12 files changed, 498 insertions, 398 deletions
diff --git a/clientapi/jsonerror/jsonerror.go b/clientapi/jsonerror/jsonerror.go
index 735de5be..85e887ae 100644
--- a/clientapi/jsonerror/jsonerror.go
+++ b/clientapi/jsonerror/jsonerror.go
@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"net/http"
 
+	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 )
 
@@ -124,6 +125,12 @@ func GuestAccessForbidden(msg string) *MatrixError {
 	return &MatrixError{"M_GUEST_ACCESS_FORBIDDEN", msg}
 }
 
+// IncompatibleRoomVersion is an error which is returned when the client
+// requests a room with a version that is unsupported.
+func IncompatibleRoomVersion(roomVersion gomatrixserverlib.RoomVersion) *MatrixError {
+	return &MatrixError{"M_INCOMPATIBLE_ROOM_VERSION", string(roomVersion)}
+}
+
 // UnsupportedRoomVersion is an error which is returned when the client
 // requests a room with a version that is unsupported.
 func UnsupportedRoomVersion(msg string) *MatrixError {
diff --git a/clientapi/routing/joinroom.go b/clientapi/routing/joinroom.go
index df83c2a9..48e42214 100644
--- a/clientapi/routing/joinroom.go
+++ b/clientapi/routing/joinroom.go
@@ -15,332 +15,48 @@
 package routing
 
 import (
-	"fmt"
 	"net/http"
-	"strings"
-	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
-	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
 	"github.com/matrix-org/dendrite/clientapi/httputil"
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
-	"github.com/matrix-org/dendrite/clientapi/producers"
-	"github.com/matrix-org/dendrite/common"
-	"github.com/matrix-org/dendrite/common/config"
-	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"
 )
 
-// JoinRoomByIDOrAlias implements the "/join/{roomIDOrAlias}" API.
-// https://matrix.org/docs/spec/client_server/r0.2.0.html#post-matrix-client-r0-join-roomidoralias
 func JoinRoomByIDOrAlias(
 	req *http.Request,
 	device *authtypes.Device,
-	roomIDOrAlias string,
-	cfg *config.Dendrite,
-	federation *gomatrixserverlib.FederationClient,
-	producer *producers.RoomserverProducer,
 	rsAPI roomserverAPI.RoomserverInternalAPI,
-	fsAPI federationSenderAPI.FederationSenderInternalAPI,
-	keyRing gomatrixserverlib.KeyRing,
-	accountDB accounts.Database,
+	roomIDOrAlias string,
 ) util.JSONResponse {
-	var content map[string]interface{} // must be a JSON object
-	if resErr := httputil.UnmarshalJSONRequest(req, &content); resErr != nil {
-		return *resErr
-	}
-
-	evTime, err := httputil.ParseTSParam(req)
-	if err != nil {
-		return util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.InvalidArgumentValue(err.Error()),
-		}
-	}
-
-	localpart, _, err := gomatrixserverlib.SplitID('@', device.UserID)
-	if err != nil {
-		util.GetLogger(req.Context()).WithError(err).Error("gomatrixserverlib.SplitID failed")
-		return jsonerror.InternalServerError()
-	}
-
-	profile, err := accountDB.GetProfileByLocalpart(req.Context(), localpart)
-	if err != nil {
-		util.GetLogger(req.Context()).WithError(err).Error("accountDB.GetProfileByLocalpart failed")
-		return jsonerror.InternalServerError()
-	}
-
-	content["membership"] = gomatrixserverlib.Join
-	content["displayname"] = profile.DisplayName
-	content["avatar_url"] = profile.AvatarURL
-
-	r := joinRoomReq{
-		req, evTime, content, device.UserID, cfg, federation, producer,
-		rsAPI, fsAPI, keyRing,
-	}
-
-	if strings.HasPrefix(roomIDOrAlias, "!") {
-		return r.joinRoomByID(roomIDOrAlias)
-	}
-	if strings.HasPrefix(roomIDOrAlias, "#") {
-		return r.joinRoomByAlias(roomIDOrAlias)
-	}
-	return util.JSONResponse{
-		Code: http.StatusBadRequest,
-		JSON: jsonerror.BadJSON(
-			fmt.Sprintf("Invalid first character '%s' for room ID or alias",
-				string([]rune(roomIDOrAlias)[0])), // Wrapping with []rune makes this call UTF-8 safe
-		),
-	}
-}
-
-type joinRoomReq struct {
-	req        *http.Request
-	evTime     time.Time
-	content    map[string]interface{}
-	userID     string
-	cfg        *config.Dendrite
-	federation *gomatrixserverlib.FederationClient
-	producer   *producers.RoomserverProducer
-	rsAPI      roomserverAPI.RoomserverInternalAPI
-	fsAPI      federationSenderAPI.FederationSenderInternalAPI
-	keyRing    gomatrixserverlib.KeyRing
-}
-
-// joinRoomByID joins a room by room ID
-func (r joinRoomReq) joinRoomByID(roomID string) util.JSONResponse {
-	// A client should only join a room by room ID when it has an invite
-	// to the room. If the server is already in the room then we can
-	// lookup the invite and process the request as a normal state event.
-	// If the server is not in the room the we will need to look up the
-	// remote server the invite came from in order to request a join event
-	// from that server.
-	queryReq := roomserverAPI.QueryInvitesForUserRequest{
-		RoomID: roomID, TargetUserID: r.userID,
-	}
-	var queryRes roomserverAPI.QueryInvitesForUserResponse
-	if err := r.rsAPI.QueryInvitesForUser(r.req.Context(), &queryReq, &queryRes); err != nil {
-		util.GetLogger(r.req.Context()).WithError(err).Error("r.queryAPI.QueryInvitesForUser failed")
-		return jsonerror.InternalServerError()
+	// Prepare to ask the roomserver to perform the room join.
+	joinReq := roomserverAPI.PerformJoinRequest{
+		RoomIDOrAlias: roomIDOrAlias,
+		UserID:        device.UserID,
 	}
+	joinRes := roomserverAPI.PerformJoinResponse{}
 
-	servers := []gomatrixserverlib.ServerName{}
-	seenInInviterIDs := map[gomatrixserverlib.ServerName]bool{}
-	for _, userID := range queryRes.InviteSenderUserIDs {
-		_, domain, err := gomatrixserverlib.SplitID('@', userID)
-		if err != nil {
-			util.GetLogger(r.req.Context()).WithError(err).Error("gomatrixserverlib.SplitID failed")
-			return jsonerror.InternalServerError()
-		}
-		if !seenInInviterIDs[domain] {
-			servers = append(servers, domain)
-			seenInInviterIDs[domain] = true
-		}
-	}
-
-	// Also add the domain extracted from the roomID as a last resort to join
-	// in case the client is erroneously trying to join by ID without an invite
-	// or all previous attempts at domains extracted from the inviter IDs fail
-	// Note: It's no guarantee we'll succeed because a room isn't bound to the domain in its ID
-	_, domain, err := gomatrixserverlib.SplitID('!', roomID)
-	if err != nil {
-		util.GetLogger(r.req.Context()).WithError(err).Error("gomatrixserverlib.SplitID failed")
-		return jsonerror.InternalServerError()
-	}
-	if domain != r.cfg.Matrix.ServerName && !seenInInviterIDs[domain] {
-		servers = append(servers, domain)
+	// If content was provided in the request then incude that
+	// in the request. It'll get used as a part of the membership
+	// event content.
+	if err := httputil.UnmarshalJSONRequest(req, &joinReq.Content); err != nil {
+		return *err
 	}
 
-	return r.joinRoomUsingServers(roomID, servers)
-
-}
-
-// joinRoomByAlias joins a room using a room alias.
-func (r joinRoomReq) joinRoomByAlias(roomAlias string) util.JSONResponse {
-	_, domain, err := gomatrixserverlib.SplitID('#', roomAlias)
-	if err != nil {
+	// Ask the roomserver to perform the join.
+	if err := rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes); err != nil {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.BadJSON("Room alias must be in the form '#localpart:domain'"),
-		}
-	}
-	if domain == r.cfg.Matrix.ServerName {
-		queryReq := roomserverAPI.GetRoomIDForAliasRequest{Alias: roomAlias}
-		var queryRes roomserverAPI.GetRoomIDForAliasResponse
-		if err = r.rsAPI.GetRoomIDForAlias(r.req.Context(), &queryReq, &queryRes); err != nil {
-			util.GetLogger(r.req.Context()).WithError(err).Error("r.aliasAPI.GetRoomIDForAlias failed")
-			return jsonerror.InternalServerError()
-		}
-
-		if len(queryRes.RoomID) > 0 {
-			return r.joinRoomUsingServers(queryRes.RoomID, []gomatrixserverlib.ServerName{r.cfg.Matrix.ServerName})
-		}
-		// If the response doesn't contain a non-empty string, return an error
-		return util.JSONResponse{
-			Code: http.StatusNotFound,
-			JSON: jsonerror.NotFound("Room alias " + roomAlias + " not found."),
-		}
-	}
-	// If the room isn't local, use federation to join
-	return r.joinRoomByRemoteAlias(domain, roomAlias)
-}
-
-func (r joinRoomReq) joinRoomByRemoteAlias(
-	domain gomatrixserverlib.ServerName, roomAlias string,
-) util.JSONResponse {
-	resp, err := r.federation.LookupRoomAlias(r.req.Context(), domain, roomAlias)
-	if err != nil {
-		switch x := err.(type) {
-		case gomatrix.HTTPError:
-			if x.Code == http.StatusNotFound {
-				return util.JSONResponse{
-					Code: http.StatusNotFound,
-					JSON: jsonerror.NotFound("Room alias not found"),
-				}
-			}
-		}
-		util.GetLogger(r.req.Context()).WithError(err).Error("r.federation.LookupRoomAlias failed")
-		return jsonerror.InternalServerError()
-	}
-
-	return r.joinRoomUsingServers(resp.RoomID, resp.Servers)
-}
-
-func (r joinRoomReq) writeToBuilder(eb *gomatrixserverlib.EventBuilder, roomID string) error {
-	eb.Type = "m.room.member"
-
-	err := eb.SetContent(r.content)
-	if err != nil {
-		return err
-	}
-
-	err = eb.SetUnsigned(struct{}{})
-	if err != nil {
-		return err
-	}
-
-	eb.Sender = r.userID
-	eb.StateKey = &r.userID
-	eb.RoomID = roomID
-	eb.Redacts = ""
-
-	return nil
-}
-
-func (r joinRoomReq) joinRoomUsingServers(
-	roomID string, servers []gomatrixserverlib.ServerName,
-) util.JSONResponse {
-	var eb gomatrixserverlib.EventBuilder
-	err := r.writeToBuilder(&eb, roomID)
-	if err != nil {
-		util.GetLogger(r.req.Context()).WithError(err).Error("r.writeToBuilder failed")
-		return jsonerror.InternalServerError()
-	}
-
-	queryRes := roomserverAPI.QueryLatestEventsAndStateResponse{}
-	event, err := common.BuildEvent(r.req.Context(), &eb, r.cfg, r.evTime, r.rsAPI, &queryRes)
-	if err == nil {
-		// If we have successfully built an event at this point then we can
-		// assert that the room is a local room, as BuildEvent was able to
-		// add prev_events etc successfully.
-		if _, err = r.producer.SendEvents(
-			r.req.Context(),
-			[]gomatrixserverlib.HeaderedEvent{
-				(*event).Headered(queryRes.RoomVersion),
-			},
-			r.cfg.Matrix.ServerName,
-			nil,
-		); err != nil {
-			util.GetLogger(r.req.Context()).WithError(err).Error("r.producer.SendEvents failed")
-			return jsonerror.InternalServerError()
+			JSON: jsonerror.Unknown(err.Error()),
 		}
-		return util.JSONResponse{
-			Code: http.StatusOK,
-			JSON: struct {
-				RoomID string `json:"room_id"`
-			}{roomID},
-		}
-	}
-
-	// Otherwise, if we've reached here, then we haven't been able to populate
-	// prev_events etc for the room, therefore the room is probably federated.
-
-	// TODO: This needs to be re-thought, as in the case of an invite, the room
-	// will exist in the database in roomserver_rooms but won't have any state
-	// events, therefore this below check fails.
-	if err != common.ErrRoomNoExists {
-		util.GetLogger(r.req.Context()).WithError(err).Error("common.BuildEvent failed")
-		return jsonerror.InternalServerError()
-	}
-
-	if len(servers) == 0 {
-		return util.JSONResponse{
-			Code: http.StatusNotFound,
-			JSON: jsonerror.NotFound("No candidate servers found for room"),
-		}
-	}
-
-	var lastErr error
-	for _, server := range servers {
-		var response *util.JSONResponse
-		response, lastErr = r.joinRoomUsingServer(roomID, server)
-		if lastErr != nil {
-			// There was a problem talking to one of the servers.
-			util.GetLogger(r.req.Context()).WithError(lastErr).WithField("server", server).Warn("Failed to join room using server")
-			// Try the next server.
-			if r.req.Context().Err() != nil {
-				// The request context has expired so don't bother trying any
-				// more servers - they will immediately fail due to the expired
-				// context.
-				break
-			} else {
-				// The request context hasn't expired yet so try the next server.
-				continue
-			}
-		}
-		return *response
-	}
-
-	// Every server we tried to join through resulted in an error.
-	// We return the error from the last server.
-
-	// TODO: Generate the correct HTTP status code for all different
-	// kinds of errors that could have happened.
-	// The possible errors include:
-	//   1) We can't connect to the remote servers.
-	//   2) None of the servers we could connect to think we are allowed
-	//	    to join the room.
-	//   3) The remote server returned something invalid.
-	//   4) We couldn't fetch the public keys needed to verify the
-	//      signatures on the state events.
-	//   5) ...
-	util.GetLogger(r.req.Context()).WithError(lastErr).Error("failed to join through any server")
-	return jsonerror.InternalServerError()
-}
-
-// joinRoomUsingServer tries to join a remote room using a given matrix server.
-// If there was a failure communicating with the server or the response from the
-// server was invalid this returns an error.
-// Otherwise this returns a JSONResponse.
-func (r joinRoomReq) joinRoomUsingServer(roomID string, server gomatrixserverlib.ServerName) (*util.JSONResponse, error) {
-	fedJoinReq := federationSenderAPI.PerformJoinRequest{
-		RoomID:     roomID,
-		UserID:     r.userID,
-		ServerName: server,
-	}
-	fedJoinRes := federationSenderAPI.PerformJoinResponse{}
-	if err := r.fsAPI.PerformJoin(r.req.Context(), &fedJoinReq, &fedJoinRes); err != nil {
-		return nil, err
 	}
 
-	return &util.JSONResponse{
+	return util.JSONResponse{
 		Code: http.StatusOK,
 		// TODO: Put the response struct somewhere common.
 		JSON: struct {
 			RoomID string `json:"room_id"`
-		}{roomID},
-	}, nil
+		}{joinReq.RoomIDOrAlias},
+	}
 }
diff --git a/clientapi/routing/routing.go b/clientapi/routing/routing.go
index 42b391de..3ceefa07 100644
--- a/clientapi/routing/routing.go
+++ b/clientapi/routing/routing.go
@@ -100,8 +100,7 @@ func Setup(
 				return util.ErrorResponse(err)
 			}
 			return JoinRoomByIDOrAlias(
-				req, device, vars["roomIDOrAlias"], cfg, federation, producer,
-				rsAPI, federationSender, keyRing, accountDB,
+				req, device, rsAPI, vars["roomIDOrAlias"],
 			)
 		}),
 	).Methods(http.MethodPost, http.MethodOptions)
diff --git a/federationapi/routing/join.go b/federationapi/routing/join.go
index be5e988a..6cadbd75 100644
--- a/federationapi/routing/join.go
+++ b/federationapi/routing/join.go
@@ -61,9 +61,7 @@ func MakeJoin(
 	if !remoteSupportsVersion {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.UnsupportedRoomVersion(
-				fmt.Sprintf("Joining server does not support room version %s", verRes.RoomVersion),
-			),
+			JSON: jsonerror.IncompatibleRoomVersion(verRes.RoomVersion),
 		}
 	}
 
@@ -132,6 +130,9 @@ func MakeJoin(
 }
 
 // SendJoin implements the /send_join API
+// The make-join send-join dance makes much more sense as a single
+// flow so the cyclomatic complexity is high:
+// nolint:gocyclo
 func SendJoin(
 	httpReq *http.Request,
 	request *gomatrixserverlib.FederationRequest,
@@ -159,6 +160,16 @@ func SendJoin(
 		}
 	}
 
+	// Check that a state key is provided.
+	if event.StateKey() == nil || (event.StateKey() != nil && *event.StateKey() == "") {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.BadJSON(
+				fmt.Sprintf("No state key was provided in the join event."),
+			),
+		}
+	}
+
 	// Check that the room ID is correct.
 	if event.RoomID() != roomID {
 		return util.JSONResponse{
@@ -234,20 +245,34 @@ func SendJoin(
 		}
 	}
 
+	// Check if the user is already in the room. If they're already in then
+	// there isn't much point in sending another join event into the room.
+	alreadyJoined := false
+	for _, se := range stateAndAuthChainResponse.StateEvents {
+		if membership, merr := se.Membership(); merr == nil {
+			if se.StateKey() != nil && *se.StateKey() == *event.StateKey() {
+				alreadyJoined = (membership == "join")
+				break
+			}
+		}
+	}
+
 	// Send the events to the room server.
 	// We are responsible for notifying other servers that the user has joined
 	// the room, so set SendAsServer to cfg.Matrix.ServerName
-	_, err = producer.SendEvents(
-		httpReq.Context(),
-		[]gomatrixserverlib.HeaderedEvent{
-			event.Headered(stateAndAuthChainResponse.RoomVersion),
-		},
-		cfg.Matrix.ServerName,
-		nil,
-	)
-	if err != nil {
-		util.GetLogger(httpReq.Context()).WithError(err).Error("producer.SendEvents failed")
-		return jsonerror.InternalServerError()
+	if !alreadyJoined {
+		_, err = producer.SendEvents(
+			httpReq.Context(),
+			[]gomatrixserverlib.HeaderedEvent{
+				event.Headered(stateAndAuthChainResponse.RoomVersion),
+			},
+			cfg.Matrix.ServerName,
+			nil,
+		)
+		if err != nil {
+			util.GetLogger(httpReq.Context()).WithError(err).Error("producer.SendEvents failed")
+			return jsonerror.InternalServerError()
+		}
 	}
 
 	return util.JSONResponse{
diff --git a/federationsender/api/api.go b/federationsender/api/api.go
index 10dc66da..678f02e6 100644
--- a/federationsender/api/api.go
+++ b/federationsender/api/api.go
@@ -8,6 +8,12 @@ import (
 
 // FederationSenderInternalAPI is used to query information from the federation sender.
 type FederationSenderInternalAPI interface {
+	// PerformDirectoryLookup looks up a remote room ID from a room alias.
+	PerformDirectoryLookup(
+		ctx context.Context,
+		request *PerformDirectoryLookupRequest,
+		response *PerformDirectoryLookupResponse,
+	) error
 	// Query the joined hosts and the membership events accounting for their participation in a room.
 	// Note that if a server has multiple users in the room, it will have multiple entries in the returned slice.
 	// See `QueryJoinedHostServerNamesInRoom` for a de-duplicated version.
diff --git a/federationsender/api/perform.go b/federationsender/api/perform.go
index 87736f29..a7b12adc 100644
--- a/federationsender/api/perform.go
+++ b/federationsender/api/perform.go
@@ -4,23 +4,50 @@ import (
 	"context"
 
 	commonHTTP "github.com/matrix-org/dendrite/common/http"
+	"github.com/matrix-org/dendrite/federationsender/types"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/opentracing/opentracing-go"
 )
 
 const (
 	// FederationSenderPerformJoinRequestPath is the HTTP path for the PerformJoinRequest API.
+	FederationSenderPerformDirectoryLookupRequestPath = "/api/federationsender/performDirectoryLookup"
+
+	// FederationSenderPerformJoinRequestPath is the HTTP path for the PerformJoinRequest API.
 	FederationSenderPerformJoinRequestPath = "/api/federationsender/performJoinRequest"
 
 	// FederationSenderPerformLeaveRequestPath is the HTTP path for the PerformLeaveRequest API.
 	FederationSenderPerformLeaveRequestPath = "/api/federationsender/performLeaveRequest"
 )
 
-type PerformJoinRequest struct {
-	RoomID     string                       `json:"room_id"`
-	UserID     string                       `json:"user_id"`
+type PerformDirectoryLookupRequest struct {
+	RoomAlias  string                       `json:"room_alias"`
 	ServerName gomatrixserverlib.ServerName `json:"server_name"`
-	Content    map[string]interface{}       `json:"content"`
+}
+
+type PerformDirectoryLookupResponse struct {
+	RoomID      string                         `json:"room_id"`
+	ServerNames []gomatrixserverlib.ServerName `json:"server_names"`
+}
+
+// Handle an instruction to make_join & send_join with a remote server.
+func (h *httpFederationSenderInternalAPI) PerformDirectoryLookup(
+	ctx context.Context,
+	request *PerformDirectoryLookupRequest,
+	response *PerformDirectoryLookupResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "PerformDirectoryLookup")
+	defer span.Finish()
+
+	apiURL := h.federationSenderURL + FederationSenderPerformDirectoryLookupRequestPath
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}
+
+type PerformJoinRequest struct {
+	RoomID      string                 `json:"room_id"`
+	UserID      string                 `json:"user_id"`
+	ServerNames types.ServerNames      `json:"server_names"`
+	Content     map[string]interface{} `json:"content"`
 }
 
 type PerformJoinResponse struct {
diff --git a/federationsender/internal/perform.go b/federationsender/internal/perform.go
index 961d8027..161b689e 100644
--- a/federationsender/internal/perform.go
+++ b/federationsender/internal/perform.go
@@ -9,8 +9,29 @@ import (
 	"github.com/matrix-org/dendrite/federationsender/internal/perform"
 	"github.com/matrix-org/dendrite/roomserver/version"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
 )
 
+// PerformLeaveRequest implements api.FederationSenderInternalAPI
+func (r *FederationSenderInternalAPI) PerformDirectoryLookup(
+	ctx context.Context,
+	request *api.PerformDirectoryLookupRequest,
+	response *api.PerformDirectoryLookupResponse,
+) (err error) {
+	dir, err := r.federation.LookupRoomAlias(
+		ctx,
+		request.ServerName,
+		request.RoomAlias,
+	)
+	if err != nil {
+		return err
+	}
+	response.RoomID = dir.RoomID
+	response.ServerNames = dir.Servers
+	return nil
+}
+
 // PerformJoinRequest implements api.FederationSenderInternalAPI
 func (r *FederationSenderInternalAPI) PerformJoin(
 	ctx context.Context,
@@ -23,91 +44,107 @@ func (r *FederationSenderInternalAPI) PerformJoin(
 		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)
-	}
+	// Deduplicate the server names we were provided.
+	util.Unique(request.ServerNames)
 
-	// 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)
-	}
+	// Try each server that we were provided until we land on one that
+	// successfully completes the make-join send-join dance.
+	for _, serverName := range request.ServerNames {
+		// Try to perform a make_join using the information supplied in the
+		// request.
+		respMakeJoin, err := r.federation.MakeJoin(
+			ctx,
+			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)
+		}
 
-	// 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)
-	}
+		// 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 = gomatrixserverlib.MRoomMember
+		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)
+		}
 
-	// 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)
-	}
+		// 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)
+		}
 
-	// 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)
-	}
+		// 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)
+		}
 
-	// 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)
-	}
+		// Try to perform a send_join using the newly built event.
+		respSendJoin, err := r.federation.SendJoin(
+			ctx,
+			serverName,
+			event,
+			respMakeJoin.RoomVersion,
+		)
+		if err != nil {
+			logrus.WithError(err).Warnf("r.federation.SendJoin failed")
+			continue
+		}
 
-	// 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)
+		// Check that the send_join response was valid.
+		joinCtx := perform.JoinContext(r.federation, r.keyRing)
+		if err = joinCtx.CheckSendJoinResponse(
+			ctx, event, serverName, respMakeJoin, respSendJoin,
+		); err != nil {
+			logrus.WithError(err).Warnf("joinCtx.CheckSendJoinResponse failed")
+			continue
+		}
+
+		// 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 {
+			logrus.WithError(err).Warnf("r.producer.SendEventWithState failed")
+			continue
+		}
+
+		// We're all good.
+		return nil
 	}
 
-	// Everything went to plan.
-	return nil
+	// If we reach here then we didn't complete a join for some reason.
+	return fmt.Errorf(
+		"failed to join user %q to room %q through %d server(s)",
+		request.UserID, request.RoomID, len(request.ServerNames),
+	)
 }
 
 // PerformLeaveRequest implements api.FederationSenderInternalAPI
diff --git a/federationsender/types/types.go b/federationsender/types/types.go
index 05ba92f7..398d3267 100644
--- a/federationsender/types/types.go
+++ b/federationsender/types/types.go
@@ -28,6 +28,12 @@ type JoinedHost struct {
 	ServerName gomatrixserverlib.ServerName
 }
 
+type ServerNames []gomatrixserverlib.ServerName
+
+func (s ServerNames) Len() int           { return len(s) }
+func (s ServerNames) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
+func (s ServerNames) Less(i, j int) bool { return s[i] < s[j] }
+
 // A EventIDMismatchError indicates that we have got out of sync with the
 // room server.
 type EventIDMismatchError struct {
diff --git a/roomserver/api/api.go b/roomserver/api/api.go
index c12dbddd..ae4beab2 100644
--- a/roomserver/api/api.go
+++ b/roomserver/api/api.go
@@ -18,6 +18,12 @@ type RoomserverInternalAPI interface {
 		response *InputRoomEventsResponse,
 	) error
 
+	PerformJoin(
+		ctx context.Context,
+		req *PerformJoinRequest,
+		res *PerformJoinResponse,
+	) error
+
 	// Query the latest events and state for a room from the room server.
 	QueryLatestEventsAndState(
 		ctx context.Context,
diff --git a/roomserver/api/perform.go b/roomserver/api/perform.go
new file mode 100644
index 00000000..e60c078b
--- /dev/null
+++ b/roomserver/api/perform.go
@@ -0,0 +1,59 @@
+package api
+
+import (
+	"context"
+
+	commonHTTP "github.com/matrix-org/dendrite/common/http"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/opentracing/opentracing-go"
+)
+
+const (
+	// RoomserverPerformJoinPath is the HTTP path for the PerformJoin API.
+	RoomserverPerformJoinPath = "/api/roomserver/performJoin"
+
+	// RoomserverPerformLeavePath is the HTTP path for the PerformLeave API.
+	RoomserverPerformLeavePath = "/api/roomserver/performLeave"
+)
+
+type PerformJoinRequest struct {
+	RoomIDOrAlias string                         `json:"room_id_or_alias"`
+	UserID        string                         `json:"user_id"`
+	Content       map[string]interface{}         `json:"content"`
+	ServerNames   []gomatrixserverlib.ServerName `json:"server_names"`
+}
+
+type PerformJoinResponse struct {
+}
+
+func (h *httpRoomserverInternalAPI) PerformJoin(
+	ctx context.Context,
+	request *PerformJoinRequest,
+	response *PerformJoinResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "PerformJoin")
+	defer span.Finish()
+
+	apiURL := h.roomserverURL + RoomserverPerformJoinPath
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}
+
+type PerformLeaveRequest struct {
+	RoomID string `json:"room_id"`
+	UserID string `json:"user_id"`
+}
+
+type PerformLeaveResponse struct {
+}
+
+func (h *httpRoomserverInternalAPI) PerformLeave(
+	ctx context.Context,
+	request *PerformLeaveRequest,
+	response *PerformLeaveResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "PerformLeave")
+	defer span.Finish()
+
+	apiURL := h.roomserverURL + RoomserverPerformLeavePath
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}
diff --git a/roomserver/internal/api.go b/roomserver/internal/api.go
index d1c443f2..1dc985ef 100644
--- a/roomserver/internal/api.go
+++ b/roomserver/internal/api.go
@@ -46,6 +46,19 @@ func (r *RoomserverInternalAPI) SetupHTTP(servMux *http.ServeMux) {
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),
 	)
+	servMux.Handle(api.RoomserverPerformJoinPath,
+		common.MakeInternalAPI("performJoin", func(req *http.Request) util.JSONResponse {
+			var request api.PerformJoinRequest
+			var response api.PerformJoinResponse
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.MessageResponse(http.StatusBadRequest, err.Error())
+			}
+			if err := r.PerformJoin(req.Context(), &request, &response); err != nil {
+				return util.ErrorResponse(err)
+			}
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
 	servMux.Handle(
 		api.RoomserverQueryLatestEventsAndStatePath,
 		common.MakeInternalAPI("queryLatestEventsAndState", func(req *http.Request) util.JSONResponse {
diff --git a/roomserver/internal/perform_join.go b/roomserver/internal/perform_join.go
new file mode 100644
index 00000000..3dfa118f
--- /dev/null
+++ b/roomserver/internal/perform_join.go
@@ -0,0 +1,199 @@
+package internal
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/matrix-org/dendrite/common"
+	fsAPI "github.com/matrix-org/dendrite/federationsender/api"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/sirupsen/logrus"
+)
+
+// WriteOutputEvents implements OutputRoomEventWriter
+func (r *RoomserverInternalAPI) PerformJoin(
+	ctx context.Context,
+	req *api.PerformJoinRequest,
+	res *api.PerformJoinResponse,
+) error {
+	_, domain, err := gomatrixserverlib.SplitID('@', req.UserID)
+	if err != nil {
+		return fmt.Errorf("Supplied user ID %q in incorrect format", req.UserID)
+	}
+	if domain != r.Cfg.Matrix.ServerName {
+		return fmt.Errorf("User %q does not belong to this homeserver", req.UserID)
+	}
+	if strings.HasPrefix(req.RoomIDOrAlias, "!") {
+		return r.performJoinRoomByID(ctx, req, res)
+	}
+	if strings.HasPrefix(req.RoomIDOrAlias, "#") {
+		return r.performJoinRoomByAlias(ctx, req, res)
+	}
+	return fmt.Errorf("Room ID or alias %q is invalid", req.RoomIDOrAlias)
+}
+
+func (r *RoomserverInternalAPI) performJoinRoomByAlias(
+	ctx context.Context,
+	req *api.PerformJoinRequest,
+	res *api.PerformJoinResponse,
+) 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)
+	}
+	req.ServerNames = append(req.ServerNames, domain)
+
+	// Check if this alias matches our own server configuration. If it
+	// doesn't then we'll need to try a federated join.
+	var roomID string
+	if domain != r.Cfg.Matrix.ServerName {
+		// The alias isn't owned by us, so we will need to try joining using
+		// a remote server.
+		dirReq := fsAPI.PerformDirectoryLookupRequest{
+			RoomAlias:  req.RoomIDOrAlias, // the room alias to lookup
+			ServerName: domain,            // the server to ask
+		}
+		dirRes := fsAPI.PerformDirectoryLookupResponse{}
+		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)
+		}
+		roomID = dirRes.RoomID
+		req.ServerNames = append(req.ServerNames, dirRes.ServerNames...)
+	} else {
+		// 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)
+		}
+	}
+
+	// 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)
+	}
+
+	// If we do, then pluck out the room ID and continue the join.
+	req.RoomIDOrAlias = roomID
+	return r.performJoinRoomByID(ctx, req, res)
+}
+
+// TODO: Break this function up a bit
+// nolint:gocyclo
+func (r *RoomserverInternalAPI) performJoinRoomByID(
+	ctx context.Context,
+	req *api.PerformJoinRequest,
+	res *api.PerformJoinResponse, // nolint:unparam
+) error {
+	// Get the domain part of the room ID.
+	_, domain, err := gomatrixserverlib.SplitID('!', req.RoomIDOrAlias)
+	if err != nil {
+		return fmt.Errorf("Room ID %q is invalid", req.RoomIDOrAlias)
+	}
+	req.ServerNames = append(req.ServerNames, domain)
+
+	// Prepare the template for the join event.
+	userID := req.UserID
+	eb := gomatrixserverlib.EventBuilder{
+		Type:     gomatrixserverlib.MRoomMember,
+		Sender:   userID,
+		StateKey: &userID,
+		RoomID:   req.RoomIDOrAlias,
+		Redacts:  "",
+	}
+	if err = eb.SetUnsigned(struct{}{}); err != nil {
+		return fmt.Errorf("eb.SetUnsigned: %w", err)
+	}
+
+	// It is possible for the request to include some "content" for the
+	// event. We'll always overwrite the "membership" key, but the rest,
+	// like "display_name" or "avatar_url", will be kept if supplied.
+	if req.Content == nil {
+		req.Content = map[string]interface{}{}
+	}
+	req.Content["membership"] = "join"
+	if err = eb.SetContent(req.Content); err != nil {
+		return fmt.Errorf("eb.SetContent: %w", err)
+	}
+
+	// Try to construct an actual join event from the template.
+	// If this succeeds then it is a sign that the room already exists
+	// locally on the homeserver.
+	// TODO: Check what happens if the room exists on the server
+	// but everyone has since left. I suspect it does the wrong thing.
+	buildRes := api.QueryLatestEventsAndStateResponse{}
+	event, err := common.BuildEvent(
+		ctx,        // the request context
+		&eb,        // the template join event
+		r.Cfg,      // the server configuration
+		time.Now(), // the event timestamp to use
+		r,          // the roomserver API to use
+		&buildRes,  // the query response
+	)
+
+	switch err {
+	case nil:
+		// The room join is local. Send the new join event into the
+		// roomserver. First of all check that the user isn't already
+		// a member of the room.
+		alreadyJoined := false
+		for _, se := range buildRes.StateEvents {
+			if membership, merr := se.Membership(); merr == nil {
+				if se.StateKey() != nil && *se.StateKey() == *event.StateKey() {
+					alreadyJoined = (membership == "join")
+					break
+				}
+			}
+		}
+
+		// If we haven't already joined the room then send an event
+		// into the room changing our membership status.
+		if !alreadyJoined {
+			inputReq := api.InputRoomEventsRequest{
+				InputRoomEvents: []api.InputRoomEvent{
+					api.InputRoomEvent{
+						Kind:         api.KindNew,
+						Event:        event.Headered(buildRes.RoomVersion),
+						AuthEventIDs: event.AuthEventIDs(),
+						SendAsServer: string(r.Cfg.Matrix.ServerName),
+					},
+				},
+			}
+			inputRes := api.InputRoomEventsResponse{}
+			if err = r.InputRoomEvents(ctx, &inputReq, &inputRes); err != nil {
+				return fmt.Errorf("r.InputRoomEvents: %w", err)
+			}
+		}
+
+	case common.ErrRoomNoExists:
+		// The room doesn't exist. First of all check if the room is a local
+		// room. If it is then there's nothing more to do - the room just
+		// hasn't been created yet.
+		if domain == r.Cfg.Matrix.ServerName {
+			return fmt.Errorf("Room ID %q does not exist", req.RoomIDOrAlias)
+		}
+
+		// Try joining by all of the supplied server names.
+		fedReq := fsAPI.PerformJoinRequest{
+			RoomID:      req.RoomIDOrAlias, // the room ID to try and join
+			UserID:      req.UserID,        // the user ID joining the room
+			ServerNames: req.ServerNames,   // the server to try joining with
+			Content:     req.Content,       // the membership event content
+		}
+		fedRes := fsAPI.PerformJoinResponse{}
+		err = r.fsAPI.PerformJoin(ctx, &fedReq, &fedRes)
+		if err != nil {
+			return fmt.Errorf("Error joining federated room: %q", err)
+		}
+
+	default:
+		return fmt.Errorf("Error joining room %q: %w", req.RoomIDOrAlias, err)
+	}
+
+	return nil
+}