proxy state requests to user IDs to sender IDs

- so state events that use user IDs translate to pseudo IDs in pseudo ID rooms
- e.g. allowing to query m.room.member by user ID in a pseudo ID room

clientapi/routing/state.go

@@ -217,6 +217,21 @@ func OnIncomingStateTypeRequest(
 	var worldReadable bool
 	var wantLatestState bool
 
+	parsedRoomID, err := spec.NewRoomID(roomID)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: spec.InvalidParam("invalid room ID"),
+		}
+	}
+
+	// Handle user ID state keys appropriately
+	newStateKey, errorResp := translateUserStateKey(ctx, rsAPI, stateKey, evType, *parsedRoomID)
+	if errorResp != nil {
+		return *errorResp
+	}
+	stateKey = *newStateKey
+
 	// Always fetch visibility so that we can work out whether to show
 	// the latest events or the last event from when the user was joined.
 	// Then include the requested event type and state key, assuming it
@@ -389,3 +404,41 @@ func OnIncomingStateTypeRequest(
 		JSON: res,
 	}
 }
+
+// If provided state key is a user ID (state keys beginning with @ are reserved for this purpose)
+// fetch it's associated sender ID and use that instead. Otherwise returns the same state key back.
+//
+// This function either returns the state key that should be used, or a JSON error response that should be returned to the user.
+//
+// TODO: if any step of this process fails, should we fail with 404, or silently continue without using sender ID?
+func translateUserStateKey(ctx context.Context, rsAPI api.ClientRoomserverAPI, stateKey string, evType string, roomID spec.RoomID) (*string, *util.JSONResponse) {
+	if len(stateKey) >= 1 && stateKey[0] == '@' {
+		parsedStateKey, err := spec.NewUserID(stateKey, true)
+		if err != nil {
+			// If invalid user ID, then there is no associated state event.
+			return nil, &util.JSONResponse{
+				Code: http.StatusNotFound,
+				JSON: spec.NotFound(fmt.Sprintf("Cannot find state event for %q", evType)),
+			}
+		}
+		senderID, err := rsAPI.QuerySenderIDForUser(ctx, roomID, *parsedStateKey)
+		if err != nil {
+			util.GetLogger(ctx).WithError(err).Error("QuerySenderIDForUser failed")
+			return nil, &util.JSONResponse{
+				Code: http.StatusInternalServerError,
+				JSON: spec.Unknown("internal server error"),
+			}
+		}
+		if senderID == nil {
+			// If no sender ID, then there is no associated state event.
+			return nil, &util.JSONResponse{
+				Code: http.StatusNotFound,
+				JSON: spec.NotFound(fmt.Sprintf("Cannot find state event for %q", evType)),
+			}
+		}
+		newStateKey := string(*senderID)
+		return &newStateKey, nil
+	} else {
+		return &stateKey, nil
+	}
+}