Proyectos de Subversion Moodle

<?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/>.

namespace core_cache;

use core\exception\coding_exception;

use stdClass;

/**

 * The main cache class.

 *

 * This class if the first class that any end developer will interact with.

 * In order to create an instance of a cache that they can work with they must call one of the static make methods belonging

 * to this class.

 *

 * @package    core_cache

 * @category   cache

 * @copyright  2012 Sam Hemelryk

 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later

 */

class cache implements loader_interface {

    /**

     * @var int Constant for cache entries that do not have a version number

     */

    const VERSION_NONE = -1;

    /**

     * We need a timestamp to use within the cache API.

     * This stamp needs to be used for all ttl and time based operations to ensure that we don't end up with

     * timing issues.

     * @var int

     */

    protected static $now;

    /**

     * A purge token used to distinguish between multiple cache purges in the same second.

     * This is in the format <microtime>-<random string>.

     *

     * @var string

     */

    protected static $purgetoken;

    /**

     * The definition used when loading this cache if there was one.

     * @var definition

     */

    private $definition = false;

    /**

     * The cache store that this loader will make use of.

     * @var store

     */

    private $store;

    /**

     * The next cache loader in the chain if there is one.

     * If a cache request misses for the store belonging to this loader then the loader

     * stored here will be checked next.

     * If there is a loader here then $datasource must be false.

     * @var loader_interface|false

     */

    private $loader = false;

    /**

     * The data source to use if we need to load data (because if doesn't exist in the cache store).

     * If there is a data source here then $loader above must be false.

     * @var data_source_interface|false

     */

    private $datasource = false;

    /**

     * Used to quickly check if the store supports key awareness.

     * This is set when the cache is initialised and is used to speed up processing.

     * @var bool

     */

    private $supportskeyawareness = null;

    /**

     * Used to quickly check if the store supports ttl natively.

     * This is set when the cache is initialised and is used to speed up processing.

     * @var bool

     */

    private $supportsnativettl = null;

    /**

     * Gets set to true if the cache is going to be using a static array for acceleration.

     * The array statically caches items used during the lifetime of the request. This greatly speeds up interaction

     * with the cache in areas where it will be repetitively hit for the same information such as with strings.

     * There are several other variables to control how this static acceleration array works.

     * @var bool

     */

    private $staticacceleration = false;

    /**

     * The static acceleration array.

     * Items will be stored in this cache as they were provided. This ensure there is no unnecessary processing taking place.

     * @var array

     */

    private $staticaccelerationarray = [];

    /**

     * The number of items in the static acceleration array. Avoids count calls like you wouldn't believe.

     * @var int

     */

    private $staticaccelerationcount = 0;

    /**

     * An array containing just the keys being used in the static acceleration array.

     * This seems redundant perhaps but is used when managing the size of the static acceleration array.

     * Items are added to the end of the array and the when we need to reduce the size of the cache we use the

     * key that is first on this array.

     * @var array

     */

    private $staticaccelerationkeys = [];

    /**

     * The maximum size of the static acceleration array.

     *

     * If set to false there is no max size.

     * Caches that make use of static acceleration should seriously consider setting this to something reasonably small, but

     * still large enough to offset repetitive calls.

     *

     * @var int|false

     */

    private $staticaccelerationsize = false;

    /**

     * Gets set to true during initialisation if the definition is making use of a ttl.

     * Used to speed up processing.

     * @var bool

     */

    private $hasattl = false;

    /**

     * Gets set to the class name of the store during initialisation. This is used several times in the cache class internally

     * and having it here helps speed up processing.

     * @var strubg

     */

    protected $storetype = 'unknown';

    /**

     * Gets set to true if we want to collect performance information about the cache API.

     * @var bool

     */

    protected $perfdebug = false;

    /**

     * Determines if this loader is a sub loader, not the top of the chain.

     * @var bool

     */

    protected $subloader = false;

    /**

     * Gets set to true if the cache writes (set|delete) must have a manual lock created first.

     * @var bool

     */

    protected $requirelockingbeforewrite = false;

    /**

     * Gets set to true if the cache's primary store natively supports locking.

     * If it does then we use that, otherwise we need to instantiate a second store to use for locking.

     * @var store|null

     */

    protected $nativelocking = null;

    /**

     * Creates a new cache instance for a pre-defined definition.

     *

     * @param string $component The component for the definition

     * @param string $area The area for the definition

     * @param array $identifiers Any additional identifiers that should be provided to the definition.

     * @param string $unused Used to be datasourceaggregate but that was removed and this is now unused.

     * @return application_cache|session_cache|store

     */

    public static function make($component, $area, array $identifiers = [], $unused = null) {

        $factory = factory::instance();

        return $factory->create_cache_from_definition($component, $area, $identifiers);

    }

    /**

     * Creates a new cache instance based upon the given params.

     *

     * @param int $mode One of store::MODE_*

     * @param string $component The component this cache relates to.

     * @param string $area The area this cache relates to.

     * @param array $identifiers Any additional identifiers that should be provided to the definition.

     * @param array $options An array of options, available options are:

     *   - simplekeys : Set to true if the keys you will use are a-zA-Z0-9_

     *   - simpledata : Set to true if the type of the data you are going to store is scalar, or an array of scalar vars

     *   - staticacceleration : If set to true the cache will hold onto data passing through it.

     *   - staticaccelerationsize : The max size for the static acceleration array.

     * @return application_cache|session_cache|request_cache

     */

    public static function make_from_params($mode, $component, $area, array $identifiers = [], array $options = []) {

        $factory = factory::instance();

        return $factory->create_cache_from_params($mode, $component, $area, $identifiers, $options);

    }

    /**

     * Constructs a new cache instance.

     *

     * You should not call this method from your code, instead you should use the cache::make methods.

     *

     * This method is public so that the factory is able to instantiate cache instances.

     * Ideally we would make this method protected and expose its construction to the factory method internally somehow.

     * The factory class is responsible for this in order to centralise the storage of instances once created. This way if needed

     * we can force a reset of the cache API (used during unit testing).

     *

     * @param definition $definition The definition for the cache instance.

     * @param store $store The store that cache should use.

     * @param loader_interface|data_source_interface $loader The next loader in the chain or the data source if there is one

     *                                                       and there are no other loader_interfaces in the chain.

     */

    public function __construct(definition $definition, store $store, $loader = null) {

        global $CFG;

        $this->definition = $definition;

        $this->store = $store;

        $this->storetype = get_class($store);

        $this->perfdebug = (!empty($CFG->perfdebug) and $CFG->perfdebug > 7);

        if ($loader instanceof loader_interface) {

            $this->set_loader($loader);

        } else if ($loader instanceof data_source_interface) {

            $this->set_data_source($loader);

        }

        $this->definition->generate_definition_hash();

        $this->staticacceleration = $this->definition->use_static_acceleration();

        if ($this->staticacceleration) {

            $this->staticaccelerationsize = $this->definition->get_static_acceleration_size();

        }

        $this->hasattl = ($this->definition->get_ttl() > 0);

    }

    /**

     * Set the loader for this cache.

     *

     * @param   loader_interface $loader

     */

    protected function set_loader(loader_interface $loader): void {

        $this->loader = $loader;

        // Mark the loader as a sub (chained) loader.

        $this->loader->set_is_sub_loader(true);

    }

    /**

     * Set the data source for this cache.

     *

     * @param   data_source_interface $datasource

     */

    protected function set_data_source(data_source_interface $datasource): void {

        $this->datasource = $datasource;

    }

    /**

     * Used to inform the loader of its state as a sub loader, or as the top of the chain.

     *

     * This is important as it ensures that we do not have more than one loader keeping static acceleration data.

     * Subloaders need to be "pure" loaders in the sense that they are used to store and retrieve information from stores or the

     * next loader/data source in the chain.

     * Nothing fancy, nothing flash.

     *

     * @param bool $setting

     */

    protected function set_is_sub_loader($setting = true) {

        if ($setting) {

            $this->subloader = true;

            // Subloaders should not keep static acceleration data.

            $this->staticacceleration = false;

            $this->staticaccelerationsize = false;

        } else {

            $this->subloader = true;

            $this->staticacceleration = $this->definition->use_static_acceleration();

            if ($this->staticacceleration) {

                $this->staticaccelerationsize = $this->definition->get_static_acceleration_size();

            }

        }

    }

    /**

     * Alters the identifiers that have been provided to the definition.

     *

     * This is an advanced method and should not be used unless really needed.

     * It allows the developer to slightly alter the definition without having to re-establish the cache.

     * It will cause more processing as the definition will need to clear and reprepare some of its properties.

     *

     * @param array $identifiers

     */

    public function set_identifiers(array $identifiers) {

        if ($this->definition->set_identifiers($identifiers)) {

            // As static acceleration uses input keys and not parsed keys

            // it much be cleared when the identifier set is changed.

            $this->staticaccelerationarray = [];

            if ($this->staticaccelerationsize !== false) {

                $this->staticaccelerationkeys = [];

                $this->staticaccelerationcount = 0;

            }

        }

    }

    /**

     * Process any outstanding invalidation events for the cache we are registering,

     *

     * Identifiers and event invalidation are not compatible with each other at this time.

     * As a result the cache does not need to consider identifiers when working out what to invalidate.

     */

    protected function handle_invalidation_events() {

        if (!$this->definition->has_invalidation_events()) {

            return;

        }

        // Each cache stores the current 'lastinvalidation' value within the cache itself.

        $lastinvalidation = $this->get('lastinvalidation');

        if ($lastinvalidation === false) {

            // There is currently no  value for the lastinvalidation token, therefore the token is not set, and there

            // can be nothing to invalidate.

            // Set the lastinvalidation value to the current purge token and return early.

            $this->set('lastinvalidation', self::get_purge_token());

            return;

        } else if ($lastinvalidation == self::get_purge_token()) {

            // The current purge request has already been fully handled by this cache.

            return;

        }

        /*

         * Now that the whole cache check is complete, we check the meaning of any specific cache invalidation events.

         * These are stored in the core/eventinvalidation cache as an multi-dimensinoal array in the form:

         *  [

         *      eventname => [

         *          keyname => purgetoken,

         *      ]

         *  ]

         *

         * The 'keyname' value is used to delete a specific key in the cache.

         * If the keyname is set to the special value 'purged', then the whole cache is purged instead.

         *

         * The 'purgetoken' is the token that this key was last purged.

         * a) If the purgetoken matches the last invalidation, then the key/cache is not purged.

         * b) If the purgetoken is newer than the last invalidation, then the key/cache is not purged.

         * c) If the purge token is older than the last invalidation, or it has a different token component, then the

         *    cache is purged.

         *

         * Option b should not happen under normal operation, but may happen in race condition whereby a long-running

         * request's cache is cleared in another process during that request, and prior to that long-running request

         * creating the cache. In such a condition, it would be incorrect to clear that cache.

         */

        $cache = self::make('core', 'eventinvalidation');

        $events = $cache->get_many($this->definition->get_invalidation_events());

        $todelete = [];

        $purgeall = false;

        // Iterate the returned data for the events.

        foreach ($events as $event => $keys) {

            if ($keys === false) {

                // No data to be invalidated yet.

                continue;

            }

            // Look at each key and check the timestamp.

            foreach ($keys as $key => $purgetoken) {

                // If the timestamp of the event is more than or equal to the last invalidation (happened between the last

                // invalidation and now), then we need to invaliate the key.

                if (self::compare_purge_tokens($purgetoken, $lastinvalidation) > 0) {

                    if ($key === 'purged') {

                        $purgeall = true;

                        break;

                    } else {

                        $todelete[] = $key;

                    }

                }

            }

        }

        if ($purgeall) {

            $this->purge();

        } else if (!empty($todelete)) {

            $todelete = array_unique($todelete);

            $this->delete_many($todelete);

        }

        // Set the time of the last invalidation.

        if ($purgeall || !empty($todelete)) {

            $this->set('lastinvalidation', self::get_purge_token(true));

        }

    }

    /**

     * Retrieves the value for the given key from the cache.

     *

     * @param string|int $key The key for the data being requested.

     *      It can be any structure although using a scalar string or int is recommended in the interests of performance.

     *      In advanced cases an array may be useful such as in situations requiring the multi-key functionality.

     * @param int $strictness One of IGNORE_MISSING | MUST_EXIST

     * @return mixed|false The data from the cache or false if the key did not exist within the cache.

     * @throws coding_exception

     */

    public function get($key, $strictness = IGNORE_MISSING) {

        return $this->get_implementation($key, self::VERSION_NONE, $strictness);

    }

    /**

     * Retrieves the value and actual version for the given key, with at least the required version.

     *

     * If there is no value for the key, or there is a value but it doesn't have the required

     * version, then this function will return null (or throw an exception if you set strictness

     * to MUST_EXIST).

     *

     * This function can be used to make it easier to support localisable caches (where the cache

     * could be stored on a local server as well as a shared cache). Specifying the version means

     * that it will automatically retrieve the correct version if available, either from the local

     * server or [if that has an older version] from the shared server.

     *

     * If the cached version is newer than specified version, it will be returned regardless. For

     * example, if you request version 4, but the locally cached version is 5, it will be returned.

     * If you request version 6, and the locally cached version is 5, then the system will look in

     * higher-level caches (if any); if there still isn't a version 6 or greater, it will return

     * null.

     *

     * You must use this function if you use set_versioned.

     *

     * @param string|int $key The key for the data being requested.

     * @param int $requiredversion Minimum required version of the data

     * @param int $strictness One of IGNORE_MISSING or MUST_EXIST.

     * @param mixed $actualversion If specified, will be set to the actual version number retrieved

     * @return mixed Data from the cache, or false if the key did not exist or was too old

     * @throws \coding_exception If you call get_versioned on a non-versioned cache key

     */

    public function get_versioned($key, int $requiredversion, int $strictness = IGNORE_MISSING, &$actualversion = null) {

        return $this->get_implementation($key, $requiredversion, $strictness, $actualversion);

    }

    /**

     * Checks returned data to see if it matches the specified version number.

     *

     * For versioned data, this returns the version_wrapper object (or false). For other

     * data, it returns the actual data (or false).

     *

     * @param mixed $result Result data

     * @param int $requiredversion Required version number or VERSION_NONE if there must be no version

     * @return bool True if version is current, false if not (or if result is false)

     * @throws \coding_exception If unexpected type of data (versioned vs non-versioned) is found

     */

    protected static function check_version($result, int $requiredversion): bool {

        if ($requiredversion === self::VERSION_NONE) {

            if ($result instanceof \core_cache\version_wrapper) {

                throw new \coding_exception('Unexpectedly found versioned cache entry');

            } else {

                // No version checks, so version is always correct.

                return true;

            }

        } else {

            // If there's no result, obviously it doesn't meet the required version.

            if (!helper::result_found($result)) {

                return false;

            }

            if (!($result instanceof \core_cache\version_wrapper)) {

                throw new \coding_exception('Unexpectedly found non-versioned cache entry');

            }

            // If the result doesn't match the required version tag, return false.

            if ($result->version < $requiredversion) {

                return false;

            }

            // The version meets the requirement.

            return true;

        }

    }

    /**

     * Retrieves the value for the given key from the cache.

     *

     * @param string|int $key The key for the data being requested.

     *      It can be any structure although using a scalar string or int is recommended in the interests of performance.

     *      In advanced cases an array may be useful such as in situations requiring the multi-key functionality.

     * @param int $requiredversion Minimum required version of the data or cache::VERSION_NONE

     * @param int $strictness One of IGNORE_MISSING | MUST_EXIST

     * @param mixed $actualversion If specified, will be set to the actual version number retrieved

     * @return mixed|false The data from the cache or false if the key did not exist within the cache.

     * @throws coding_exception

     */

    protected function get_implementation($key, int $requiredversion, int $strictness, &$actualversion = null) {

        // 1. Get it from the static acceleration array if we can (only when it is enabled and it has already been requested/set).

        $usesstaticacceleration = $this->use_static_acceleration();

        if ($usesstaticacceleration) {

            $result = $this->static_acceleration_get($key);

            if (helper::result_found($result) && self::check_version($result, $requiredversion)) {

                if ($requiredversion === self::VERSION_NONE) {

                    return $result;

                } else {

                    $actualversion = $result->version;

                    return $result->data;

                }

            }

        }

        // 2. Parse the key.

        $parsedkey = $this->parse_key($key);

        // 3. Get it from the store. Obviously wasn't in the static acceleration array.

        $result = $this->store->get($parsedkey);

        if (helper::result_found($result)) {

            // Check the result has at least the required version.

            try {

                $validversion = self::check_version($result, $requiredversion);

            } catch (\coding_exception $e) {

                // In certain circumstances this could happen before users are taken to the upgrade

                // screen when upgrading from an earlier Moodle version that didn't use a versioned

                // cache for this item, so redirect instead of showing error if that's the case.

                redirect_if_major_upgrade_required();

                // If we still get an exception because there is incorrect data in the cache (not

                // versioned when it ought to be), delete it so this exception goes away next time.

                // The exception should only happen if there is a code bug (which is why we still

                // throw it) but there are unusual scenarios in development where it might happen

                // and that would be annoying if it doesn't fix itself.

                $this->store->delete($parsedkey);

                throw $e;

            }

            if (!$validversion) {

                // If the result was too old, don't use it.

                $result = false;

                // Also delete it immediately. This improves performance in the

                // case when the cache item is large and there may be multiple clients simultaneously

                // requesting it - they won't all have to do a megabyte of IO just in order to find

                // that it's out of date.

                $this->store->delete($parsedkey);

            }

        }

        if (helper::result_found($result)) {

            // Look to see if there's a TTL wrapper. It might be inside a version wrapper.

            if ($requiredversion !== self::VERSION_NONE) {

                $ttlconsider = $result->data;

            } else {

                $ttlconsider = $result;

            }

            if ($ttlconsider instanceof ttl_wrapper) {

                if ($ttlconsider->has_expired()) {

                    $this->store->delete($parsedkey);

                    $result = false;

                } else if ($requiredversion === self::VERSION_NONE) {

                    // Use the data inside the TTL wrapper as the result.

                    $result = $ttlconsider->data;

                } else {

                    // Put the data from the TTL wrapper directly inside the version wrapper.

                    $result->data = $ttlconsider->data;

                }

            }

            if ($usesstaticacceleration) {

                $this->static_acceleration_set($key, $result);

            }

            // Remove version wrapper if necessary.

            if ($requiredversion !== self::VERSION_NONE) {

                $actualversion = $result->version;

                $result = $result->data;

            }

            if ($result instanceof cached_object) {

                $result = $result->restore_object();

            }

        }

        // 4. Load if from the loader/datasource if we don't already have it.

        $setaftervalidation = false;

        if (!helper::result_found($result)) {

            if ($this->perfdebug) {

                helper::record_cache_miss($this->store, $this->definition);

            }

            if ($this->loader !== false) {

                // We must pass the original (unparsed) key to the next loader in the chain.

                // The next loader will parse the key as it sees fit. It may be parsed differently

                // depending upon the capabilities of the store associated with the loader.

                if ($requiredversion === self::VERSION_NONE) {

                    $result = $this->loader->get($key);

                } else {

                    $result = $this->loader->get_versioned($key, $requiredversion, IGNORE_MISSING, $actualversion);

                }

            } else if ($this->datasource !== false) {

                if ($requiredversion === self::VERSION_NONE) {

                    $result = $this->datasource->load_for_cache($key);

                } else {

                    if (!$this->datasource instanceof versionable_data_source_interface) {

                        throw new \coding_exception('Data source is not versionable');

                    }

                    $result = $this->datasource->load_for_cache_versioned($key, $requiredversion, $actualversion);

                    if ($result && $actualversion < $requiredversion) {

                        throw new \coding_exception('Data source returned outdated version');

                    }

                }

            }

            $setaftervalidation = (helper::result_found($result));

        } else if ($this->perfdebug) {

            $readbytes = $this->store->get_last_io_bytes();

            helper::record_cache_hit($this->store, $this->definition, 1, $readbytes);

        }

        // 5. Validate strictness.

        if ($strictness === MUST_EXIST && !helper::result_found($result)) {

            throw new coding_exception('Requested key did not exist in any cache stores and could not be loaded.');

        }

        // 6. Set it to the store if we got it from the loader/datasource. Only set to this direct

        // store; parent method will have set it to all stores if needed.

        if ($setaftervalidation) {

            $lock = false;

            try {

                // Only try to acquire a lock for this cache if we do not already have one.

                if (!empty($this->requirelockingbeforewrite) && !$this->check_lock_state($key)) {

                    $this->acquire_lock($key);

                    $lock = true;

                }

                if ($requiredversion === self::VERSION_NONE) {

                    $this->set_implementation($key, self::VERSION_NONE, $result, false);

                } else {

                    $this->set_implementation($key, $actualversion, $result, false);

                }

            } finally {

                if ($lock) {

                    $this->release_lock($key);

                }

            }

        }

        // 7. Make sure we don't pass back anything that could be a reference.

        // We don't want people modifying the data in the cache.

        if (!$this->store->supports_dereferencing_objects() && !is_scalar($result)) {

            // If data is an object it will be a reference.

            // If data is an array if may contain references.

            // We want to break references so that the cache cannot be modified outside of itself.

            // Call the function to unreference it (in the best way possible).

            $result = $this->unref($result);

        }

        return $result;

    }

    /**

     * Retrieves an array of values for an array of keys.

     *

     * Using this function comes with potential performance implications.

     * Not all cache stores will support get_many/set_many operations and in order to replicate this functionality will call

     * the equivalent singular method for each item provided.

     * This should not deter you from using this function as there is a performance benefit in situations where the cache store

     * does support it, but you should be aware of this fact.

     *

     * @param array $keys The keys of the data being requested.

     *      Each key can be any structure although using a scalar string or int is recommended in the interests of performance.

     *      In advanced cases an array may be useful such as in situations requiring the multi-key functionality.

     * @param int $strictness One of IGNORE_MISSING or MUST_EXIST.

     * @return array An array of key value pairs for the items that could be retrieved from the cache.

     *      If MUST_EXIST was used and not all keys existed within the cache then an exception will be thrown.

     *      Otherwise any key that did not exist will have a data value of false within the results.

     * @throws coding_exception

     */

    public function get_many(array $keys, $strictness = IGNORE_MISSING) {

        $keysparsed = [];

        $parsedkeys = [];

        $resultpersist = [];

        $resultstore = [];

        $keystofind = [];

        $readbytes = store::IO_BYTES_NOT_SUPPORTED;

        // First up check the persist cache for each key.

        $isusingpersist = $this->use_static_acceleration();

        foreach ($keys as $key) {

            $pkey = $this->parse_key($key);

            if (is_array($pkey)) {

                $pkey = $pkey['key'];

            }

            $keysparsed[$key] = $pkey;

            $parsedkeys[$pkey] = $key;

            $keystofind[$pkey] = $key;

            if ($isusingpersist) {

                $value = $this->static_acceleration_get($key);

                if ($value !== false) {

                    $resultpersist[$pkey] = $value;

                    unset($keystofind[$pkey]);

                }

            }

        }

        // Next assuming we didn't find all of the keys in the persist cache try loading them from the store.

        if (count($keystofind)) {

            $resultstore = $this->store->get_many(array_keys($keystofind));

            if ($this->perfdebug) {

                $readbytes = $this->store->get_last_io_bytes();

            }

            // Process each item in the result to "unwrap" it.

            foreach ($resultstore as $key => $value) {

                if ($value instanceof ttl_wrapper) {

                    if ($value->has_expired()) {

                        $value = false;

                    } else {

                        $value = $value->data;

                    }

                }

                if ($value !== false && $this->use_static_acceleration()) {

                    $this->static_acceleration_set($keystofind[$key], $value);

                }

                if ($value instanceof cached_object) {

                    $value = $value->restore_object();

                }

                $resultstore[$key] = $value;

            }

        }

        // Merge the result from the persis cache with the results from the store load.

        $result = $resultpersist + $resultstore;

        unset($resultpersist);

        unset($resultstore);

        // Next we need to find any missing values and load them from the loader/datasource next in the chain.

        $usingloader = ($this->loader !== false);

        $usingsource = (!$usingloader && ($this->datasource !== false));

        if ($usingloader || $usingsource) {

            $missingkeys = [];

            foreach ($result as $key => $value) {

                if ($value === false) {

                    $missingkeys[] = $parsedkeys[$key];

                }

            }

            if (!empty($missingkeys)) {

                if ($usingloader) {

                    $resultmissing = $this->loader->get_many($missingkeys);

                } else {

                    $resultmissing = $this->datasource->load_many_for_cache($missingkeys);

                }

                foreach ($resultmissing as $key => $value) {

                    $result[$keysparsed[$key]] = $value;

                    $lock = false;

                    try {

                        if (!empty($this->requirelockingbeforewrite)) {

                            $this->acquire_lock($key);

                            $lock = true;

                        }

                        if ($value !== false) {

                            $this->set($key, $value);

                        }

                    } finally {

                        if ($lock) {

                            $this->release_lock($key);

                        }

                    }

                }

                unset($resultmissing);

            }

            unset($missingkeys);

        }

        // Create an array with the original keys and the found values. This will be what we return.

        $fullresult = [];

        foreach ($result as $key => $value) {

            if (!is_scalar($value)) {

                // If data is an object it will be a reference.

                // If data is an array if may contain references.

                // We want to break references so that the cache cannot be modified outside of itself.

                // Call the function to unreference it (in the best way possible).

                $value = $this->unref($value);

            }

            $fullresult[$parsedkeys[$key]] = $value;

        }

        unset($result);

        // Final step is to check strictness.

        if ($strictness === MUST_EXIST) {

            foreach ($keys as $key) {

                if (!array_key_exists($key, $fullresult)) {

                    throw new coding_exception('Not all the requested keys existed within the cache stores.');

                }

            }

        }

        if ($this->perfdebug) {

            $hits = 0;

            $misses = 0;

            foreach ($fullresult as $value) {

                if ($value === false) {

                    $misses++;

                } else {

                    $hits++;

                }

            }

            helper::record_cache_hit($this->store, $this->definition, $hits, $readbytes);

            helper::record_cache_miss($this->store, $this->definition, $misses);

        }

        // Return the result. Phew!

        return $fullresult;

    }

    /**

     * Sends a key => value pair to the cache.

     *

     * <code>

     * // This code will add four entries to the cache, one for each url.

     * $cache->set('main', 'http://moodle.org');

     * $cache->set('docs', 'http://docs.moodle.org');

     * $cache->set('tracker', 'http://tracker.moodle.org');

     * $cache->set('qa', 'http://qa.moodle.net');

     * </code>

     *

     * @param string|int $key The key for the data being requested.

     *      It can be any structure although using a scalar string or int is recommended in the interests of performance.

     *      In advanced cases an array may be useful such as in situations requiring the multi-key functionality.

     * @param mixed $data The data to set against the key.

     * @return bool True on success, false otherwise.

     */

    public function set($key, $data) {

        return $this->set_implementation($key, self::VERSION_NONE, $data);

    }

    /**

     * Sets the value for the given key with the given version.

     *

     * The cache does not store multiple versions - any existing version will be overwritten with

     * this one. This function should only be used if there is a known 'current version' (e.g.

     * stored in a database table). It only ensures that the cache does not return outdated data.

     *

     * This function can be used to help implement localisable caches (where the cache could be

     * stored on a local server as well as a shared cache). The version will be recorded alongside

     * the item and get_versioned will always return the correct version.

     *

     * The version number must be an integer that always increases. This could be based on the

     * current time, or a stored value that increases by 1 each time it changes, etc.

     *

     * If you use this function you must use get_versioned to retrieve the data.

     *

     * @param string|int $key The key for the data being set.

     * @param int $version Integer for the version of the data

     * @param mixed $data The data to set against the key.

     * @return bool True on success, false otherwise.

     */

    public function set_versioned($key, int $version, $data): bool {

        return $this->set_implementation($key, $version, $data);

    }

    /**

     * Sets the value for the given key, optionally with a version tag.

     *

     * @param string|int $key The key for the data being set.

     * @param int $version Version number for the data or cache::VERSION_NONE if none

     * @param mixed $data The data to set against the key.

     * @param bool $setparents If true, sets all parent loaders, otherwise only this one

     * @return bool True on success, false otherwise.

     */

    protected function set_implementation($key, int $version, $data, bool $setparents = true): bool {

        if ($this->loader !== false && $setparents) {

            // We have a loader available set it there as well.

            // We have to let the loader do its own parsing of data as it may be unique.

            if ($version === self::VERSION_NONE) {

                $this->loader->set($key, $data);

            } else {

                $this->loader->set_versioned($key, $version, $data);

            }

        }

        $usestaticacceleration = $this->use_static_acceleration();

        if (is_object($data) && $data instanceof cacheable_object_interface) {

            $data = new cached_object($data);

        } else if (!$this->store->supports_dereferencing_objects() && !is_scalar($data)) {

            // If data is an object it will be a reference.

            // If data is an array if may contain references.

            // We want to break references so that the cache cannot be modified outside of itself.

            // Call the function to unreference it (in the best way possible).

            $data = $this->unref($data);

        }

        if ($usestaticacceleration) {

            // Static acceleration cache should include the cache version wrapper, but not TTL.

            if ($version === self::VERSION_NONE) {

                $this->static_acceleration_set($key, $data);

            } else {

                $this->static_acceleration_set($key, new \core_cache\version_wrapper($data, $version));

            }

        }

        if ($this->has_a_ttl() && !$this->store_supports_native_ttl()) {

            $data = new ttl_wrapper($data, $this->definition->get_ttl());

        }

        $parsedkey = $this->parse_key($key);

        if ($version !== self::VERSION_NONE) {

            $data = new \core_cache\version_wrapper($data, $version);

        }

        $success = $this->store->set($parsedkey, $data);

        if ($this->perfdebug) {

            helper::record_cache_set(

                $this->store,

                $this->definition,

                1,

                $this->store->get_last_io_bytes()

            );

        }

        return $success;

    }

    /**

     * Removes references where required.

     *

     * @param stdClass|array $data

     * @return mixed What ever was put in but without any references.

     */

    protected function unref($data) {

        if ($this->definition->uses_simple_data()) {

            return $data;

        }

        // Check if it requires serialisation in order to produce a reference free copy.

        if ($this->requires_serialisation($data)) {

            // Damn, its going to have to be serialise.

            $data = serialize($data);

            // We unserialise immediately so that we don't have to do it every time on get.

            $data = unserialize($data);

        } else if (!is_scalar($data)) {

            // Its safe to clone, lets do it, its going to beat the pants of serialisation.

            $data = $this->deep_clone($data);

        }

        return $data;

    }

    /**

     * Checks to see if a var requires serialisation.

     *

     * @param mixed $value The value to check.

     * @param int $depth Used to ensure we don't enter an endless loop (think recursion).

     * @return bool Returns true if the value is going to require serialisation in order to ensure a reference free copy

     *      or false if its safe to clone.

     */

    protected function requires_serialisation($value, $depth = 1) {

        if (is_scalar($value)) {

            return false;

        } else if (is_array($value) || $value instanceof stdClass || $value instanceof Traversable) {

            if ($depth > 5) {

                // Skrew it, mega-deep object, developer you suck, we're just going to serialise.

                return true;

            }

            foreach ($value as $key => $subvalue) {

                if ($this->requires_serialisation($subvalue, $depth++)) {

                    return true;

                }

            }

        }

        // Its not scalar, array, or stdClass so we'll need to serialise.

        return true;

    }

    /**

     * Creates a reference free clone of the given value.

     *

     * @param mixed $value

     * @return mixed

     */

    protected function deep_clone($value) {

        if (is_object($value)) {

            // Objects must be cloned to begin with.

            $value = clone $value;

        }

        if (is_array($value) || is_object($value)) {

            foreach ($value as $key => $subvalue) {

                $value[$key] = $this->deep_clone($subvalue);

            }

        }

        return $value;

    }

    /**

     * Sends several key => value pairs to the cache.

     *

     * Using this function comes with potential performance implications.

     * Not all cache stores will support get_many/set_many operations and in order to replicate this functionality will call

     * the equivalent singular method for each item provided.

     * This should not deter you from using this function as there is a performance benefit in situations where the cache store

     * does support it, but you should be aware of this fact.

     *

     * <code>

     * // This code will add four entries to the cache, one for each url.

     * $cache->set_many(array(

     *     'main' => 'http://moodle.org',

     *     'docs' => 'http://docs.moodle.org',

     *     'tracker' => 'http://tracker.moodle.org',

     *     'qa' => ''http://qa.moodle.net'

     * ));

     * </code>

     *

     * @param array $keyvaluearray An array of key => value pairs to send to the cache.

     * @return int The number of items successfully set. It is up to the developer to check this matches the number of items.

     *      ... if they care that is.

     */

    public function set_many(array $keyvaluearray) {