Redact on ban: Client implementation (#4867)

* First pass implementation

* fix naming/docs

* apply lint

* Add test for existing behaviour

* Add happy path tests

* Fix bug identified by tests

* ... and this is why we add negative tests too

* Add some sanity tests

* Apply linter

spec/unit/models/room.spec.ts

@@ -0,0 +1,188 @@
+/*
+Copyright 2025 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { Direction, type MatrixClient, MatrixEvent, Room } from "../../../src";
+import type { MockedObject } from "jest-mock";
+
+const CREATOR_USER_ID = "@creator:example.org";
+const MODERATOR_USER_ID = "@moderator:example.org";
+
+describe("Room", () => {
+    function createMockClient(): MatrixClient {
+        return {
+            supportsThreads: jest.fn().mockReturnValue(true),
+            decryptEventIfNeeded: jest.fn().mockReturnThis(),
+            getUserId: jest.fn().mockReturnValue(CREATOR_USER_ID),
+        } as unknown as MockedObject<MatrixClient>;
+    }
+
+    function createEvent(eventId: string): MatrixEvent {
+        return new MatrixEvent({
+            type: "m.room.message",
+            content: {
+                body: eventId, // we do this for ease of use, not practicality
+            },
+            event_id: eventId,
+            sender: CREATOR_USER_ID,
+        });
+    }
+
+    function createRedaction(redactsEventId: string): MatrixEvent {
+        return new MatrixEvent({
+            type: "m.room.redaction",
+            redacts: redactsEventId,
+            event_id: "$redacts_" + redactsEventId.substring(1),
+            sender: CREATOR_USER_ID,
+        });
+    }
+
+    function getNonStateMainTimelineLiveEvents(room: Room): Array<MatrixEvent> {
+        return room
+            .getLiveTimeline()
+            .getEvents()
+            .filter((e) => !e.isState());
+    }
+
+    it("should apply redactions locally", async () => {
+        const mockClient = createMockClient();
+        const room = new Room("!room:example.org", mockClient, CREATOR_USER_ID);
+        const messageEvent = createEvent("$message_event");
+
+        // Set up the room
+        await room.addLiveEvents([messageEvent], { addToState: false });
+        let timeline = getNonStateMainTimelineLiveEvents(room);
+        expect(timeline.length).toEqual(1);
+        expect(timeline[0].getId()).toEqual(messageEvent.getId());
+        expect(timeline[0].isRedacted()).toEqual(false); // "should never happen"
+
+        // Now redact
+        const redactionEvent = createRedaction(messageEvent.getId()!);
+        await room.addLiveEvents([redactionEvent], { addToState: false });
+        timeline = getNonStateMainTimelineLiveEvents(room);
+        expect(timeline.length).toEqual(2);
+        expect(timeline[0].getId()).toEqual(messageEvent.getId());
+        expect(timeline[0].isRedacted()).toEqual(true); // test case
+        expect(timeline[1].getId()).toEqual(redactionEvent.getId());
+        expect(timeline[1].isRedacted()).toEqual(false); // "should never happen"
+    });
+
+    describe("MSC4293: Redact on ban", () => {
+        async function setupRoom(andGrantPermissions: boolean): Promise<{ room: Room; messageEvents: MatrixEvent[] }> {
+            const mockClient = createMockClient();
+            const room = new Room("!room:example.org", mockClient, CREATOR_USER_ID);
+
+            // Pre-populate room
+            const messageEvents: MatrixEvent[] = [];
+            for (let i = 0; i < 3; i++) {
+                messageEvents.push(createEvent(`$message_${i}`));
+            }
+            await room.addLiveEvents(messageEvents, { addToState: false });
+
+            if (andGrantPermissions) {
+                room.getLiveTimeline().getState(Direction.Forward)!.maySendRedactionForEvent = (ev, userId) => {
+                    return true;
+                };
+            }
+
+            return { room, messageEvents };
+        }
+
+        function createRedactOnMembershipChange(
+            targetUserId: string,
+            senderUserId: string,
+            membership: string,
+        ): MatrixEvent {
+            return new MatrixEvent({
+                type: "m.room.member",
+                state_key: targetUserId,
+                content: {
+                    "membership": membership,
+                    "org.matrix.msc4293.redact_events": true,
+                },
+                sender: senderUserId,
+            });
+        }
+
+        function expectRedacted(messageEvents: MatrixEvent[], room: Room, shouldAllBeRedacted: boolean) {
+            const actualEvents = getNonStateMainTimelineLiveEvents(room).filter((e) =>
+                messageEvents.find((e2) => e2.getId() === e.getId()),
+            );
+            expect(actualEvents.length).toEqual(messageEvents.length);
+            const redactedEvents = actualEvents.filter((e) => e.isRedacted());
+            if (shouldAllBeRedacted) {
+                expect(redactedEvents.length).toEqual(messageEvents.length);
+            } else {
+                expect(redactedEvents.length).toEqual(0);
+            }
+        }
+
+        it("should apply on ban", async () => {
+            const { room, messageEvents } = await setupRoom(true);
+            const banEvent = createRedactOnMembershipChange(CREATOR_USER_ID, MODERATOR_USER_ID, "ban");
+            await room.addLiveEvents([banEvent], { addToState: true });
+
+            expectRedacted(messageEvents, room, true);
+        });
+
+        it("should apply on kick", async () => {
+            const { room, messageEvents } = await setupRoom(true);
+            const kickEvent = createRedactOnMembershipChange(CREATOR_USER_ID, MODERATOR_USER_ID, "leave");
+            await room.addLiveEvents([kickEvent], { addToState: true });
+
+            expectRedacted(messageEvents, room, true);
+        });
+
+        it("should not apply if the user doesn't have permission to redact", async () => {
+            const { room, messageEvents } = await setupRoom(false); // difference from other tests here
+            const banEvent = createRedactOnMembershipChange(CREATOR_USER_ID, MODERATOR_USER_ID, "ban");
+            await room.addLiveEvents([banEvent], { addToState: true });
+
+            expectRedacted(messageEvents, room, false);
+        });
+
+        it("should not apply to self-leaves", async () => {
+            const { room, messageEvents } = await setupRoom(true);
+            const leaveEvent = createRedactOnMembershipChange(CREATOR_USER_ID, CREATOR_USER_ID, "leave");
+            await room.addLiveEvents([leaveEvent], { addToState: true });
+
+            expectRedacted(messageEvents, room, false);
+        });
+
+        it("should not apply to invites", async () => {
+            const { room, messageEvents } = await setupRoom(true);
+            const leaveEvent = createRedactOnMembershipChange(CREATOR_USER_ID, CREATOR_USER_ID, "invite");
+            await room.addLiveEvents([leaveEvent], { addToState: true });
+
+            expectRedacted(messageEvents, room, false);
+        });
+
+        it("should not apply to joins", async () => {
+            const { room, messageEvents } = await setupRoom(true);
+            const leaveEvent = createRedactOnMembershipChange(CREATOR_USER_ID, CREATOR_USER_ID, "join");
+            await room.addLiveEvents([leaveEvent], { addToState: true });
+
+            expectRedacted(messageEvents, room, false);
+        });
+
+        it("should not apply to knocks", async () => {
+            const { room, messageEvents } = await setupRoom(true);
+            const leaveEvent = createRedactOnMembershipChange(CREATOR_USER_ID, CREATOR_USER_ID, "knock");
+            await room.addLiveEvents([leaveEvent], { addToState: true });
+
+            expectRedacted(messageEvents, room, false);
+        });
+    });
+});