Add PerformInvite and refactor how errors get handled (#1158)

* Add PerformInvite and refactor how errors get handled

- Rename `JoinError` to `PerformError`
- Remove `error` from the API function signature entirely. This forces
  errors to be bundled into `PerformError` which makes it easier for callers
  to detect and handle errors. On network errors, HTTP clients will make a
  `PerformError`.

* Unbreak everything; thanks Go!

* Send back JSONResponse according to the PerformError

* Update federation invite code too

16 files changed, 469 insertions, 332 deletions

diff --git a/clientapi/routing/createroom.go b/clientapi/routing/createroom.go
index be712482..8682b03a 100644
--- a/clientapi/routing/createroom.go
+++ b/clientapi/routing/createroom.go
@@ -403,15 +403,15 @@ func createRoom(
 			}
 		}
 		// Send the invite event to the roomserver.
-		if err = roomserverAPI.SendInvite(
+		if perr := roomserverAPI.SendInvite(
 			req.Context(), rsAPI,
 			inviteEvent.Headered(roomVersion),
 			strippedState,         // invite room state
 			cfg.Matrix.ServerName, // send as server
 			nil,                   // transaction ID
-		); err != nil {
-			util.GetLogger(req.Context()).WithError(err).Error("SendInvite failed")
-			return jsonerror.InternalServerError()
+		); perr != nil {
+			util.GetLogger(req.Context()).WithError(perr).Error("SendInvite failed")
+			return perr.JSONResponse()
 		}
 	}
 
diff --git a/clientapi/routing/joinroom.go b/clientapi/routing/joinroom.go
index e606e35f..cb68fe19 100644
--- a/clientapi/routing/joinroom.go
+++ b/clientapi/routing/joinroom.go
@@ -15,12 +15,10 @@
 package routing
 
 import (
-	"errors"
 	"net/http"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
 	"github.com/matrix-org/dendrite/clientapi/httputil"
-	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/userapi/api"
 	"github.com/matrix-org/dendrite/userapi/storage/accounts"
@@ -65,32 +63,9 @@ func JoinRoomByIDOrAlias(
 	}
 
 	// Ask the roomserver to perform the join.
-	err = rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes)
-	// Handle known errors first, if this is 0 then there will be no matches (eg on success)
-	switch joinRes.Error {
-	case roomserverAPI.JoinErrorBadRequest:
-		return util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.Unknown(joinRes.ErrMsg),
-		}
-	case roomserverAPI.JoinErrorNoRoom:
-		return util.JSONResponse{
-			Code: http.StatusNotFound,
-			JSON: jsonerror.NotFound(joinRes.ErrMsg),
-		}
-	case roomserverAPI.JoinErrorNotAllowed:
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: jsonerror.Forbidden(joinRes.ErrMsg),
-		}
-	}
-	// this is always populated on generic errors
-	if joinRes.ErrMsg != "" {
-		return util.ErrorResponse(errors.New(joinRes.ErrMsg))
-	}
-	// this is set on network errors in polylith mode
-	if err != nil {
-		return util.ErrorResponse(err)
+	rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes)
+	if joinRes.Error != nil {
+		return joinRes.Error.JSONResponse()
 	}
 
 	return util.JSONResponse{
diff --git a/clientapi/routing/membership.go b/clientapi/routing/membership.go
index 0d4b0d88..aff1730c 100644
--- a/clientapi/routing/membership.go
+++ b/clientapi/routing/membership.go
@@ -111,16 +111,16 @@ func SendMembership(
 	switch membership {
 	case gomatrixserverlib.Invite:
 		// Invites need to be handled specially
-		err = roomserverAPI.SendInvite(
+		perr := roomserverAPI.SendInvite(
 			req.Context(), rsAPI,
 			event.Headered(verRes.RoomVersion),
 			nil, // ask the roomserver to draw up invite room state for us
 			cfg.Matrix.ServerName,
 			nil,
 		)
-		if err != nil {
-			util.GetLogger(req.Context()).WithError(err).Error("producer.SendInvite failed")
-			return jsonerror.InternalServerError()
+		if perr != nil {
+			util.GetLogger(req.Context()).WithError(perr).Error("producer.SendInvite failed")
+			return perr.JSONResponse()
 		}
 	case gomatrixserverlib.Join:
 		// The join membership requires the room id to be sent in the response
diff --git a/federationapi/routing/invite.go b/federationapi/routing/invite.go
index 7d02bc1d..b1d84f25 100644
--- a/federationapi/routing/invite.go
+++ b/federationapi/routing/invite.go
@@ -98,15 +98,15 @@ func Invite(
 	)
 
 	// Add the invite event to the roomserver.
-	if err = api.SendInvite(
+	if perr := api.SendInvite(
 		httpReq.Context(), rsAPI,
 		signedEvent.Headered(inviteReq.RoomVersion()),
 		inviteReq.InviteRoomState(),
 		event.Origin(),
 		nil,
-	); err != nil {
+	); perr != nil {
 		util.GetLogger(httpReq.Context()).WithError(err).Error("producer.SendInvite failed")
-		return jsonerror.InternalServerError()
+		return perr.JSONResponse()
 	}
 
 	// Return the signed event to the originating server, it should then tell
diff --git a/federationapi/routing/send_test.go b/federationapi/routing/send_test.go
index e512f4b4..3f5d5f4e 100644
--- a/federationapi/routing/send_test.go
+++ b/federationapi/routing/send_test.go
@@ -97,12 +97,18 @@ func (t *testRoomserverAPI) InputRoomEvents(
 	return nil
 }
 
+func (t *testRoomserverAPI) PerformInvite(
+	ctx context.Context,
+	req *api.PerformInviteRequest,
+	res *api.PerformInviteResponse,
+) {
+}
+
 func (t *testRoomserverAPI) PerformJoin(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
 	res *api.PerformJoinResponse,
-) error {
-	return nil
+) {
 }
 
 func (t *testRoomserverAPI) PerformLeave(
diff --git a/roomserver/api/api.go b/roomserver/api/api.go
index 967f58ba..26ec8ca1 100644
--- a/roomserver/api/api.go
+++ b/roomserver/api/api.go
@@ -18,11 +18,17 @@ type RoomserverInternalAPI interface {
 		response *InputRoomEventsResponse,
 	) error
 
+	PerformInvite(
+		ctx context.Context,
+		req *PerformInviteRequest,
+		res *PerformInviteResponse,
+	)
+
 	PerformJoin(
 		ctx context.Context,
 		req *PerformJoinRequest,
 		res *PerformJoinResponse,
-	) error
+	)
 
 	PerformLeave(
 		ctx context.Context,
diff --git a/roomserver/api/api_trace.go b/roomserver/api/api_trace.go
index a478eeb9..8645b6f2 100644
--- a/roomserver/api/api_trace.go
+++ b/roomserver/api/api_trace.go
@@ -29,14 +29,22 @@ func (t *RoomserverInternalAPITrace) InputRoomEvents(
 	return err
 }
 
+func (t *RoomserverInternalAPITrace) PerformInvite(
+	ctx context.Context,
+	req *PerformInviteRequest,
+	res *PerformInviteResponse,
+) {
+	t.Impl.PerformInvite(ctx, req, res)
+	util.GetLogger(ctx).Infof("PerformInvite req=%+v res=%+v", js(req), js(res))
+}
+
 func (t *RoomserverInternalAPITrace) PerformJoin(
 	ctx context.Context,
 	req *PerformJoinRequest,
 	res *PerformJoinResponse,
-) error {
-	err := t.Impl.PerformJoin(ctx, req, res)
-	util.GetLogger(ctx).WithError(err).Infof("PerformJoin req=%+v res=%+v", js(req), js(res))
-	return err
+) {
+	t.Impl.PerformJoin(ctx, req, res)
+	util.GetLogger(ctx).Infof("PerformJoin req=%+v res=%+v", js(req), js(res))
 }
 
 func (t *RoomserverInternalAPITrace) PerformLeave(
diff --git a/roomserver/api/input.go b/roomserver/api/input.go
index 6c3c8941..05c981df 100644
--- a/roomserver/api/input.go
+++ b/roomserver/api/input.go
@@ -76,21 +76,9 @@ type TransactionID struct {
 	TransactionID string `json:"id"`
 }
 
-// InputInviteEvent is a matrix invite event received over federation without
-// the usual context a matrix room event would have. We usually do not have
-// access to the events needed to check the event auth rules for the invite.
-type InputInviteEvent struct {
-	RoomVersion     gomatrixserverlib.RoomVersion             `json:"room_version"`
-	Event           gomatrixserverlib.HeaderedEvent           `json:"event"`
-	InviteRoomState []gomatrixserverlib.InviteV2StrippedState `json:"invite_room_state"`
-	SendAsServer    string                                    `json:"send_as_server"`
-	TransactionID   *TransactionID                            `json:"transaction_id"`
-}
-
 // InputRoomEventsRequest is a request to InputRoomEvents
 type InputRoomEventsRequest struct {
-	InputRoomEvents   []InputRoomEvent   `json:"input_room_events"`
-	InputInviteEvents []InputInviteEvent `json:"input_invite_events"`
+	InputRoomEvents []InputRoomEvent `json:"input_room_events"`
 }
 
 // InputRoomEventsResponse is a response to InputRoomEvents
diff --git a/roomserver/api/perform.go b/roomserver/api/perform.go
index 0f5394c9..0b8e6df2 100644
--- a/roomserver/api/perform.go
+++ b/roomserver/api/perform.go
@@ -1,19 +1,57 @@
 package api
 
 import (
+	"fmt"
+	"net/http"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 )
 
-type JoinError int
+type PerformErrorCode int
+
+type PerformError struct {
+	Msg  string
+	Code PerformErrorCode
+}
+
+func (p *PerformError) Error() string {
+	return fmt.Sprintf("%d : %s", p.Code, p.Msg)
+}
+
+// JSONResponse maps error codes to suitable HTTP error codes, defaulting to 500.
+func (p *PerformError) JSONResponse() util.JSONResponse {
+	switch p.Code {
+	case PerformErrorBadRequest:
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.Unknown(p.Msg),
+		}
+	case PerformErrorNoRoom:
+		return util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound(p.Msg),
+		}
+	case PerformErrorNotAllowed:
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: jsonerror.Forbidden(p.Msg),
+		}
+	default:
+		return util.ErrorResponse(p)
+	}
+}
 
 const (
-	// JoinErrorNotAllowed means the user is not allowed to join this room (e.g join_rule:invite or banned)
-	JoinErrorNotAllowed JoinError = 1
-	// JoinErrorBadRequest means the request was wrong in some way (invalid user ID, wrong server, etc)
-	JoinErrorBadRequest JoinError = 2
-	// JoinErrorNoRoom means that the room being joined doesn't exist.
-	JoinErrorNoRoom JoinError = 3
+	// PerformErrorNotAllowed means the user is not allowed to invite/join/etc this room (e.g join_rule:invite or banned)
+	PerformErrorNotAllowed PerformErrorCode = 1
+	// PerformErrorBadRequest means the request was wrong in some way (invalid user ID, wrong server, etc)
+	PerformErrorBadRequest PerformErrorCode = 2
+	// PerformErrorNoRoom means that the room being joined doesn't exist.
+	PerformErrorNoRoom PerformErrorCode = 3
+	// PerformErrorNoOperation means that the request resulted in nothing happening e.g invite->invite or leave->leave.
+	PerformErrorNoOperation PerformErrorCode = 4
 )
 
 type PerformJoinRequest struct {
@@ -26,10 +64,8 @@ type PerformJoinRequest struct {
 type PerformJoinResponse struct {
 	// The room ID, populated on success.
 	RoomID string `json:"room_id"`
-	// The reason why the join failed. Can be blank.
-	Error JoinError `json:"error"`
-	// Debugging description of the error. Always present on failure.
-	ErrMsg string `json:"err_msg"`
+	// If non-nil, the join request failed. Contains more information why it failed.
+	Error *PerformError
 }
 
 type PerformLeaveRequest struct {
@@ -40,6 +76,19 @@ type PerformLeaveRequest struct {
 type PerformLeaveResponse struct {
 }
 
+type PerformInviteRequest struct {
+	RoomVersion     gomatrixserverlib.RoomVersion             `json:"room_version"`
+	Event           gomatrixserverlib.HeaderedEvent           `json:"event"`
+	InviteRoomState []gomatrixserverlib.InviteV2StrippedState `json:"invite_room_state"`
+	SendAsServer    string                                    `json:"send_as_server"`
+	TransactionID   *TransactionID                            `json:"transaction_id"`
+}
+
+type PerformInviteResponse struct {
+	// If non-nil, the invite request failed. Contains more information why it failed.
+	Error *PerformError
+}
+
 // PerformBackfillRequest is a request to PerformBackfill.
 type PerformBackfillRequest struct {
 	// The room to backfill
diff --git a/roomserver/api/wrapper.go b/roomserver/api/wrapper.go
index 97940e0c..b73cd190 100644
--- a/roomserver/api/wrapper.go
+++ b/roomserver/api/wrapper.go
@@ -98,16 +98,20 @@ func SendInvite(
 	rsAPI RoomserverInternalAPI, inviteEvent gomatrixserverlib.HeaderedEvent,
 	inviteRoomState []gomatrixserverlib.InviteV2StrippedState,
 	sendAsServer gomatrixserverlib.ServerName, txnID *TransactionID,
-) error {
-	request := InputRoomEventsRequest{
-		InputInviteEvents: []InputInviteEvent{{
-			Event:           inviteEvent,
-			InviteRoomState: inviteRoomState,
-			RoomVersion:     inviteEvent.RoomVersion,
-			SendAsServer:    string(sendAsServer),
-			TransactionID:   txnID,
-		}},
+) *PerformError {
+	request := PerformInviteRequest{
+		Event:           inviteEvent,
+		InviteRoomState: inviteRoomState,
+		RoomVersion:     inviteEvent.RoomVersion,
+		SendAsServer:    string(sendAsServer),
+		TransactionID:   txnID,
 	}
-	var response InputRoomEventsResponse
-	return rsAPI.InputRoomEvents(ctx, &request, &response)
+	var response PerformInviteResponse
+	rsAPI.PerformInvite(ctx, &request, &response)
+	// we need to do this because many places people will use `var err error` as the return
+	// arg and a nil interface != nil pointer to a concrete interface (in this case PerformError)
+	if response.Error != nil && response.Error.Msg != "" {
+		return response.Error
+	}
+	return nil
 }
diff --git a/roomserver/internal/input.go b/roomserver/internal/input.go
index e863af95..2af3e62d 100644
--- a/roomserver/internal/input.go
+++ b/roomserver/internal/input.go
@@ -74,18 +74,6 @@ func (r *RoomserverInternalAPI) InputRoomEvents(
 	// We lock as processRoomEvent can only be called once at a time
 	r.mutex.Lock()
 	defer r.mutex.Unlock()
-	for i := range request.InputInviteEvents {
-		var loopback *api.InputRoomEvent
-		if loopback, err = r.processInviteEvent(ctx, r, request.InputInviteEvents[i]); err != nil {
-			return err
-		}
-		// The processInviteEvent function can optionally return a
-		// loopback room event containing the invite, for local invites.
-		// If it does, we should process it with the room events below.
-		if loopback != nil {
-			request.InputRoomEvents = append(request.InputRoomEvents, *loopback)
-		}
-	}
 	for i := range request.InputRoomEvents {
 		if response.EventID, err = r.processRoomEvent(ctx, request.InputRoomEvents[i]); err != nil {
 			return err
diff --git a/roomserver/internal/input_events.go b/roomserver/internal/input_events.go
index fe3bdf4b..ae57f2e7 100644
--- a/roomserver/internal/input_events.go
+++ b/roomserver/internal/input_events.go
@@ -18,17 +18,12 @@ package internal
 
 import (
 	"context"
-	"errors"
-	"fmt"
 
-	"github.com/matrix-org/dendrite/internal/sqlutil"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/state"
-	"github.com/matrix-org/dendrite/roomserver/storage"
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/sirupsen/logrus"
-	log "github.com/sirupsen/logrus"
 )
 
 // processRoomEvent can only be called once at a time
@@ -148,193 +143,3 @@ func (r *RoomserverInternalAPI) calculateAndSetState(
 	}
 	return r.DB.SetState(ctx, stateAtEvent.EventNID, stateAtEvent.BeforeStateSnapshotNID)
 }
-
-func (r *RoomserverInternalAPI) processInviteEvent(
-	ctx context.Context,
-	ow *RoomserverInternalAPI,
-	input api.InputInviteEvent,
-) (*api.InputRoomEvent, error) {
-	if input.Event.StateKey() == nil {
-		return nil, fmt.Errorf("invite must be a state event")
-	}
-
-	roomID := input.Event.RoomID()
-	targetUserID := *input.Event.StateKey()
-
-	log.WithFields(log.Fields{
-		"event_id":       input.Event.EventID(),
-		"room_id":        roomID,
-		"room_version":   input.RoomVersion,
-		"target_user_id": targetUserID,
-	}).Info("processing invite event")
-
-	_, domain, _ := gomatrixserverlib.SplitID('@', targetUserID)
-	isTargetLocalUser := domain == r.Cfg.Matrix.ServerName
-
-	updater, err := r.DB.MembershipUpdater(ctx, roomID, targetUserID, isTargetLocalUser, input.RoomVersion)
-	if err != nil {
-		return nil, err
-	}
-	succeeded := false
-	defer func() {
-		txerr := sqlutil.EndTransaction(updater, &succeeded)
-		if err == nil && txerr != nil {
-			err = txerr
-		}
-	}()
-
-	if updater.IsJoin() {
-		// If the user is joined to the room then that takes precedence over this
-		// invite event. It makes little sense to move a user that is already
-		// joined to the room into the invite state.
-		// This could plausibly happen if an invite request raced with a join
-		// request for a user. For example if a user was invited to a public
-		// room and they joined the room at the same time as the invite was sent.
-		// The other way this could plausibly happen is if an invite raced with
-		// a kick. For example if a user was kicked from a room in error and in
-		// response someone else in the room re-invited them then it is possible
-		// for the invite request to race with the leave event so that the
-		// target receives invite before it learns that it has been kicked.
-		// There are a few ways this could be plausibly handled in the roomserver.
-		// 1) Store the invite, but mark it as retired. That will result in the
-		//    permanent rejection of that invite event. So even if the target
-		//    user leaves the room and the invite is retransmitted it will be
-		//    ignored. However a new invite with a new event ID would still be
-		//    accepted.
-		// 2) Silently discard the invite event. This means that if the event
-		//    was retransmitted at a later date after the target user had left
-		//    the room we would accept the invite. However since we hadn't told
-		//    the sending server that the invite had been discarded it would
-		//    have no reason to attempt to retry.
-		// 3) Signal the sending server that the user is already joined to the
-		//    room.
-		// For now we will implement option 2. Since in the abesence of a retry
-		// mechanism it will be equivalent to option 1, and we don't have a
-		// signalling mechanism to implement option 3.
-		return nil, nil
-	}
-
-	// Normally, with a federated invite, the federation sender would do
-	// the /v2/invite request (in which the remote server signs the invite)
-	// and then the signed event gets sent back to the roomserver as an input
-	// event. When the invite is local, we don't interact with the federation
-	// sender therefore we need to generate the loopback invite event for
-	// the room ourselves.
-	loopback, err := localInviteLoopback(ow, input)
-	if err != nil {
-		return nil, err
-	}
-
-	event := input.Event.Unwrap()
-	if len(input.InviteRoomState) > 0 {
-		// If we were supplied with some invite room state already (which is
-		// most likely to be if the event came in over federation) then use
-		// that.
-		if err = event.SetUnsignedField("invite_room_state", input.InviteRoomState); err != nil {
-			return nil, err
-		}
-	} else {
-		// There's no invite room state, so let's have a go at building it
-		// up from local data (which is most likely to be if the event came
-		// from the CS API). If we know about the room then we can insert
-		// the invite room state, if we don't then we just fail quietly.
-		if irs, ierr := buildInviteStrippedState(ctx, r.DB, input); ierr == nil {
-			if err = event.SetUnsignedField("invite_room_state", irs); err != nil {
-				return nil, err
-			}
-		}
-	}
-
-	outputUpdates, err := updateToInviteMembership(updater, &event, nil, input.Event.RoomVersion)
-	if err != nil {
-		return nil, err
-	}
-
-	if err = ow.WriteOutputEvents(roomID, outputUpdates); err != nil {
-		return nil, err
-	}
-
-	succeeded = true
-	return loopback, nil
-}
-
-func localInviteLoopback(
-	ow *RoomserverInternalAPI,
-	input api.InputInviteEvent,
-) (ire *api.InputRoomEvent, err error) {
-	if input.Event.StateKey() == nil {
-		return nil, errors.New("no state key on invite event")
-	}
-	ourServerName := string(ow.Cfg.Matrix.ServerName)
-	_, theirServerName, err := gomatrixserverlib.SplitID('@', *input.Event.StateKey())
-	if err != nil {
-		return nil, err
-	}
-	// Check if the invite originated locally and is destined locally.
-	if input.Event.Origin() == ow.Cfg.Matrix.ServerName && string(theirServerName) == ourServerName {
-		rsEvent := input.Event.Sign(
-			ourServerName,
-			ow.Cfg.Matrix.KeyID,
-			ow.Cfg.Matrix.PrivateKey,
-		).Headered(input.RoomVersion)
-		ire = &api.InputRoomEvent{
-			Kind:          api.KindNew,
-			Event:         rsEvent,
-			AuthEventIDs:  rsEvent.AuthEventIDs(),
-			SendAsServer:  ourServerName,
-			TransactionID: nil,
-		}
-	}
-	return ire, nil
-}
-
-func buildInviteStrippedState(
-	ctx context.Context,
-	db storage.Database,
-	input api.InputInviteEvent,
-) ([]gomatrixserverlib.InviteV2StrippedState, error) {
-	roomNID, err := db.RoomNID(ctx, input.Event.RoomID())
-	if err != nil || roomNID == 0 {
-		return nil, fmt.Errorf("room %q unknown", input.Event.RoomID())
-	}
-	stateWanted := []gomatrixserverlib.StateKeyTuple{}
-	// "If they are set on the room, at least the state for m.room.avatar, m.room.canonical_alias, m.room.join_rules, and m.room.name SHOULD be included."
-	// https://matrix.org/docs/spec/client_server/r0.6.0#m-room-member
-	for _, t := range []string{
-		gomatrixserverlib.MRoomName, gomatrixserverlib.MRoomCanonicalAlias,
-		gomatrixserverlib.MRoomAliases, gomatrixserverlib.MRoomJoinRules,
-		"m.room.avatar",
-	} {
-		stateWanted = append(stateWanted, gomatrixserverlib.StateKeyTuple{
-			EventType: t,
-			StateKey:  "",
-		})
-	}
-	_, currentStateSnapshotNID, _, err := db.LatestEventIDs(ctx, roomNID)
-	if err != nil {
-		return nil, err
-	}
-	roomState := state.NewStateResolution(db)
-	stateEntries, err := roomState.LoadStateAtSnapshotForStringTuples(
-		ctx, currentStateSnapshotNID, stateWanted,
-	)
-	if err != nil {
-		return nil, err
-	}
-	stateNIDs := []types.EventNID{}
-	for _, stateNID := range stateEntries {
-		stateNIDs = append(stateNIDs, stateNID.EventNID)
-	}
-	stateEvents, err := db.Events(ctx, stateNIDs)
-	if err != nil {
-		return nil, err
-	}
-	inviteState := []gomatrixserverlib.InviteV2StrippedState{
-		gomatrixserverlib.NewInviteV2StrippedState(&input.Event.Event),
-	}
-	stateEvents = append(stateEvents, types.Event{Event: input.Event.Unwrap()})
-	for _, event := range stateEvents {
-		inviteState = append(inviteState, gomatrixserverlib.NewInviteV2StrippedState(&event.Event))
-	}
-	return inviteState, nil
-}
diff --git a/roomserver/internal/perform_invite.go b/roomserver/internal/perform_invite.go
new file mode 100644
index 00000000..c65c87f9
--- /dev/null
+++ b/roomserver/internal/perform_invite.go
@@ -0,0 +1,249 @@
+package internal
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/roomserver/state"
+	"github.com/matrix-org/dendrite/roomserver/storage"
+	"github.com/matrix-org/dendrite/roomserver/types"
+	"github.com/matrix-org/gomatrixserverlib"
+	log "github.com/sirupsen/logrus"
+)
+
+// PerformInvite handles inviting to matrix rooms, including over federation by talking to the federationsender.
+func (r *RoomserverInternalAPI) PerformInvite(
+	ctx context.Context,
+	req *api.PerformInviteRequest,
+	res *api.PerformInviteResponse,
+) {
+	err := r.performInvite(ctx, req)
+	if err != nil {
+		perr, ok := err.(*api.PerformError)
+		if ok {
+			res.Error = perr
+		} else {
+			res.Error = &api.PerformError{
+				Msg: err.Error(),
+			}
+		}
+	}
+}
+
+func (r *RoomserverInternalAPI) performInvite(ctx context.Context,
+	req *api.PerformInviteRequest,
+) error {
+	loopback, err := r.processInviteEvent(ctx, r, req)
+	if err != nil {
+		return err
+	}
+	// The processInviteEvent function can optionally return a
+	// loopback room event containing the invite, for local invites.
+	// If it does, we should process it with the room events below.
+	if loopback != nil {
+		var loopbackRes api.InputRoomEventsResponse
+		err := r.InputRoomEvents(ctx, &api.InputRoomEventsRequest{
+			InputRoomEvents: []api.InputRoomEvent{*loopback},
+		}, &loopbackRes)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (r *RoomserverInternalAPI) processInviteEvent(
+	ctx context.Context,
+	ow *RoomserverInternalAPI,
+	input *api.PerformInviteRequest,
+) (*api.InputRoomEvent, error) {
+	if input.Event.StateKey() == nil {
+		return nil, fmt.Errorf("invite must be a state event")
+	}
+
+	roomID := input.Event.RoomID()
+	targetUserID := *input.Event.StateKey()
+
+	log.WithFields(log.Fields{
+		"event_id":       input.Event.EventID(),
+		"room_id":        roomID,
+		"room_version":   input.RoomVersion,
+		"target_user_id": targetUserID,
+	}).Info("processing invite event")
+
+	_, domain, _ := gomatrixserverlib.SplitID('@', targetUserID)
+	isTargetLocalUser := domain == r.Cfg.Matrix.ServerName
+
+	updater, err := r.DB.MembershipUpdater(ctx, roomID, targetUserID, isTargetLocalUser, input.RoomVersion)
+	if err != nil {
+		return nil, err
+	}
+	succeeded := false
+	defer func() {
+		txerr := sqlutil.EndTransaction(updater, &succeeded)
+		if err == nil && txerr != nil {
+			err = txerr
+		}
+	}()
+
+	if updater.IsJoin() {
+		// If the user is joined to the room then that takes precedence over this
+		// invite event. It makes little sense to move a user that is already
+		// joined to the room into the invite state.
+		// This could plausibly happen if an invite request raced with a join
+		// request for a user. For example if a user was invited to a public
+		// room and they joined the room at the same time as the invite was sent.
+		// The other way this could plausibly happen is if an invite raced with
+		// a kick. For example if a user was kicked from a room in error and in
+		// response someone else in the room re-invited them then it is possible
+		// for the invite request to race with the leave event so that the
+		// target receives invite before it learns that it has been kicked.
+		// There are a few ways this could be plausibly handled in the roomserver.
+		// 1) Store the invite, but mark it as retired. That will result in the
+		//    permanent rejection of that invite event. So even if the target
+		//    user leaves the room and the invite is retransmitted it will be
+		//    ignored. However a new invite with a new event ID would still be
+		//    accepted.
+		// 2) Silently discard the invite event. This means that if the event
+		//    was retransmitted at a later date after the target user had left
+		//    the room we would accept the invite. However since we hadn't told
+		//    the sending server that the invite had been discarded it would
+		//    have no reason to attempt to retry.
+		// 3) Signal the sending server that the user is already joined to the
+		//    room.
+		// For now we will implement option 2. Since in the abesence of a retry
+		// mechanism it will be equivalent to option 1, and we don't have a
+		// signalling mechanism to implement option 3.
+		return nil, &api.PerformError{
+			Code: api.PerformErrorNoOperation,
+			Msg:  "user is already joined to room",
+		}
+	}
+
+	// Normally, with a federated invite, the federation sender would do
+	// the /v2/invite request (in which the remote server signs the invite)
+	// and then the signed event gets sent back to the roomserver as an input
+	// event. When the invite is local, we don't interact with the federation
+	// sender therefore we need to generate the loopback invite event for
+	// the room ourselves.
+	loopback, err := localInviteLoopback(ow, input)
+	if err != nil {
+		return nil, err
+	}
+
+	event := input.Event.Unwrap()
+	if len(input.InviteRoomState) > 0 {
+		// If we were supplied with some invite room state already (which is
+		// most likely to be if the event came in over federation) then use
+		// that.
+		if err = event.SetUnsignedField("invite_room_state", input.InviteRoomState); err != nil {
+			return nil, err
+		}
+	} else {
+		// There's no invite room state, so let's have a go at building it
+		// up from local data (which is most likely to be if the event came
+		// from the CS API). If we know about the room then we can insert
+		// the invite room state, if we don't then we just fail quietly.
+		if irs, ierr := buildInviteStrippedState(ctx, r.DB, input); ierr == nil {
+			if err = event.SetUnsignedField("invite_room_state", irs); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	outputUpdates, err := updateToInviteMembership(updater, &event, nil, input.Event.RoomVersion)
+	if err != nil {
+		return nil, err
+	}
+
+	if err = ow.WriteOutputEvents(roomID, outputUpdates); err != nil {
+		return nil, err
+	}
+
+	succeeded = true
+	return loopback, nil
+}
+
+func localInviteLoopback(
+	ow *RoomserverInternalAPI,
+	input *api.PerformInviteRequest,
+) (ire *api.InputRoomEvent, err error) {
+	if input.Event.StateKey() == nil {
+		return nil, errors.New("no state key on invite event")
+	}
+	ourServerName := string(ow.Cfg.Matrix.ServerName)
+	_, theirServerName, err := gomatrixserverlib.SplitID('@', *input.Event.StateKey())
+	if err != nil {
+		return nil, err
+	}
+	// Check if the invite originated locally and is destined locally.
+	if input.Event.Origin() == ow.Cfg.Matrix.ServerName && string(theirServerName) == ourServerName {
+		rsEvent := input.Event.Sign(
+			ourServerName,
+			ow.Cfg.Matrix.KeyID,
+			ow.Cfg.Matrix.PrivateKey,
+		).Headered(input.RoomVersion)
+		ire = &api.InputRoomEvent{
+			Kind:          api.KindNew,
+			Event:         rsEvent,
+			AuthEventIDs:  rsEvent.AuthEventIDs(),
+			SendAsServer:  ourServerName,
+			TransactionID: nil,
+		}
+	}
+	return ire, nil
+}
+
+func buildInviteStrippedState(
+	ctx context.Context,
+	db storage.Database,
+	input *api.PerformInviteRequest,
+) ([]gomatrixserverlib.InviteV2StrippedState, error) {
+	roomNID, err := db.RoomNID(ctx, input.Event.RoomID())
+	if err != nil || roomNID == 0 {
+		return nil, fmt.Errorf("room %q unknown", input.Event.RoomID())
+	}
+	stateWanted := []gomatrixserverlib.StateKeyTuple{}
+	// "If they are set on the room, at least the state for m.room.avatar, m.room.canonical_alias, m.room.join_rules, and m.room.name SHOULD be included."
+	// https://matrix.org/docs/spec/client_server/r0.6.0#m-room-member
+	for _, t := range []string{
+		gomatrixserverlib.MRoomName, gomatrixserverlib.MRoomCanonicalAlias,
+		gomatrixserverlib.MRoomAliases, gomatrixserverlib.MRoomJoinRules,
+		"m.room.avatar",
+	} {
+		stateWanted = append(stateWanted, gomatrixserverlib.StateKeyTuple{
+			EventType: t,
+			StateKey:  "",
+		})
+	}
+	_, currentStateSnapshotNID, _, err := db.LatestEventIDs(ctx, roomNID)
+	if err != nil {
+		return nil, err
+	}
+	roomState := state.NewStateResolution(db)
+	stateEntries, err := roomState.LoadStateAtSnapshotForStringTuples(
+		ctx, currentStateSnapshotNID, stateWanted,
+	)
+	if err != nil {
+		return nil, err
+	}
+	stateNIDs := []types.EventNID{}
+	for _, stateNID := range stateEntries {
+		stateNIDs = append(stateNIDs, stateNID.EventNID)
+	}
+	stateEvents, err := db.Events(ctx, stateNIDs)
+	if err != nil {
+		return nil, err
+	}
+	inviteState := []gomatrixserverlib.InviteV2StrippedState{
+		gomatrixserverlib.NewInviteV2StrippedState(&input.Event.Event),
+	}
+	stateEvents = append(stateEvents, types.Event{Event: input.Event.Unwrap()})
+	for _, event := range stateEvents {
+		inviteState = append(inviteState, gomatrixserverlib.NewInviteV2StrippedState(&event.Event))
+	}
+	return inviteState, nil
+}
diff --git a/roomserver/internal/perform_join.go b/roomserver/internal/perform_join.go
index 44f84240..d409b684 100644
--- a/roomserver/internal/perform_join.go
+++ b/roomserver/internal/perform_join.go
@@ -14,40 +14,63 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-// WriteOutputEvents implements OutputRoomEventWriter
+// PerformJoin handles joining matrix rooms, including over federation by talking to the federationsender.
 func (r *RoomserverInternalAPI) PerformJoin(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
 	res *api.PerformJoinResponse,
-) error {
+) {
+	roomID, err := r.performJoin(ctx, req)
+	if err != nil {
+		perr, ok := err.(*api.PerformError)
+		if ok {
+			res.Error = perr
+		} else {
+			res.Error = &api.PerformError{
+				Msg: err.Error(),
+			}
+		}
+	}
+	res.RoomID = roomID
+}
+
+func (r *RoomserverInternalAPI) performJoin(
+	ctx context.Context,
+	req *api.PerformJoinRequest,
+) (string, error) {
 	_, domain, err := gomatrixserverlib.SplitID('@', req.UserID)
 	if err != nil {
-		res.Error = api.JoinErrorBadRequest
-		return fmt.Errorf("Supplied user ID %q in incorrect format", req.UserID)
+		return "", &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  fmt.Sprintf("Supplied user ID %q in incorrect format", req.UserID),
+		}
 	}
 	if domain != r.Cfg.Matrix.ServerName {
-		res.Error = api.JoinErrorBadRequest
-		return fmt.Errorf("User %q does not belong to this homeserver", req.UserID)
+		return "", &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  fmt.Sprintf("User %q does not belong to this homeserver", req.UserID),
+		}
 	}
 	if strings.HasPrefix(req.RoomIDOrAlias, "!") {
-		return r.performJoinRoomByID(ctx, req, res)
+		return r.performJoinRoomByID(ctx, req)
 	}
 	if strings.HasPrefix(req.RoomIDOrAlias, "#") {
-		return r.performJoinRoomByAlias(ctx, req, res)
+		return r.performJoinRoomByAlias(ctx, req)
+	}
+	return "", &api.PerformError{
+		Code: api.PerformErrorBadRequest,
+		Msg:  fmt.Sprintf("Room ID or alias %q is invalid", req.RoomIDOrAlias),
 	}
-	res.Error = api.JoinErrorBadRequest
-	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 {
+) (string, 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)
 
@@ -65,7 +88,7 @@ func (r *RoomserverInternalAPI) 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...)
@@ -73,18 +96,18 @@ func (r *RoomserverInternalAPI) 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.
 	req.RoomIDOrAlias = roomID
-	return r.performJoinRoomByID(ctx, req, res)
+	return r.performJoinRoomByID(ctx, req)
 }
 
 // TODO: Break this function up a bit
@@ -92,19 +115,14 @@ func (r *RoomserverInternalAPI) performJoinRoomByAlias(
 func (r *RoomserverInternalAPI) performJoinRoomByID(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
-	res *api.PerformJoinResponse, // nolint:unparam
-) error {
-	// 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.
-	res.RoomID = req.RoomIDOrAlias
-
+) (string, error) {
 	// Get the domain part of the room ID.
 	_, domain, err := gomatrixserverlib.SplitID('!', req.RoomIDOrAlias)
 	if err != nil {
-		res.Error = api.JoinErrorBadRequest
-		return fmt.Errorf("Room ID %q is invalid", req.RoomIDOrAlias)
+		return "", &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  fmt.Sprintf("Room ID %q is invalid: %s", req.RoomIDOrAlias, err),
+		}
 	}
 	req.ServerNames = append(req.ServerNames, domain)
 
@@ -118,7 +136,7 @@ func (r *RoomserverInternalAPI) 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
@@ -129,7 +147,7 @@ func (r *RoomserverInternalAPI) 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)
 	}
 
 	// First work out if this is in response to an existing invite
@@ -142,7 +160,7 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 		// Check if there's an invite pending.
 		_, inviterDomain, ierr := gomatrixserverlib.SplitID('@', inviteSender)
 		if ierr != nil {
-			return fmt.Errorf("gomatrixserverlib.SplitID: %w", err)
+			return "", fmt.Errorf("gomatrixserverlib.SplitID: %w", err)
 		}
 
 		// Check that the domain isn't ours. If it's local then we don't
@@ -154,7 +172,7 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 			req.ServerNames = append(req.ServerNames, inviterDomain)
 
 			// Perform a federated room join.
-			return r.performFederatedJoinRoomByID(ctx, req, res)
+			return req.RoomIDOrAlias, r.performFederatedJoinRoomByID(ctx, req)
 		}
 	}
 
@@ -205,9 +223,12 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 			if err = r.InputRoomEvents(ctx, &inputReq, &inputRes); err != nil {
 				var notAllowed *gomatrixserverlib.NotAllowed
 				if errors.As(err, &notAllowed) {
-					res.Error = api.JoinErrorNotAllowed
+					return "", &api.PerformError{
+						Code: api.PerformErrorNotAllowed,
+						Msg:  fmt.Sprintf("InputRoomEvents auth failed: %s", err),
+					}
 				}
-				return fmt.Errorf("r.InputRoomEvents: %w", err)
+				return "", fmt.Errorf("r.InputRoomEvents: %w", err)
 			}
 		}
 
@@ -216,25 +237,30 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 		// 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 {
-			res.Error = api.JoinErrorNoRoom
-			return fmt.Errorf("Room ID %q does not exist", req.RoomIDOrAlias)
+			return "", &api.PerformError{
+				Code: api.PerformErrorNoRoom,
+				Msg:  fmt.Sprintf("Room ID %q does not exist", req.RoomIDOrAlias),
+			}
 		}
 
 		// Perform a federated room join.
-		return r.performFederatedJoinRoomByID(ctx, req, res)
+		return req.RoomIDOrAlias, r.performFederatedJoinRoomByID(ctx, req)
 
 	default:
 		// Something else went wrong.
-		return fmt.Errorf("Error joining local room: %q", err)
+		return "", fmt.Errorf("Error joining local room: %q", err)
 	}
 
-	return nil
+	// 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
 }
 
 func (r *RoomserverInternalAPI) performFederatedJoinRoomByID(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
-	res *api.PerformJoinResponse, // nolint:unparam
 ) error {
 	// Try joining by all of the supplied server names.
 	fedReq := fsAPI.PerformJoinRequest{
diff --git a/roomserver/inthttp/client.go b/roomserver/inthttp/client.go
index e41adb99..8a2b1204 100644
--- a/roomserver/inthttp/client.go
+++ b/roomserver/inthttp/client.go
@@ -3,6 +3,7 @@ package inthttp
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net/http"
 
 	fsInputAPI "github.com/matrix-org/dendrite/federationsender/api"
@@ -24,6 +25,7 @@ const (
 	RoomserverInputRoomEventsPath = "/roomserver/inputRoomEvents"
 
 	// Perform operations
+	RoomserverPerformInvitePath   = "/roomserver/performInvite"
 	RoomserverPerformJoinPath     = "/roomserver/performJoin"
 	RoomserverPerformLeavePath    = "/roomserver/performLeave"
 	RoomserverPerformBackfillPath = "/roomserver/performBackfill"
@@ -146,16 +148,38 @@ func (h *httpRoomserverInternalAPI) InputRoomEvents(
 	return httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
 }
 
+func (h *httpRoomserverInternalAPI) PerformInvite(
+	ctx context.Context,
+	request *api.PerformInviteRequest,
+	response *api.PerformInviteResponse,
+) {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "PerformInvite")
+	defer span.Finish()
+
+	apiURL := h.roomserverURL + RoomserverPerformInvitePath
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	if err != nil {
+		response.Error = &api.PerformError{
+			Msg: fmt.Sprintf("failed to communicate with roomserver: %s", err),
+		}
+	}
+}
+
 func (h *httpRoomserverInternalAPI) PerformJoin(
 	ctx context.Context,
 	request *api.PerformJoinRequest,
 	response *api.PerformJoinResponse,
-) error {
+) {
 	span, ctx := opentracing.StartSpanFromContext(ctx, "PerformJoin")
 	defer span.Finish()
 
 	apiURL := h.roomserverURL + RoomserverPerformJoinPath
-	return httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	if err != nil {
+		response.Error = &api.PerformError{
+			Msg: fmt.Sprintf("failed to communicate with roomserver: %s", err),
+		}
+	}
 }
 
 func (h *httpRoomserverInternalAPI) PerformLeave(
diff --git a/roomserver/inthttp/server.go b/roomserver/inthttp/server.go
index e3b81daa..1c47e87e 100644
--- a/roomserver/inthttp/server.go
+++ b/roomserver/inthttp/server.go
@@ -26,6 +26,17 @@ func AddRoutes(r api.RoomserverInternalAPI, internalAPIMux *mux.Router) {
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),
 	)
+	internalAPIMux.Handle(RoomserverPerformInvitePath,
+		httputil.MakeInternalAPI("performInvite", func(req *http.Request) util.JSONResponse {
+			var request api.PerformInviteRequest
+			var response api.PerformInviteResponse
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.MessageResponse(http.StatusBadRequest, err.Error())
+			}
+			r.PerformInvite(req.Context(), &request, &response)
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
 	internalAPIMux.Handle(RoomserverPerformJoinPath,
 		httputil.MakeInternalAPI("performJoin", func(req *http.Request) util.JSONResponse {
 			var request api.PerformJoinRequest
@@ -33,9 +44,7 @@ func AddRoutes(r api.RoomserverInternalAPI, internalAPIMux *mux.Router) {
 			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 {
-				response.ErrMsg = err.Error()
-			}
+			r.PerformJoin(req.Context(), &request, &response)
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),
 	)