os-lock

os-lock
=======

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

Install
-------

npm install os-lock

Usage
-----

$3

``javascript
await lock(fd, options)
await lock(
fd, start = 0, length = 0, options = { exclusive: false, immediate: false }
)
await unlock(fd, start = 0, length = 0)
`

$3

- 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.

$3

`javascript
const cluster = require('cluster');
const fs = require('fs');
const util = require('util');

const { lock, unlock } = require('os-lock');

const fsOpen = util.promisify(fs.open);
const fsClose = util.promisify(fs.close);
const fsUnlink = util.promisify(fs.unlink);

const filename = 'file';

async function master() {
const fd = await fsOpen(filename, 'wx');
// Get an exclusive lock
await lock(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(fd); await fsClose(fd) }, 3000);
// Delete the file after the worker is done with it
worker.on('disconnect', async () => await fsUnlink(filename));
}

async function worker() {
const fd = await fsOpen(filename, 'r+');
console.time('got lock after');
try {
// Try (and fail) to get a lock immediately
await lock(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(fd);
console.timeEnd('got lock after');
await unlock(fd);
cluster.worker.disconnect();
}

if (cluster.isMaster) master();
else worker();
``