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<?php
// This file is part of Moodle - http://moodle.org/
//
// Moodle is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Moodle is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Moodle. If not, see <http://www.gnu.org/licenses/>.
/**
* File locking for the Cache API
*
* @package cachelock_file
* @category cache
* @copyright 2012 Sam Hemelryk
* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
*/
defined('MOODLE_INTERNAL') || die();
/**
* File locking plugin
*
* @copyright 2012 Sam Hemelryk
* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
*/
class cachelock_file implements cache_lock_interface {
/**
* The name of the cache lock instance
* @var string
*/
protected $name;
/**
* The absolute directory in which lock files will be created and looked for.
* @var string
*/
protected $cachedir;
/**
* The maximum life in seconds for a lock file. By default null for none.
* @var int|null
*/
protected $maxlife = null;
/**
* The number of attempts to acquire a lock when blocking is required before throwing an exception.
* @var int
*/
protected $blockattempts = 100;
/**
* An array containing the locks that have been acquired but not released so far.
* @var array Array of key => lock file path
*/
protected $locks = array();
/**
* Initialises the cache lock instance.
*
* @param string $name The name of the cache lock
* @param array $configuration
*/
public function __construct($name, array $configuration = array()) {
$this->name = $name;
if (!array_key_exists('dir', $configuration)) {
$this->cachedir = make_cache_directory(md5($name));
} else {
$dir = $configuration['dir'];
if (strpos($dir, '/') !== false && strpos($dir, '.') !== 0) {
// This looks like an absolute path.
if (file_exists($dir) && is_dir($dir) && is_writable($dir)) {
$this->cachedir = $dir;
}
}
if (empty($this->cachedir)) {
$dir = preg_replace('#[^a-zA-Z0-9_]#', '_', $dir);
$this->cachedir = make_cache_directory($dir);
}
}
if (array_key_exists('maxlife', $configuration) && is_number($configuration['maxlife'])) {
$maxlife = (int)$configuration['maxlife'];
// Minimum lock time is 60 seconds.
$this->maxlife = max($maxlife, 60);
}
if (array_key_exists('blockattempts', $configuration) && is_number($configuration['blockattempts'])) {
$this->blockattempts = (int)$configuration['blockattempts'];
}
}
/**
* Acquire a lock.
*
* If the lock can be acquired:
* This function will return true.
*
* If the lock cannot be acquired the result of this method is determined by the block param:
* $block = true (default)
* The function will block any further execution unti the lock can be acquired.
* This involves the function attempting to acquire the lock and the sleeping for a period of time. This process
* will be repeated until the lock is required or until a limit is hit (100 by default) in which case a cache
* exception will be thrown.
* $block = false
* The function will return false immediately.
*
* If a max life has been specified and the lock can not be acquired then the lock file will be checked against this time.
* In the case that the file exceeds that max time it will be forcefully deleted.
* Because this can obviously be a dangerous thing it is not used by default. If it is used it should be set high enough that
* we can be as sure as possible that the executing code has completed.
*
* @param string $key The key that we want to lock
* @param string $ownerid A unique identifier for the owner of this lock. Not used by default.
* @param bool $block True if we want the program block further execution until the lock has been acquired.
* @return bool
* @throws cache_exception If block is set to true and more than 100 attempts have been made to acquire a lock.
*/
public function lock($key, $ownerid, $block = false) {
// Get the name of the lock file we want to use.
$lockfile = $this->get_lock_file($key);
// Attempt to create a handle to the lock file.
// Mode xb is the secret to this whole function.
// x = Creates the file and opens it for writing. If the file already exists fopen returns false and a warning is thrown.
// b = Forces binary mode.
$result = @fopen($lockfile, 'xb');
// Check if we could create the file or not.
if ($result === false) {
// Lock exists already.
if ($this->maxlife !== null && !array_key_exists($key, $this->locks)) {
$mtime = filemtime($lockfile);
if ($mtime < time() - $this->maxlife) {
$this->unlock($key, true);
$result = $this->lock($key, false);
if ($result) {
return true;
}
}
}
if ($block) {
// OK we are blocking. We had better sleep and then retry to lock.
$iterations = 0;
$maxiterations = $this->blockattempts;
while (($result = $this->lock($key, false)) === false) {
// Usleep causes the application to cleep to x microseconds.
// Before anyone asks there are 1'000'000 microseconds to a second.
usleep(rand(1000, 50000)); // Sleep between 1 and 50 milliseconds.
$iterations++;
if ($iterations > $maxiterations) {
// BOOM! We've exceeded the maximum number of iterations we want to block for.
throw new cache_exception('ex_unabletolock');
}
}
}
return false;
} else {
// We have the lock.
fclose($result);
$this->locks[$key] = $lockfile;
return true;
}
}
/**
* Releases an acquired lock.
*
* For more details see {@link cache_lock::unlock()}
*
* @param string $key
* @param string $ownerid A unique identifier for the owner of this lock. Not used by default.
* @param bool $forceunlock If set to true the lock will be removed if it exists regardless of whether or not we own it.
* @return bool
*/
public function unlock($key, $ownerid, $forceunlock = false) {
if (array_key_exists($key, $this->locks)) {
@unlink($this->locks[$key]);
unset($this->locks[$key]);
return true;
} else if ($forceunlock) {
$lockfile = $this->get_lock_file($key);
if (file_exists($lockfile)) {
@unlink($lockfile);
}
return true;
}
// You cannot unlock a file you didn't lock.
return false;
}
/**
* Checks if the given key is locked.
*
* @param string $key
* @param string $ownerid
*/
public function check_state($key, $ownerid) {
if (array_key_exists($key, $this->locks)) {
// The key is locked and we own it.
return true;
}
$lockfile = $this->get_lock_file($key);
if (file_exists($lockfile)) {
// The key is locked and we don't own it.
return false;
}
return null;
}
/**
* Gets the name to use for a lock file.
*
* @param string $key
* @return string
*/
protected function get_lock_file($key) {
return $this->cachedir.'/'. $key .'.lock';
}
/**
* Cleans up the instance what it is no longer needed.
*/
public function __destruct() {
foreach ($this->locks as $lockfile) {
// Naught, naughty developers.
@unlink($lockfile);
}
}
}