Fix finding event read up to if stable private read receipts is missing (#2585) (#2588)

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>
(cherry picked from commit 478270b225)

Co-authored-by: Šimon Brandner <simon.bra.ag@gmail.com>

spec/unit/room.spec.ts

@@ -2458,7 +2458,8 @@ describe("Room", function() {
                 }
             });
 
-            it("should compare correctly by timestamp", () => {
+            describe("correctly compares by timestamp", () => {
+                it("should correctly compare, if we have all receipts", () => {
                     for (let i = 1; i <= 3; i++) {
                         room.getUnfilteredTimelineSet = () => ({
                             compareEventOrdering: (_1, _2) => null,
@@ -2479,6 +2480,23 @@ describe("Room", function() {
                     }
                 });
 
+                it("should correctly compare, if private read receipt is missing", () => {
+                    room.getUnfilteredTimelineSet = () => ({
+                        compareEventOrdering: (_1, _2) => null,
+                    } as EventTimelineSet);
+                    room.getReadReceiptForUserId = (userId, ignore, receiptType) => {
+                        if (receiptType === ReceiptType.UnstableReadPrivate) {
+                            return { eventId: "eventId1", data: { ts: 0 } } as IWrappedReceipt;
+                        }
+                        if (receiptType === ReceiptType.Read) {
+                            return { eventId: "eventId2", data: { ts: 1 } } as IWrappedReceipt;
+                        }
+                    };
+
+                    expect(room.getEventReadUpTo(userA)).toEqual(`eventId2`);
+                });
+            });
+
             describe("fallback precedence", () => {
                 beforeAll(() => {
                     room.getUnfilteredTimelineSet = () => ({

src/models/room.ts

@@ -2592,7 +2592,7 @@ export class Room extends TypedEventEmitter<EmittedEvents, RoomEventHandlerMap>
 
         let latest = privateReadReceipt;
         [unstablePrivateReadReceipt, publicReadReceipt].forEach((receipt) => {
-            if (receipt?.data?.ts > latest?.data?.ts) {
+            if (receipt?.data?.ts > latest?.data?.ts || !latest) {
                 latest = receipt;
             }
         });