Add mechanism to locally enforce MSC1763 retention rules (#33772)

* First pass at new retention

* Add more test cases

* fixup

* Fix TODO

* Disable retention explicitly from the start

* fixup

* Just apply retention as-is

* Fixup

apps/web/src/MatrixClientPeg.ts

@@ -441,6 +441,7 @@ class MatrixClientPegClass implements IMatrixClientPeg {
             // can toggle on without reloading and also be accessed immediately after login.
             cryptoCallbacks: { ...crossSigningCallbacks },
             enableEncryptedStateEvents: SettingsStore.getValue("feature_msc4362_encrypted_state_events"),
+            unstableMSC1763Retention: SettingsStore.getValue("feature_retention"),
             roomNameGenerator: (_: string, state: RoomNameState) => {
                 switch (state.type) {
                     case RoomNameType.Generated:

apps/web/src/i18n/strings/en_EN.json

@@ -1542,6 +1542,11 @@
         "experimental_section": "Early previews",
         "extended_profiles_msc_support": "Requires your server to support MSC4133",
         "feature_disable_call_per_sender_encryption": "Disable per-sender encryption for Element Call",
+        "feature_retention": {
+            "description": "Automatically remove messages from a room when they reach a configured retention period. Currently only supports reading settings.",
+            "disabled_sliding_sync": "Retention cannot be enabled when Sliding Sync is enabled.",
+            "display_name": "Message retention"
+        },
         "feature_user_status": {
             "description": "Enables being able to see and set a current status.",
             "display_name": "User status",

apps/web/src/settings/Settings.tsx

@@ -225,6 +225,7 @@ export interface Settings {
     "feature_dynamic_room_predecessors": IFeature;
     "feature_render_reaction_images": IFeature;
     "feature_new_room_list": IFeature;
+    "feature_retention": IFeature;
     "feature_room_list_sections": IFeature;
     "feature_ask_to_join": IFeature;
     "feature_notifications": IFeature;
@@ -807,6 +808,22 @@ export const SETTINGS: Settings = {
         ),
         default: false,
     },
+    "feature_retention": {
+        isFeature: true,
+        labsGroup: LabGroup.Messaging,
+        displayName: _td("labs|feature_retention|display_name"),
+        description: _td("labs|feature_retention|description"),
+        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG_PRIORITISED,
+        supportedLevelsAreOrdered: true,
+        controller: new IncompatibleController(
+            "feature_sliding_sync",
+            false,
+            true,
+            _td("labs|feature_retention|disabled_sliding_sync"),
+            true,
+        ),
+        default: false,
+    },
     "useCompactLayout": {
         supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS,
         displayName: _td("settings|preferences|compact_modern"),

apps/web/src/settings/controllers/IncompatibleController.ts

@@ -10,6 +10,7 @@ import SettingController from "./SettingController";
 import { type SettingLevel } from "../SettingLevel";
 import SettingsStore from "../SettingsStore";
 import { type BooleanSettingKey } from "../Settings.tsx";
+import PlatformPeg from "../../PlatformPeg.ts";
 
 /**
  * Enforces that a boolean setting cannot be enabled if the incompatible setting
@@ -21,6 +22,8 @@ export default class IncompatibleController extends SettingController {
         private settingName: BooleanSettingKey,
         private forcedValue: any = false,
         private incompatibleValue: any | ((v: any) => boolean) = true,
+        private readonly disabledMessage?: string,
+        private readonly forceReload = false,
     ) {
         super();
     }
@@ -37,8 +40,11 @@ export default class IncompatibleController extends SettingController {
         return null; // no override
     }
 
-    public get settingDisabled(): boolean {
-        return this.incompatibleSetting;
+    public get settingDisabled(): boolean | string {
+        if (this.incompatibleSetting) {
+            return this.disabledMessage ?? true;
+        }
+        return false;
     }
 
     public get incompatibleSetting(): boolean {
@@ -47,4 +53,10 @@ export default class IncompatibleController extends SettingController {
         }
         return SettingsStore.getValue(this.settingName) === this.incompatibleValue;
     }
+
+    public onChange(): void {
+        if (this.forceReload) {
+            PlatformPeg.get()?.reload();
+        }
+    }
 }