#12: Add expiration time for memcache lock storage

[arvenil]

TLDR

->setExpiration($expirationInSeconds)

<?php
require 'vendor/autoload.php';

use NinjaMutex\Lock\MemcacheLock;
use NinjaMutex\Mutex;

$memcache = new Memcache();
$memcache->connect('127.0.0.1', 11211);
$lock = new MemcacheLock($memcache);
$lock->setExpiration(1800) // 1800s = 30 minutes
$mutex = new Mutex('very-critical-stuff', $lock);
if ($mutex->acquireLock(1000)) {
    // Do some very critical stuff

    // and release lock after you finish
    $mutex->releaseLock();
} else {
    throw new Exception('Unable to gain lock!');
}

Explanation

If you gain lock with Memcache or Memcached and the program crashes it may lead to situation where the lock is not released as it is not automatically released when program dies (in theory there is __destruct that should release the lock but this may not always work). Flock and MySQL implementation doesn't have this drawback as the lock is released when file handle is released (flock) or connection is closed (MySQL) and those both things happen when php script dies.

To bypass the issue for Memcache and Memcached you can now use ->setExpiration($expirationInSeconds) method to allow lock to expire.

Be aware this method is highly unrecommended as it may lead to race conditions if lock expires and it was still actually held by still working process. This is for users who knows what they do ;) I would suggest high values for ->setExpiration($expiration) and adding logging to your script if locking took more time than $expiration so you know the $expiration time was set too low.

If script which gained the lock is still running while the lock expires then when it will try to release the lock it will fail or throw UnrecoverableMutexException in __destructor (or if lock was gained by another script already - race condition - then the second one may fail this way). In other words if any of your scripts fails with this exception or while executing ->releaseLock() then a) you screwed up with setting ->setExpiration() :) b) Memcache or Memcached became unavailable service (#2).

Why so huge PR?

Numerous problems and bugs encountered while trying to write tests for this (some of the issues)

• https://github.com/facebook/hhvm/blob/af329776c9f740cc1c8c4791f673ba5aa49042ce/hphp/doc/inconsistencies#L40-L45
• User-flags related changes cause issues during inheritance php-memcached-dev/php-memcached#126
• Serialization of 'Closure' is not allowed sebastianbergmann/phpunit#451