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proper-lockfile

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An inter-process and inter-machine lockfile utility that works on a local or network file system.

Installation

$ npm install proper-lockfile

Design

There are various ways to achieve file locking.

This library utilizes the mkdir strategy which works atomically on any kind of file system, even network based ones. The lockfile path is based on the file path you are trying to lock by suffixing it with .lock.

When a lock is successfully acquired, the lockfile's mtime (modified time) is periodically updated to prevent staleness. This allows to effectively check if a lock is stale by checking its mtime against a stale threshold. If the update of the mtime fails several times, the lock might be compromised. The mtime is supported in almost every filesystem.

Comparison

This library is similar to lockfile but the latter has some drawbacks:

O_EXCL is broken on NFS file systems; programs which rely on it for performing locking tasks will contain a race condition.

Compromised

proper-lockfile does not detect cases in which:

proper-lockfile detects cases in which:

As you see, the first two are a consequence of bad usage. Technically, it was possible to detect the first two but it would introduce complexity and eventual race conditions.

Usage

.lock(file, [options])

Tries to acquire a lock on file or rejects the promise on error.

If the lock succeeds, a release function is provided that should be called when you want to release the lock. The release function also rejects the promise on error (e.g. when the lock was already compromised).

Available options:

const lockfile = require('proper-lockfile');

lockfile.lock('some/file')
.then((release) => {
    // Do something while the file is locked

    // Call the provided release function when you're done,
    // which will also return a promise
    return release();
})
.catch((e) => {
    // either lock could not be acquired
    // or releasing it failed
    console.error(e)
});

// Alternatively, you may use lockfile('some/file') directly.

.unlock(file, [options])

Releases a previously acquired lock on file or rejects the promise on error.

Whenever possible you should use the release function instead (as exemplified above). Still there are cases in which it's hard to keep a reference to it around code. In those cases unlock() might be handy.

Available options:

const lockfile = require('proper-lockfile');

lockfile.lock('some/file')
.then(() => {
    // Do something while the file is locked

    // Later..
    return lockfile.unlock('some/file');
});

.check(file, [options])

Check if the file is locked and its lockfile is not stale, rejects the promise on error.

Available options:

const lockfile = require('proper-lockfile');

lockfile.check('some/file')
.then((isLocked) => {
    // isLocked will be true if 'some/file' is locked, false otherwise
});

.lockSync(file, [options])

Sync version of .lock().
Returns the release function or throws on error.

.unlockSync(file, [options])

Sync version of .unlock().
Throws on error.

.checkSync(file, [options])

Sync version of .check(). Returns a boolean or throws on error.

Graceful exit

proper-lockfile automatically removes locks if the process exits, except if the process is killed with SIGKILL or it crashes due to a VM fatal error (e.g.: out of memory).

Tests

$ npm test
$ npm test -- --watch during development

The test suite is very extensive. There's even a stress test to guarantee exclusiveness of locks.

License

Released under the MIT License.