Provide eventId as well as roomId from Room.findPredecessor

spec/unit/room.spec.ts

@@ -3282,19 +3282,19 @@ describe("Room", function () {
 
         it("Returns null if there is no create event", () => {
             const room = new Room("roomid", client!, "@u:example.com");
-            expect(room.findPredecessorRoomId()).toBeNull();
+            expect(room.findPredecessor()).toBeNull();
         });
 
         it("Returns null if the create event has no predecessor", () => {
             const room = new Room("roomid", client!, "@u:example.com");
             room.addLiveEvents([roomCreateEvent("roomid", null)]);
-            expect(room.findPredecessorRoomId()).toBeNull();
+            expect(room.findPredecessor()).toBeNull();
         });
 
         it("Returns the predecessor ID if one is provided via create event", () => {
             const room = new Room("roomid", client!, "@u:example.com");
             room.addLiveEvents([roomCreateEvent("roomid", "replacedroomid")]);
-            expect(room.findPredecessorRoomId()).toBe("replacedroomid");
+            expect(room.findPredecessor()).toEqual({ roomId: "replacedroomid", eventId: "id_of_last_known_event" });
         });
 
         it("Prefers the m.predecessor event if one exists", () => {
@@ -3304,7 +3304,10 @@ describe("Room", function () {
                 predecessorEvent("roomid", "otherreplacedroomid"),
             ]);
             const useMsc3946 = true;
-            expect(room.findPredecessorRoomId(useMsc3946)).toBe("otherreplacedroomid");
+            expect(room.findPredecessor(useMsc3946)).toEqual({
+                roomId: "otherreplacedroomid",
+                eventId: null, // m.predecessor does not include an event_id
+            });
         });
 
         it("Ignores the m.predecessor event if we don't ask to use it", () => {
@@ -3315,7 +3318,7 @@ describe("Room", function () {
             ]);
             // Don't provide an argument for msc3946ProcessDynamicPredecessor -
             // we should ignore the predecessor event.
-            expect(room.findPredecessorRoomId()).toBe("replacedroomid");
+            expect(room.findPredecessor()).toEqual({ roomId: "replacedroomid", eventId: "id_of_last_known_event" });
         });
 
         it("Ignores the m.predecessor event and returns null if we don't ask to use it", () => {
@@ -3326,7 +3329,7 @@ describe("Room", function () {
             ]);
             // Don't provide an argument for msc3946ProcessDynamicPredecessor -
             // we should ignore the predecessor event.
-            expect(room.findPredecessorRoomId()).toBeNull();
+            expect(room.findPredecessor()).toBeNull();
         });
     });
 });

src/client.ts

@@ -3791,7 +3791,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
 
         const replacedRooms = new Set();
         for (const r of allRooms) {
-            const predecessor = r.findPredecessorRoomId(msc3946ProcessDynamicPredecessor);
+            const predecessor = r.findPredecessor(msc3946ProcessDynamicPredecessor)?.roomId;
             if (predecessor) {
                 replacedRooms.add(predecessor);
             }
@@ -5014,7 +5014,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         const ret: Room[] = [];
 
         // Work backwards from newer to older rooms
-        let predecessorRoomId = room.findPredecessorRoomId(msc3946ProcessDynamicPredecessor);
+        let predecessorRoomId = room.findPredecessor(msc3946ProcessDynamicPredecessor)?.roomId;
         while (predecessorRoomId !== null) {
             const predecessorRoom = this.getRoom(predecessorRoomId);
             if (predecessorRoom === null) {
@@ -5031,7 +5031,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             ret.splice(0, 0, predecessorRoom);
 
             room = predecessorRoom;
-            predecessorRoomId = room.findPredecessorRoomId(msc3946ProcessDynamicPredecessor);
+            predecessorRoomId = room.findPredecessor(msc3946ProcessDynamicPredecessor)?.roomId;
         }
         return ret;
     }
@@ -5047,7 +5047,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             if (successorRoom.roomId === room.roomId) break; // Tombstone is referencing its own room
 
             if (verifyLinks) {
-                const predecessorRoomId = successorRoom.findPredecessorRoomId(msc3946ProcessDynamicPredecessor);
+                const predecessorRoomId = successorRoom.findPredecessor(msc3946ProcessDynamicPredecessor)?.roomId;
                 if (!predecessorRoomId || predecessorRoomId !== room.roomId) {
                     break;
                 }

src/models/room.ts

@@ -2992,12 +2992,17 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
 
     /**
      * @param msc3946ProcessDynamicPredecessor - if true, look for an
-     *                                           m.room.predecessor state event and
-     *                                           use it if found (MSC3946).
-     * @returns the ID of the room that was this room's predecessor, or null if
-     *          this room has no predecessor.
-     */
-    public findPredecessorRoomId(msc3946ProcessDynamicPredecessor = false): string | null {
+     * m.room.predecessor state event and use it if found (MSC3946).
+     * @returns null if this room has no predecessor. Otherwise, returns
+     * the roomId and last eventId of the predecessor room.
+     * If msc3946ProcessDynamicPredecessor is true, use m.predecessor events
+     * as well as m.room.create events to find predecessors.
+     * Note: if an m.predecessor event is used, eventId is null since those
+     * events do not include an event_id property.
+     */
+    public findPredecessor(
+        msc3946ProcessDynamicPredecessor = false,
+    ): { roomId: string; eventId: string | null } | null {
         const currentState = this.getLiveTimeline().getState(EventTimeline.FORWARDS);
         if (!currentState) {
             return null;
@@ -3007,7 +3012,7 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
             if (predecessorEvent) {
                 const roomId = predecessorEvent.getContent()["predecessor_room_id"];
                 if (roomId) {
-                    return roomId;
+                    return { roomId, eventId: null };
                 }
             }
         }
@@ -3018,7 +3023,8 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
             if (predecessor) {
                 const roomId = predecessor["room_id"];
                 if (roomId) {
-                    return roomId;
+                    const eventId = predecessor["event_id"] || null;
+                    return { roomId, eventId };
                 }
             }
         }