objectcache: Run preemptive WAN cache refreshes post-send

[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
 */

use Liuggio\StatsdClient\Factory\StatsdDataFactoryInterface;
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().
 * The preferred way to do this logic is through getWithSetCallback().
 * 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 three 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 one of the memcached
 *         BagOStuff classes as 'cache'. Use OperationSelectorRoute in the mcrouter settings
 *         to 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).
 *   - c) Ignore the 'purge' EventRelayer configuration (default is NullEventRelayer)
 *         and set up dynomite as cache middleware between the web servers and either
 *         memcached or redis. This will also broadcast all key setting operations, not just purges,
 *         which can be useful for cache warming. Writes are eventually consistent via the
 *         Dynamo replication model (see https://github.com/Netflix/dynomite).
 *
 * 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 that 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, except when reading immutable data.
 *
 * 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[] Map of group PHP instance caches */
        protected $processCaches = [];
        /** @var string Purge channel name */
        protected $purgeChannel;
        /** @var EventRelayer Bus that handles purge broadcasts */
        protected $purgeRelayer;
        /** @var LoggerInterface */
        protected $logger;
        /** @var StatsdDataFactoryInterface */
        protected $stats;
        /** @var bool Whether to use "interim" caching while keys are tombstoned */
        protected $useInterimHoldOffCaching = true;
        /** @var callable|null Function that takes a WAN cache callback and runs it later */
        protected $asyncHandler;

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

        /** @var int Callback stack depth for getWithSetCallback() */
        private $callbackDepth = 0;
        /** @var mixed[] Temporary warm-up cache */
        private $warmupCache = [];
        /** @var int Key fetched */
        private $warmupKeyMisses = 0;

        /** 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 interim value keys for tombstoned keys around */
        const INTERIM_KEY_TTL = 1;

        /** 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;

        /** Never consider performing "popularity" refreshes until a key reaches this age */
        const AGE_NEW = 60;
        /** The time length of the "popularity" refresh window for hot keys */
        const HOT_TTR = 900;
        /** Hits/second for a refresh to be expected within the "popularity" window */
        const HIT_RATE_HIGH = 1;
        /** Seconds to ramp up to the "popularity" refresh chance after a key is no longer new */
        const RAMPUP_TTL = 30;

        /** 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;
        /** Idiom for set()/getWithSetCallback() for "do not augment the storage medium TTL" */
        const STALE_TTL_NONE = 0;
        /** Idiom for set()/getWithSetCallback() for "no post-expired grace period" */
        const GRACE_TTL_NONE = 0;

        /** Idiom for getWithSetCallback() for "no minimum required as-of timestamp" */
        const MIN_TIMESTAMP_NONE = 0.0;

        /** 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 int 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 PC_PRIMARY = 'primary:1000'; // process cache name and max key count

        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
         *   - stats    : LoggerInterface object
         *   - asyncHandler : A function that takes a callback and runs it later. If supplied,
         *       whenever a preemptive refresh would be triggered in getWithSetCallback(), the
         *       current cache value is still used instead. However, the async-handler function
         *       receives a WAN cache callback that, when run, will execute the value generation
         *       callback supplied by the getWithSetCallback() caller. The result will be saved
         *       as normal. The handler is expected to call the WAN cache callback at an opportune
         *       time (e.g. HTTP post-send), though generally within a few 100ms. [optional]
         */
        public function __construct( array $params ) {
                $this->cache = $params['cache'];
                $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() );
                $this->stats = isset( $params['stats'] ) ? $params['stats'] : new NullStatsdDataFactory();
                $this->asyncHandler = isset( $params['asyncHandler'] ) ? $params['asyncHandler'] : null;
        }

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

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

        /**
         * 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 $key Cache key made from makeKey() or makeGlobalKey()
         * @param mixed &$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 $keys List of cache keys made from makeKey() or makeGlobalKey()
         * @param array &$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 and are not tombstoned
         */
        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 => $checkKeyGroup ) {
                        $prefixed = self::prefixCacheKeys( (array)$checkKeyGroup, 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
                $keysGet = array_merge( $valueKeys, $checkKeysFlat );
                if ( $this->warmupCache ) {
                        $wrappedValues = array_intersect_key( $this->warmupCache, array_flip( $keysGet ) );
                        $keysGet = array_diff( $keysGet, array_keys( $wrappedValues ) ); // keys left to fetch
                        $this->warmupKeyMisses += count( $keysGet );
                } else {
                        $wrappedValues = [];
                }
                if ( $keysGet ) {
                        $wrappedValues += $this->cache->getMulti( $keysGet );
                }
                // Time used to compare/init "check" keys (derived after getMulti() to be pessimistic)
                $now = $this->getCurrentTime();

                // 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_REPLICA );
         *     $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 int $ttl Seconds to live. Special values are:
         *   - WANObjectCache::TTL_INDEFINITE: Cache forever
         * @param array $opts Options map:
         *   - lag : Seconds of replica DB lag. Typically, this is either the replica DB lag
         *      before the data was read or, if applicable, the replica DB lag before
         *      the snapshot-isolated transaction the data was read from started.
         *      Use false to indicate that replication is not running.
         *      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. Note that if "staleTTL" is set
         *      then it will still add on to this TTL in the excessive lag scenario.
         *      Default: WANObjectCache::TSE_NONE
         *   - staleTTL : Seconds to keep the key around if it is stale. The get()/getMulti()
         *      methods return such stale values with a $curTTL of 0, and getWithSetCallback()
         *      will call the regeneration callback in such cases, passing in the old value
         *      and its as-of time to the callback. This is useful if adaptiveTTL() is used
         *      on the old value's as-of time when it is verified as still being correct.
         *      Default: WANObjectCache::STALE_TTL_NONE.
         * @note Options added in 1.28: staleTTL
         * @return bool Success
         */
        final public function set( $key, $value, $ttl = 0, array $opts = [] ) {
                $now = $this->getCurrentTime();
                $lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
                $staleTTL = isset( $opts['staleTTL'] ) ? $opts['staleTTL'] : self::STALE_TTL_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 {cachekey} due to pending writes.',
                                [ 'cachekey' => $key ] );

                        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->info( 'Rejected set() for {cachekey} due to snapshot lag.',
                                        [ 'cachekey' => $key ] );

                                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 {cachekey} due to replication lag.',
                                        [ 'cachekey' => $key ] );
                        // Case D: medium length request with medium replication lag; ignore this set()
                        } else {
                                $this->logger->info( 'Rejected set() for {cachekey} due to high read lag.',
                                        [ 'cachekey' => $key ] );

                                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 + $staleTTL, 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->startAtomic( __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->endAtomic( __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 int $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
                        $now = (string)$this->getCurrentTime();
                        $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 time of an entity.
         * When the key is touched, the timestamp will be updated to the current time.
         * Keys using the "check" key via get(), getMulti(), or getWithSetCallback() will
         * be invalidated. This approach is useful if many keys depend on a single entity.
         *
         * The timestamp of the "check" key is treated as being HOLDOFF_TTL seconds in the
         * future by get*() methods in order to avoid race conditions where keys are updated
         * with stale values (e.g. from a lagged replica DB). A high TTL is set on the "check"
         * key, making it possible to know the timestamp of the last change to the corresponding
         * entities in most cases. This might use more cache space than resetCheckKey().
         *
         * 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 here is that the "check" keys, which have high TTLs, will only
         * be created when a get*() method actually uses that key. This is better when
         * a large number of "check" keys are invalided in a short period of time.
         *
         * 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
         *   - $oldAsOf : generation UNIX timestamp of $oldValue or null if not present (since 1.28)
         *
         * 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 $ttl argument and "hotTTR" option (in $opts) use time-dependant randomization
         * to avoid stampedes. Keys that are slow to regenerate and either heavily used
         * or subject to explicit (unpredictable) purges, may need additional mechanisms.
         * The simplest way to avoid stampedes for such keys is to use 'lockTSE' (in $opts).
         * If explicit 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_REPLICA );
         *             // Account for any snapshot/replica DB 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_REPLICA );
         *             // Account for any snapshot/replica DB 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,
         *             // Avoid querying cache servers multiple times in a web request
         *             'pcTTL' => $cache::TTL_PROC_LONG
         *         ]
         *     );
         * @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_REPLICA );
         *             // Account for any snapshot/replica DB 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_REPLICA );
         *             // Account for any snapshot/replica DB 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,
         *             // Use a magic value when no cache value is ready rather than stampeding
         *             'busyValue' => 'computing'
         *        ]
         *     );
         * @endcode
         *
         * @see WANObjectCache::get()
         * @see WANObjectCache::set()
         *
         * @param string $key Cache key made from makeKey() or makeGlobalKey()
         * @param int $ttl Seconds to live for key updates. Special values are:
         *   - WANObjectCache::TTL_INDEFINITE: Cache forever (subject to LRU-style evictions)
         *   - WANObjectCache::TTL_UNCACHEABLE: Do not cache (if the key exists, it is not deleted)
         * @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 the keys in this list. This
         *      is useful if thousands or millions of keys depend on the same entity. The entity can
         *      simply have its "check" key updated whenever the entity is modified.
         *      Default: [].
         *   - graceTTL: Consider reusing expired values instead of refreshing them if they expired
         *      less than this many seconds ago. The odds of a refresh becomes more likely over time,
         *      becoming certain once the grace period is reached. This can reduce traffic spikes
         *      when millions of keys are compared to the same "check" key and touchCheckKey()
         *      or resetCheckKey() is called on that "check" key.
         *      Default: WANObjectCache::GRACE_TTL_NONE.
         *   - 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 replica DBs and they may be lagged or have snapshot
         *      isolation anyway, this should not typically matter.
         *      Default: WANObjectCache::TTL_UNCACHEABLE.
         *   - pcGroup: Process cache group to use instead of the primary one. If set, this must be
         *      of the format ALPHANUMERIC_NAME:MAX_KEY_SIZE, e.g. "mydata:10". Use this for storing
         *      large values, small yet numerous values, or some values with a high cost of eviction.
         *      It is generally preferable to use a class constant when setting this value.
         *      This has no effect unless pcTTL is used.
         *      Default: WANObjectCache::PC_PRIMARY.
         *   - 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.
         *   - minAsOf: Reject values if they were generated before this UNIX timestamp.
         *      This is useful if the source of a key is suspected of having possibly changed
         *      recently, and the caller wants any such changes to be reflected.
         *      Default: WANObjectCache::MIN_TIMESTAMP_NONE.
         *   - hotTTR: Expected time-till-refresh (TTR) in seconds for keys that average ~1 hit per
         *      second (e.g. 1Hz). Keys with a hit rate higher than 1Hz will refresh sooner than this
         *      TTR and vise versa. Such refreshes won't happen until keys are "ageNew" seconds old.
         *      This uses randomization to avoid triggering cache stampedes. The TTR is useful at
         *      reducing the impact of missed cache purges, since the effect of a heavily referenced
         *      key being stale is worse than that of a rarely referenced key. Unlike simply lowering
         *      $ttl, seldomly used keys are largely unaffected by this option, which makes it
         *      possible to have a high hit rate for the "long-tail" of less-used keys.
         *      Default: WANObjectCache::HOT_TTR.
         *   - 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.
         *      This helps avoid cache stampedes that might be triggered due to the key expiring.
         *      Default: WANObjectCache::LOW_TTL.
         *   - ageNew: Consider popularity refreshes only once a key reaches this age in seconds.
         *      Default: WANObjectCache::AGE_NEW.
         *   - staleTTL: Seconds to keep the key around if it is stale. This means that on cache
         *      miss the callback may get $oldValue/$oldAsOf values for keys that have already been
         *      expired for this specified time. This is useful if adaptiveTTL() is used on the old
         *      value's as-of time when it is verified as still being correct.
         *      Default: WANObjectCache::STALE_TTL_NONE
         * @return mixed Value found or written to the key
         * @note Options added in 1.28: version, busyValue, hotTTR, ageNew, pcGroup, minAsOf
         * @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 and the cache callback is not within a cache callback.
                // Process cache use in nested callbacks is not lag-safe with regard to HOLDOFF_TTL since
                // the in-memory value is further lagged than the shared one since it uses a blind TTL.
                if ( $pcTTL >= 0 && $this->callbackDepth == 0 ) {
                        $group = isset( $opts['pcGroup'] ) ? $opts['pcGroup'] : self::PC_PRIMARY;
                        $procCache = $this->getProcessCache( $group );
                        $value = $procCache->get( $key );
                } else {
                        $procCache = false;
                        $value = false;
                }

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

                                                return [
                                                        self::VFLD_DATA => $callback( $oldData, $ttl, $setOpts, $oldAsOf ),
                                                        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(
                                                $this->makeGlobalKey( 'WANCache-key-variant', md5( $key ), $version ),
                                                $ttl,
                                                $callback,
                                                // Regenerate value if not newer than $key
                                                [ 'version' => null, 'minAsOf' => $asOf ] + $opts
                                        );
                                }
                        } else {
                                $value = $this->doGetWithSetCallback( $key, $ttl, $callback, $opts );
                        }

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

                return $value;
        }

        /**
         * Do the actual I/O for getWithSetCallback() when needed
         *
         * @see WANObjectCache::getWithSetCallback()
         *
         * @param string $key
         * @param int $ttl
         * @param callback $callback
         * @param array $opts Options map for getWithSetCallback()
         * @param float &$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;
                $staleTTL = isset( $opts['staleTTL'] ) ? $opts['staleTTL'] : self::STALE_TTL_NONE;
                $graceTTL = isset( $opts['graceTTL'] ) ? $opts['graceTTL'] : self::GRACE_TTL_NONE;
                $checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];
                $busyValue = isset( $opts['busyValue'] ) ? $opts['busyValue'] : null;
                $popWindow = isset( $opts['hotTTR'] ) ? $opts['hotTTR'] : self::HOT_TTR;
                $ageNew = isset( $opts['ageNew'] ) ? $opts['ageNew'] : self::AGE_NEW;
                $minTime = isset( $opts['minAsOf'] ) ? $opts['minAsOf'] : self::MIN_TIMESTAMP_NONE;
                $versioned = isset( $opts['version'] );

                // Get a collection name to describe this class of key
                $kClass = $this->determineKeyClass( $key );

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

                $preCallbackTime = $this->getCurrentTime();
                // Determine if a cached value regeneration is needed or desired
                if ( $value !== false
                        && $this->isAliveOrInGracePeriod( $curTTL, $graceTTL )
                        && $this->isValid( $value, $versioned, $asOf, $minTime )
                ) {
                        $preemptiveRefresh = (
                                $this->worthRefreshExpiring( $curTTL, $lowTTL ) ||
                                $this->worthRefreshPopular( $asOf, $ageNew, $popWindow, $preCallbackTime )
                        );

                        if ( !$preemptiveRefresh ) {
                                $this->stats->increment( "wanobjectcache.$kClass.hit.good" );

                                return $value;
                        } elseif ( $this->asyncHandler ) {
                                // Update the cache value later, such during post-send of an HTTP request
                                $func = $this->asyncHandler;
                                $func( function () use ( $key, $ttl, $callback, $opts, $asOf ) {
                                        $opts['minAsOf'] = INF; // force a refresh
                                        $this->doGetWithSetCallback( $key, $ttl, $callback, $opts, $asOf );
                                } );
                                $this->stats->increment( "wanobjectcache.$kClass.hit.refresh" );

                                return $value;
                        }
                }

                // A deleted key with a negative TTL left must be tombstoned
                $isTombstone = ( $curTTL !== null && $value === false );
                if ( $isTombstone && $lockTSE <= 0 ) {
                        // Use the INTERIM value for tombstoned keys to reduce regeneration load
                        $lockTSE = self::INTERIM_KEY_TTL;
                }
                // 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 ) ) {
                                $this->stats->increment( "wanobjectcache.$kClass.hit.stale" );
                                // 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.
                                $value = $this->getInterimValue( $key, $versioned, $minTime, $asOf );
                                if ( $value !== false ) {
                                        $this->stats->increment( "wanobjectcache.$kClass.hit.volatile" );

                                        return $value;
                                }
                                // Use the busy fallback value if nothing else
                                if ( $busyValue !== null ) {
                                        $this->stats->increment( "wanobjectcache.$kClass.miss.busy" );

                                        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...
                $setOpts = [];
                ++$this->callbackDepth;
                try {
                        $value = call_user_func_array( $callback, [ $cValue, &$ttl, &$setOpts, $asOf ] );
                } finally {
                        --$this->callbackDepth;
                }
                $valueIsCacheable = ( $value !== false && $ttl >= 0 );

                // 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 ) && $valueIsCacheable ) {
                        $tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
                        $newAsOf = $this->getCurrentTime();
                        $wrapped = $this->wrap( $value, $tempTTL, $newAsOf );
                        // Avoid using set() to avoid pointless mcrouter broadcasting
                        $this->setInterimValue( $key, $wrapped, $tempTTL );
                }

                if ( $valueIsCacheable ) {
                        $setOpts['lockTSE'] = $lockTSE;
                        $setOpts['staleTTL'] = $staleTTL;
                        // Use best known "since" timestamp if not provided
                        $setOpts += [ 'since' => $preCallbackTime ];
                        // Update the cache; this will fail if the key is tombstoned
                        $this->set( $key, $value, $ttl, $setOpts );
                }

                if ( $lockAcquired ) {
                        // Avoid using delete() to avoid pointless mcrouter broadcasting
                        $this->cache->changeTTL( self::MUTEX_KEY_PREFIX . $key, (int)$preCallbackTime - 60 );
                }

                $this->stats->increment( "wanobjectcache.$kClass.miss.compute" );

                return $value;
        }

        /**
         * @param string $key
         * @param bool $versioned
         * @param float $minTime
         * @param mixed $asOf
         * @return mixed
         */
        protected function getInterimValue( $key, $versioned, $minTime, &$asOf ) {
                if ( !$this->useInterimHoldOffCaching ) {
                        return false; // disabled
                }

                $wrapped = $this->cache->get( self::INTERIM_KEY_PREFIX . $key );
                list( $value ) = $this->unwrap( $wrapped, $this->getCurrentTime() );
                if ( $value !== false && $this->isValid( $value, $versioned, $asOf, $minTime ) ) {
                        $asOf = $wrapped[self::FLD_TIME];

                        return $value;
                }

                return false;
        }

        /**
         * @param string $key
         * @param array $wrapped
         * @param int $tempTTL
         */
        protected function setInterimValue( $key, $wrapped, $tempTTL ) {
                $this->cache->merge(
                        self::INTERIM_KEY_PREFIX . $key,
                        function () use ( $wrapped ) {
                                return $wrapped;
                        },
                        $tempTTL,
                        1
                );
        }

        /**
         * Method to fetch multiple cache keys at once with regeneration
         *
         * This works the same as getWithSetCallback() except:
         *   - a) The $keys argument expects the result of WANObjectCache::makeMultiKeys()
         *   - b) The $callback argument expects a callback taking the following arguments:
         *         - $id: ID of an entity to query
         *         - $oldValue : the prior cache value or false if none was present
         *         - &$ttl : a reference to the new value TTL in seconds
         *         - &$setOpts : a reference to options for set() which can be altered
         *         - $oldAsOf : generation UNIX timestamp of $oldValue or null if not present
         *        Aside from the additional $id argument, the other arguments function the same
         *        way they do in getWithSetCallback().
         *   - c) The return value is a map of (cache key => value) in the order of $keyedIds
         *
         * @see WANObjectCache::getWithSetCallback()
         * @see WANObjectCache::getMultiWithUnionSetCallback()
         *
         * Example usage:
         * @code
         *     $rows = $cache->getMultiWithSetCallback(
         *         // Map of cache keys to entity IDs
         *         $cache->makeMultiKeys(
         *             $this->fileVersionIds(),
         *             function ( $id, WANObjectCache $cache ) {
         *                 return $cache->makeKey( 'file-version', $id );
         *             }
         *         ),
         *         // Time-to-live (in seconds)
         *         $cache::TTL_DAY,
         *         // Function that derives the new key value
         *         function ( $id, $oldValue, &$ttl, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_REPLICA );
         *             // Account for any snapshot/replica DB lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             // Load the row for this file
         *             $queryInfo = File::getQueryInfo();
         *             $row = $dbr->selectRow(
         *                 $queryInfo['tables'],
         *                 $queryInfo['fields'],
         *                 [ 'id' => $id ],
         *                 __METHOD__,
         *                 [],
         *                 $queryInfo['joins']
         *             );
         *
         *             return $row ? (array)$row : false;
         *         },
         *         [
         *             // Process cache for 30 seconds
         *             'pcTTL' => 30,
         *             // Use a dedicated 500 item cache (initialized on-the-fly)
         *             'pcGroup' => 'file-versions:500'
         *         ]
         *     );
         *     $files = array_map( [ __CLASS__, 'newFromRow' ], $rows );
         * @endcode
         *
         * @param ArrayIterator $keyedIds Result of WANObjectCache::makeMultiKeys()
         * @param int $ttl Seconds to live for key updates
         * @param callable $callback Callback the yields entity regeneration callbacks
         * @param array $opts Options map
         * @return array Map of (cache key => value) in the same order as $keyedIds
         * @since 1.28
         */
        final public function getMultiWithSetCallback(
                ArrayIterator $keyedIds, $ttl, callable $callback, array $opts = []
        ) {
                $valueKeys = array_keys( $keyedIds->getArrayCopy() );
                $checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];

                // Load required keys into process cache in one go
                $this->warmupCache = $this->getRawKeysForWarmup(
                        $this->getNonProcessCachedKeys( $valueKeys, $opts ),
                        $checkKeys
                );
                $this->warmupKeyMisses = 0;

                // Wrap $callback to match the getWithSetCallback() format while passing $id to $callback
                $id = null; // current entity ID
                $func = function ( $oldValue, &$ttl, &$setOpts, $oldAsOf ) use ( $callback, &$id ) {
                        return $callback( $id, $oldValue, $ttl, $setOpts, $oldAsOf );
                };

                $values = [];
                foreach ( $keyedIds as $key => $id ) { // preserve order
                        $values[$key] = $this->getWithSetCallback( $key, $ttl, $func, $opts );
                }

                $this->warmupCache = [];

                return $values;
        }

        /**
         * Method to fetch/regenerate multiple cache keys at once
         *
         * This works the same as getWithSetCallback() except:
         *   - a) The $keys argument expects the result of WANObjectCache::makeMultiKeys()
         *   - b) The $callback argument expects a callback returning a map of (ID => new value)
         *        for all entity IDs in $regenById and it takes the following arguments:
         *          - $ids: a list of entity IDs to regenerate
         *          - &$ttls: a reference to the (entity ID => new TTL) map
         *          - &$setOpts: a reference to options for set() which can be altered
         *   - c) The return value is a map of (cache key => value) in the order of $keyedIds
         *   - d) The "lockTSE" and "busyValue" options are ignored
         *
         * @see WANObjectCache::getWithSetCallback()
         * @see WANObjectCache::getMultiWithSetCallback()
         *
         * Example usage:
         * @code
         *     $rows = $cache->getMultiWithUnionSetCallback(
         *         // Map of cache keys to entity IDs
         *         $cache->makeMultiKeys(
         *             $this->fileVersionIds(),
         *             function ( $id, WANObjectCache $cache ) {
         *                 return $cache->makeKey( 'file-version', $id );
         *             }
         *         ),
         *         // Time-to-live (in seconds)
         *         $cache::TTL_DAY,
         *         // Function that derives the new key value
         *         function ( array $ids, array &$ttls, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_REPLICA );
         *             // Account for any snapshot/replica DB lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             // Load the rows for these files
         *             $rows = [];
         *             $queryInfo = File::getQueryInfo();
         *             $res = $dbr->select(
         *                 $queryInfo['tables'],
         *                 $queryInfo['fields'],
         *                 [ 'id' => $ids ],
         *                 __METHOD__,
         *                 [],
         *                 $queryInfo['joins']
         *             );
         *             foreach ( $res as $row ) {
         *                 $rows[$row->id] = $row;
         *                 $mtime = wfTimestamp( TS_UNIX, $row->timestamp );
         *                 $ttls[$row->id] = $this->adaptiveTTL( $mtime, $ttls[$row->id] );
         *             }
         *
         *             return $rows;
         *         },
         *         ]
         *     );
         *     $files = array_map( [ __CLASS__, 'newFromRow' ], $rows );
         * @endcode
         *
         * @param ArrayIterator $keyedIds Result of WANObjectCache::makeMultiKeys()
         * @param int $ttl Seconds to live for key updates
         * @param callable $callback Callback the yields entity regeneration callbacks
         * @param array $opts Options map
         * @return array Map of (cache key => value) in the same order as $keyedIds
         * @since 1.30
         */
        final public function getMultiWithUnionSetCallback(
                ArrayIterator $keyedIds, $ttl, callable $callback, array $opts = []
        ) {
                $idsByValueKey = $keyedIds->getArrayCopy();
                $valueKeys = array_keys( $idsByValueKey );
                $checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];
                unset( $opts['lockTSE'] ); // incompatible
                unset( $opts['busyValue'] ); // incompatible

                // Load required keys into process cache in one go
                $keysGet = $this->getNonProcessCachedKeys( $valueKeys, $opts );
                $this->warmupCache = $this->getRawKeysForWarmup( $keysGet, $checkKeys );
                $this->warmupKeyMisses = 0;

                // IDs of entities known to be in need of regeneration
                $idsRegen = [];

                // Find out which keys are missing/deleted/stale
                $curTTLs = [];
                $asOfs = [];
                $curByKey = $this->getMulti( $keysGet, $curTTLs, $checkKeys, $asOfs );
                foreach ( $keysGet as $key ) {
                        if ( !array_key_exists( $key, $curByKey ) || $curTTLs[$key] < 0 ) {
                                $idsRegen[] = $idsByValueKey[$key];
                        }
                }

                // Run the callback to populate the regeneration value map for all required IDs
                $newSetOpts = [];
                $newTTLsById = array_fill_keys( $idsRegen, $ttl );
                $newValsById = $idsRegen ? $callback( $idsRegen, $newTTLsById, $newSetOpts ) : [];

                // Wrap $callback to match the getWithSetCallback() format while passing $id to $callback
                $id = null; // current entity ID
                $func = function ( $oldValue, &$ttl, &$setOpts, $oldAsOf )
                        use ( $callback, &$id, $newValsById, $newTTLsById, $newSetOpts )
                {
                        if ( array_key_exists( $id, $newValsById ) ) {
                                // Value was already regerated as expected, so use the value in $newValsById
                                $newValue = $newValsById[$id];
                                $ttl = $newTTLsById[$id];
                                $setOpts = $newSetOpts;
                        } else {
                                // Pre-emptive/popularity refresh and version mismatch cases are not detected
                                // above and thus $newValsById has no entry. Run $callback on this single entity.
                                $ttls = [ $id => $ttl ];
                                $newValue = $callback( [ $id ], $ttls, $setOpts )[$id];
                                $ttl = $ttls[$id];
                        }

                        return $newValue;
                };

                // Run the cache-aside logic using warmupCache instead of persistent cache queries
                $values = [];
                foreach ( $idsByValueKey as $key => $id ) { // preserve order
                        $values[$key] = $this->getWithSetCallback( $key, $ttl, $func, $opts );
                }

                $this->warmupCache = [];

                return $values;
        }

        /**
         * Locally set a key to expire soon if it is stale based on $purgeTimestamp
         *
         * This sets stale keys' time-to-live at HOLDOFF_TTL seconds, which both avoids
         * broadcasting in mcrouter setups and also avoids races with new tombstones.
         *
         * @param string $key Cache key
         * @param int $purgeTimestamp UNIX timestamp of purge
         * @param bool &$isStale Whether the key is stale
         * @return bool Success
         * @since 1.28
         */
        public function reap( $key, $purgeTimestamp, &$isStale = false ) {
                $minAsOf = $purgeTimestamp + self::HOLDOFF_TTL;
                $wrapped = $this->cache->get( self::VALUE_KEY_PREFIX . $key );
                if ( is_array( $wrapped ) && $wrapped[self::FLD_TIME] < $minAsOf ) {
                        $isStale = true;
                        $this->logger->warning( "Reaping stale value key '$key'." );
                        $ttlReap = self::HOLDOFF_TTL; // avoids races with tombstone creation
                        $ok = $this->cache->changeTTL( self::VALUE_KEY_PREFIX . $key, $ttlReap );
                        if ( !$ok ) {
                                $this->logger->error( "Could not complete reap of key '$key'." );
                        }

                        return $ok;
                }

                $isStale = false;

                return true;
        }

        /**
         * Locally set a "check" key to expire soon if it is stale based on $purgeTimestamp
         *
         * @param string $key Cache key
         * @param int $purgeTimestamp UNIX timestamp of purge
         * @param bool &$isStale Whether the key is stale
         * @return bool Success
         * @since 1.28
         */
        public function reapCheckKey( $key, $purgeTimestamp, &$isStale = false ) {
                $purge = $this->parsePurgeValue( $this->cache->get( self::TIME_KEY_PREFIX . $key ) );
                if ( $purge && $purge[self::FLD_TIME] < $purgeTimestamp ) {
                        $isStale = true;
                        $this->logger->warning( "Reaping stale check key '$key'." );
                        $ok = $this->cache->changeTTL( self::TIME_KEY_PREFIX . $key, 1 );
                        if ( !$ok ) {
                                $this->logger->error( "Could not complete reap of check key '$key'." );
                        }

                        return $ok;
                }

                $isStale = false;

                return false;
        }

        /**
         * @see BagOStuff::makeKey()
         * @param string $class Key class
         * @param string $component [optional] Key component (starting with a key collection name)
         * @return string Colon-delimited list of $keyspace followed by escaped components of $args
         * @since 1.27
         */
        public function makeKey( $class, $component = null ) {
                return call_user_func_array( [ $this->cache, __FUNCTION__ ], func_get_args() );
        }

        /**
         * @see BagOStuff::makeGlobalKey()
         * @param string $class Key class
         * @param string $component [optional] Key component (starting with a key collection name)
         * @return string Colon-delimited list of $keyspace followed by escaped components of $args
         * @since 1.27
         */
        public function makeGlobalKey( $class, $component = null ) {
                return call_user_func_array( [ $this->cache, __FUNCTION__ ], func_get_args() );
        }

        /**
         * @param array $entities List of entity IDs
         * @param callable $keyFunc Callback yielding a key from (entity ID, this WANObjectCache)
         * @return ArrayIterator Iterator yielding (cache key => entity ID) in $entities order
         * @since 1.28
         */
        public function makeMultiKeys( array $entities, callable $keyFunc ) {
                $map = [];
                foreach ( $entities as $entity ) {
                        $map[$keyFunc( $entity, $this )] = $entity;
                }

                return new ArrayIterator( $map );
        }

        /**
         * 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 latter.
                        // 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->processCaches = [];
        }

        /**
         * Disable the use of brief caching for tombstoned keys
         *
         * When a key is purged via delete(), there normally is a period where caching
         * is hold-off limited to an extremely short time. This method will disable that
         * caching, forcing the callback to run for any of:
         *   - WANObjectCache::getWithSetCallback()
         *   - WANObjectCache::getMultiWithSetCallback()
         *   - WANObjectCache::getMultiWithUnionSetCallback()
         *
         * This is useful when both:
         *   - a) the database used by the callback is known to be up-to-date enough
         *        for some particular purpose (e.g. replica DB has applied transaction X)
         *   - b) the caller needs to exploit that fact, and therefore needs to avoid the
         *        use of inherently volatile and possibly stale interim keys
         *
         * @see WANObjectCache::delete()
         * @param bool $enabled Whether to enable interim caching
         * @since 1.31
         */
        public function useInterimHoldOffCaching( $enabled ) {
                $this->useInterimHoldOffCaching = $enabled;
        }

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

        /**
         * Get a TTL that is higher for objects that have not changed recently
         *
         * This is useful for keys that get explicit purges and DB or purge relay
         * lag is a potential concern (especially how it interacts with CDN cache)
         *
         * Example usage:
         * @code
         *     // Last-modified time of page
         *     $mtime = wfTimestamp( TS_UNIX, $page->getTimestamp() );
         *     // Get adjusted TTL. If $mtime is 3600 seconds ago and $minTTL/$factor left at
         *     // defaults, then $ttl is 3600 * .2 = 720. If $minTTL was greater than 720, then
         *     // $ttl would be $minTTL. If $maxTTL was smaller than 720, $ttl would be $maxTTL.
         *     $ttl = $cache->adaptiveTTL( $mtime, $cache::TTL_DAY );
         * @endcode
         *
         * Another use case is when there are no applicable "last modified" fields in the DB,
         * and there are too many dependencies for explicit purges to be viable, and the rate of
         * change to relevant content is unstable, and it is highly valued to have the cached value
         * be as up-to-date as possible.
         *
         * Example usage:
         * @code
         *     $query = "<some complex query>";
         *     $idListFromComplexQuery = $cache->getWithSetCallback(
         *         $cache->makeKey( 'complex-graph-query', $hashOfQuery ),
         *         GraphQueryClass::STARTING_TTL,
         *         function ( $oldValue, &$ttl, array &$setOpts, $oldAsOf ) use ( $query, $cache ) {
         *             $gdb = $this->getReplicaGraphDbConnection();
         *             // Account for any snapshot/replica DB lag
         *             $setOpts += GraphDatabase::getCacheSetOptions( $gdb );
         *
         *             $newList = iterator_to_array( $gdb->query( $query ) );
         *             sort( $newList, SORT_NUMERIC ); // normalize
         *
         *             $minTTL = GraphQueryClass::MIN_TTL;
         *             $maxTTL = GraphQueryClass::MAX_TTL;
         *             if ( $oldValue !== false ) {
         *                 // Note that $oldAsOf is the last time this callback ran
         *                 $ttl = ( $newList === $oldValue )
         *                     // No change: cache for 150% of the age of $oldValue
         *                     ? $cache->adaptiveTTL( $oldAsOf, $maxTTL, $minTTL, 1.5 )
         *                     // Changed: cache for %50 of the age of $oldValue
         *                     : $cache->adaptiveTTL( $oldAsOf, $maxTTL, $minTTL, .5 );
         *             }
         *
         *             return $newList;
         *        },
         *        [
         *             // Keep stale values around for doing comparisons for TTL calculations.
         *             // High values improve long-tail keys hit-rates, though might waste space.
         *             'staleTTL' => GraphQueryClass::GRACE_TTL
         *        ]
         *     );
         * @endcode
         *
         * @param int|float $mtime UNIX timestamp
         * @param int $maxTTL Maximum TTL (seconds)
         * @param int $minTTL Minimum TTL (seconds); Default: 30
         * @param float $factor Value in the range (0,1); Default: .2
         * @return int Adaptive TTL
         * @since 1.28
         */
        public function adaptiveTTL( $mtime, $maxTTL, $minTTL = 30, $factor = 0.2 ) {
                if ( is_float( $mtime ) || ctype_digit( $mtime ) ) {
                        $mtime = (int)$mtime; // handle fractional seconds and string integers
                }

                if ( !is_int( $mtime ) || $mtime <= 0 ) {
                        return $minTTL; // no last-modified time provided
                }

                $age = $this->getCurrentTime() - $mtime;

                return (int)min( $maxTTL, max( $minTTL, $factor * $age ) );
        }

        /**
         * @return int Number of warmup key cache misses last round
         * @since 1.30
         */
        public function getWarmupKeyMisses() {
                return $this->warmupKeyMisses;
        }

        /**
         * 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 int $ttl How long to keep the tombstone [seconds]
         * @param int $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( $this->getCurrentTime(), 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 is fresh or in the grace window and thus due for randomized reuse
         *
         * If $curTTL > 0 (e.g. not expired) this returns true. Otherwise, the chance of returning
         * true decrease steadily from 100% to 0% as the |$curTTL| moves from 0 to $graceTTL seconds.
         * This handles widely varying levels of cache access traffic.
         *
         * If $curTTL <= -$graceTTL (e.g. already expired), then this returns false.
         *
         * @param float $curTTL Approximate TTL left on the key if present
         * @param int $graceTTL Consider using stale values if $curTTL is greater than this
         * @return bool
         */
        protected function isAliveOrInGracePeriod( $curTTL, $graceTTL ) {
                if ( $curTTL > 0 ) {
                        return true;
                } elseif ( $graceTTL <= 0 ) {
                        return false;
                }

                $ageStale = abs( $curTTL ); // seconds of staleness
                $curGTTL = ( $graceTTL - $ageStale ); // current grace-time-to-live
                if ( $curGTTL <= 0 ) {
                        return false; //  already out of grace period
                }

                // Chance of using a stale value is the complement of the chance of refreshing it
                return !$this->worthRefreshExpiring( $curGTTL, $graceTTL );
        }

        /**
         * Check if a key is nearing expiration and thus due for randomized regeneration
         *
         * 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.
         *
         * If $curTTL <= 0 (e.g. already expired), then this returns false.
         *
         * @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 worthRefreshExpiring( $curTTL, $lowTTL ) {
                if ( $lowTTL <= 0 ) {
                        return false;
                } elseif ( $curTTL >= $lowTTL ) {
                        return false;
                } elseif ( $curTTL <= 0 ) {
                        return false;
                }

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

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

        /**
         * Check if a key is due for randomized regeneration due to its popularity
         *
         * This is used so that popular keys can preemptively refresh themselves for higher
         * consistency (especially in the case of purge loss/delay). Unpopular keys can remain
         * in cache with their high nominal TTL. This means popular keys keep good consistency,
         * whether the data changes frequently or not, and long-tail keys get to stay in cache
         * and get hits too. Similar to worthRefreshExpiring(), randomization is used.
         *
         * @param float $asOf UNIX timestamp of the value
         * @param int $ageNew Age of key when this might recommend refreshing (seconds)
         * @param int $timeTillRefresh Age of key when it should be refreshed if popular (seconds)
         * @param float $now The current UNIX timestamp
         * @return bool
         */
        protected function worthRefreshPopular( $asOf, $ageNew, $timeTillRefresh, $now ) {
                if ( $ageNew < 0 || $timeTillRefresh <= 0 ) {
                        return false;
                }

                $age = $now - $asOf;
                $timeOld = $age - $ageNew;
                if ( $timeOld <= 0 ) {
                        return false;
                }

                // Lifecycle is: new, ramp-up refresh chance, full refresh chance.
                // Note that the "expected # of refreshes" for the ramp-up time range is half of what it
                // would be if P(refresh) was at its full value during that time range.
                $refreshWindowSec = max( $timeTillRefresh - $ageNew - self::RAMPUP_TTL / 2, 1 );
                // P(refresh) * (# hits in $refreshWindowSec) = (expected # of refreshes)
                // P(refresh) * ($refreshWindowSec * $popularHitsPerSec) = 1
                // P(refresh) = 1/($refreshWindowSec * $popularHitsPerSec)
                $chance = 1 / ( self::HIT_RATE_HIGH * $refreshWindowSec );

                // Ramp up $chance from 0 to its nominal value over RAMPUP_TTL seconds to avoid stampedes
                $chance *= ( $timeOld <= self::RAMPUP_TTL ) ? $timeOld / self::RAMPUP_TTL : 1;

                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 int $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/tombstoned/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 $key String of the format <scope>:<class>[:<class or variable>]...
         * @return string
         */
        protected function determineKeyClass( $key ) {
                $parts = explode( ':', $key );

                return isset( $parts[1] ) ? $parts[1] : $parts[0]; // sanity
        }

        /**
         * @return float UNIX timestamp
         * @codeCoverageIgnore
         */
        protected function getCurrentTime() {
                return microtime( true );
        }

        /**
         * @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;
        }

        /**
         * @param string $group
         * @return HashBagOStuff
         */
        protected function getProcessCache( $group ) {
                if ( !isset( $this->processCaches[$group] ) ) {
                        list( , $n ) = explode( ':', $group );
                        $this->processCaches[$group] = new HashBagOStuff( [ 'maxKeys' => (int)$n ] );
                }

                return $this->processCaches[$group];
        }

        /**
         * @param array $keys
         * @param array $opts
         * @return array List of keys
         */
        private function getNonProcessCachedKeys( array $keys, array $opts ) {
                $keysFound = [];
                if ( isset( $opts['pcTTL'] ) && $opts['pcTTL'] > 0 && $this->callbackDepth == 0 ) {
                        $pcGroup = isset( $opts['pcGroup'] ) ? $opts['pcGroup'] : self::PC_PRIMARY;
                        $procCache = $this->getProcessCache( $pcGroup );
                        foreach ( $keys as $key ) {
                                if ( $procCache->get( $key ) !== false ) {
                                        $keysFound[] = $key;
                                }
                        }
                }

                return array_diff( $keys, $keysFound );
        }

        /**
         * @param array $keys
         * @param array $checkKeys
         * @return array Map of (cache key => mixed)
         */
        private function getRawKeysForWarmup( array $keys, array $checkKeys ) {
                if ( !$keys ) {
                        return [];
                }

                $keysWarmUp = [];
                // Get all the value keys to fetch...
                foreach ( $keys as $key ) {
                        $keysWarmUp[] = self::VALUE_KEY_PREFIX . $key;
                }
                // Get all the check keys to fetch...
                foreach ( $checkKeys as $i => $checkKeyOrKeys ) {
                        if ( is_int( $i ) ) {
                                // Single check key that applies to all value keys
                                $keysWarmUp[] = self::TIME_KEY_PREFIX . $checkKeyOrKeys;
                        } else {
                                // List of check keys that apply to value key $i
                                $keysWarmUp = array_merge(
                                        $keysWarmUp,
                                        self::prefixCacheKeys( $checkKeyOrKeys, self::TIME_KEY_PREFIX )
                                );
                        }
                }

                $warmupCache = $this->cache->getMulti( $keysWarmUp );
                $warmupCache += array_fill_keys( $keysWarmUp, false );

                return $warmupCache;
        }
}