pi-gui/src/bridge/instance-lock.js

import { randomUUID } from "node:crypto";
import { chmod, open, readFile, unlink } from "node:fs/promises";

const LOCK_MODE = 0o600;

export class BridgeAlreadyRunningError extends Error {
	constructor(metadata) {
		super(`bridge instance is already running with pid ${metadata.pid}`);
		this.name = "BridgeAlreadyRunningError";
		this.metadata = metadata;
	}
}

export class BridgeLockError extends Error {
	constructor(message) {
		super(message);
		this.name = "BridgeLockError";
	}
}

function parseLockMetadata(text, lockPath) {
	let metadata;
	try {
		metadata = JSON.parse(text);
	} catch {
		throw new BridgeLockError(`lock file is unreadable: ${lockPath}`);
	}
	if (
		metadata === null ||
		typeof metadata !== "object" ||
		!Number.isSafeInteger(metadata.pid) ||
		metadata.pid < 1 ||
		typeof metadata.token !== "string" ||
		metadata.token.length < 1 ||
		typeof metadata.socketPath !== "string" ||
		metadata.socketPath.length < 1 ||
		typeof metadata.startedAt !== "string" ||
		Number.isNaN(Date.parse(metadata.startedAt))
	) {
		throw new BridgeLockError(`lock file has an invalid format: ${lockPath}`);
	}
	return metadata;
}

export function isProcessAlive(pid) {
	try {
		process.kill(pid, 0);
		return true;
	} catch (error) {
		if (error?.code === "ESRCH") return false;
		if (error?.code === "EPERM") return true;
		throw error;
	}
}

async function readLockMetadata(lockPath) {
	try {
		return parseLockMetadata(await readFile(lockPath, "utf8"), lockPath);
	} catch (error) {
		if (error?.code === "ENOENT") return undefined;
		throw error;
	}
}

export async function acquireBridgeLock({
	lockPath,
	socketPath,
	pid = process.pid,
	now = () => new Date(),
	processAlive = isProcessAlive,
}) {
	if (!lockPath || !socketPath)
		throw new TypeError("lockPath and socketPath are required");

	const metadata = {
		pid,
		socketPath,
		startedAt: now().toISOString(),
		token: randomUUID(),
	};

	for (let attempt = 0; attempt < 2; attempt += 1) {
		let handle;
		let createdLock = false;
		try {
			handle = await open(lockPath, "wx", LOCK_MODE);
			createdLock = true;
			await handle.writeFile(`${JSON.stringify(metadata)}\n`, "utf8");
			await handle.sync();
			await chmod(lockPath, LOCK_MODE);
			await handle.close();
			return {
				metadata,
				async release() {
					const current = await readLockMetadata(lockPath);
					if (current?.token !== metadata.token) return false;
					await unlink(lockPath);
					return true;
				},
			};
		} catch (error) {
			await handle?.close().catch(() => {});
			if (createdLock) {
				await unlink(lockPath).catch(() => {});
				throw error;
			}
			if (error?.code !== "EEXIST") throw error;

			const existing = await readLockMetadata(lockPath);
			if (!existing) continue;
			if (processAlive(existing.pid))
				throw new BridgeAlreadyRunningError(existing);

			// Only a lock with a readable PID that has exited may be reclaimed.
			await unlink(lockPath).catch((unlinkError) => {
				if (unlinkError?.code !== "ENOENT") throw unlinkError;
			});
		}
	}

	throw new BridgeLockError(
		`could not acquire lock after stale-owner recovery: ${lockPath}`,
	);
}

export const lockMode = LOCK_MODE;