Restricted join support on /make_join, /send_join (#2478)

* Add `QueryRestrictedJoinAllowed`

* Add `Resident` flag to `QueryRestrictedJoinAllowedResponse`

* Check restricted joins on federation API

* Return `Restricted` to determine if the room was restricted or not

* Populate `AuthorisedVia` properly

* Sign the event on `/send_join`, return it in the `/send_join` response in the `"event"` key

* Kick back joins with invalid authorising user IDs, use event from `"event"` key if returned in `RespSendJoin`

* Use invite helper in `QueryRestrictedJoinAllowed`

* Only use users with the power to invite, change error bubbling a bit

* Placate the almighty linter

One day I will nuke `gocyclo` from orbit and everything in the world will be much better for it.

* Review comments

roomserver/api/api.go

@@ -184,6 +184,7 @@ type FederationRoomserverAPI interface {
 	// Query whether a server is allowed to see an event
 	QueryServerAllowedToSeeEvent(ctx context.Context, req *QueryServerAllowedToSeeEventRequest, res *QueryServerAllowedToSeeEventResponse) error
 	QueryRoomsForUser(ctx context.Context, req *QueryRoomsForUserRequest, res *QueryRoomsForUserResponse) error
+	QueryRestrictedJoinAllowed(ctx context.Context, req *QueryRestrictedJoinAllowedRequest, res *QueryRestrictedJoinAllowedResponse) error
 	PerformInboundPeek(ctx context.Context, req *PerformInboundPeekRequest, res *PerformInboundPeekResponse) error
 	PerformInvite(ctx context.Context, req *PerformInviteRequest, res *PerformInviteResponse) error
 	// Query a given amount (or less) of events prior to a given set of events.

roomserver/api/api_trace.go

@@ -354,6 +354,16 @@ func (t *RoomserverInternalAPITrace) QueryAuthChain(
 	return err
 }
 
+func (t *RoomserverInternalAPITrace) QueryRestrictedJoinAllowed(
+	ctx context.Context,
+	request *QueryRestrictedJoinAllowedRequest,
+	response *QueryRestrictedJoinAllowedResponse,
+) error {
+	err := t.Impl.QueryRestrictedJoinAllowed(ctx, request, response)
+	util.GetLogger(ctx).WithError(err).Infof("QueryRestrictedJoinAllowed req=%+v res=%+v", js(request), js(response))
+	return err
+}
+
 func js(thing interface{}) string {
 	b, err := json.Marshal(thing)
 	if err != nil {

roomserver/api/query.go

@@ -348,6 +348,26 @@ type QueryServerBannedFromRoomResponse struct {
 	Banned bool `json:"banned"`
 }
 
+type QueryRestrictedJoinAllowedRequest struct {
+	UserID string `json:"user_id"`
+	RoomID string `json:"room_id"`
+}
+
+type QueryRestrictedJoinAllowedResponse struct {
+	// True if the room membership is restricted by the join rule being set to "restricted"
+	Restricted bool `json:"restricted"`
+	// True if our local server is joined to all of the allowed rooms specified in the "allow"
+	// key of the join rule, false if we are missing from some of them and therefore can't
+	// reliably decide whether or not we can satisfy the join
+	Resident bool `json:"resident"`
+	// True if the restricted join is allowed because we found the membership in one of the
+	// allowed rooms from the join rule, false if not
+	Allowed bool `json:"allowed"`
+	// Contains the user ID of the selected user ID that has power to issue invites, this will
+	// get populated into the "join_authorised_via_users_server" content in the membership
+	AuthorisedVia string `json:"authorised_via,omitempty"`
+}
+
 // MarshalJSON stringifies the room ID and StateKeyTuple keys so they can be sent over the wire in HTTP API mode.
 func (r *QueryBulkStateContentResponse) MarshalJSON() ([]byte, error) {
 	se := make(map[string]string)