perf(app): stop unbounded WASM heap growth and idle-CPU churn (#1721)

fluxer_app/src/features/channel/state/ChannelDisplayName.ts

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
+import {trackedRecipientNameKey} from '@app/features/channel/state/ChannelDisplayNameTracking';
 import {onLocaleChange} from '@app/features/i18n/utils/LocaleChangeListener';
 import {deferUntilModulesLoaded} from '@app/features/platform/utils/DeferUntilModulesLoaded';
 import {noteText} from '@app/features/theme/fonts/ScriptFontLoader';
@@ -48,12 +49,8 @@ class ChannelDisplayName {
 		deferUntilModulesLoaded(() => {
 			reaction(
 				() => {
-					if (!Users) return [];
-					return Users.usersList.map((user) => ({
-						id: user.id,
-						username: user.username,
-						globalName: user.globalName,
-					}));
+					if (!Users) return '';
+					return trackedRecipientNameKey(this.channelSnapshots, Users.users);
 				},
 				() => this.recomputeAll(),
 			);

fluxer_app/src/features/channel/state/ChannelDisplayNameTracking.test.ts

@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+import {
+	type TrackedRecipientSnapshot,
+	type TrackedRecipientUser,
+	trackedRecipientNameKey,
+} from '@app/features/channel/state/ChannelDisplayNameTracking';
+import {autorun, observable, runInAction} from 'mobx';
+import {describe, expect, it} from 'vitest';
+
+const snapshots = (entries: Record<string, ReadonlyArray<string>>): ReadonlyMap<string, TrackedRecipientSnapshot> =>
+	new Map(Object.entries(entries).map(([id, recipientIds]) => [id, {recipientIds}]));
+
+const userTable = (count: number): Record<string, TrackedRecipientUser> => {
+	const users: Record<string, TrackedRecipientUser> = {};
+	for (let i = 0; i < count; i++) {
+		users[`u${i}`] = {username: `user${i}`, globalName: null};
+	}
+	return users;
+};
+
+/** Wraps a user table so we can count exactly which keys get read. */
+const countingUsers = (
+	users: Record<string, TrackedRecipientUser | undefined>,
+): {proxy: Record<string, TrackedRecipientUser | undefined>; reads: Array<string>} => {
+	const reads: Array<string> = [];
+	const proxy = new Proxy(users, {
+		get(target, key, receiver) {
+			if (typeof key === 'string') reads.push(key);
+			return Reflect.get(target, key, receiver);
+		},
+	});
+	return {proxy, reads};
+};
+
+describe('trackedRecipientNameKey', () => {
+	it('reads only the recipients of tracked group DMs, not the whole user cache', () => {
+		const {proxy, reads} = countingUsers(userTable(10_000));
+		trackedRecipientNameKey(snapshots({c1: ['u1', 'u2'], c2: ['u3']}), proxy);
+
+		// The cost must be a function of tracked recipients, never of cache size.
+		expect(reads).toEqual(['u1', 'u2', 'u3']);
+	});
+
+	it('cost is independent of how large the user cache grows', () => {
+		const small = countingUsers(userTable(10));
+		const huge = countingUsers(userTable(100_000));
+		const tracked = snapshots({c1: ['u1', 'u2']});
+
+		trackedRecipientNameKey(tracked, small.proxy);
+		trackedRecipientNameKey(tracked, huge.proxy);
+
+		expect(small.reads.length).toBe(2);
+		expect(huge.reads.length).toBe(2);
+	});
+
+	it('changes when a tracked recipient is renamed', () => {
+		const users = userTable(3);
+		const tracked = snapshots({c1: ['u1']});
+		const before = trackedRecipientNameKey(tracked, users);
+
+		users.u1 = {username: 'renamed', globalName: null};
+
+		expect(trackedRecipientNameKey(tracked, users)).not.toBe(before);
+	});
+
+	it('does not change when an untracked user is renamed', () => {
+		const users = userTable(3);
+		const tracked = snapshots({c1: ['u1']});
+		const before = trackedRecipientNameKey(tracked, users);
+
+		users.u2 = {username: 'unrelated-change', globalName: 'also changed'};
+
+		expect(trackedRecipientNameKey(tracked, users)).toBe(before);
+	});
+
+	it('distinguishes a missing recipient from one that arrives later', () => {
+		const users: Record<string, TrackedRecipientUser | undefined> = {};
+		const tracked = snapshots({c1: ['u1']});
+		const before = trackedRecipientNameKey(tracked, users);
+
+		users.u1 = {username: 'arrived', globalName: null};
+
+		expect(trackedRecipientNameKey(tracked, users)).not.toBe(before);
+	});
+
+	it('is not confused by usernames containing the separator characters', () => {
+		const tracked = snapshots({c1: ['a', 'b']});
+		const collidingLeft: Record<string, TrackedRecipientUser> = {
+			a: {username: 'x", "y', globalName: null},
+			b: {username: '', globalName: null},
+		};
+		const collidingRight: Record<string, TrackedRecipientUser> = {
+			a: {username: '', globalName: null},
+			b: {username: 'x", "y', globalName: null},
+		};
+
+		expect(trackedRecipientNameKey(tracked, collidingLeft)).not.toBe(trackedRecipientNameKey(tracked, collidingRight));
+	});
+
+	it('drives a MobX reaction on tracked renames but stays quiet on untracked ones', () => {
+		const users = observable.object<Record<string, TrackedRecipientUser | undefined>>({
+			u1: {username: 'one', globalName: null},
+			u2: {username: 'two', globalName: null},
+		});
+		const tracked = snapshots({c1: ['u1']});
+
+		let runs = 0;
+		const dispose = autorun(() => {
+			trackedRecipientNameKey(tracked, users);
+			runs++;
+		});
+		expect(runs).toBe(1);
+
+		// An untracked user changing must not wake the reaction.
+		runInAction(() => {
+			users.u2 = {username: 'two-renamed', globalName: null};
+		});
+		expect(runs).toBe(1);
+
+		// A tracked user changing must.
+		runInAction(() => {
+			users.u1 = {username: 'one-renamed', globalName: null};
+		});
+		expect(runs).toBe(2);
+
+		dispose();
+	});
+});

fluxer_app/src/features/channel/state/ChannelDisplayNameTracking.ts

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+export interface TrackedRecipientSnapshot {
+	readonly recipientIds: ReadonlyArray<string>;
+}
+
+export interface TrackedRecipientUser {
+	readonly username: string;
+	readonly globalName?: string | null;
+}
+
+/**
+ * Builds the MobX tracking key for group-DM display names.
+ *
+ * Only the recipients of tracked group DMs are read, so the observing reaction
+ * depends on those users alone rather than on the whole (unbounded) user cache.
+ * Reading a recipient that is not cached yet still registers a dependency on
+ * that key, so a user arriving later re-triggers the reaction.
+ *
+ * Kept as a standalone function rather than a class method because
+ * makeAutoObservable converts methods into actions, and actions run untracked.
+ */
+export const trackedRecipientNameKey = (
+	snapshots: ReadonlyMap<string, TrackedRecipientSnapshot>,
+	users: Record<string, TrackedRecipientUser | undefined>,
+): string => {
+	const parts: Array<[string, string, string]> = [];
+	for (const snapshot of snapshots.values()) {
+		for (const recipientId of snapshot.recipientIds) {
+			const user = users[recipientId];
+			parts.push([recipientId, user?.username ?? '', user?.globalName ?? '']);
+		}
+	}
+	return JSON.stringify(parts);
+};