<?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/>.

use core_cache\configurable_cache_interface;
use core_cache\definition;
use core_cache\key_aware_cache_interface;
use core_cache\lockable_cache_interface;
use core_cache\searchable_cache_interface;
use core_cache\store;

/**
 * The file store class.
 *
 * Configuration options
 *      path:           string: path to the cache directory, if left empty one will be created in the cache directory
 *      autocreate:     true, false
 *      prescan:        true, false
 *
 * @package    cachestore_file
 * @copyright  2012 Sam Hemelryk
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
class cachestore_file extends store implements
    key_aware_cache_interface,
    configurable_cache_interface,
    searchable_cache_interface,
    lockable_cache_interface
{
    /**
     * Value to represent use of the PHP serializer.
     */
    public const SERIALIZER_PHP = 'php';

    /**
     * Value to represent use of the Igbinary serializer.
     */
    public const SERIALIZER_IGBINARY = 'igbinary';

    /**
     * The name of the store.
     * @var string
     */
    protected $name;

    /**
     * The path used to store files for this store and the definition it was initialised with.
     * @var string
     */
    protected $path = false;

    /**
     * The path in which definition specific sub directories will be created for caching.
     * @var string
     */
    protected $filestorepath = false;

    /**
     * Set to true when a prescan has been performed.
     * @var bool
     */
    protected $prescan = false;

    /**
     * Set to true if we should store files within a single directory.
     * By default we use a nested structure in order to reduce the chance of conflicts and avoid any file system
     * limitations such as maximum files per directory.
     * @var bool
     */
    protected $singledirectory = false;

    /**
     * Set to true when the path should be automatically created if it does not yet exist.
     * @var bool
     */
    protected $autocreate = false;

    /**
     * Set to true if new cache revision directory needs to be created. Old directory will be purged asynchronously
     * via Schedule task.
     * @var bool
     */
    protected $asyncpurge = false;

    /**
     * Set to true if a custom path is being used.
     * @var bool
     */
    protected $custompath = false;

    /**
     * An array of keys we are sure about presently.
     * @var array
     */
    protected $keys = array();

    /**
     * True when the store is ready to be initialised.
     * @var bool
     */
    protected $isready = false;

    /**
     * The cache definition this instance has been initialised with.
     * @var definition
     */
    protected $definition;

    /**
     * Bytes read or written by last call to set()/get() or set_many()/get_many().
     *
     * @var int
     */
    protected $lastiobytes = 0;

    /**
     * A reference to the global $CFG object.
     *
     * You may be asking yourself why on earth this is here, but there is a good reason.
     * By holding onto a reference of the $CFG object we can be absolutely sure that it won't be destroyed before
     * we are done with it.
     * This makes it possible to use a cache within a destructor method for the purposes of
     * delayed writes. Like how the session mechanisms work.
     *
     * @var stdClass
     */
    private $cfg = null;

    /** @var int Maximum number of seconds to wait for a lock before giving up. */
    protected $lockwait = 60;

    /**
     * Instance of file_lock_factory configured to create locks in the cache directory.
     *
     * @var \core\lock\file_lock_factory $lockfactory
     */
    protected $lockfactory = null;

    /**
     * List of current locks.
     *
     * @var array $locks
     */
    protected $locks = [];

    /**
     * Serializer for this store.
     *
     * @var string
     */
    protected $serializer = self::SERIALIZER_PHP;

    /**
     * Determine if igbinary functions are available for use.
     *
     * @return boolean
     */
    public static function igbinary_available(): bool {
        return function_exists('igbinary_serialize');
    }

    /**
     * Gets an array of options to use as the serialiser.
     *
     * @return array
     */
    public static function config_get_serializer_options(): array {
        $options = [
            self::SERIALIZER_PHP => get_string('serializer_php', 'cachestore_file'),
        ];
        if (self::igbinary_available()) {
            $options[self::SERIALIZER_IGBINARY] = get_string('serializer_igbinary', 'cachestore_file');
        }
        return $options;
    }

    /**
     * Constructs the store instance.
     *
     * Noting that this function is not an initialisation. It is used to prepare the store for use.
     * The store will be initialised when required and will be provided with a definition at that time.
     *
     * @param string $name
     * @param array $configuration
     */
    public function __construct($name, array $configuration = array()) {
        global $CFG;

        if (isset($CFG)) {
            // Hold onto a reference of the global $CFG object.
            $this->cfg = $CFG;
        }

        $this->name = $name;
        if (array_key_exists('path', $configuration) && $configuration['path'] !== '') {
            $this->custompath = true;
            $this->autocreate = !empty($configuration['autocreate']);
            $path = (string)$configuration['path'];
            if (!is_dir($path)) {
                if ($this->autocreate) {
                    if (!make_writable_directory($path, false)) {
                        $path = false;
                        debugging('Error trying to autocreate file store path. '.$path, DEBUG_DEVELOPER);
                    }
                } else {
                    $path = false;
                    debugging('The given file cache store path does not exist. '.$path, DEBUG_DEVELOPER);
                }
            }
            if ($path !== false && !is_writable($path)) {
                $path = false;
                debugging('The file cache store path is not writable for `'.$name.'`', DEBUG_DEVELOPER);
            }
        } else {
            $path = make_cache_directory('cachestore_file/'.preg_replace('#[^a-zA-Z0-9\.\-_]+#', '', $name));
        }
        $this->isready = $path !== false;
        $this->filestorepath = $path;
        // This will be updated once the store has been initialised for a definition.
        $this->path = $path;

        // Check if we should prescan the directory.
        if (array_key_exists('prescan', $configuration)) {
            $this->prescan = (bool)$configuration['prescan'];
        } else {
            // Default is no, we should not prescan.
            $this->prescan = false;
        }
        // Check if we should be storing in a single directory.
        if (array_key_exists('singledirectory', $configuration)) {
            $this->singledirectory = (bool)$configuration['singledirectory'];
        } else {
            // Default: No, we will use multiple directories.
            $this->singledirectory = false;
        }
        // Check if directory needs to be purged asynchronously.
        if (array_key_exists('asyncpurge', $configuration)) {
            $this->asyncpurge = (bool)$configuration['asyncpurge'];
        } else {
            $this->asyncpurge = false;
        }

        // Leverage cachelock_file to provide native locking, to avoid duplicating logic.
        // This will store locks alongside the cache, so local cache uses local locks.
        $lockdir = $path . '/filelocks';
        if (!file_exists($lockdir)) {
            make_writable_directory($lockdir);
        }
        if (array_key_exists('lockwait', $configuration)) {
            $this->lockwait = (int)$configuration['lockwait'];
        }
        $this->lockfactory = new \core\lock\file_lock_factory('cachestore_file', $lockdir);
        if (!$this->lockfactory->is_available()) {
            // File locking is disabled in config, fall back to default lock factory.
            $this->lockfactory = \core\lock\lock_config::get_lock_factory('cachestore_file');
        }

        // Set the serializer to use based on configuration.
        if (array_key_exists('serializer', $configuration)) {
            $this->serializer = (string)$configuration['serializer'];
        }
    }

    /**
     * Performs any necessary operation when the file store instance has been created.
     */
    public function instance_created() {
        if ($this->isready && !$this->prescan) {
            // It is supposed the store instance to expect an empty folder.
            $this->purge_all_definitions();
        }
    }

    /**
     * Returns true if this store instance is ready to be used.
     * @return bool
     */
    public function is_ready() {
        return $this->isready;
    }

    /**
     * Returns true once this instance has been initialised.
     *
     * @return bool
     */
    public function is_initialised() {
        return true;
    }

    /**
     * Returns the supported features as a combined int.
     *
     * @param array $configuration
     * @return int
     */
    public static function get_supported_features(array $configuration = array()) {
        $supported = self::SUPPORTS_DATA_GUARANTEE +
                     self::SUPPORTS_NATIVE_TTL +
                     self::IS_SEARCHABLE +
                     self::DEREFERENCES_OBJECTS;
        return $supported;
    }

    /**
     * Returns false as this store does not support multiple identifiers.
     * (This optional function is a performance optimisation; it must be
     * consistent with the value from get_supported_features.)
     *
     * @return bool False
     */
    public function supports_multiple_identifiers() {
        return false;
    }

    /**
     * Returns the supported modes as a combined int.
     *
     * @param array $configuration
     * @return int
     */
    public static function get_supported_modes(array $configuration = array()) {
        return self::MODE_APPLICATION + self::MODE_SESSION;
    }

    /**
     * Returns true if the store requirements are met.
     *
     * @return bool
     */
    public static function are_requirements_met() {
        return true;
    }

    /**
     * Returns true if the given mode is supported by this store.
     *
     * @param int $mode One of store::MODE_*
     * @return bool
     */
    public static function is_supported_mode($mode) {
        return ($mode === static::MODE_APPLICATION || $mode === static::MODE_SESSION);
    }

    /**
     * Initialises the cache.
     *
     * Once this has been done the cache is all set to be used.
     *
     * @param definition $definition
     */
    public function initialise(definition $definition) {
        global $CFG;

        $this->definition = $definition;
        $hash = preg_replace('#[^a-zA-Z0-9]+#', '_', $this->definition->get_id());
        $this->path = $this->filestorepath.'/'.$hash;
        make_writable_directory($this->path, false);

        if ($this->asyncpurge) {
            $timestampfile = $this->path . '/.lastpurged';
            if (!file_exists($timestampfile)) {
                touch($timestampfile);
                @chmod($timestampfile, $CFG->filepermissions);
            }
            $cacherev = gmdate("YmdHis", filemtime($timestampfile));
            // Update file path with new cache revision.
            $this->path .= '/' . $cacherev;
            make_writable_directory($this->path, false);
        }

        if ($this->prescan && $definition->get_mode() !== self::MODE_REQUEST) {
            $this->prescan = false;
        }
        if ($this->prescan) {
            $this->prescan_keys();
        }
    }

    /**
     * Pre-scan the cache to see which keys are present.
     */
    protected function prescan_keys() {
        $files = glob($this->glob_keys_pattern(), GLOB_MARK | GLOB_NOSORT);
        if (is_array($files)) {
            foreach ($files as $filename) {
                $this->keys[basename($filename)] = filemtime($filename);
            }
        }
    }

    /**
     * Gets a pattern suitable for use with glob to find all keys in the cache.
     *
     * @param string $prefix A prefix to use.
     * @return string The pattern.
     */
    protected function glob_keys_pattern($prefix = '') {
        if ($this->singledirectory) {
            return $this->path . '/'.$prefix.'*.cache';
        } else {
            return $this->path . '/*/'.$prefix.'*.cache';
        }
    }

    /**
     * Returns the file path to use for the given key.
     *
     * @param string $key The key to generate a file path for.
     * @param bool $create If set to the true the directory structure the key requires will be created.
     * @return string The full path to the file that stores a particular cache key.
     */
    protected function file_path_for_key($key, $create = false) {
        if ($this->singledirectory) {
            // Its a single directory, easy, just the store instances path + the file name.
            return $this->path . '/' . $key . '.cache';
        } else {
            // We are using a single subdirectory to achieve 1 level.
           // We suffix the subdir so it does not clash with any windows
           // reserved filenames like 'con'.
            $subdir = substr($key, 0, 3) . '-cache';
            $dir = $this->path . '/' . $subdir;
            if ($create) {
                // Create the directory. This function does it recursivily!
                make_writable_directory($dir, false);
            }
            return $dir . '/' . $key . '.cache';
        }
    }

    /**
     * Retrieves an item from the cache store given its key.
     *
     * @param string $key The key to retrieve
     * @return mixed The data that was associated with the key, or false if the key did not exist.
     */
    public function get($key) {
        $this->lastiobytes = 0;
        $filename = $key.'.cache';
        $file = $this->file_path_for_key($key);
        $ttl = $this->definition->get_ttl();
        $maxtime = 0;
        if ($ttl) {
            $maxtime = cache::now() - $ttl;
        }
        $readfile = false;
        if ($this->prescan && array_key_exists($filename, $this->keys)) {
            if ((!$ttl || $this->keys[$filename] >= $maxtime) && file_exists($file)) {
                $readfile = true;
            } else {
                $this->delete($key);
            }
        } else if (file_exists($file) && (!$ttl || filemtime($file) >= $maxtime)) {
            $readfile = true;
        }
        if (!$readfile) {
            return false;
        }
        // Open ensuring the file for reading in binary format.
        if (!$handle = @fopen($file, 'rb')) {
            return false;
        }

        // Note: There is no need to perform any file locking here.
        // The cache file is only ever written to in the `write_file` function, where it does so by writing to a temp
        // file and performing an atomic rename of that file. The target file is never locked, so there is no benefit to
        // obtaining a lock (shared or exclusive) here.

        $data = '';
        // Read the data in 1Mb chunks. Small caches will not loop more than once.  We don't use filesize as it may
        // be cached with a different value than what we need to read from the file.
        do {
            $data .= @fread($handle, 1048576);
        } while (!feof($handle));
        $this->lastiobytes = strlen($data);

        if ($this->lastiobytes == 0) {
            // Potentially statcache is stale. File can be deleted, let's clear cache and recheck.
            clearstatcache(true, $file);
            if (!file_exists($file)) {
                // It's a completely normal condition. Just ignore and keep going.
                return false;
            }
        }

        // Return it unserialised.
        return $this->prep_data_after_read($data, $file);
    }

    /**
     * Retrieves several items from the cache store in a single transaction.
     *
     * If not all of the items are available in the cache then the data value for those that are missing will be set to false.
     *
     * @param array $keys The array of keys to retrieve
     * @return array An array of items from the cache. There will be an item for each key, those that were not in the store will
     *      be set to false.
     */
    public function get_many($keys) {
        $result = array();
        $total = 0;
        foreach ($keys as $key) {
            $result[$key] = $this->get($key);
            $total += $this->lastiobytes;
        }
        $this->lastiobytes = $total;
        return $result;
    }

    /**
     * Gets bytes read by last get() or get_many(), or written by set() or set_many().
     *
     * @return int Bytes read or written
     * @since Moodle 4.0
     */
    public function get_last_io_bytes(): int {
        return $this->lastiobytes;
    }

    /**
     * Deletes an item from the cache store.
     *
     * @param string $key The key to delete.
     * @return bool Returns true if the operation was a success, false otherwise.
     */
    public function delete($key) {
        $filename = $key.'.cache';
        $file = $this->file_path_for_key($key);
        if (file_exists($file) && @unlink($file)) {
            unset($this->keys[$filename]);
            return true;
        }

        return false;
    }

    /**
     * Deletes several keys from the cache in a single action.
     *
     * @param array $keys The keys to delete
     * @return int The number of items successfully deleted.
     */
    public function delete_many(array $keys) {
        $count = 0;
        foreach ($keys as $key) {
            if ($this->delete($key)) {
                $count++;
            }
        }
        return $count;
    }

    /**
     * Sets an item in the cache given its key and data value.
     *
     * @param string $key The key to use.
     * @param mixed $data The data to set.
     * @return bool True if the operation was a success false otherwise.
     */
    public function set($key, $data) {
        $this->ensure_path_exists();
        $filename = $key.'.cache';
        $file = $this->file_path_for_key($key, true);
        $serialized = $this->prep_data_before_save($data);
        $this->lastiobytes = strlen($serialized);
        $result = $this->write_file($file, $serialized);
        if (!$result) {
            // Couldn't write the file.
            return false;
        }
        // Record the key if required.
        if ($this->prescan) {
            $this->keys[$filename] = cache::now() + 1;
        }
        // Return true.. it all worked **miracles**.
        return true;
    }

    /**
     * Prepares data to be stored in a file.
     *
     * @param mixed $data
     * @return string
     */
    protected function prep_data_before_save($data) {
        return $this->serialize($data);
    }

    /**
     * Prepares the data it has been read from the cache. Undoing what was done in prep_data_before_save.
     *
     * @param string $data
     * @param string $path
     * @return mixed
     */
    protected function prep_data_after_read($data, $path) {
        $result = @$this->unserialize($data);
        if ($result === false && $data != @$this->serialize(false)) {
            debugging('Failed to unserialise data from cache file: ' . $path . '. Data: ' . $data, DEBUG_DEVELOPER);
            return false;
        }
        return $result;
    }

    /**
     * Sets many items in the cache in a single transaction.
     *
     * @param array $keyvaluearray An array of key value pairs. Each item in the array will be an associative array with two
     *      keys, 'key' and 'value'.
     * @return int The number of items successfully set. It is up to the developer to check this matches the number of items
     *      sent ... if they care that is.
     */
    public function set_many(array $keyvaluearray) {
        $count = 0;
        $totaliobytes = 0;
        foreach ($keyvaluearray as $pair) {
            if ($this->set($pair['key'], $pair['value'])) {
                $totaliobytes += $this->lastiobytes;
                $count++;
            }
        }
        $this->lastiobytes = $totaliobytes;
        return $count;
    }

    /**
     * Checks if the store has a record for the given key and returns true if so.
     *
     * @param string $key
     * @return bool
     */
    public function has($key) {
        $filename = $key.'.cache';
        $maxtime = cache::now() - $this->definition->get_ttl();
        if ($this->prescan) {
            return array_key_exists($filename, $this->keys) && $this->keys[$filename] >= $maxtime;
        }
        $file = $this->file_path_for_key($key);
        return (file_exists($file) && ($this->definition->get_ttl() == 0 || filemtime($file) >= $maxtime));
    }

    /**
     * Returns true if the store contains records for all of the given keys.
     *
     * @param array $keys
     * @return bool
     */
    public function has_all(array $keys) {
        foreach ($keys as $key) {
            if (!$this->has($key)) {
                return false;
            }
        }
        return true;
    }

    /**
     * Returns true if the store contains records for any of the given keys.
     *
     * @param array $keys
     * @return bool
     */
    public function has_any(array $keys) {
        foreach ($keys as $key) {
            if ($this->has($key)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Purges the cache definition deleting all the items within it.
     *
     * @return boolean True on success. False otherwise.
     */
    public function purge() {
        global $CFG;
        if ($this->isready) {
            // If asyncpurge = true, create a new cache revision directory and adhoc task to delete old directory.
            if ($this->asyncpurge && isset($this->definition)) {
                $hash = preg_replace('#[^a-zA-Z0-9]+#', '_', $this->definition->get_id());
                $filepath = $this->filestorepath . '/' . $hash;
                $timestampfile = $filepath . '/.lastpurged';
                if (file_exists($timestampfile)) {
                    $oldcacherev = gmdate("YmdHis", filemtime($timestampfile));
                    $oldcacherevpath = $filepath . '/' . $oldcacherev;
                    // Delete old cache revision file.
                    @unlink($timestampfile);

                    // Create adhoc task to delete old cache revision folder.
                    $purgeoldcacherev = new \cachestore_file\task\asyncpurge();
                    $purgeoldcacherev->set_custom_data(['path' => $oldcacherevpath]);
                    \core\task\manager::queue_adhoc_task($purgeoldcacherev);
                }
                touch($timestampfile, time());
                @chmod($timestampfile, $CFG->filepermissions);
                $newcacherev = gmdate("YmdHis", filemtime($timestampfile));
                $filepath .= '/' . $newcacherev;
                make_writable_directory($filepath, false);
            } else {
                $files = glob($this->glob_keys_pattern(), GLOB_MARK | GLOB_NOSORT);
                if (is_array($files)) {
                    foreach ($files as $filename) {
                        @unlink($filename);
                    }
                }
                $this->keys = [];
            }
        }
        return true;
    }

    /**
     * Purges all the cache definitions deleting all items within them.
     *
     * @return boolean True on success. False otherwise.
     */
    protected function purge_all_definitions() {
        // Warning: limit the deletion to what file store is actually able
        // to create using the internal {@link purge()} providing the
        // {@link $path} with a wildcard to perform a purge action over all the definitions.
        $currpath = $this->path;
        $this->path = $this->filestorepath.'/*';
        $result = $this->purge();
        $this->path = $currpath;
        return $result;
    }

    /**
     * Given the data from the add instance form this function creates a configuration array.
     *
     * @param stdClass $data
     * @return array
     */
    public static function config_get_configuration_array($data) {
        $config = array();

        if (isset($data->path)) {
            $config['path'] = $data->path;
        }
        if (isset($data->autocreate)) {
            $config['autocreate'] = $data->autocreate;
        }
        if (isset($data->singledirectory)) {
            $config['singledirectory'] = $data->singledirectory;
        }
        if (isset($data->prescan)) {
            $config['prescan'] = $data->prescan;
        }
        if (isset($data->asyncpurge)) {
            $config['asyncpurge'] = $data->asyncpurge;
        }
        if (isset($data->lockwait)) {
            $config['lockwait'] = $data->lockwait;
        }
        if (isset($data->serializer)) {
            $config['serializer'] = $data->serializer;
        }

        return $config;
    }

    /**
     * Allows the cache store to set its data against the edit form before it is shown to the user.
     *
     * @param moodleform $editform
     * @param array $config
     */
    public static function config_set_edit_form_data(moodleform $editform, array $config) {
        $data = array();
        if (!empty($config['path'])) {
            $data['path'] = $config['path'];
        }
        if (isset($config['autocreate'])) {
            $data['autocreate'] = (bool)$config['autocreate'];
        }
        if (isset($config['singledirectory'])) {
            $data['singledirectory'] = (bool)$config['singledirectory'];
        }
        if (isset($config['prescan'])) {
            $data['prescan'] = (bool)$config['prescan'];
        }
        if (isset($config['asyncpurge'])) {
            $data['asyncpurge'] = (bool)$config['asyncpurge'];
        }
        if (isset($config['lockwait'])) {
            $data['lockwait'] = (int)$config['lockwait'];
        }
        if (isset($config['serializer'])) {
            $data['serializer'] = (string)$config['serializer'];
        }
        $editform->set_data($data);
    }

    /**
     * Checks to make sure that the path for the file cache exists.
     *
     * @return bool
     * @throws coding_exception
     */
    protected function ensure_path_exists() {
        global $CFG;
        if (!is_writable($this->path)) {
            if ($this->custompath && !$this->autocreate) {
                throw new coding_exception('File store path does not exist. It must exist and be writable by the web server.');
            }
            $createdcfg = false;
            if (!isset($CFG)) {
                // This can only happen during destruction of objects.
                // A cache is being used within a destructor, php is ending a request and $CFG has
                // already being cleaned up.
                // Rebuild $CFG with directory permissions just to complete this write.
                $CFG = $this->cfg;
                $createdcfg = true;
            }
            if (!make_writable_directory($this->path, false)) {
                throw new coding_exception('File store path does not exist and can not be created.');
            }
            if ($createdcfg) {
                // We re-created it so we'll clean it up.
                unset($CFG);
            }
        }
        return true;
    }

    /**
     * Performs any necessary clean up when the file store instance is being deleted.
     *
     * 1. Purges the cache directory.
     * 2. Deletes the directory we created for the given definition.
     */
    public function instance_deleted() {
        $this->purge_all_definitions();
        @rmdir($this->filestorepath);
    }

    /**
     * Generates an instance of the cache store that can be used for testing.
     *
     * Returns an instance of the cache store, or false if one cannot be created.
     *
     * @param definition $definition
     * @return cachestore_file
     */
    public static function initialise_test_instance(definition $definition) {
        $name = 'File test';
        $path = make_cache_directory('cachestore_file_test');
        $cache = new cachestore_file($name, array('path' => $path));
        if ($cache->is_ready()) {
            $cache->initialise($definition);
        }
        return $cache;
    }

    /**
     * Generates the appropriate configuration required for unit testing.
     *
     * @return array Array of unit test configuration data to be used by initialise().
     */
    public static function unit_test_configuration() {
        return array();
    }

    /**
     * Writes your madness to a file.
     *
     * There are several things going on in this function to try to ensure what we don't end up with partial writes etc.
     *   1. Files for writing are opened with the mode xb, the file must be created and can not already exist.
     *   2. Renaming, data is written to a temporary file, where it can be verified using md5 and is then renamed.
     *
     * @param string $file Absolute file path
     * @param string $content The content to write.
     * @return bool
     */
    protected function write_file($file, $content) {
        // Generate a temp file that is going to be unique. We'll rename it at the end to the desired file name.
        // in this way we avoid partial writes.
        $path = dirname($file);
        while (true) {
            $tempfile = $path.'/'.uniqid(sesskey().'.', true) . '.temp';
            if (!file_exists($tempfile)) {
                break;
            }
        }

        // Open the file with mode=x. This acts to create and open the file for writing only.
        // If the file already exists this will return false.
        // We also force binary.
        $handle = @fopen($tempfile, 'xb+');
        if ($handle === false) {
            // File already exists... lock already exists, return false.
            return false;
        }
        fwrite($handle, $content);
        fflush($handle);
        // Close the handle, we're done.
        fclose($handle);

        if (md5_file($tempfile) !== md5($content)) {
            // The md5 of the content of the file must match the md5 of the content given to be written.
            @unlink($tempfile);
            return false;
        }

        // Finally rename the temp file to the desired file, returning the true|false result.
        $result = rename($tempfile, $file);
        @chmod($file, $this->cfg->filepermissions);
        if (!$result) {
            // Failed to rename, don't leave files lying around.
            @unlink($tempfile);
        }
        return $result;
    }

    /**
     * Returns the name of this instance.
     * @return string
     */
    public function my_name() {
        return $this->name;
    }

    /**
     * Finds all of the keys being used by this cache store instance.
     *
     * @return array
     */
    public function find_all() {
        $this->ensure_path_exists();
        $files = glob($this->glob_keys_pattern(), GLOB_MARK | GLOB_NOSORT);
        $return = array();
        if ($files === false) {
            return $return;
        }
        foreach ($files as $file) {
            $return[] = substr(basename($file), 0, -6);
        }
        return $return;
    }

    /**
     * Finds all of the keys whose keys start with the given prefix.
     *
     * @param string $prefix
     */
    public function find_by_prefix($prefix) {
        $this->ensure_path_exists();
        $prefix = preg_replace('#(\*|\?|\[)#', '[$1]', $prefix);
        $files = glob($this->glob_keys_pattern($prefix), GLOB_MARK | GLOB_NOSORT);
        $return = array();
        if ($files === false) {
            return $return;
        }
        foreach ($files as $file) {
            // Trim off ".cache" from the end.
            $return[] = substr(basename($file), 0, -6);
        }
        return $return;
    }

    /**
     * Gets total size for the directory used by the cache store.
     *
     * @return int Total size in bytes
     */
    public function store_total_size(): ?int {
        return get_directory_size($this->filestorepath);
    }

    /**
     * Gets total size for a specific cache.
     *
     * With the file cache we can just look at the directory listing without having to
     * actually load any files, so the $samplekeys parameter is ignored.
     *
     * @param int $samplekeys Unused
     * @return stdClass Cache details
     */
    public function cache_size_details(int $samplekeys = 50): stdClass {
        $result = (object)[
            'supported' => true,
            'items' => 0,
            'mean' => 0,
            'sd' => 0,
            'margin' => 0
        ];

        // Find all the files in this cache.
        $this->ensure_path_exists();
        $files = glob($this->glob_keys_pattern(), GLOB_MARK | GLOB_NOSORT);
        if ($files === false || count($files) === 0) {
            return $result;
        }

        // Get the sizes and count of files.
        $sizes = [];
        foreach ($files as $file) {
            $result->items++;
            $sizes[] = filesize($file);
        }

        // Work out mean and standard deviation.
        $total = array_sum($sizes);
        $result->mean = $total / $result->items;
        $squarediff = 0;
        foreach ($sizes as $size) {
            $squarediff += ($size - $result->mean) ** 2;
        }
        $squarediff /= $result->items;
        $result->sd = sqrt($squarediff);
        return $result;
    }

    /**
     * Use lock factory to determine the lock state.
     *
     * @param string $key Lock identifier
     * @param string $ownerid Cache identifier
     * @return bool|null
     */
    public function check_lock_state($key, $ownerid): ?bool {
        if (!array_key_exists($key, $this->locks)) {
            return null; // Lock does not exist.
        }
        if (!array_key_exists($ownerid, $this->locks[$key])) {
            return false; // Lock exists, but belongs to someone else.
        }
        if ($this->locks[$key][$ownerid] instanceof \core\lock\lock) {
            return true; // Lock exists, and we own it.
        }
        // Try to get the lock with an immediate timeout. If this succeeds, the lock does not currently exist.
        $lock = $this->lockfactory->get_lock($key, 0);
        if ($lock) {
            // Lock was not already held.
            $lock->release();
            return null;
        } else {
            // Lock is held by someone else.
            return false;
        }
    }

    /**
     * Use lock factory to acquire a lock.
     *
     * @param string $key Lock identifier
     * @param string $ownerid Cache identifier
     * @return bool
     */
    public function acquire_lock($key, $ownerid): bool {
        $lock = $this->lockfactory->get_lock($key, $this->lockwait);
        if ($lock) {
            $this->locks[$key][$ownerid] = $lock;
        }
        return (bool)$lock;
    }

    /**
     * Use lock factory to release a lock.
     *
     * @param string $key Lock identifier
     * @param string $ownerid Cache identifier
     * @return bool
     */
    public function release_lock($key, $ownerid): bool {
        if (!array_key_exists($key, $this->locks)) {
            return false; // No lock to release.
        }
        if (!array_key_exists($ownerid, $this->locks[$key])) {
            return false; // Tried to release someone else's lock.
        }
        $unlocked = $this->locks[$key][$ownerid]->release();
        if ($unlocked) {
            unset($this->locks[$key]);
        }
        return $unlocked;
    }

    /**
     * Serializes the data according to the configured serializer.
     *
     * @param mixed $value
     * @return string
     */
    protected function serialize($value): string {
        switch ($this->serializer) {
            case self::SERIALIZER_PHP:
                return serialize($value);
            case self::SERIALIZER_IGBINARY:
                if (self::igbinary_available()) {
                    return igbinary_serialize($value);
                }
        }
        debugging("Unknown or unavailable serializer {$this->serializer}");
        return serialize($value);
    }

    /**
     * Unserializes the data according to the configured serializer.
     *
     * @param mixed $value
     * @return mixed
     */
    protected function unserialize($value) {
        switch ($this->serializer) {
            case self::SERIALIZER_PHP:
                return unserialize($value);
            case self::SERIALIZER_IGBINARY:
                if (self::igbinary_available()) {
                    return igbinary_unserialize($value);
                }
        }
        debugging("Unknown or unavailable serializer: {$this->serializer}");
        return unserialize($value);
    }
}
