validate room and return proper error in case it is unknown

Author: sampaiodiego

ee/packages/federation-matrix/src/api/_matrix/client/_shared.ts

@@ -16,6 +16,23 @@ export const internalError = (msg: string, err?: unknown, context?: Record<strin
 	};
 };
 
+// The federation SDK throws an Error with name 'UnknownRoomError' (message `Room <id> does not exist`)
+// when a room isn't known to this homeserver. The class isn't exported for `instanceof`, so we match on
+// `.name` — the same check the SDK uses internally.
+export const isUnknownRoomError = (err: unknown): err is Error => err instanceof Error && err.name === 'UnknownRoomError';
+
+// Matrix response for a room this homeserver doesn't know about. Mirrors Synapse: it must be a 4xx (never
+// 5xx) so bridges like matrix-bifrost treat it as terminal and stop retrying, instead of hammering the
+// endpoint every 100ms as if it were a transient server fault.
+export const roomNotFound = () =>
+	({
+		statusCode: 403 as const,
+		body: {
+			errcode: 'M_FORBIDDEN',
+			error: "You aren't a member of the room and weren't previously a member of the room.",
+		},
+	}) as const;
+
 // Logs a warning and returns the matching Matrix 501 response. Use for endpoints/branches
 // that are deliberately not implemented yet, so hits on those paths stay visible in the logs.
 export const notImplemented = (msg: string, context?: Record<string, unknown>) => {

ee/packages/federation-matrix/src/api/_matrix/client/rooms-state.ts

@@ -6,7 +6,9 @@ import type { ClientRouter } from './_shared';
 import {
 	MATRIX_ROOM_ID_PATTERN,
 	internalError,
+	isUnknownRoomError,
 	notImplemented,
+	roomNotFound,
 	isImpersonationQueryProps,
 	isMatrixErrorProps,
 	isRoomIdParamsProps,
@@ -105,6 +107,9 @@ const getRoomStateEvent = async (roomId: RoomID, eventType: string, stateKey = '
 			body: pe.getContent(),
 		};
 	} catch (error) {
+		if (isUnknownRoomError(error)) {
+			return roomNotFound();
+		}
 		return internalError('Failed to fetch state event', error);
 	}
 };
@@ -119,7 +124,7 @@ export const addRoomsStateRoutes = (router: ClientRouter) => {
 				response: {
 					200: isJoinedMembersResponseProps,
 					401: isMatrixErrorProps,
-					404: isMatrixErrorProps,
+					403: isMatrixErrorProps,
 					500: isMatrixErrorProps,
 				},
 				tags,
@@ -128,6 +133,7 @@ export const addRoomsStateRoutes = (router: ClientRouter) => {
 			isAppServiceAuthenticatedMiddleware(),
 			async (c) => {
 				const roomId = c.req.param('roomId') as RoomID;
+
 				try {
 					const state = await federationSDK.getLatestRoomState(roomId);
 					const joined: Record<string, { display_name?: string; avatar_url?: string }> = {};
@@ -147,6 +153,9 @@ export const addRoomsStateRoutes = (router: ClientRouter) => {
 						body: { joined },
 					};
 				} catch (error) {
+					if (isUnknownRoomError(error)) {
+						return roomNotFound();
+					}
 					return internalError('Failed to fetch joined members', error, { roomId });
 				}
 			},
@@ -160,7 +169,7 @@ export const addRoomsStateRoutes = (router: ClientRouter) => {
 				response: {
 					200: isStateArrayResponseProps,
 					401: isMatrixErrorProps,
-					404: isMatrixErrorProps,
+					403: isMatrixErrorProps,
 					500: isMatrixErrorProps,
 				},
 				tags,
@@ -180,6 +189,9 @@ export const addRoomsStateRoutes = (router: ClientRouter) => {
 						body: events,
 					};
 				} catch (error) {
+					if (isUnknownRoomError(error)) {
+						return roomNotFound();
+					}
 					return internalError('Failed to fetch room state', error, { roomId });
 				}
 			},
@@ -197,6 +209,7 @@ export const addRoomsStateRoutes = (router: ClientRouter) => {
 				response: {
 					200: isStateContentResponseProps,
 					401: isMatrixErrorProps,
+					403: isMatrixErrorProps,
 					404: isMatrixErrorProps,
 					500: isMatrixErrorProps,
 				},
@@ -220,6 +233,7 @@ export const addRoomsStateRoutes = (router: ClientRouter) => {
 				response: {
 					200: isStateContentResponseProps,
 					401: isMatrixErrorProps,
+					403: isMatrixErrorProps,
 					404: isMatrixErrorProps,
 					500: isMatrixErrorProps,
 				},