Add Experimental MentionLimitProtection. (#518)

Based on https://github.com/the-draupnir-project/Draupnir/pull/495/files.
Documentation pending.

src/config.ts

@@ -84,6 +84,10 @@ export interface IConfig {
       serverNames: string[];
       banMessage: string;
     };
+    mentionLimitProtection: {
+      maxMentions: number;
+      redactReason: string;
+    };
   };
   health: {
     healthz: {
@@ -178,6 +182,11 @@ const defaultConfig: IConfig = {
       banMessage:
         "Unfortunately we cannot accept new users from your homeserver at this time.",
     },
+    mentionLimitProtection: {
+      maxMentions: 3,
+      redactReason:
+        "You have mentioned too many users in this message, so we have had to redact it.",
+    },
   },
   health: {
     healthz: {

src/protections/DraupnirProtectionsIndex.ts

@@ -14,6 +14,7 @@ import "./BasicFlooding";
 import "./FirstMessageIsImage";
 import "./JoinWaveShortCircuit";
 import "./RedactionSynchronisation";
+import "./MentionLimitProtection";
 import "./MessageIsMedia";
 import "./MessageIsVoice";
 import "./NewJoinerProtection";

src/protections/MentionLimitProtection.ts

@@ -0,0 +1,113 @@
+// Copyright 2024 Gnuxie <[email protected]>
+//
+// SPDX-License-Identifier: AFL-3.0
+
+import {
+  AbstractProtection,
+  ActionResult,
+  EventConsequences,
+  Ok,
+  ProtectedRoomsSet,
+  Protection,
+  ProtectionDescription,
+  RoomEvent,
+  UnknownSettings,
+  Value,
+  describeProtection,
+} from "matrix-protection-suite";
+import { Draupnir } from "../Draupnir";
+import { IConfig } from "../config";
+import { MatrixRoomID } from "@the-draupnir-project/matrix-basic-types";
+import { Type } from "@sinclair/typebox";
+
+const MentionsContentSchema = Type.Object({
+  "m.mentions": Type.Object({
+    user_ids: Type.Array(Type.String()),
+  }),
+});
+
+const NewContentMentionsSchema = Type.Object({
+  "m.new_content": MentionsContentSchema,
+});
+
+export type MentionLimitProtectionDescription = ProtectionDescription<
+  unknown,
+  UnknownSettings<string>,
+  MentionLimitProtectionCapabilities
+>;
+
+export class MentionLimitProtection
+  extends AbstractProtection<MentionLimitProtectionDescription>
+  implements Protection<MentionLimitProtectionDescription>
+{
+  private readonly eventConsequences: EventConsequences;
+  private readonly redactReason: string;
+  private readonly maxMentions: number;
+  constructor(
+    description: MentionLimitProtectionDescription,
+    capabilities: MentionLimitProtectionCapabilities,
+    protectedRoomsSet: ProtectedRoomsSet,
+    draupnirConfig: IConfig
+  ) {
+    super(description, capabilities, protectedRoomsSet, {});
+    this.eventConsequences = capabilities.eventConsequences;
+    this.maxMentions =
+      draupnirConfig.protections.mentionLimitProtection.maxMentions;
+    this.redactReason =
+      draupnirConfig.protections.mentionLimitProtection.redactReason;
+  }
+  public async handleTimelineEvent(
+    _room: MatrixRoomID,
+    event: RoomEvent
+  ): Promise<ActionResult<void>> {
+    const isOverLimit = (user_ids: string[]): boolean =>
+      user_ids.length > this.maxMentions;
+    if (
+      Value.Check(NewContentMentionsSchema, event.content) &&
+      isOverLimit(event.content["m.new_content"]["m.mentions"].user_ids)
+    ) {
+      return await this.eventConsequences.consequenceForEvent(
+        event.room_id,
+        event.event_id,
+        this.redactReason
+      );
+    } else if (
+      Value.Check(MentionsContentSchema, event.content) &&
+      isOverLimit(event.content["m.mentions"].user_ids)
+    ) {
+      return await this.eventConsequences.consequenceForEvent(
+        event.room_id,
+        event.event_id,
+        this.redactReason
+      );
+    } else {
+      return Ok(undefined);
+    }
+  }
+}
+
+export type MentionLimitProtectionCapabilities = {
+  eventConsequences: EventConsequences;
+};
+
+describeProtection<MentionLimitProtectionCapabilities, Draupnir>({
+  name: "MentionLimitProtection",
+  description: `Highly experimental protection that will remove any messages with
+    a number of mentions over a preconfigured limit.
+    Please read the documentation https://the-draupnir-project.github.io/draupnir-documentation/protections/mention-limit-protection.`,
+  capabilityInterfaces: {
+    eventConsequences: "EventConsequences",
+  },
+  defaultCapabilities: {
+    eventConsequences: "StandardEventConsequences",
+  },
+  factory: (decription, protectedRoomsSet, draupnir, capabilitySet) =>
+    Ok(
+      new MentionLimitProtection(
+        decription,
+        capabilitySet,
+        protectedRoomsSet,
+        draupnir.config
+      )
+    ),
+});