Ignore m.replace relations on state events, they're invalid (#2306)

* Ignore m.replace relations on state events, they're invalid

* Add tests

* Fix test

Co-authored-by: Kerry <kerrya@element.io>

spec/unit/relations.spec.ts

@@ -130,4 +130,49 @@ describe("Relations", function() {
             await relationsCreated;
         }
     });
+
+    it("should ignore m.replace for state events", async () => {
+        const userId = "@bob:example.com";
+        const room = new Room("room123", null, userId);
+        const relations = new Relations("m.replace", "m.room.topic", room);
+
+        // Create an instance of a state event with rel_type m.replace
+        const originalTopic = new MatrixEvent({
+            "sender": userId,
+            "type": "m.room.topic",
+            "event_id": "$orig",
+            "room_id": room.roomId,
+            "content": {
+                "topic": "orig",
+            },
+            "state_key": "",
+        });
+        const badlyEditedTopic = new MatrixEvent({
+            "sender": userId,
+            "type": "m.room.topic",
+            "event_id": "$orig",
+            "room_id": room.roomId,
+            "content": {
+                "topic": "topic",
+                "m.new_content": {
+                    "topic": "edit",
+                },
+                "m.relates_to": {
+                    "event_id": "$orig",
+                    "rel_type": "m.replace",
+                },
+            },
+            "state_key": "",
+        });
+
+        await relations.setTargetEvent(originalTopic);
+        expect(originalTopic.replacingEvent()).toBe(null);
+        expect(originalTopic.getContent().topic).toBe("orig");
+
+        await relations.addEvent(badlyEditedTopic);
+        expect(originalTopic.replacingEvent()).toBe(null);
+        expect(originalTopic.getContent().topic).toBe("orig");
+        expect(badlyEditedTopic.replacingEvent()).toBe(null);
+        expect(badlyEditedTopic.getContent().topic).toBe("topic");
+    });
 });

src/models/event.ts

@@ -1336,6 +1336,10 @@ export class MatrixEvent extends TypedEventEmitter<EmittedEvents, MatrixEventHan
         if (this.isRedacted() && newEvent) {
             return;
         }
+        // don't allow state events to be replaced using this mechanism as per MSC2676
+        if (this.isState()) {
+            return;
+        }
         if (this._replacingEvent !== newEvent) {
             this._replacingEvent = newEvent;
             this.emit(MatrixEventEvent.Replaced, this);