os-lock

Cross-platform file locking. Uses fcntl on UNIX and LockFileEx/UnlockFileEx on Windows.

Install

npm install os-lock

Usage

Synopsis

async function lock(fd, options);
async function lock(
  fd,
  start = 0,
  length = 0,
  options = { exclusive: false, immediate: false }
);
async function unlock(fd, start = 0, length = 0);

Arguments

• fd - File descriptor.
• start - Starting offset for lock.
• length - Number of bytes to lock. If 0, lock till end of file.
• exclusive - Acquire an exclusive lock (i.e. write lock).
• immediate - Throw if a conflicting lock is present instead of waiting.

An error thrown due to immediate will have its code property set to EACCES, EAGAIN or EBUSY.

Example

import cluster from "cluster";
import fs from "fs/promises";

import { lock, unlock } from "os-lock";

const filename = "file";

async function primary() {
  const file = await fs.open(filename, "wx");
  // Get an exclusive lock
  await lock(file.fd, { exclusive: true });
  // Fork a new process which will attempt to get a lock for itself
  // (see the worker() function below)
  const worker = cluster.fork();
  // Pretend to do work and unlock the file after 3 seconds
  setTimeout(async () => {
    await unlock(file.fd);
    await file.close();
  }, 3000);
  // Delete the file after the worker is done with it
  worker.on("disconnect", async () => await fs.unlink(filename));
}

async function worker() {
  const file = await fs.open(filename, "r+");
  console.time("got lock after");
  try {
    // Try (and fail) to get a lock immediately
    await lock(file.fd, { immediate: true });
  } catch (e) {
    if (!/^(EACCES|EAGAIN|EBUSY)$/.test(e.code)) throw e;
    console.warn("failed to get lock immediately");
  }
  // Try again, this time with patience
  await lock(file.fd);
  console.timeEnd("got lock after");
  await unlock(file.fd);
  cluster.worker.disconnect();
}

if (cluster.isPrimary) primary();
else worker();