Implement MSC3912: Relation based redactions

src/@types/event.ts

@@ -165,6 +165,15 @@ export const UNSTABLE_MSC3089_BRANCH = new UnstableValue("m.branch", "org.matrix
  */
 export const UNSTABLE_MSC2716_MARKER = new UnstableValue("m.room.marker", "org.matrix.msc2716.marker");
 
+/**
+ * Server-side feature to also redact related events by providing `with_relations` in the redaction event content.
+ * {@link https://github.com/matrix-org/matrix-spec-proposals/pull/3912}
+ */
+export const UNSTABLE_MSC3912_RELATION_BASED_REDACTIONS = new UnstableValue(
+    "org.matrix.msc3912.stable",
+    "org.matrix.msc3912",
+);
+
 /**
  * Functional members type for declaring a purpose of room members (e.g. helpful bots).
  * Note that this reference is UNSTABLE and subject to breaking changes, including its

src/@types/requests.ts

@@ -21,7 +21,7 @@ import { IRoomEventFilter } from "../filter";
 import { Direction } from "../models/event-timeline";
 import { PushRuleAction } from "./PushRules";
 import { IRoomEvent } from "../sync-accumulator";
-import { EventType, RoomType } from "./event";
+import { EventType, RelationType, RoomType } from "./event";
 
 // allow camelcase as these are things that go onto the wire
 /* eslint-disable camelcase */
@@ -47,6 +47,8 @@ export interface IJoinRoomOpts {
 
 export interface IRedactOpts {
     reason?: string;
+    // also delete related events, if the server supports it (MSC3912)
+    with_relations?: Array<RelationType | string>;
 }
 
 export interface ISendEventResponse {

src/client.ts

@@ -154,6 +154,7 @@ import {
     UNSTABLE_MSC3088_ENABLED,
     UNSTABLE_MSC3088_PURPOSE,
     UNSTABLE_MSC3089_TREE_SUBTYPE,
+    UNSTABLE_MSC3912_RELATION_BASED_REDACTIONS,
 } from "./@types/event";
 import { IdServerUnbindResult, IImageInfo, Preset, Visibility } from "./@types/partials";
 import { EventMapper, eventMapperFor, MapperOpts } from "./event-mapper";
@@ -4355,20 +4356,20 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @returns Promise which resolves: TODO
      * @returns Rejects: with an error response.
      */
-    public redactEvent(
+    public async redactEvent(
         roomId: string,
         eventId: string,
         txnId?: string | undefined,
         opts?: IRedactOpts,
     ): Promise<ISendEventResponse>;
-    public redactEvent(
+    public async redactEvent(
         roomId: string,
         threadId: string | null,
         eventId: string,
         txnId?: string | undefined,
         opts?: IRedactOpts,
     ): Promise<ISendEventResponse>;
-    public redactEvent(
+    public async redactEvent(
         roomId: string,
         threadId: string | null,
         eventId?: string,
@@ -4382,9 +4383,20 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             threadId = null;
         }
         const reason = opts?.reason;
+
+        if (opts?.with_relations && !await this.supportsRelationBasedRedactions()) {
+            throw new Error(
+                "Server does not support relation based redactions "
+                + `roomId ${roomId} eventId ${eventId} txnId: ${txnId} threadId ${threadId}`,
+            );
+        }
+
         return this.sendCompleteEvent(roomId, threadId, {
             type: EventType.RoomRedaction,
-            content: { reason },
+            content: {
+                ...(opts?.with_relations ? { with_relations: opts?.with_relations } : {}),
+                reason,
+            },
             redacts: eventId,
         }, txnId as string);
     }
@@ -9265,6 +9277,11 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         return this.clientOpts?.experimentalThreadSupport || false;
     }
 
+    public async supportsRelationBasedRedactions(): Promise<boolean> {
+        return await this.doesServerSupportUnstableFeature(UNSTABLE_MSC3912_RELATION_BASED_REDACTIONS.stable!)
+            || await this.doesServerSupportUnstableFeature(UNSTABLE_MSC3912_RELATION_BASED_REDACTIONS.unstable!);
+    }
+
     /**
      * Fetches the summary of a room as defined by an initial version of MSC3266 and implemented in Synapse
      * Proposed at https://github.com/matrix-org/matrix-doc/pull/3266