clientapi/routing/joinroom.go b/clientapi/routing/joinroom.go
index d0dee7c2..f55d1b6a 100644
--- a/clientapi/routing/joinroom.go
+++ b/clientapi/routing/joinroom.go
@@ -27,12 +27,11 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/producers"
 	"github.com/matrix-org/dendrite/common"
 	"github.com/matrix-org/dendrite/common/config"
-	"github.com/matrix-org/dendrite/roomserver/api"
+	federationSenderAPI "github.com/matrix-org/dendrite/federationsender/api"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrix"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
-	"github.com/sirupsen/logrus"
 )
 
 // JoinRoomByIDOrAlias implements the "/join/{roomIDOrAlias}" API.
@@ -46,6 +45,7 @@ func JoinRoomByIDOrAlias(
 	producer *producers.RoomserverProducer,
 	queryAPI roomserverAPI.RoomserverQueryAPI,
 	aliasAPI roomserverAPI.RoomserverAliasAPI,
+	fsAPI federationSenderAPI.FederationSenderInternalAPI,
 	keyRing gomatrixserverlib.KeyRing,
 	accountDB accounts.Database,
 ) util.JSONResponse {
@@ -79,7 +79,8 @@ func JoinRoomByIDOrAlias(
 	content["avatar_url"] = profile.AvatarURL
 
 	r := joinRoomReq{
-		req, evTime, content, device.UserID, cfg, federation, producer, queryAPI, aliasAPI, keyRing,
+		req, evTime, content, device.UserID, cfg, federation, producer,
+		queryAPI, aliasAPI, fsAPI, keyRing,
 	}
 
 	if strings.HasPrefix(roomIDOrAlias, "!") {
@@ -107,6 +108,7 @@ type joinRoomReq struct {
 	producer   *producers.RoomserverProducer
 	queryAPI   roomserverAPI.RoomserverQueryAPI
 	aliasAPI   roomserverAPI.RoomserverAliasAPI
+	fsAPI      federationSenderAPI.FederationSenderInternalAPI
 	keyRing    gomatrixserverlib.KeyRing
 }
 
@@ -326,72 +328,16 @@ func (r joinRoomReq) joinRoomUsingServers(
 // server was invalid this returns an error.
 // Otherwise this returns a JSONResponse.
 func (r joinRoomReq) joinRoomUsingServer(roomID string, server gomatrixserverlib.ServerName) (*util.JSONResponse, error) {
-	// Ask the room server for information about room versions.
-	var request api.QueryRoomVersionCapabilitiesRequest
-	var response api.QueryRoomVersionCapabilitiesResponse
-	if err := r.queryAPI.QueryRoomVersionCapabilities(r.req.Context(), &request, &response); err != nil {
-		return nil, err
-	}
-	var supportedVersions []gomatrixserverlib.RoomVersion
-	for version := range response.AvailableRoomVersions {
-		supportedVersions = append(supportedVersions, version)
-	}
-	respMakeJoin, err := r.federation.MakeJoin(r.req.Context(), server, roomID, r.userID, supportedVersions)
-	if err != nil {
-		// TODO: Check if the user was not allowed to join the room.
-		return nil, fmt.Errorf("r.federation.MakeJoin: %w", err)
-	}
-
-	// Set all the fields to be what they should be, this should be a no-op
-	// but it's possible that the remote server returned us something "odd"
-	err = r.writeToBuilder(&respMakeJoin.JoinEvent, roomID)
-	if err != nil {
-		return nil, fmt.Errorf("r.writeToBuilder: %w", err)
-	}
-
-	if respMakeJoin.RoomVersion == "" {
-		respMakeJoin.RoomVersion = gomatrixserverlib.RoomVersionV1
-	}
-	if _, err = respMakeJoin.RoomVersion.EventFormat(); err != nil {
-		return &util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.UnsupportedRoomVersion(
-				fmt.Sprintf("Room version '%s' is not supported", respMakeJoin.RoomVersion),
-			),
-		}, nil
-	}
-
-	event, err := respMakeJoin.JoinEvent.Build(
-		r.evTime, r.cfg.Matrix.ServerName, r.cfg.Matrix.KeyID,
-		r.cfg.Matrix.PrivateKey, respMakeJoin.RoomVersion,
-	)
-	if err != nil {
-		return nil, fmt.Errorf("respMakeJoin.JoinEvent.Build: %w", err)
+	fedJoinReq := federationSenderAPI.PerformJoinRequest{
+		RoomID:     roomID,
+		UserID:     r.userID,
+		ServerName: server,
 	}
-
-	respSendJoin, err := r.federation.SendJoin(r.req.Context(), server, event, respMakeJoin.RoomVersion)
-	if err != nil {
-		return nil, fmt.Errorf("r.federation.SendJoin: %w", err)
-	}
-
-	if err = r.checkSendJoinResponse(event, server, respMakeJoin, respSendJoin); err != nil {
+	fedJoinRes := federationSenderAPI.PerformJoinResponse{}
+	if err := r.fsAPI.PerformJoin(r.req.Context(), &fedJoinReq, &fedJoinRes); err != nil {
 		return nil, err
 	}
 
-	util.GetLogger(r.req.Context()).WithFields(logrus.Fields{
-		"room_id":          roomID,
-		"num_auth_events":  len(respSendJoin.AuthEvents),
-		"num_state_events": len(respSendJoin.StateEvents),
-	}).Info("Room join signature and auth verification passed")
-
-	if err = r.producer.SendEventWithState(
-		r.req.Context(),
-		respSendJoin.ToRespState(),
-		event.Headered(respMakeJoin.RoomVersion),
-	); err != nil {
-		util.GetLogger(r.req.Context()).WithError(err).Error("r.producer.SendEventWithState")
-	}
-
 	return &util.JSONResponse{
 		Code: http.StatusOK,
 		// TODO: Put the response struct somewhere common.
@@ -400,49 +346,3 @@ func (r joinRoomReq) joinRoomUsingServer(roomID string, server gomatrixserverlib
 		}{roomID},
 	}, nil
 }
-
-// checkSendJoinResponse checks that all of the signatures are correct
-// and that the join is allowed by the supplied state.
-func (r joinRoomReq) checkSendJoinResponse(
-	event gomatrixserverlib.Event,
-	server gomatrixserverlib.ServerName,
-	respMakeJoin gomatrixserverlib.RespMakeJoin,
-	respSendJoin gomatrixserverlib.RespSendJoin,
-) error {
-	// A list of events that we have retried, if they were not included in
-	// the auth events supplied in the send_join.
-	retries := map[string]bool{}
-
-retryCheck:
-	// TODO: Can we expand Check here to return a list of missing auth
-	// events rather than failing one at a time?
-	if err := respSendJoin.Check(r.req.Context(), r.keyRing, event); err != nil {
-		switch e := err.(type) {
-		case gomatrixserverlib.MissingAuthEventError:
-			// Check that we haven't already retried for this event, prevents
-			// us from ending up in endless loops
-			if !retries[e.AuthEventID] {
-				// Ask the server that we're talking to right now for the event
-				tx, txerr := r.federation.GetEvent(r.req.Context(), server, e.AuthEventID)
-				if txerr != nil {
-					return fmt.Errorf("r.federation.GetEvent: %w", txerr)
-				}
-				// For each event returned, add it to the auth events.
-				for _, pdu := range tx.PDUs {
-					ev, everr := gomatrixserverlib.NewEventFromUntrustedJSON(pdu, respMakeJoin.RoomVersion)
-					if everr != nil {
-						return fmt.Errorf("gomatrixserverlib.NewEventFromUntrustedJSON: %w", everr)
-					}
-					respSendJoin.AuthEvents = append(respSendJoin.AuthEvents, ev)
-				}
-				// Mark the event as retried and then give the check another go.
-				retries[e.AuthEventID] = true
-				goto retryCheck
-			}
-			return fmt.Errorf("respSendJoin (after retries): %w", e)
-		default:
-			return fmt.Errorf("respSendJoin: %w", err)
-		}
-	}
-	return nil
-}
diff --git a/clientapi/routing/routing.go b/clientapi/routing/routing.go
index 9ab22cbe..e62b5193 100644
--- a/clientapi/routing/routing.go
+++ b/clientapi/routing/routing.go
@@ -101,7 +101,8 @@ func Setup(
 				return util.ErrorResponse(err)
 			}
 			return JoinRoomByIDOrAlias(
-				req, device, vars["roomIDOrAlias"], cfg, federation, producer, queryAPI, aliasAPI, keyRing, accountDB,
+				req, device, vars["roomIDOrAlias"], cfg, federation, producer,
+				queryAPI, aliasAPI, federationSender, keyRing, accountDB,
 			)
 		}),
 	).Methods(http.MethodPost, http.MethodOptions)
diff --git a/cmd/dendrite-demo-libp2p/main.go b/cmd/dendrite-demo-libp2p/main.go
index b9fbfc53..a2a4675b 100644
--- a/cmd/dendrite-demo-libp2p/main.go
+++ b/cmd/dendrite-demo-libp2p/main.go
@@ -153,7 +153,7 @@ func main() {
 	asQuery := appservice.SetupAppServiceAPIComponent(
 		&base.Base, accountDB, deviceDB, federation, alias, query, transactions.New(),
 	)
-	fsAPI := federationsender.SetupFederationSenderComponent(&base.Base, federation, query, input)
+	fsAPI := federationsender.SetupFederationSenderComponent(&base.Base, federation, query, input, &keyRing)
 
 	clientapi.SetupClientAPIComponent(
 		&base.Base, deviceDB, accountDB,
diff --git a/cmd/dendrite-federation-sender-server/main.go b/cmd/dendrite-federation-sender-server/main.go
index 1593afaa..f8d43b99 100644
--- a/cmd/dendrite-federation-sender-server/main.go
+++ b/cmd/dendrite-federation-sender-server/main.go
@@ -16,6 +16,7 @@ package main
 
 import (
 	"github.com/matrix-org/dendrite/common/basecomponent"
+	"github.com/matrix-org/dendrite/common/keydb"
 	"github.com/matrix-org/dendrite/federationsender"
 )
 
@@ -25,11 +26,13 @@ func main() {
 	defer base.Close() // nolint: errcheck
 
 	federation := base.CreateFederationClient()
+	keyDB := base.CreateKeyDB()
+	keyRing := keydb.CreateKeyRing(federation.Client, keyDB, cfg.Matrix.KeyPerspectives)
 
 	_, input, query := base.CreateHTTPRoomserverAPIs()
 
 	federationsender.SetupFederationSenderComponent(
-		base, federation, query, input,
+		base, federation, query, input, &keyRing,
 	)
 
 	base.SetupAndServeHTTP(string(base.Cfg.Bind.FederationSender), string(base.Cfg.Listen.FederationSender))
diff --git a/cmd/dendrite-monolith-server/main.go b/cmd/dendrite-monolith-server/main.go
index e806f6f3..f43f8b04 100644
--- a/cmd/dendrite-monolith-server/main.go
+++ b/cmd/dendrite-monolith-server/main.go
@@ -62,7 +62,7 @@ func main() {
 	asQuery := appservice.SetupAppServiceAPIComponent(
 		base, accountDB, deviceDB, federation, alias, query, transactions.New(),
 	)
-	fsAPI := federationsender.SetupFederationSenderComponent(base, federation, query, input)
+	fsAPI := federationsender.SetupFederationSenderComponent(base, federation, query, input, &keyRing)
 	input.SetFederationSenderAPI(fsAPI)
 
 	clientapi.SetupClientAPIComponent(
diff --git a/cmd/dendritejs/main.go b/cmd/dendritejs/main.go
index 7665138e..1f2f20fb 100644
--- a/cmd/dendritejs/main.go
+++ b/cmd/dendritejs/main.go
@@ -128,7 +128,7 @@ func main() {
 	asQuery := appservice.SetupAppServiceAPIComponent(
 		base, accountDB, deviceDB, federation, alias, query, transactions.New(),
 	)
-	fedSenderAPI := federationsender.SetupFederationSenderComponent(base, federation, query, input)
+	fedSenderAPI := federationsender.SetupFederationSenderComponent(base, federation, query, input, &keyRing)
 	input.SetFederationSenderAPI(fedSenderAPI)
 
 	clientapi.SetupClientAPIComponent(
diff --git a/federationsender/api/api.go b/federationsender/api/api.go
index 1340179e..10dc66da 100644
--- a/federationsender/api/api.go
+++ b/federationsender/api/api.go
@@ -25,13 +25,13 @@ type FederationSenderInternalAPI interface {
 		response *QueryJoinedHostServerNamesInRoomResponse,
 	) error
 	// Handle an instruction to make_join & send_join with a remote server.
-	PerformJoinRequest(
+	PerformJoin(
 		ctx context.Context,
 		request *PerformJoinRequest,
 		response *PerformJoinResponse,
 	) error
 	// Handle an instruction to make_leave & send_leave with a remote server.
-	PerformLeaveRequest(
+	PerformLeave(
 		ctx context.Context,
 		request *PerformLeaveRequest,
 		response *PerformLeaveResponse,
diff --git a/federationsender/api/perform.go b/federationsender/api/perform.go
index 8c30ecbe..87736f29 100644
--- a/federationsender/api/perform.go
+++ b/federationsender/api/perform.go
@@ -4,6 +4,7 @@ import (
 	"context"
 
 	commonHTTP "github.com/matrix-org/dendrite/common/http"
+	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/opentracing/opentracing-go"
 )
 
@@ -16,14 +17,17 @@ const (
 )
 
 type PerformJoinRequest struct {
-	RoomID string `json:"room_id"`
+	RoomID     string                       `json:"room_id"`
+	UserID     string                       `json:"user_id"`
+	ServerName gomatrixserverlib.ServerName `json:"server_name"`
+	Content    map[string]interface{}       `json:"content"`
 }
 
 type PerformJoinResponse struct {
 }
 
 // Handle an instruction to make_join & send_join with a remote server.
-func (h *httpFederationSenderInternalAPI) PerformJoinRequest(
+func (h *httpFederationSenderInternalAPI) PerformJoin(
 	ctx context.Context,
 	request *PerformJoinRequest,
 	response *PerformJoinResponse,
@@ -43,7 +47,7 @@ type PerformLeaveResponse struct {
 }
 
 // Handle an instruction to make_leave & send_leave with a remote server.
-func (h *httpFederationSenderInternalAPI) PerformLeaveRequest(
+func (h *httpFederationSenderInternalAPI) PerformLeave(
 	ctx context.Context,
 	request *PerformLeaveRequest,
 	response *PerformLeaveResponse,
diff --git a/federationsender/federationsender.go b/federationsender/federationsender.go
index 355775f8..aa9a7bc9 100644
--- a/federationsender/federationsender.go
+++ b/federationsender/federationsender.go
@@ -36,6 +36,7 @@ func SetupFederationSenderComponent(
 	federation *gomatrixserverlib.FederationClient,
 	rsQueryAPI roomserverAPI.RoomserverQueryAPI,
 	rsInputAPI roomserverAPI.RoomserverInputAPI,
+	keyRing *gomatrixserverlib.KeyRing,
 ) api.FederationSenderInternalAPI {
 	federationSenderDB, err := storage.NewDatabase(string(base.Cfg.Database.FederationSender))
 	if err != nil {
@@ -61,10 +62,10 @@ func SetupFederationSenderComponent(
 		logrus.WithError(err).Panic("failed to start typing server consumer")
 	}
 
-	queryAPI := query.FederationSenderInternalAPI{
-		DB: federationSenderDB,
-	}
+	queryAPI := query.NewFederationSenderInternalAPI(
+		federationSenderDB, base.Cfg, roomserverProducer, federation, keyRing,
+	)
 	queryAPI.SetupHTTP(http.DefaultServeMux)
 
-	return &queryAPI
+	return queryAPI
 }
diff --git a/federationsender/producers/roomserver.go b/federationsender/producers/roomserver.go
index 0395f962..ff4cda5b 100644
--- a/federationsender/producers/roomserver.go
+++ b/federationsender/producers/roomserver.go
@@ -54,6 +54,42 @@ func (c *RoomserverProducer) SendInviteResponse(
 	return c.SendInputRoomEvents(ctx, []api.InputRoomEvent{ire})
 }
 
+// SendEventWithState writes an event with KindNew to the roomserver input log
+// with the state at the event as KindOutlier before it.
+func (c *RoomserverProducer) SendEventWithState(
+	ctx context.Context, state gomatrixserverlib.RespState, event gomatrixserverlib.HeaderedEvent,
+) error {
+	outliers, err := state.Events()
+	if err != nil {
+		return err
+	}
+
+	var ires []api.InputRoomEvent
+	for _, outlier := range outliers {
+		ires = append(ires, api.InputRoomEvent{
+			Kind:         api.KindOutlier,
+			Event:        outlier.Headered(event.RoomVersion),
+			AuthEventIDs: outlier.AuthEventIDs(),
+		})
+	}
+
+	stateEventIDs := make([]string, len(state.StateEvents))
+	for i := range state.StateEvents {
+		stateEventIDs[i] = state.StateEvents[i].EventID()
+	}
+
+	ires = append(ires, api.InputRoomEvent{
+		Kind:          api.KindNew,
+		Event:         event,
+		AuthEventIDs:  event.AuthEventIDs(),
+		HasState:      true,
+		StateEventIDs: stateEventIDs,
+	})
+
+	_, err = c.SendInputRoomEvents(ctx, ires)
+	return err
+}
+
 // SendInputRoomEvents writes the given input room events to the roomserver input API.
 func (c *RoomserverProducer) SendInputRoomEvents(
 	ctx context.Context, ires []api.InputRoomEvent,
diff --git a/federationsender/query/api.go b/federationsender/query/api.go
index e33bcc11..e92453f9 100644
--- a/federationsender/query/api.go
+++ b/federationsender/query/api.go
@@ -5,17 +5,37 @@ import (
 	"net/http"
 
 	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/config"
 	"github.com/matrix-org/dendrite/federationsender/api"
+	"github.com/matrix-org/dendrite/federationsender/producers"
 	"github.com/matrix-org/dendrite/federationsender/storage"
-	rsAPI "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 )
 
 // FederationSenderInternalAPI is an implementation of api.FederationSenderInternalAPI
 type FederationSenderInternalAPI struct {
 	api.FederationSenderInternalAPI
-	DB                 storage.Database
-	RoomserverInputAPI rsAPI.RoomserverInputAPI
+	db         storage.Database
+	cfg        *config.Dendrite
+	producer   *producers.RoomserverProducer
+	federation *gomatrixserverlib.FederationClient
+	keyRing    *gomatrixserverlib.KeyRing
+}
+
+func NewFederationSenderInternalAPI(
+	db storage.Database, cfg *config.Dendrite,
+	producer *producers.RoomserverProducer,
+	federation *gomatrixserverlib.FederationClient,
+	keyRing *gomatrixserverlib.KeyRing,
+) *FederationSenderInternalAPI {
+	return &FederationSenderInternalAPI{
+		db:         db,
+		cfg:        cfg,
+		producer:   producer,
+		federation: federation,
+		keyRing:    keyRing,
+	}
 }
 
 // SetupHTTP adds the FederationSenderInternalAPI handlers to the http.ServeMux.
@@ -55,7 +75,7 @@ func (f *FederationSenderInternalAPI) SetupHTTP(servMux *http.ServeMux) {
 			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
 				return util.MessageResponse(http.StatusBadRequest, err.Error())
 			}
-			if err := f.PerformJoinRequest(req.Context(), &request, &response); err != nil {
+			if err := f.PerformJoin(req.Context(), &request, &response); err != nil {
 				return util.ErrorResponse(err)
 			}
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
@@ -68,7 +88,7 @@ func (f *FederationSenderInternalAPI) SetupHTTP(servMux *http.ServeMux) {
 			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
 				return util.MessageResponse(http.StatusBadRequest, err.Error())
 			}
-			if err := f.PerformLeaveRequest(req.Context(), &request, &response); err != nil {
+			if err := f.PerformLeave(req.Context(), &request, &response); err != nil {
 				return util.ErrorResponse(err)
 			}
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
diff --git a/federationsender/query/perform.go b/federationsender/query/perform.go
index 2486873c..d39fef5e 100644
--- a/federationsender/query/perform.go
+++ b/federationsender/query/perform.go
@@ -2,21 +2,116 @@ package query
 
 import (
 	"context"
+	"fmt"
+	"time"
 
 	"github.com/matrix-org/dendrite/federationsender/api"
+	"github.com/matrix-org/dendrite/federationsender/query/perform"
+	"github.com/matrix-org/dendrite/roomserver/version"
+	"github.com/matrix-org/gomatrixserverlib"
 )
 
 // PerformJoinRequest implements api.FederationSenderInternalAPI
-func (r *FederationSenderInternalAPI) PerformJoinRequest(
+func (r *FederationSenderInternalAPI) PerformJoin(
 	ctx context.Context,
 	request *api.PerformJoinRequest,
 	response *api.PerformJoinResponse,
 ) (err error) {
+	// Look up the supported room versions.
+	var supportedVersions []gomatrixserverlib.RoomVersion
+	for version := range version.SupportedRoomVersions() {
+		supportedVersions = append(supportedVersions, version)
+	}
+
+	// Try to perform a make_join using the information supplied in the
+	// request.
+	respMakeJoin, err := r.federation.MakeJoin(
+		ctx,
+		request.ServerName,
+		request.RoomID,
+		request.UserID,
+		supportedVersions,
+	)
+	if err != nil {
+		// TODO: Check if the user was not allowed to join the room.
+		return fmt.Errorf("r.federation.MakeJoin: %w", err)
+	}
+
+	// Set all the fields to be what they should be, this should be a no-op
+	// but it's possible that the remote server returned us something "odd"
+	respMakeJoin.JoinEvent.Type = "m.room.member"
+	respMakeJoin.JoinEvent.Sender = request.UserID
+	respMakeJoin.JoinEvent.StateKey = &request.UserID
+	respMakeJoin.JoinEvent.RoomID = request.RoomID
+	respMakeJoin.JoinEvent.Redacts = ""
+	if request.Content == nil {
+		request.Content = map[string]interface{}{}
+	}
+	request.Content["membership"] = "join"
+	if err = respMakeJoin.JoinEvent.SetContent(request.Content); err != nil {
+		return fmt.Errorf("respMakeJoin.JoinEvent.SetContent: %w", err)
+	}
+	if err = respMakeJoin.JoinEvent.SetUnsigned(struct{}{}); err != nil {
+		return fmt.Errorf("respMakeJoin.JoinEvent.SetUnsigned: %w", err)
+	}
+
+	// Work out if we support the room version that has been supplied in
+	// the make_join response.
+	if respMakeJoin.RoomVersion == "" {
+		respMakeJoin.RoomVersion = gomatrixserverlib.RoomVersionV1
+	}
+	if _, err = respMakeJoin.RoomVersion.EventFormat(); err != nil {
+		return fmt.Errorf("respMakeJoin.RoomVersion.EventFormat: %w", err)
+	}
+
+	// Build the join event.
+	event, err := respMakeJoin.JoinEvent.Build(
+		time.Now(),
+		r.cfg.Matrix.ServerName,
+		r.cfg.Matrix.KeyID,
+		r.cfg.Matrix.PrivateKey,
+		respMakeJoin.RoomVersion,
+	)
+	if err != nil {
+		return fmt.Errorf("respMakeJoin.JoinEvent.Build: %w", err)
+	}
+
+	// Try to perform a send_join using the newly built event.
+	respSendJoin, err := r.federation.SendJoin(
+		ctx,
+		request.ServerName,
+		event,
+		respMakeJoin.RoomVersion,
+	)
+	if err != nil {
+		return fmt.Errorf("r.federation.SendJoin: %w", err)
+	}
+
+	// Check that the send_join response was valid.
+	joinCtx := perform.JoinContext(r.federation, r.keyRing)
+	if err = joinCtx.CheckSendJoinResponse(
+		ctx, event, request.ServerName, respMakeJoin, respSendJoin,
+	); err != nil {
+		return fmt.Errorf("perform.JoinRequest.CheckSendJoinResponse: %w", err)
+	}
+
+	// If we successfully performed a send_join above then the other
+	// server now thinks we're a part of the room. Send the newly
+	// returned state to the roomserver to update our local view.
+	if err = r.producer.SendEventWithState(
+		ctx,
+		respSendJoin.ToRespState(),
+		event.Headered(respMakeJoin.RoomVersion),
+	); err != nil {
+		return fmt.Errorf("r.producer.SendEventWithState: %w", err)
+	}
+
+	// Everything went to plan.
 	return nil
 }
 
 // PerformLeaveRequest implements api.FederationSenderInternalAPI
-func (r *FederationSenderInternalAPI) PerformLeaveRequest(
+func (r *FederationSenderInternalAPI) PerformLeave(
 	ctx context.Context,
 	request *api.PerformLeaveRequest,
 	response *api.PerformLeaveResponse,
diff --git a/federationsender/query/perform/join.go b/federationsender/query/perform/join.go
new file mode 100644
index 00000000..3c7ef076
--- /dev/null
+++ b/federationsender/query/perform/join.go
@@ -0,0 +1,70 @@
+package perform
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// This file contains helpers for the PerformJoin function.
+
+type joinContext struct {
+	federation *gomatrixserverlib.FederationClient
+	keyRing    *gomatrixserverlib.KeyRing
+}
+
+// Returns a new join context.
+func JoinContext(f *gomatrixserverlib.FederationClient, k *gomatrixserverlib.KeyRing) *joinContext {
+	return &joinContext{
+		federation: f,
+		keyRing:    k,
+	}
+}
+
+// checkSendJoinResponse checks that all of the signatures are correct
+// and that the join is allowed by the supplied state.
+func (r joinContext) CheckSendJoinResponse(
+	ctx context.Context,
+	event gomatrixserverlib.Event,
+	server gomatrixserverlib.ServerName,
+	respMakeJoin gomatrixserverlib.RespMakeJoin,
+	respSendJoin gomatrixserverlib.RespSendJoin,
+) error {
+	// A list of events that we have retried, if they were not included in
+	// the auth events supplied in the send_join.
+	retries := map[string]bool{}
+
+retryCheck:
+	// TODO: Can we expand Check here to return a list of missing auth
+	// events rather than failing one at a time?
+	if err := respSendJoin.Check(ctx, r.keyRing, event); err != nil {
+		switch e := err.(type) {
+		case gomatrixserverlib.MissingAuthEventError:
+			// Check that we haven't already retried for this event, prevents
+			// us from ending up in endless loops
+			if !retries[e.AuthEventID] {
+				// Ask the server that we're talking to right now for the event
+				tx, txerr := r.federation.GetEvent(ctx, server, e.AuthEventID)
+				if txerr != nil {
+					return fmt.Errorf("r.federation.GetEvent: %w", txerr)
+				}
+				// For each event returned, add it to the auth events.
+				for _, pdu := range tx.PDUs {
+					ev, everr := gomatrixserverlib.NewEventFromUntrustedJSON(pdu, respMakeJoin.RoomVersion)
+					if everr != nil {
+						return fmt.Errorf("gomatrixserverlib.NewEventFromUntrustedJSON: %w", everr)
+					}
+					respSendJoin.AuthEvents = append(respSendJoin.AuthEvents, ev)
+				}
+				// Mark the event as retried and then give the check another go.
+				retries[e.AuthEventID] = true
+				goto retryCheck
+			}
+			return fmt.Errorf("respSendJoin (after retries): %w", e)
+		default:
+			return fmt.Errorf("respSendJoin: %w", err)
+		}
+	}
+	return nil
+}
diff --git a/federationsender/query/query.go b/federationsender/query/query.go
index ec668204..004ad156 100644
--- a/federationsender/query/query.go
+++ b/federationsender/query/query.go
@@ -13,7 +13,7 @@ func (f *FederationSenderInternalAPI) QueryJoinedHostsInRoom(
 	request *api.QueryJoinedHostsInRoomRequest,
 	response *api.QueryJoinedHostsInRoomResponse,
 ) (err error) {
-	response.JoinedHosts, err = f.DB.GetJoinedHosts(ctx, request.RoomID)
+	response.JoinedHosts, err = f.db.GetJoinedHosts(ctx, request.RoomID)
 	return
 }
 
@@ -23,7 +23,7 @@ func (f *FederationSenderInternalAPI) QueryJoinedHostServerNamesInRoom(
 	request *api.QueryJoinedHostServerNamesInRoomRequest,
 	response *api.QueryJoinedHostServerNamesInRoomResponse,
 ) (err error) {
-	joinedHosts, err := f.DB.GetJoinedHosts(ctx, request.RoomID)
+	joinedHosts, err := f.db.GetJoinedHosts(ctx, request.RoomID)
 	if err != nil {
 		return
 	}