src/lib/server/app.ts

import { randomUUID } from 'node:crypto';
import { RateLimiter } from './auth/rate-limit';
import { loadConfig, type Config } from './config';
import { Ledger, type CompactionResult } from './ledger/ledger';
import { LOCK_PATH, WriterGuard } from './ledger/writer-guard';
import { HfBucketStore } from './storage/hf-bucket-store';
import { LocalFsStore } from './storage/local-fs-store';
import { PrefixedStore } from './storage/prefixed-store';
import type { BucketStore } from './storage/store';
import { UsersRepo } from './users/repository';

export interface CompactionStatus {
	lastRunAt: string | null;
	lastResult: CompactionResult | null;
	lastError: string | null;
	running: boolean;
}

export interface App {
	config: Config;
	store: BucketStore;
	users: UsersRepo;
	ledger: Ledger;
	guard: WriterGuard;
	loginLimiter: RateLimiter;
	/** Second login gate, keyed by client IP, so one host cannot spray many usernames. */
	ipLimiter: RateLimiter;
	compaction: CompactionStatus;
	/** Aligns the ledger's read-only flag with the writer lock, logging any transition. */
	syncReadOnly(): void;
	runCompaction(): Promise<CompactionResult>;
}

const BUILD_RETRY_AFTER_MS = 10_000;

let instance: Promise<App> | null = null;
let failedAt = 0;

/** Builds the singleton on first call; subsequent calls return the same promise. */
export function getApp(): Promise<App> {
	if (instance && (failedAt === 0 || Date.now() - failedAt < BUILD_RETRY_AFTER_MS)) return instance;
	failedAt = 0;
	instance = build().catch((e) => {
		failedAt = Date.now();
		throw e;
	});
	return instance;
}

async function build(): Promise<App> {
	const config = loadConfig();
	const base: BucketStore =
		config.storage === 'hf' ? new HfBucketStore(config.hfBucket, config.hfToken) : new LocalFsStore(config.localDataDir);
	const store: BucketStore = config.hfPrefix ? new PrefixedStore(base, config.hfPrefix) : base;
	console.log(
		`[app] storage=${config.storage}${config.storage === 'hf' ? ` bucket=${config.hfBucket}` : ` dir=${config.localDataDir}`}${config.hfPrefix ? ` prefix=${config.hfPrefix}/` : ''}`
	);

	const users = await UsersRepo.load(store);
	const guard = new WriterGuard(store, randomUUID());
	const acquired = await guard.acquire();
	if (!acquired) console.error('[app] another instance holds the writer lock; starting READ-ONLY until it goes stale');

	try {
		const ledger = await Ledger.load(store);
		ledger.readOnly = !guard.canWrite;
		guard.start();

		const compaction: CompactionStatus = { lastRunAt: null, lastResult: null, lastError: null, running: false };
		let inFlight: Promise<CompactionResult> | null = null;

		const compactOnce = async (): Promise<CompactionResult> => {
			compaction.running = true;
			try {
				const result = await ledger.compact();
				compaction.lastResult = result;
				compaction.lastError = null;
				return result;
			} catch (e) {
				compaction.lastError = e instanceof Error ? e.message : String(e);
				throw e;
			} finally {
				compaction.running = false;
				compaction.lastRunAt = new Date().toISOString();
			}
		};

		const app: App = {
			config,
			store,
			users,
			ledger,
			guard,
			loginLimiter: new RateLimiter(5, 60_000),
			ipLimiter: new RateLimiter(30, 60_000),
			compaction,
			syncReadOnly() {
				const readOnly = !guard.canWrite;
				if (readOnly === ledger.readOnly) return;
				ledger.readOnly = readOnly;
				console.log(readOnly ? '[app] entering READ-ONLY mode' : '[app] writer lock acquired, leaving read-only mode');
			},
			runCompaction() {
				inFlight ??= compactOnce().finally(() => {
					inFlight = null;
				});
				return inFlight;
			}
		};

		const timer = setInterval(() => {
			app.syncReadOnly();
			if (ledger.readOnly) return;
			app.runCompaction().catch((e) => console.error('[compaction] failed', e));
		}, config.compactionIntervalMs);
		timer.unref?.();

		registerShutdown(app, timer);

		if (guard.canWrite && ledger.stats().pendingEvents > 0) {
			app.runCompaction().catch((e) => console.error('[compaction] boot compaction failed', e));
		}
		console.log(
			`[app] ready: ${users.all().length} users, ${ledger.listExpenses().length} expenses, ${ledger.stats().pendingEvents} pending events`
		);
		return app;
	} catch (e) {
		guard.stop();
		throw e;
	}
}

let shutdownRegistered = false;

/**
 * Hands the writer lock back on the way out so a redeploy does not have to wait for the
 * lock to go stale. adapter-node emits `sveltekit:shutdown` once the HTTP server is closed;
 * the signal handlers are a fallback for hosts that do not (e.g. `vite dev`).
 */
function registerShutdown(app: App, timer: ReturnType<typeof setInterval>): void {
	if (shutdownRegistered) return;
	shutdownRegistered = true;

	let released: Promise<void> | null = null;
	const release = (): Promise<void> => {
		released ??= (async () => {
			clearInterval(timer);
			app.guard.stop();
			if (!app.guard.canWrite) return;
			try {
				const bail = new Promise<never>((_, reject) => {
					const t = setTimeout(() => reject(new Error('timed out')), 5_000);
					t.unref?.();
				});
				await Promise.race([app.store.delete([LOCK_PATH]), bail]);
				console.log('[app] released writer lock');
			} catch (e) {
				console.warn('[app] could not release writer lock', e);
			}
		})();
		return released;
	};

	const emitter = process as NodeJS.EventEmitter;
	emitter.on('sveltekit:shutdown', () => void release());

	for (const signal of ['SIGTERM', 'SIGINT'] as const) {
		process.once(signal, () => {
			// If nothing else listens for this signal our handler would otherwise swallow it.
			const sole = process.listenerCount(signal) === 0;
			release().finally(() => {
				if (sole) process.exit(0);
			});
		});
	}
}