/**
 * One osu!tourney + tosu stack per PC. tosu reads every osu! process on the
 * machine and serves the first tourney manager it finds, so two instances
 * running osu!tourney at once would mix up their scores and rooms. An
 * instance holds this lock while it runs the stack; others wait and say why.
 */
import { open, readFile, rm } from 'node:fs/promises';

export interface LockOwner {
	instance: string;
	pid: number;
	at: string;
}

function alive(pid: number) {
	try {
		process.kill(pid, 0);
		return true;
	} catch (err) {
		// EPERM: exists but belongs to someone else.
		return (err as NodeJS.ErrnoException).code === 'EPERM';
	}
}

export async function lockOwner(path: string): Promise<LockOwner | null> {
	try {
		const owner = JSON.parse(await readFile(path, 'utf8')) as LockOwner;
		return owner && alive(owner.pid) ? owner : null;
	} catch {
		return null;
	}
}

/** Takes the lock for this process, or returns who holds it. */
export async function acquireLock(
	path: string,
	instance: string,
	pid = process.pid
): Promise<{ ok: true } | { ok: false; owner: LockOwner }> {
	for (let attempt = 0; attempt < 3; attempt++) {
		try {
			const file = await open(path, 'wx');
			await file.writeFile(JSON.stringify({ instance, pid, at: new Date().toISOString() }));
			await file.close();
			return { ok: true };
		} catch (err) {
			if ((err as NodeJS.ErrnoException).code !== 'EEXIST') throw err;
		}
		const owner = await lockOwner(path);
		if (owner && owner.pid === pid) return { ok: true };
		if (owner) return { ok: false, owner };
		// The holder is gone; clear its lock and try again.
		await rm(path, { force: true });
	}
	const owner = await lockOwner(path);
	return owner
		? { ok: false, owner }
		: { ok: false, owner: { instance: 'unknown', pid: 0, at: '' } };
}

export async function releaseLock(path: string, pid = process.pid) {
	try {
		const owner = JSON.parse(await readFile(path, 'utf8')) as LockOwner;
		if (owner.pid === pid) await rm(path, { force: true });
	} catch {
		/* not ours or already gone */
	}
}