objectcache: add mcrouter support to WANObjectCache

[lhc/web/wiklou.git] / includes / libs / objectcache / WANObjectCache.php

<?php
/**
 * This program 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 2 of the License, or
 * (at your option) any later version.
 *
 * This program 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 this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Cache
 * @author Aaron Schulz
 */

use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;

/**
 * Multi-datacenter aware caching interface
 *
 * All operations go to the local datacenter cache, except for delete(),
 * touchCheckKey(), and resetCheckKey(), which broadcast to all datacenters.
 *
 * This class is intended for caching data from primary stores.
 * If the get() method does not return a value, then the caller
 * should query the new value and backfill the cache using set().
 * When querying the store on cache miss, the closest DB replica
 * should be used. Try to avoid heavyweight DB master or quorum reads.
 * When the source data changes, a purge method should be called.
 * Since purges are expensive, they should be avoided. One can do so if:
 *   - a) The object cached is immutable; or
 *   - b) Validity is checked against the source after get(); or
 *   - c) Using a modest TTL is reasonably correct and performant
 *
 * The simplest purge method is delete().
 *
 * There are two supported ways to handle broadcasted operations:
 *   - a) Configure the 'purge' EventRelayer to point to a valid PubSub endpoint
 *        that has subscribed listeners on the cache servers applying the cache updates.
 *   - b) Ignore the 'purge' EventRelayer configuration (default is NullEventRelayer)
 *        and set up mcrouter as the underlying cache backend. Using OperationSelectorRoute,
 *        configure 'set' and 'delete' operations to go to all DCs via AllAsyncRoute and
 *        configure other operations to go to the local DC via PoolRoute (for reference,
 *        see https://github.com/facebook/mcrouter/wiki/List-of-Route-Handles).
 *
 * Broadcasted operations like delete() and touchCheckKey() are done asynchronously
 * in all datacenters this way, though the local one should likely be near immediate.
 *
 * This means that callers in all datacenters may see older values for however many
 * milliseconds the the purge took to reach that datacenter. As with any cache, this
 * should not be relied on for cases where reads are used to determine writes to source
 * (e.g. non-cache) data stores.
 *
 * All values are wrapped in metadata arrays. Keys use a "WANCache:" prefix
 * to avoid collisions with keys that are not wrapped as metadata arrays. The
 * prefixes are as follows:
 *   - a) "WANCache:v" : used for regular value keys
 *   - b) "WANCache:i" : used for temporarily storing values of tombstoned keys
 *   - c) "WANCache:t" : used for storing timestamp "check" keys
 *   - d) "WANCache:m" : used for temporary mutex keys to avoid cache stampedes
 *
 * @ingroup Cache
 * @since 1.26
 */
class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
        /** @var BagOStuff The local datacenter cache */
        protected $cache;
        /** @var HashBagOStuff Script instance PHP cache */
        protected $procCache;
        /** @var string Purge channel name */
        protected $purgeChannel;
        /** @var EventRelayer Bus that handles purge broadcasts */
        protected $purgeRelayer;
        /** @var LoggerInterface */
        protected $logger;

        /** @var int ERR_* constant for the "last error" registry */
        protected $lastRelayError = self::ERR_NONE;

        /** Max time expected to pass between delete() and DB commit finishing */
        const MAX_COMMIT_DELAY = 3;
        /** Max replication+snapshot lag before applying TTL_LAGGED or disallowing set() */
        const MAX_READ_LAG = 7;
        /** Seconds to tombstone keys on delete() */
        const HOLDOFF_TTL = 11; // MAX_COMMIT_DELAY + MAX_READ_LAG + 1

        /** Seconds to keep dependency purge keys around */
        const CHECK_KEY_TTL = self::TTL_YEAR;
        /** Seconds to keep lock keys around */
        const LOCK_TTL = 10;
        /** Default remaining TTL at which to consider pre-emptive regeneration */
        const LOW_TTL = 30;
        /** Default time-since-expiry on a miss that makes a key "hot" */
        const LOCK_TSE = 1;

        /** Idiom for getWithSetCallback() callbacks to avoid calling set() */
        const TTL_UNCACHEABLE = -1;
        /** Idiom for getWithSetCallback() callbacks to 'lockTSE' logic */
        const TSE_NONE = -1;
        /** Max TTL to store keys when a data sourced is lagged */
        const TTL_LAGGED = 30;
        /** Idiom for delete() for "no hold-off" */
        const HOLDOFF_NONE = 0;

        /** Tiny negative float to use when CTL comes up >= 0 due to clock skew */
        const TINY_NEGATIVE = -0.000001;

        /** Cache format version number */
        const VERSION = 1;

        const FLD_VERSION = 0; // key to cache version number
        const FLD_VALUE = 1; // key to the cached value
        const FLD_TTL = 2; // key to the original TTL
        const FLD_TIME = 3; // key to the cache time
        const FLD_FLAGS = 4; // key to the flags bitfield
        const FLD_HOLDOFF = 5; // key to any hold-off TTL

        /** @var integer Treat this value as expired-on-arrival */
        const FLG_STALE = 1;

        const ERR_NONE = 0; // no error
        const ERR_NO_RESPONSE = 1; // no response
        const ERR_UNREACHABLE = 2; // can't connect
        const ERR_UNEXPECTED = 3; // response gave some error
        const ERR_RELAY = 4; // relay broadcast failed

        const VALUE_KEY_PREFIX = 'WANCache:v:';
        const INTERIM_KEY_PREFIX = 'WANCache:i:';
        const TIME_KEY_PREFIX = 'WANCache:t:';
        const MUTEX_KEY_PREFIX = 'WANCache:m:';

        const PURGE_VAL_PREFIX = 'PURGED:';

        const VFLD_DATA = 'WOC:d'; // key to the value of versioned data
        const VFLD_VERSION = 'WOC:v'; // key to the version of the value present

        const MAX_PC_KEYS = 1000; // max keys to keep in process cache

        const DEFAULT_PURGE_CHANNEL = 'wancache-purge';

        /**
         * @param array $params
         *   - cache    : BagOStuff object for a persistent cache
         *   - channels : Map of (action => channel string). Actions include "purge".
         *   - relayers : Map of (action => EventRelayer object). Actions include "purge".
         *   - logger   : LoggerInterface object
         */
        public function __construct( array $params ) {
                $this->cache = $params['cache'];
                $this->procCache = new HashBagOStuff( [ 'maxKeys' => self::MAX_PC_KEYS ] );
                $this->purgeChannel = isset( $params['channels']['purge'] )
                        ? $params['channels']['purge']
                        : self::DEFAULT_PURGE_CHANNEL;
                $this->purgeRelayer = isset( $params['relayers']['purge'] )
                        ? $params['relayers']['purge']
                        : new EventRelayerNull( [] );
                $this->setLogger( isset( $params['logger'] ) ? $params['logger'] : new NullLogger() );
        }

        public function setLogger( LoggerInterface $logger ) {
                $this->logger = $logger;
        }

        /**
         * Get an instance that wraps EmptyBagOStuff
         *
         * @return WANObjectCache
         */
        public static function newEmpty() {
                return new self( [
                        'cache'   => new EmptyBagOStuff(),
                        'pool'    => 'empty',
                        'relayer' => new EventRelayerNull( [] )
                ] );
        }

        /**
         * Fetch the value of a key from cache
         *
         * If supplied, $curTTL is set to the remaining TTL (current time left):
         *   - a) INF; if $key exists, has no TTL, and is not expired by $checkKeys
         *   - b) float (>=0); if $key exists, has a TTL, and is not expired by $checkKeys
         *   - c) float (<0); if $key is tombstoned, stale, or existing but expired by $checkKeys
         *   - d) null; if $key does not exist and is not tombstoned
         *
         * If a key is tombstoned, $curTTL will reflect the time since delete().
         *
         * The timestamp of $key will be checked against the last-purge timestamp
         * of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
         * initialized to the current timestamp. If any of $checkKeys have a timestamp
         * greater than that of $key, then $curTTL will reflect how long ago $key
         * became invalid. Callers can use $curTTL to know when the value is stale.
         * The $checkKeys parameter allow mass invalidations by updating a single key:
         *   - a) Each "check" key represents "last purged" of some source data
         *   - b) Callers pass in relevant "check" keys as $checkKeys in get()
         *   - c) When the source data that "check" keys represent changes,
         *        the touchCheckKey() method is called on them
         *
         * Source data entities might exists in a DB that uses snapshot isolation
         * (e.g. the default REPEATABLE-READ in innoDB). Even for mutable data, that
         * isolation can largely be maintained by doing the following:
         *   - a) Calling delete() on entity change *and* creation, before DB commit
         *   - b) Keeping transaction duration shorter than delete() hold-off TTL
         *
         * However, pre-snapshot values might still be seen if an update was made
         * in a remote datacenter but the purge from delete() didn't relay yet.
         *
         * Consider using getWithSetCallback() instead of get() and set() cycles.
         * That method has cache slam avoiding features for hot/expensive keys.
         *
         * @param string $key Cache key
         * @param mixed $curTTL Approximate TTL left on the key if present/tombstoned [returned]
         * @param array $checkKeys List of "check" keys
         * @param float &$asOf UNIX timestamp of cached value; null on failure [returned]
         * @return mixed Value of cache key or false on failure
         */
        final public function get( $key, &$curTTL = null, array $checkKeys = [], &$asOf = null ) {
                $curTTLs = [];
                $asOfs = [];
                $values = $this->getMulti( [ $key ], $curTTLs, $checkKeys, $asOfs );
                $curTTL = isset( $curTTLs[$key] ) ? $curTTLs[$key] : null;
                $asOf = isset( $asOfs[$key] ) ? $asOfs[$key] : null;

                return isset( $values[$key] ) ? $values[$key] : false;
        }

        /**
         * Fetch the value of several keys from cache
         *
         * @see WANObjectCache::get()
         *
         * @param array $keys List of cache keys
         * @param array $curTTLs Map of (key => approximate TTL left) for existing keys [returned]
         * @param array $checkKeys List of check keys to apply to all $keys. May also apply "check"
         *  keys to specific cache keys only by using cache keys as keys in the $checkKeys array.
         * @param float[] &$asOfs Map of (key =>  UNIX timestamp of cached value; null on failure)
         * @return array Map of (key => value) for keys that exist
         */
        final public function getMulti(
                array $keys, &$curTTLs = [], array $checkKeys = [], array &$asOfs = []
        ) {
                $result = [];
                $curTTLs = [];
                $asOfs = [];

                $vPrefixLen = strlen( self::VALUE_KEY_PREFIX );
                $valueKeys = self::prefixCacheKeys( $keys, self::VALUE_KEY_PREFIX );

                $checkKeysForAll = [];
                $checkKeysByKey = [];
                $checkKeysFlat = [];
                foreach ( $checkKeys as $i => $keys ) {
                        $prefixed = self::prefixCacheKeys( (array)$keys, self::TIME_KEY_PREFIX );
                        $checkKeysFlat = array_merge( $checkKeysFlat, $prefixed );
                        // Is this check keys for a specific cache key, or for all keys being fetched?
                        if ( is_int( $i ) ) {
                                $checkKeysForAll = array_merge( $checkKeysForAll, $prefixed );
                        } else {
                                $checkKeysByKey[$i] = isset( $checkKeysByKey[$i] )
                                        ? array_merge( $checkKeysByKey[$i], $prefixed )
                                        : $prefixed;
                        }
                }

                // Fetch all of the raw values
                $wrappedValues = $this->cache->getMulti( array_merge( $valueKeys, $checkKeysFlat ) );
                // Time used to compare/init "check" keys (derived after getMulti() to be pessimistic)
                $now = microtime( true );

                // Collect timestamps from all "check" keys
                $purgeValuesForAll = $this->processCheckKeys( $checkKeysForAll, $wrappedValues, $now );
                $purgeValuesByKey = [];
                foreach ( $checkKeysByKey as $cacheKey => $checks ) {
                        $purgeValuesByKey[$cacheKey] =
                                $this->processCheckKeys( $checks, $wrappedValues, $now );
                }

                // Get the main cache value for each key and validate them
                foreach ( $valueKeys as $vKey ) {
                        if ( !isset( $wrappedValues[$vKey] ) ) {
                                continue; // not found
                        }

                        $key = substr( $vKey, $vPrefixLen ); // unprefix

                        list( $value, $curTTL ) = $this->unwrap( $wrappedValues[$vKey], $now );
                        if ( $value !== false ) {
                                $result[$key] = $value;

                                // Force dependant keys to be invalid for a while after purging
                                // to reduce race conditions involving stale data getting cached
                                $purgeValues = $purgeValuesForAll;
                                if ( isset( $purgeValuesByKey[$key] ) ) {
                                        $purgeValues = array_merge( $purgeValues, $purgeValuesByKey[$key] );
                                }
                                foreach ( $purgeValues as $purge ) {
                                        $safeTimestamp = $purge[self::FLD_TIME] + $purge[self::FLD_HOLDOFF];
                                        if ( $safeTimestamp >= $wrappedValues[$vKey][self::FLD_TIME] ) {
                                                // How long ago this value was expired by *this* check key
                                                $ago = min( $purge[self::FLD_TIME] - $now, self::TINY_NEGATIVE );
                                                // How long ago this value was expired by *any* known check key
                                                $curTTL = min( $curTTL, $ago );
                                        }
                                }
                        }
                        $curTTLs[$key] = $curTTL;
                        $asOfs[$key] = ( $value !== false ) ? $wrappedValues[$vKey][self::FLD_TIME] : null;
                }

                return $result;
        }

        /**
         * @since 1.27
         * @param array $timeKeys List of prefixed time check keys
         * @param array $wrappedValues
         * @param float $now
         * @return array List of purge value arrays
         */
        private function processCheckKeys( array $timeKeys, array $wrappedValues, $now ) {
                $purgeValues = [];
                foreach ( $timeKeys as $timeKey ) {
                        $purge = isset( $wrappedValues[$timeKey] )
                                ? self::parsePurgeValue( $wrappedValues[$timeKey] )
                                : false;
                        if ( $purge === false ) {
                                // Key is not set or invalid; regenerate
                                $newVal = $this->makePurgeValue( $now, self::HOLDOFF_TTL );
                                $this->cache->add( $timeKey, $newVal, self::CHECK_KEY_TTL );
                                $purge = self::parsePurgeValue( $newVal );
                        }
                        $purgeValues[] = $purge;
                }
                return $purgeValues;
        }

        /**
         * Set the value of a key in cache
         *
         * Simply calling this method when source data changes is not valid because
         * the changes do not replicate to the other WAN sites. In that case, delete()
         * should be used instead. This method is intended for use on cache misses.
         *
         * If the data was read from a snapshot-isolated transactions (e.g. the default
         * REPEATABLE-READ in innoDB), use 'since' to avoid the following race condition:
         *   - a) T1 starts
         *   - b) T2 updates a row, calls delete(), and commits
         *   - c) The HOLDOFF_TTL passes, expiring the delete() tombstone
         *   - d) T1 reads the row and calls set() due to a cache miss
         *   - e) Stale value is stuck in cache
         *
         * Setting 'lag' and 'since' help avoids keys getting stuck in stale states.
         *
         * Example usage:
         * @code
         *     $dbr = wfGetDB( DB_SLAVE );
         *     $setOpts = Database::getCacheSetOptions( $dbr );
         *     // Fetch the row from the DB
         *     $row = $dbr->selectRow( ... );
         *     $key = $cache->makeKey( 'building', $buildingId );
         *     $cache->set( $key, $row, $cache::TTL_DAY, $setOpts );
         * @endcode
         *
         * @param string $key Cache key
         * @param mixed $value
         * @param integer $ttl Seconds to live. Special values are:
         *   - WANObjectCache::TTL_INDEFINITE: Cache forever
         * @param array $opts Options map:
         *   - lag     : Seconds of slave lag. Typically, this is either the slave lag
         *               before the data was read or, if applicable, the slave lag before
         *               the snapshot-isolated transaction the data was read from started.
         *               Default: 0 seconds
         *   - since   : UNIX timestamp of the data in $value. Typically, this is either
         *               the current time the data was read or (if applicable) the time when
         *               the snapshot-isolated transaction the data was read from started.
         *               Default: 0 seconds
         *   - pending : Whether this data is possibly from an uncommitted write transaction.
         *               Generally, other threads should not see values from the future and
         *               they certainly should not see ones that ended up getting rolled back.
         *               Default: false
         *   - lockTSE : if excessive replication/snapshot lag is detected, then store the value
         *               with this TTL and flag it as stale. This is only useful if the reads for
         *               this key use getWithSetCallback() with "lockTSE" set.
         *               Default: WANObjectCache::TSE_NONE
         * @return bool Success
         */
        final public function set( $key, $value, $ttl = 0, array $opts = [] ) {
                $now = microtime( true );
                $lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
                $age = isset( $opts['since'] ) ? max( 0, $now - $opts['since'] ) : 0;
                $lag = isset( $opts['lag'] ) ? $opts['lag'] : 0;

                // Do not cache potentially uncommitted data as it might get rolled back
                if ( !empty( $opts['pending'] ) ) {
                        $this->logger->info( "Rejected set() for $key due to pending writes." );

                        return true; // no-op the write for being unsafe
                }

                $wrapExtra = []; // additional wrapped value fields
                // Check if there's a risk of writing stale data after the purge tombstone expired
                if ( $lag === false || ( $lag + $age ) > self::MAX_READ_LAG ) {
                        // Case A: read lag with "lockTSE"; save but record value as stale
                        if ( $lockTSE >= 0 ) {
                                $ttl = max( 1, (int)$lockTSE ); // set() expects seconds
                                $wrapExtra[self::FLD_FLAGS] = self::FLG_STALE; // mark as stale
                        // Case B: any long-running transaction; ignore this set()
                        } elseif ( $age > self::MAX_READ_LAG ) {
                                $this->logger->warning( "Rejected set() for $key due to snapshot lag." );

                                return true; // no-op the write for being unsafe
                        // Case C: high replication lag; lower TTL instead of ignoring all set()s
                        } elseif ( $lag === false || $lag > self::MAX_READ_LAG ) {
                                $ttl = $ttl ? min( $ttl, self::TTL_LAGGED ) : self::TTL_LAGGED;
                                $this->logger->warning( "Lowered set() TTL for $key due to replication lag." );
                        // Case D: medium length request with medium replication lag; ignore this set()
                        } else {
                                $this->logger->warning( "Rejected set() for $key due to high read lag." );

                                return true; // no-op the write for being unsafe
                        }
                }

                // Wrap that value with time/TTL/version metadata
                $wrapped = $this->wrap( $value, $ttl, $now ) + $wrapExtra;

                $func = function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
                        return ( is_string( $cWrapped ) )
                                ? false // key is tombstoned; do nothing
                                : $wrapped;
                };

                return $this->cache->merge( self::VALUE_KEY_PREFIX . $key, $func, $ttl, 1 );
        }

        /**
         * Purge a key from all datacenters
         *
         * This should only be called when the underlying data (being cached)
         * changes in a significant way. This deletes the key and starts a hold-off
         * period where the key cannot be written to for a few seconds (HOLDOFF_TTL).
         * This is done to avoid the following race condition:
         *   - a) Some DB data changes and delete() is called on a corresponding key
         *   - b) A request refills the key with a stale value from a lagged DB
         *   - c) The stale value is stuck there until the key is expired/evicted
         *
         * This is implemented by storing a special "tombstone" value at the cache
         * key that this class recognizes; get() calls will return false for the key
         * and any set() calls will refuse to replace tombstone values at the key.
         * For this to always avoid stale value writes, the following must hold:
         *   - a) Replication lag is bounded to being less than HOLDOFF_TTL; or
         *   - b) If lag is higher, the DB will have gone into read-only mode already
         *
         * Note that set() can also be lag-aware and lower the TTL if it's high.
         *
         * When using potentially long-running ACID transactions, a good pattern is
         * to use a pre-commit hook to issue the delete. This means that immediately
         * after commit, callers will see the tombstone in cache upon purge relay.
         * It also avoids the following race condition:
         *   - a) T1 begins, changes a row, and calls delete()
         *   - b) The HOLDOFF_TTL passes, expiring the delete() tombstone
         *   - c) T2 starts, reads the row and calls set() due to a cache miss
         *   - d) T1 finally commits
         *   - e) Stale value is stuck in cache
         *
         * Example usage:
         * @code
         *     $dbw->begin( __METHOD__ ); // start of request
         *     ... <execute some stuff> ...
         *     // Update the row in the DB
         *     $dbw->update( ... );
         *     $key = $cache->makeKey( 'homes', $homeId );
         *     // Purge the corresponding cache entry just before committing
         *     $dbw->onTransactionPreCommitOrIdle( function() use ( $cache, $key ) {
         *         $cache->delete( $key );
         *     } );
         *     ... <execute some stuff> ...
         *     $dbw->commit( __METHOD__ ); // end of request
         * @endcode
         *
         * The $ttl parameter can be used when purging values that have not actually changed
         * recently. For example, a cleanup script to purge cache entries does not really need
         * a hold-off period, so it can use HOLDOFF_NONE. Likewise for user-requested purge.
         * Note that $ttl limits the effective range of 'lockTSE' for getWithSetCallback().
         *
         * If called twice on the same key, then the last hold-off TTL takes precedence. For
         * idempotence, the $ttl should not vary for different delete() calls on the same key.
         *
         * @param string $key Cache key
         * @param integer $ttl Tombstone TTL; Default: WANObjectCache::HOLDOFF_TTL
         * @return bool True if the item was purged or not found, false on failure
         */
        final public function delete( $key, $ttl = self::HOLDOFF_TTL ) {
                $key = self::VALUE_KEY_PREFIX . $key;

                if ( $ttl <= 0 ) {
                        // Publish the purge to all datacenters
                        $ok = $this->relayDelete( $key );
                } else {
                        // Publish the purge to all datacenters
                        $ok = $this->relayPurge( $key, $ttl, self::HOLDOFF_NONE );
                }

                return $ok;
        }

        /**
         * Fetch the value of a timestamp "check" key
         *
         * The key will be *initialized* to the current time if not set,
         * so only call this method if this behavior is actually desired
         *
         * The timestamp can be used to check whether a cached value is valid.
         * Callers should not assume that this returns the same timestamp in
         * all datacenters due to relay delays.
         *
         * The level of staleness can roughly be estimated from this key, but
         * if the key was evicted from cache, such calculations may show the
         * time since expiry as ~0 seconds.
         *
         * Note that "check" keys won't collide with other regular keys.
         *
         * @param string $key
         * @return float UNIX timestamp of the check key
         */
        final public function getCheckKeyTime( $key ) {
                $key = self::TIME_KEY_PREFIX . $key;

                $purge = self::parsePurgeValue( $this->cache->get( $key ) );
                if ( $purge !== false ) {
                        $time = $purge[self::FLD_TIME];
                } else {
                        // Casting assures identical floats for the next getCheckKeyTime() calls
                        $now = (string)microtime( true );
                        $this->cache->add( $key,
                                $this->makePurgeValue( $now, self::HOLDOFF_TTL ),
                                self::CHECK_KEY_TTL
                        );
                        $time = (float)$now;
                }

                return $time;
        }

        /**
         * Purge a "check" key from all datacenters, invalidating keys that use it
         *
         * This should only be called when the underlying data (being cached)
         * changes in a significant way, and it is impractical to call delete()
         * on all keys that should be changed. When get() is called on those
         * keys, the relevant "check" keys must be supplied for this to work.
         *
         * The "check" key essentially represents a last-modified field.
         * When touched, the field will be updated on all cache servers.
         * Keys using it via get(), getMulti(), or getWithSetCallback() will
         * be invalidated. It is treated as being HOLDOFF_TTL seconds in the future
         * by those methods to avoid race conditions where dependent keys get updated
         * with stale values (e.g. from a DB slave).
         *
         * This is typically useful for keys with hardcoded names or in some cases
         * dynamically generated names where a low number of combinations exist.
         * When a few important keys get a large number of hits, a high cache
         * time is usually desired as well as "lockTSE" logic. The resetCheckKey()
         * method is less appropriate in such cases since the "time since expiry"
         * cannot be inferred, causing any get() after the reset to treat the key
         * as being "hot", resulting in more stale value usage.
         *
         * Note that "check" keys won't collide with other regular keys.
         *
         * @see WANObjectCache::get()
         * @see WANObjectCache::getWithSetCallback()
         * @see WANObjectCache::resetCheckKey()
         *
         * @param string $key Cache key
         * @param int $holdoff HOLDOFF_TTL or HOLDOFF_NONE constant
         * @return bool True if the item was purged or not found, false on failure
         */
        final public function touchCheckKey( $key, $holdoff = self::HOLDOFF_TTL ) {
                // Publish the purge to all datacenters
                return $this->relayPurge( self::TIME_KEY_PREFIX . $key, self::CHECK_KEY_TTL, $holdoff );
        }

        /**
         * Delete a "check" key from all datacenters, invalidating keys that use it
         *
         * This is similar to touchCheckKey() in that keys using it via get(), getMulti(),
         * or getWithSetCallback() will be invalidated. The differences are:
         *   - a) The "check" key will be deleted from all caches and lazily
         *        re-initialized when accessed (rather than set everywhere)
         *   - b) Thus, dependent keys will be known to be invalid, but not
         *        for how long (they are treated as "just" purged), which
         *        effects any lockTSE logic in getWithSetCallback()
         *   - c) Since "check" keys are initialized only on the server the key hashes
         *        to, any temporary ejection of that server will cause the value to be
         *        seen as purged as a new server will initialize the "check" key.
         *
         * The advantage is that this does not place high TTL keys on every cache
         * server, making it better for code that will cache many different keys
         * and either does not use lockTSE or uses a low enough TTL anyway.
         *
         * This is typically useful for keys with dynamically generated names
         * where a high number of combinations exist.
         *
         * Note that "check" keys won't collide with other regular keys.
         *
         * @see WANObjectCache::get()
         * @see WANObjectCache::getWithSetCallback()
         * @see WANObjectCache::touchCheckKey()
         *
         * @param string $key Cache key
         * @return bool True if the item was purged or not found, false on failure
         */
        final public function resetCheckKey( $key ) {
                // Publish the purge to all datacenters
                return $this->relayDelete( self::TIME_KEY_PREFIX . $key );
        }

        /**
         * Method to fetch/regenerate cache keys
         *
         * On cache miss, the key will be set to the callback result via set()
         * (unless the callback returns false) and that result will be returned.
         * The arguments supplied to the callback are:
         *   - $oldValue : current cache value or false if not present
         *   - &$ttl : a reference to the TTL which can be altered
         *   - &$setOpts : a reference to options for set() which can be altered
         *
         * It is strongly recommended to set the 'lag' and 'since' fields to avoid race conditions
         * that can cause stale values to get stuck at keys. Usually, callbacks ignore the current
         * value, but it can be used to maintain "most recent X" values that come from time or
         * sequence based source data, provided that the "as of" id/time is tracked. Note that
         * preemptive regeneration and $checkKeys can result in a non-false current value.
         *
         * Usage of $checkKeys is similar to get() and getMulti(). However, rather than the caller
         * having to inspect a "current time left" variable (e.g. $curTTL, $curTTLs), a cache
         * regeneration will automatically be triggered using the callback.
         *
         * The simplest way to avoid stampedes for hot keys is to use
         * the 'lockTSE' option in $opts. If cache purges are needed, also:
         *   - a) Pass $key into $checkKeys
         *   - b) Use touchCheckKey( $key ) instead of delete( $key )
         *
         * Example usage (typical key):
         * @code
         *     $catInfo = $cache->getWithSetCallback(
         *         // Key to store the cached value under
         *         $cache->makeKey( 'cat-attributes', $catId ),
         *         // Time-to-live (in seconds)
         *         $cache::TTL_MINUTE,
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             return $dbr->selectRow( ... );
         *        }
         *     );
         * @endcode
         *
         * Example usage (key that is expensive and hot):
         * @code
         *     $catConfig = $cache->getWithSetCallback(
         *         // Key to store the cached value under
         *         $cache->makeKey( 'site-cat-config' ),
         *         // Time-to-live (in seconds)
         *         $cache::TTL_DAY,
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             return CatConfig::newFromRow( $dbr->selectRow( ... ) );
         *         },
         *         [
         *             // Calling touchCheckKey() on this key invalidates the cache
         *             'checkKeys' => [ $cache->makeKey( 'site-cat-config' ) ],
         *             // Try to only let one datacenter thread manage cache updates at a time
         *             'lockTSE' => 30
         *         ]
         *     );
         * @endcode
         *
         * Example usage (key with dynamic dependencies):
         * @code
         *     $catState = $cache->getWithSetCallback(
         *         // Key to store the cached value under
         *         $cache->makeKey( 'cat-state', $cat->getId() ),
         *         // Time-to-live (seconds)
         *         $cache::TTL_HOUR,
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             // Determine new value from the DB
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             return CatState::newFromResults( $dbr->select( ... ) );
         *         },
         *         [
         *              // The "check" keys that represent things the value depends on;
         *              // Calling touchCheckKey() on any of them invalidates the cache
         *             'checkKeys' => [
         *                 $cache->makeKey( 'sustenance-bowls', $cat->getRoomId() ),
         *                 $cache->makeKey( 'people-present', $cat->getHouseId() ),
         *                 $cache->makeKey( 'cat-laws', $cat->getCityId() ),
         *             ]
         *         ]
         *     );
         * @endcode
         *
         * Example usage (hot key holding most recent 100 events):
         * @code
         *     $lastCatActions = $cache->getWithSetCallback(
         *         // Key to store the cached value under
         *         $cache->makeKey( 'cat-last-actions', 100 ),
         *         // Time-to-live (in seconds)
         *         10,
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             // Start off with the last cached list
         *             $list = $oldValue ?: [];
         *             // Fetch the last 100 relevant rows in descending order;
         *             // only fetch rows newer than $list[0] to reduce scanning
         *             $rows = iterator_to_array( $dbr->select( ... ) );
         *             // Merge them and get the new "last 100" rows
         *             return array_slice( array_merge( $new, $list ), 0, 100 );
         *        },
         *        // Try to only let one datacenter thread manage cache updates at a time
         *        [ 'lockTSE' => 30 ]
         *     );
         * @endcode
         *
         * @see WANObjectCache::get()
         * @see WANObjectCache::set()
         *
         * @param string $key Cache key
         * @param integer $ttl Seconds to live for key updates. Special values are:
         *   - WANObjectCache::TTL_INDEFINITE: Cache forever
         *   - WANObjectCache::TTL_UNCACHEABLE: Do not cache at all
         * @param callable $callback Value generation function
         * @param array $opts Options map:
         *   - checkKeys: List of "check" keys. The key at $key will be seen as invalid when either
         *      touchCheckKey() or resetCheckKey() is called on any of these keys.
         *      Default: [].
         *   - lowTTL: Consider pre-emptive updates when the current TTL (seconds) of the key is less
         *      than this. It becomes more likely over time, becoming certain once the key is expired.
         *      Default: WANObjectCache::LOW_TTL.
         *   - lockTSE: If the key is tombstoned or expired (by checkKeys) less than this many seconds
         *      ago, then try to have a single thread handle cache regeneration at any given time.
         *      Other threads will try to use stale values if possible. If, on miss, the time since
         *      expiration is low, the assumption is that the key is hot and that a stampede is worth
         *      avoiding. Setting this above WANObjectCache::HOLDOFF_TTL makes no difference. The
         *      higher this is set, the higher the worst-case staleness can be.
         *      Use WANObjectCache::TSE_NONE to disable this logic.
         *      Default: WANObjectCache::TSE_NONE.
         *   - busyValue: If no value exists and another thread is currently regenerating it, use this
         *      as a fallback value (or a callback to generate such a value). This assures that cache
         *      stampedes cannot happen if the value falls out of cache. This can be used as insurance
         *      against cache regeneration becoming very slow for some reason (greater than the TTL).
         *      Default: null.
         *   - pcTTL: Process cache the value in this PHP instance for this many seconds. This avoids
         *      network I/O when a key is read several times. This will not cache when the callback
         *      returns false, however. Note that any purges will not be seen while process cached;
         *      since the callback should use slave DBs and they may be lagged or have snapshot
         *      isolation anyway, this should not typically matter.
         *      Default: WANObjectCache::TTL_UNCACHEABLE.
         *   - version: Integer version number. This allows for callers to make breaking changes to
         *      how values are stored while maintaining compatability and correct cache purges. New
         *      versions are stored alongside older versions concurrently. Avoid storing class objects
         *      however, as this reduces compatibility (due to serialization).
         *      Default: null.
         * @return mixed Value found or written to the key
         * @note Callable type hints are not used to avoid class-autoloading
         */
        final public function getWithSetCallback( $key, $ttl, $callback, array $opts = [] ) {
                $pcTTL = isset( $opts['pcTTL'] ) ? $opts['pcTTL'] : self::TTL_UNCACHEABLE;

                // Try the process cache if enabled
                $value = ( $pcTTL >= 0 ) ? $this->procCache->get( $key ) : false;

                if ( $value === false ) {
                        unset( $opts['minTime'] ); // not a public feature

                        // Fetch the value over the network
                        if ( isset( $opts['version'] ) ) {
                                $version = $opts['version'];
                                $asOf = null;
                                $cur = $this->doGetWithSetCallback(
                                        $key,
                                        $ttl,
                                        function ( $oldValue, &$ttl, &$setOpts ) use ( $callback, $version ) {
                                                if ( is_array( $oldValue )
                                                        && array_key_exists( self::VFLD_DATA, $oldValue )
                                                ) {
                                                        $oldData = $oldValue[self::VFLD_DATA];
                                                } else {
                                                        // VFLD_DATA is not set if an old, unversioned, key is present
                                                        $oldData = false;
                                                }

                                                return [
                                                        self::VFLD_DATA => $callback( $oldData, $ttl, $setOpts ),
                                                        self::VFLD_VERSION => $version
                                                ];
                                        },
                                        $opts,
                                        $asOf
                                );
                                if ( $cur[self::VFLD_VERSION] === $version ) {
                                        // Value created or existed before with version; use it
                                        $value = $cur[self::VFLD_DATA];
                                } else {
                                        // Value existed before with a different version; use variant key.
                                        // Reflect purges to $key by requiring that this key value be newer.
                                        $value = $this->doGetWithSetCallback(
                                                'cache-variant:' . md5( $key ) . ":$version",
                                                $ttl,
                                                $callback,
                                                // Regenerate value if not newer than $key
                                                [ 'version' => null, 'minTime' => $asOf ] + $opts
                                        );
                                }
                        } else {
                                $value = $this->doGetWithSetCallback( $key, $ttl, $callback, $opts );
                        }

                        // Update the process cache if enabled
                        if ( $pcTTL >= 0 && $value !== false ) {
                                $this->procCache->set( $key, $value, $pcTTL );
                        }
                }

                return $value;
        }

        /**
         * Do the actual I/O for getWithSetCallback() when needed
         *
         * @see WANObjectCache::getWithSetCallback()
         *
         * @param string $key
         * @param integer $ttl
         * @param callback $callback
         * @param array $opts Options map for getWithSetCallback() which also includes:
         *   - minTime: Treat values older than this UNIX timestamp as not existing. Default: null.
         * @param float &$asOf Cache generation timestamp of returned value [returned]
         * @return mixed
         * @note Callable type hints are not used to avoid class-autoloading
         */
        protected function doGetWithSetCallback( $key, $ttl, $callback, array $opts, &$asOf = null ) {
                $lowTTL = isset( $opts['lowTTL'] ) ? $opts['lowTTL'] : min( self::LOW_TTL, $ttl );
                $lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
                $checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];
                $busyValue = isset( $opts['busyValue'] ) ? $opts['busyValue'] : null;
                $minTime = isset( $opts['minTime'] ) ? $opts['minTime'] : 0.0;
                $versioned = isset( $opts['version'] );

                // Get the current key value
                $curTTL = null;
                $cValue = $this->get( $key, $curTTL, $checkKeys, $asOf ); // current value
                $value = $cValue; // return value

                // Determine if a regeneration is desired
                if ( $value !== false
                        && $curTTL > 0
                        && $this->isValid( $value, $versioned, $asOf, $minTime )
                        && !$this->worthRefresh( $curTTL, $lowTTL )
                ) {
                        return $value;
                }

                // A deleted key with a negative TTL left must be tombstoned
                $isTombstone = ( $curTTL !== null && $value === false );
                // Assume a key is hot if requested soon after invalidation
                $isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
                // Use the mutex if there is no value and a busy fallback is given
                $checkBusy = ( $busyValue !== null && $value === false );
                // Decide whether a single thread should handle regenerations.
                // This avoids stampedes when $checkKeys are bumped and when preemptive
                // renegerations take too long. It also reduces regenerations while $key
                // is tombstoned. This balances cache freshness with avoiding DB load.
                $useMutex = ( $isHot || ( $isTombstone && $lockTSE > 0 ) || $checkBusy );

                $lockAcquired = false;
                if ( $useMutex ) {
                        // Acquire a datacenter-local non-blocking lock
                        if ( $this->cache->add( self::MUTEX_KEY_PREFIX . $key, 1, self::LOCK_TTL ) ) {
                                // Lock acquired; this thread should update the key
                                $lockAcquired = true;
                        } elseif ( $value !== false && $this->isValid( $value, $versioned, $asOf, $minTime ) ) {
                                // If it cannot be acquired; then the stale value can be used
                                return $value;
                        } else {
                                // Use the INTERIM value for tombstoned keys to reduce regeneration load.
                                // For hot keys, either another thread has the lock or the lock failed;
                                // use the INTERIM value from the last thread that regenerated it.
                                $wrapped = $this->cache->get( self::INTERIM_KEY_PREFIX . $key );
                                list( $value ) = $this->unwrap( $wrapped, microtime( true ) );
                                if ( $value !== false && $this->isValid( $value, $versioned, $asOf, $minTime ) ) {
                                        $asOf = $wrapped[self::FLD_TIME];

                                        return $value;
                                }
                                // Use the busy fallback value if nothing else
                                if ( $busyValue !== null ) {
                                        return is_callable( $busyValue ) ? $busyValue() : $busyValue;
                                }
                        }
                }

                if ( !is_callable( $callback ) ) {
                        throw new InvalidArgumentException( "Invalid cache miss callback provided." );
                }

                // Generate the new value from the callback...
                $setOpts = [];
                $value = call_user_func_array( $callback, [ $cValue, &$ttl, &$setOpts ] );
                $asOf = microtime( true );
                // When delete() is called, writes are write-holed by the tombstone,
                // so use a special INTERIM key to pass the new value around threads.
                if ( ( $isTombstone && $lockTSE > 0 ) && $value !== false && $ttl >= 0 ) {
                        $tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
                        $wrapped = $this->wrap( $value, $tempTTL, $asOf );
                        // Avoid using set() to avoid pointless mcrouter broadcasting
                        $this->cache->merge(
                                self::INTERIM_KEY_PREFIX . $key,
                                function () use ( $wrapped ) {
                                        return $wrapped;
                                },
                                $tempTTL,
                                1
                        );
                }

                if ( $value !== false && $ttl >= 0 ) {
                        // Update the cache; this will fail if the key is tombstoned
                        $setOpts['lockTSE'] = $lockTSE;
                        $this->set( $key, $value, $ttl, $setOpts );
                }

                if ( $lockAcquired ) {
                        // Avoid using delete() to avoid pointless mcrouter broadcasting
                        $this->cache->changeTTL( self::MUTEX_KEY_PREFIX . $key, 1 );
                }

                return $value;
        }

        /**
         * @see BagOStuff::makeKey()
         * @param string ... Key component
         * @return string
         * @since 1.27
         */
        public function makeKey() {
                return call_user_func_array( [ $this->cache, __FUNCTION__ ], func_get_args() );
        }

        /**
         * @see BagOStuff::makeGlobalKey()
         * @param string ... Key component
         * @return string
         * @since 1.27
         */
        public function makeGlobalKey() {
                return call_user_func_array( [ $this->cache, __FUNCTION__ ], func_get_args() );
        }

        /**
         * Get the "last error" registered; clearLastError() should be called manually
         * @return int ERR_* class constant for the "last error" registry
         */
        final public function getLastError() {
                if ( $this->lastRelayError ) {
                        // If the cache and the relayer failed, focus on the later.
                        // An update not making it to the relayer means it won't show up
                        // in other DCs (nor will consistent re-hashing see up-to-date values).
                        // On the other hand, if just the cache update failed, then it should
                        // eventually be applied by the relayer.
                        return $this->lastRelayError;
                }

                $code = $this->cache->getLastError();
                switch ( $code ) {
                        case BagOStuff::ERR_NONE:
                                return self::ERR_NONE;
                        case BagOStuff::ERR_NO_RESPONSE:
                                return self::ERR_NO_RESPONSE;
                        case BagOStuff::ERR_UNREACHABLE:
                                return self::ERR_UNREACHABLE;
                        default:
                                return self::ERR_UNEXPECTED;
                }
        }

        /**
         * Clear the "last error" registry
         */
        final public function clearLastError() {
                $this->cache->clearLastError();
                $this->lastRelayError = self::ERR_NONE;
        }

        /**
         * Clear the in-process caches; useful for testing
         *
         * @since 1.27
         */
        public function clearProcessCache() {
                $this->procCache->clear();
        }

        /**
         * @param integer $flag ATTR_* class constant
         * @return integer QOS_* class constant
         * @since 1.28
         */
        public function getQoS( $flag ) {
                return $this->cache->getQoS( $flag );
        }

        /**
         * Do the actual async bus purge of a key
         *
         * This must set the key to "PURGED:<UNIX timestamp>:<holdoff>"
         *
         * @param string $key Cache key
         * @param integer $ttl How long to keep the tombstone [seconds]
         * @param integer $holdoff HOLDOFF_* constant controlling how long to ignore sets for this key
         * @return bool Success
         */
        protected function relayPurge( $key, $ttl, $holdoff ) {
                if ( $this->purgeRelayer instanceof EventRelayerNull ) {
                        // This handles the mcrouter and the single-DC case
                        $ok = $this->cache->set( $key,
                                $this->makePurgeValue( microtime( true ), self::HOLDOFF_NONE ),
                                $ttl
                        );
                } else {
                        $event = $this->cache->modifySimpleRelayEvent( [
                                'cmd' => 'set',
                                'key' => $key,
                                'val' => 'PURGED:$UNIXTIME$:' . (int)$holdoff,
                                'ttl' => max( $ttl, 1 ),
                                'sbt' => true, // substitute $UNIXTIME$ with actual microtime
                        ] );

                        $ok = $this->purgeRelayer->notify( $this->purgeChannel, $event );
                        if ( !$ok ) {
                                $this->lastRelayError = self::ERR_RELAY;
                        }
                }

                return $ok;
        }

        /**
         * Do the actual async bus delete of a key
         *
         * @param string $key Cache key
         * @return bool Success
         */
        protected function relayDelete( $key ) {
                if ( $this->purgeRelayer instanceof EventRelayerNull ) {
                        // This handles the mcrouter and the single-DC case
                        $ok = $this->cache->delete( $key );
                } else {
                        $event = $this->cache->modifySimpleRelayEvent( [
                                'cmd' => 'delete',
                                'key' => $key,
                        ] );

                        $ok = $this->purgeRelayer->notify( $this->purgeChannel, $event );
                        if ( !$ok ) {
                                $this->lastRelayError = self::ERR_RELAY;
                        }
                }

                return $ok;
        }

        /**
         * Check if a key should be regenerated (using random probability)
         *
         * This returns false if $curTTL >= $lowTTL. Otherwise, the chance
         * of returning true increases steadily from 0% to 100% as the $curTTL
         * moves from $lowTTL to 0 seconds. This handles widely varying
         * levels of cache access traffic.
         *
         * @param float $curTTL Approximate TTL left on the key if present
         * @param float $lowTTL Consider a refresh when $curTTL is less than this
         * @return bool
         */
        protected function worthRefresh( $curTTL, $lowTTL ) {
                if ( $curTTL >= $lowTTL ) {
                        return false;
                } elseif ( $curTTL <= 0 ) {
                        return true;
                }

                $chance = ( 1 - $curTTL / $lowTTL );

                return mt_rand( 1, 1e9 ) <= 1e9 * $chance;
        }

        /**
         * Check whether $value is appropriately versioned and not older than $minTime (if set)
         *
         * @param array $value
         * @param bool $versioned
         * @param float $asOf The time $value was generated
         * @param float $minTime The last time the main value was generated (0.0 if unknown)
         * @return bool
         */
        protected function isValid( $value, $versioned, $asOf, $minTime ) {
                if ( $versioned && !isset( $value[self::VFLD_VERSION] ) ) {
                        return false;
                } elseif ( $minTime > 0 && $asOf < $minTime ) {
                        return false;
                }

                return true;
        }

        /**
         * Do not use this method outside WANObjectCache
         *
         * @param mixed $value
         * @param integer $ttl [0=forever]
         * @param float $now Unix Current timestamp just before calling set()
         * @return array
         */
        protected function wrap( $value, $ttl, $now ) {
                return [
                        self::FLD_VERSION => self::VERSION,
                        self::FLD_VALUE => $value,
                        self::FLD_TTL => $ttl,
                        self::FLD_TIME => $now
                ];
        }

        /**
         * Do not use this method outside WANObjectCache
         *
         * @param array|string|bool $wrapped
         * @param float $now Unix Current timestamp (preferrably pre-query)
         * @return array (mixed; false if absent/invalid, current time left)
         */
        protected function unwrap( $wrapped, $now ) {
                // Check if the value is a tombstone
                $purge = self::parsePurgeValue( $wrapped );
                if ( $purge !== false ) {
                        // Purged values should always have a negative current $ttl
                        $curTTL = min( $purge[self::FLD_TIME] - $now, self::TINY_NEGATIVE );
                        return [ false, $curTTL ];
                }

                if ( !is_array( $wrapped ) // not found
                        || !isset( $wrapped[self::FLD_VERSION] ) // wrong format
                        || $wrapped[self::FLD_VERSION] !== self::VERSION // wrong version
                ) {
                        return [ false, null ];
                }

                $flags = isset( $wrapped[self::FLD_FLAGS] ) ? $wrapped[self::FLD_FLAGS] : 0;
                if ( ( $flags & self::FLG_STALE ) == self::FLG_STALE ) {
                        // Treat as expired, with the cache time as the expiration
                        $age = $now - $wrapped[self::FLD_TIME];
                        $curTTL = min( -$age, self::TINY_NEGATIVE );
                } elseif ( $wrapped[self::FLD_TTL] > 0 ) {
                        // Get the approximate time left on the key
                        $age = $now - $wrapped[self::FLD_TIME];
                        $curTTL = max( $wrapped[self::FLD_TTL] - $age, 0.0 );
                } else {
                        // Key had no TTL, so the time left is unbounded
                        $curTTL = INF;
                }

                return [ $wrapped[self::FLD_VALUE], $curTTL ];
        }

        /**
         * @param array $keys
         * @param string $prefix
         * @return string[]
         */
        protected static function prefixCacheKeys( array $keys, $prefix ) {
                $res = [];
                foreach ( $keys as $key ) {
                        $res[] = $prefix . $key;
                }

                return $res;
        }

        /**
         * @param string $value Wrapped value like "PURGED:<timestamp>:<holdoff>"
         * @return array|bool Array containing a UNIX timestamp (float) and holdoff period (integer),
         *  or false if value isn't a valid purge value
         */
        protected static function parsePurgeValue( $value ) {
                if ( !is_string( $value ) ) {
                        return false;
                }
                $segments = explode( ':', $value, 3 );
                if ( !isset( $segments[0] ) || !isset( $segments[1] )
                        || "{$segments[0]}:" !== self::PURGE_VAL_PREFIX
                ) {
                        return false;
                }
                if ( !isset( $segments[2] ) ) {
                        // Back-compat with old purge values without holdoff
                        $segments[2] = self::HOLDOFF_TTL;
                }
                return [
                        self::FLD_TIME => (float)$segments[1],
                        self::FLD_HOLDOFF => (int)$segments[2],
                ];
        }

        /**
         * @param float $timestamp
         * @param int $holdoff In seconds
         * @return string Wrapped purge value
         */
        protected function makePurgeValue( $timestamp, $holdoff ) {
                return self::PURGE_VAL_PREFIX . (float)$timestamp . ':' . (int)$holdoff;
        }
}