os-lock/README.md

Cross-platform file locking/unlocking

os-lock
=======

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

Install
-------

    npm install os-lock

Usage
-----

### Synopsis

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

```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();
```