Merge "Fix call to non-existing TempFSFileFactory::getTempFSFile()"

[lhc/web/wiklou.git] / includes / libs / objectcache / wancache / 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
 *
 * ### Using WANObjectCache
 *
 * 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.
 *
 * To ensure consumers of the cache see new values in a timely manner,
 * you either need to follow either the validation strategy, or the
 * purge strategy.
 *
 * The validation strategy refers to the natural avoidance of stale data
 * by one of the following means:
 *
 *   - A) The cached value is immutable.
 *        If the consumer has access to an identifier that uniquely describes a value,
 *        cached value need not change. Instead, the key can change. This also allows
 *        all servers to access their perceived current version. This is important
 *        in context of multiple deployed versions of your application and/or cross-dc
 *        database replication, to ensure deterministic values without oscillation.
 *   - B) Validity is checked against the source after get().
 *        This is the inverse of A. The unique identifier is embedded inside the value
 *        and validated after on retreival. If outdated, the value is recomputed.
 *   - C) The value is cached with a modest TTL (without validation).
 *        If value recomputation is reasonably performant, and the value is allowed to
 *        be stale, one should consider using TTL only – using the value's age as
 *        method of validation.
 *
 * The purge strategy refers to the the approach whereby your application knows that
 * source data has changed and can react by purging the relevant cache keys.
 * As purges are expensive, this strategy should be avoided if possible.
 * The simplest purge method is delete().
 *
 * No matter which strategy you choose, callers must not rely on updates or purges
 * being immediately visible to other servers. It should be treated similarly as
 * one would a database replica.
 *
 * The need for immediate updates should be avoided. If needed, solutions must be
 * sought outside WANObjectCache.
 *
 * ### Deploying WANObjectCache
 *
 * There are two supported ways to set up broadcasted operations:
 *
 *   - A) Set up mcrouter as the underlying cache backend, using a memcached BagOStuff class
 *        for the 'cache' parameter. The 'region' and 'cluster' parameters must be provided
 *        and 'mcrouterAware' must be set to `true`.
 *        Configure mcrouter as follows:
 *          - 1) Use Route Prefixing based on region (datacenter) and cache cluster.
 *               See https://github.com/facebook/mcrouter/wiki/Routing-Prefix and
 *               https://github.com/facebook/mcrouter/wiki/Multi-cluster-broadcast-setup.
 *          - 2) To increase the consistency of delete() and touchCheckKey() during cache
 *               server membership changes, you can use the OperationSelectorRoute to
 *               configure 'set' and 'delete' operations to go to all servers in the cache
 *               cluster, instead of just one server determined by hashing.
 *               See https://github.com/facebook/mcrouter/wiki/List-of-Route-Handles.
 *   - B) Set up dynomite as a cache middleware between the web servers and either memcached
 *        or redis and use it as the underlying cache backend, using a memcached BagOStuff
 *        class for the 'cache' parameter. This will 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 intended to run
 * immediately in the local datacenter and asynchronously in remote datacenters.
 *
 * 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, IStoreKeyEncoder, LoggerAwareInterface {
        /** @var BagOStuff The local datacenter cache */
        protected $cache;
        /** @var MapCacheLRU[] Map of group PHP instance caches */
        protected $processCaches = [];
        /** @var LoggerInterface */
        protected $logger;
        /** @var StatsdDataFactoryInterface */
        protected $stats;
        /** @var callable|null Function that takes a WAN cache callback and runs it later */
        protected $asyncHandler;

        /** @bar bool Whether to use mcrouter key prefixing for routing */
        protected $mcrouterAware;
        /** @var string Physical region for mcrouter use */
        protected $region;
        /** @var string Cache cluster name for mcrouter use */
        protected $cluster;
        /** @var bool Whether to use "interim" caching while keys are tombstoned */
        protected $useInterimHoldOffCaching = true;
        /** @var float Unix timestamp of the oldest possible valid values */
        protected $epoch;
        /** @var string Stable secret used for hasing long strings into key components */
        protected $secret;

        /** @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;

        /** @var float|null */
        private $wallClockOverride;

        /** @var int Max expected seconds to pass between delete() and DB commit finishing */
        const MAX_COMMIT_DELAY = 3;
        /** @var int Max expected seconds of combined lag from replication and view snapshots */
        const MAX_READ_LAG = 7;
        /** @var int Seconds to tombstone keys on delete() and treat as volatile after invalidation */
        const HOLDOFF_TTL = self::MAX_COMMIT_DELAY + self::MAX_READ_LAG + 1;

        /** @var int Idiom for getWithSetCallback() meaning "do not store the callback result" */
        const TTL_UNCACHEABLE = -1;

        /** @var int Consider regeneration if the key will expire within this many seconds */
        const LOW_TTL = 30;
        /** @var int Max TTL, in seconds, to store keys when a data sourced is lagged */
        const TTL_LAGGED = 30;

        /** @var int Expected time-till-refresh, in seconds, if the key is accessed once per second */
        const HOT_TTR = 900;
        /** @var int Minimum key age, in seconds, for expected time-till-refresh to be considered */
        const AGE_NEW = 60;

        /** @var int Idiom for getWithSetCallback() meaning "no cache stampede mutex required" */
        const TSE_NONE = -1;

        /** @var int Idiom for set()/getWithSetCallback() meaning "no post-expiration persistence" */
        const STALE_TTL_NONE = 0;
        /** @var int Idiom for set()/getWithSetCallback() meaning "no post-expiration grace period" */
        const GRACE_TTL_NONE = 0;
        /** @var int Idiom for delete()/touchCheckKey() meaning "no hold-off period" */
        const HOLDOFF_TTL_NONE = 0;
        /** @var int Alias for HOLDOFF_TTL_NONE (b/c) (deprecated since 1.34) */
        const HOLDOFF_NONE = self::HOLDOFF_TTL_NONE;

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

        /** @var string Default process cache name and max key count */
        const PC_PRIMARY = 'primary:1000';

        /** @var int Idion for get()/getMulti() to return extra information by reference */
        const PASS_BY_REF = -1;

        /** @var int Seconds to keep dependency purge keys around */
        private static $CHECK_KEY_TTL = self::TTL_YEAR;
        /** @var int Seconds to keep interim value keys for tombstoned keys around */
        private static $INTERIM_KEY_TTL = 1;

        /** @var int Seconds to keep lock keys around */
        private static $LOCK_TTL = 10;
        /** @var int Seconds to no-op key set() calls to avoid large blob I/O stampedes */
        private static $COOLOFF_TTL = 1;
        /** @var int Seconds to ramp up the chance of regeneration due to expected time-till-refresh */
        private static $RAMPUP_TTL = 30;

        /** @var float Tiny negative float to use when CTL comes up >= 0 due to clock skew */
        private static $TINY_NEGATIVE = -0.000001;
        /** @var float Tiny positive float to use when using "minTime" to assert an inequality */
        private static $TINY_POSTIVE = 0.000001;

        /** @var int Milliseconds of key fetch/validate/regenerate delay prone to set() stampedes */
        private static $SET_DELAY_HIGH_MS = 50;
        /** @var int Min millisecond set() backoff during hold-off (far less than INTERIM_KEY_TTL) */
        private static $RECENT_SET_LOW_MS = 50;
        /** @var int Max millisecond set() backoff during hold-off (far less than INTERIM_KEY_TTL) */
        private static $RECENT_SET_HIGH_MS = 100;

        /** @var int Consider value generation slow if it takes more than this many seconds */
        private static $GENERATION_SLOW_SEC = 3;

        /** @var int Key to the tombstone entry timestamp */
        private static $PURGE_TIME = 0;
        /** @var int Key to the tombstone entry hold-off TTL */
        private static $PURGE_HOLDOFF = 1;

        /** @var int Cache format version number */
        private static $VERSION = 1;

        /** @var int Key to WAN cache version number */
        private static $FLD_FORMAT_VERSION = 0;
        /** @var int Key to the cached value */
        private static $FLD_VALUE = 1;
        /** @var int Key to the original TTL */
        private static $FLD_TTL = 2;
        /** @var int Key to the cache timestamp */
        private static $FLD_TIME = 3;
        /** @var int Key to the flags bit field (reserved number) */
        private static /** @noinspection PhpUnusedPrivateFieldInspection */ $FLD_FLAGS = 4;
        /** @var int Key to collection cache version number */
        private static $FLD_VALUE_VERSION = 5;
        /** @var int Key to how long it took to generate the value */
        private static $FLD_GENERATION_TIME = 6;

        private static $VALUE_KEY_PREFIX = 'WANCache:v:';
        private static $INTERIM_KEY_PREFIX = 'WANCache:i:';
        private static $TIME_KEY_PREFIX = 'WANCache:t:';
        private static $MUTEX_KEY_PREFIX = 'WANCache:m:';
        private static $COOLOFF_KEY_PREFIX = 'WANCache:c:';

        private static $PURGE_VAL_PREFIX = 'PURGED:';

        /**
         * @param array $params
         *   - cache    : BagOStuff object for a persistent cache
         *   - logger   : LoggerInterface object
         *   - stats    : StatsdDataFactoryInterface 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]
         *   - region: the current physical region. This is required when using mcrouter as the
         *       backing store proxy. [optional]
         *   - cluster: name of the cache cluster used by this WAN cache. The name must be the
         *       same in all datacenters; the ("region","cluster") tuple is what distinguishes
         *       the counterpart cache clusters among all the datacenter. The contents of
         *       https://github.com/facebook/mcrouter/wiki/Config-Files give background on this.
         *       This is required when using mcrouter as the backing store proxy. [optional]
         *   - mcrouterAware: set as true if mcrouter is the backing store proxy and mcrouter
         *       is configured to interpret /<region>/<cluster>/ key prefixes as routes. This
         *       requires that "region" and "cluster" are both set above. [optional]
         *   - epoch: lowest UNIX timestamp a value/tombstone must have to be valid. [optional]
         *   - secret: stable secret used for hashing long strings into key components. [optional]
         */
        public function __construct( array $params ) {
                $this->cache = $params['cache'];
                $this->region = $params['region'] ?? 'main';
                $this->cluster = $params['cluster'] ?? 'wan-main';
                $this->mcrouterAware = !empty( $params['mcrouterAware'] );
                $this->epoch = $params['epoch'] ?? 0;
                $this->secret = $params['secret'] ?? (string)$this->epoch;

                $this->setLogger( $params['logger'] ?? new NullLogger() );
                $this->stats = $params['stats'] ?? new NullStatsdDataFactory();
                $this->asyncHandler = $params['asyncHandler'] ?? null;
        }

        /**
         * @param LoggerInterface $logger
         */
        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() ] );
        }

        /**
         * 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 invalidated by $checkKeys
         *   - b) float (>=0); if $key exists, has a TTL, and is not invalidated by $checkKeys
         *   - c) float (<0); if $key is tombstoned, stale, or existing but invalidated 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 the delete() hold-off TTL
         *   - c) Disabling interim key caching via useInterimHoldOffCaching() before get() calls
         *
         * 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.
         *
         * Pass $info as WANObjectCache::PASS_BY_REF to transform it into a cache key metadata map.
         * This map includes the following metadata:
         *   - asOf: UNIX timestamp of the value or null if the key is nonexistant
         *   - tombAsOf: UNIX timestamp of the tombstone or null if the key is not tombstoned
         *   - lastCKPurge: UNIX timestamp of the highest check key or null if none provided
         *   - version: cached value version number or null if the key is nonexistant
         *
         * Otherwise, $info will transform into the cached value timestamp.
         *
         * @param string $key Cache key made from makeKey() or makeGlobalKey()
         * @param mixed|null &$curTTL Approximate TTL left on the key if present/tombstoned [returned]
         * @param string[] $checkKeys The "check" keys used to validate the value
         * @param mixed|null &$info Key info if WANObjectCache::PASS_BY_REF [returned]
         * @return mixed Value of cache key or false on failure
         */
        final public function get(
                $key, &$curTTL = null, array $checkKeys = [], &$info = null
        ) {
                $curTTLs = self::PASS_BY_REF;
                $infoByKey = self::PASS_BY_REF;
                $values = $this->getMulti( [ $key ], $curTTLs, $checkKeys, $infoByKey );
                $curTTL = $curTTLs[$key] ?? null;
                if ( $info === self::PASS_BY_REF ) {
                        $info = [
                                'asOf' => $infoByKey[$key]['asOf'] ?? null,
                                'tombAsOf' => $infoByKey[$key]['tombAsOf'] ?? null,
                                'lastCKPurge' => $infoByKey[$key]['lastCKPurge'] ?? null,
                                'version' => $infoByKey[$key]['version'] ?? null
                        ];
                } else {
                        $info = $infoByKey[$key]['asOf'] ?? null; // b/c
                }

                return $values[$key] ?? false;
        }

        /**
         * Fetch the value of several keys from cache
         *
         * Pass $info as WANObjectCache::PASS_BY_REF to transform it into a map of cache keys
         * to cache key metadata maps, each having the same style as those of WANObjectCache::get().
         * All the cache keys listed in $keys will have an entry.
         *
         * Othwerwise, $info will transform into a map of (cache key => cached value timestamp).
         * Only the cache keys listed in $keys that exists or are tombstoned will have an entry.
         *
         * $checkKeys holds the "check" keys used to validate values of applicable keys. The integer
         * indexes hold "check" keys that apply to all of $keys while the string indexes hold "check"
         * keys that only apply to the cache key with that name.
         *
         * @see WANObjectCache::get()
         *
         * @param string[] $keys List of cache keys made from makeKey() or makeGlobalKey()
         * @param mixed|null &$curTTLs Map of (key => TTL left) for existing/tombstoned keys [returned]
         * @param string[]|string[][] $checkKeys Map of (integer or cache key => "check" key(s))
         * @param mixed|null &$info Map of (key => info) if WANObjectCache::PASS_BY_REF [returned]
         * @return mixed[] Map of (key => value) for existing values; order of $keys is preserved
         */
        final public function getMulti(
                array $keys,
                &$curTTLs = [],
                array $checkKeys = [],
                &$info = null
        ) {
                $result = [];
                $curTTLs = [];
                $infoByKey = [];

                $vPrefixLen = strlen( self::$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 );
                        // Are these 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] = $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 ) {
                        $key = substr( $vKey, $vPrefixLen ); // unprefix
                        list( $value, $keyInfo ) = $this->unwrap( $wrappedValues[$vKey] ?? false, $now );
                        // Force dependent keys to be seen as stale 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] );
                        }

                        $lastCKPurge = null; // timestamp of the highest check key
                        foreach ( $purgeValues as $purge ) {
                                $lastCKPurge = max( $purge[self::$PURGE_TIME], $lastCKPurge );
                                $safeTimestamp = $purge[self::$PURGE_TIME] + $purge[self::$PURGE_HOLDOFF];
                                if ( $value !== false && $safeTimestamp >= $keyInfo['asOf'] ) {
                                        // How long ago this value was invalidated by *this* check key
                                        $ago = min( $purge[self::$PURGE_TIME] - $now, self::$TINY_NEGATIVE );
                                        // How long ago this value was invalidated by *any* known check key
                                        $keyInfo['curTTL'] = min( $keyInfo['curTTL'], $ago );
                                }
                        }
                        $keyInfo[ 'lastCKPurge'] = $lastCKPurge;

                        if ( $value !== false ) {
                                $result[$key] = $value;
                        }
                        if ( $keyInfo['curTTL'] !== null ) {
                                $curTTLs[$key] = $keyInfo['curTTL'];
                        }

                        $infoByKey[$key] = ( $info === self::PASS_BY_REF )
                                ? $keyInfo
                                : $keyInfo['asOf']; // b/c
                }

                $info = $infoByKey;

                return $result;
        }

        /**
         * @since 1.27
         * @param string[] $timeKeys List of prefixed time check keys
         * @param mixed[] $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] )
                                ? $this->parsePurgeValue( $wrappedValues[$timeKey] )
                                : false;
                        if ( $purge === false ) {
                                // Key is not set or malformed; regenerate
                                $newVal = $this->makePurgeValue( $now, self::HOLDOFF_TTL );
                                $this->cache->add( $timeKey, $newVal, self::$CHECK_KEY_TTL );
                                $purge = $this->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.
         *
         * Be aware that this does not update the process cache for getWithSetCallback()
         * callers. Keys accessed via that method are not generally meant to also be set
         * using this primitive method.
         *
         * Do not use this method on versioned keys accessed via getWithSetCallback().
         *
         * 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 (default)
         *   - WANObjectCache::TTL_UNCACHEABLE: Do not cache (if the key exists, it is not deleted)
         * @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
         *   - creating: Optimize for the case where the key does not already exist.
         *      Default: false
         *   - version: Integer version number signifiying the format of the value.
         *      Default: null
         *   - walltime: How long the value took to generate in seconds. Default: 0.0
         * @note Options added in 1.28: staleTTL
         * @note Options added in 1.33: creating
         * @note Options added in 1.34: version, walltime
         * @return bool Success
         */
        final public function set( $key, $value, $ttl = self::TTL_INDEFINITE, array $opts = [] ) {
                $now = $this->getCurrentTime();
                $lag = $opts['lag'] ?? 0;
                $age = isset( $opts['since'] ) ? max( 0, $now - $opts['since'] ) : 0;
                $pending = $opts['pending'] ?? false;
                $lockTSE = $opts['lockTSE'] ?? self::TSE_NONE;
                $staleTTL = $opts['staleTTL'] ?? self::STALE_TTL_NONE;
                $creating = $opts['creating'] ?? false;
                $version = $opts['version'] ?? null;
                $walltime = $opts['walltime'] ?? 0.0;

                if ( $ttl < 0 ) {
                        return true;
                }

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

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

                $logicalTTL = null; // logical TTL override
                // 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: any long-running transaction
                        if ( $age > self::MAX_READ_LAG ) {
                                if ( $lockTSE >= 0 ) {
                                        // Store value as *almost* stale to avoid cache and mutex stampedes
                                        $logicalTTL = self::TTL_SECOND;
                                        $this->logger->info(
                                                'Lowered set() TTL for {cachekey} due to snapshot lag.',
                                                [ 'cachekey' => $key, 'lag' => $lag, 'age' => $age ]
                                        );
                                } else {
                                        $this->logger->info(
                                                'Rejected set() for {cachekey} due to snapshot lag.',
                                                [ 'cachekey' => $key, 'lag' => $lag, 'age' => $age ]
                                        );

                                        return true; // no-op the write for being unsafe
                                }
                        // Case B: high replication lag; lower TTL instead of ignoring all set()s
                        } elseif ( $lag === false || $lag > self::MAX_READ_LAG ) {
                                if ( $lockTSE >= 0 ) {
                                        $logicalTTL = min( $ttl ?: INF, self::TTL_LAGGED );
                                } else {
                                        $ttl = min( $ttl ?: INF, self::TTL_LAGGED );
                                }
                                $this->logger->warning(
                                        'Lowered set() TTL for {cachekey} due to replication lag.',
                                        [ 'cachekey' => $key, 'lag' => $lag, 'age' => $age ]
                                );
                        // Case C: medium length request with medium replication lag
                        } elseif ( $lockTSE >= 0 ) {
                                // Store value as *almost* stale to avoid cache and mutex stampedes
                                $logicalTTL = self::TTL_SECOND;
                                $this->logger->info(
                                        'Lowered set() TTL for {cachekey} due to high read lag.',
                                        [ 'cachekey' => $key, 'lag' => $lag, 'age' => $age ]
                                );
                        } else {
                                $this->logger->info(
                                        'Rejected set() for {cachekey} due to high read lag.',
                                        [ 'cachekey' => $key, 'lag' => $lag, 'age' => $age ]
                                );

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

                // Wrap that value with time/TTL/version metadata
                $wrapped = $this->wrap( $value, $logicalTTL ?: $ttl, $version, $now, $walltime );
                $storeTTL = $ttl + $staleTTL;

                if ( $creating ) {
                        $ok = $this->cache->add( self::$VALUE_KEY_PREFIX . $key, $wrapped, $storeTTL );
                } else {
                        $ok = $this->cache->merge(
                                self::$VALUE_KEY_PREFIX . $key,
                                function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
                                        // A string value means that it is a tombstone; do nothing in that case
                                        return ( is_string( $cWrapped ) ) ? false : $wrapped;
                                },
                                $storeTTL,
                                1 // 1 attempt
                        );
                }

                return $ok;
        }

        /**
         * 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.
         *
         * Be aware that this does not clear the process cache. Even if it did, callbacks
         * used by getWithSetCallback() might still return stale data in the case of either
         * uncommitted or not-yet-replicated changes (callback generally use replica DBs).
         *
         * 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_TTL_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 ) {
                if ( $ttl <= 0 ) {
                        // Publish the purge to all datacenters
                        $ok = $this->relayDelete( self::$VALUE_KEY_PREFIX . $key );
                } else {
                        // Publish the purge to all datacenters
                        $ok = $this->relayPurge( self::$VALUE_KEY_PREFIX . $key, $ttl, self::HOLDOFF_TTL_NONE );
                }

                $kClass = $this->determineKeyClassForStats( $key );
                $this->stats->increment( "wanobjectcache.$kClass.delete." . ( $ok ? 'ok' : 'error' ) );

                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
         */
        final public function getCheckKeyTime( $key ) {
                return $this->getMultiCheckKeyTime( [ $key ] )[$key];
        }

        /**
         * Fetch the values of each timestamp "check" key
         *
         * This works like getCheckKeyTime() except it takes a list of keys
         * and returns a map of timestamps instead of just that of one key
         *
         * This might be useful if both:
         *   - a) a class of entities each depend on hundreds of other entities
         *   - b) these other entities are depended upon by millions of entities
         *
         * The later entities can each use a "check" key to invalidate their dependee entities.
         * However, it is expensive for the former entities to verify against all of the relevant
         * "check" keys during each getWithSetCallback() call. A less expensive approach is to do
         * these verifications only after a "time-till-verify" (TTV) has passed. This is a middle
         * ground between using blind TTLs and using constant verification. The adaptiveTTL() method
         * can be used to dynamically adjust the TTV. Also, the initial TTV can make use of the
         * last-modified times of the dependant entities (either from the DB or the "check" keys).
         *
         * Example usage:
         * @code
         *     $value = $cache->getWithSetCallback(
         *         $cache->makeGlobalKey( 'wikibase-item', $id ),
         *         self::INITIAL_TTV, // initial time-till-verify
         *         function ( $oldValue, &$ttv, &$setOpts, $oldAsOf ) use ( $checkKeys, $cache ) {
         *             $now = microtime( true );
         *             // Use $oldValue if it passes max ultimate age and "check" key comparisons
         *             if ( $oldValue &&
         *                 $oldAsOf > max( $cache->getMultiCheckKeyTime( $checkKeys ) ) &&
         *                 ( $now - $oldValue['ctime'] ) <= self::MAX_CACHE_AGE
         *             ) {
         *                 // Increase time-till-verify by 50% of last time to reduce overhead
         *                 $ttv = $cache->adaptiveTTL( $oldAsOf, self::MAX_TTV, self::MIN_TTV, 1.5 );
         *                 // Unlike $oldAsOf, "ctime" is the ultimate age of the cached data
         *                 return $oldValue;
         *             }
         *
         *             $mtimes = []; // dependency last-modified times; passed by reference
         *             $value = [ 'data' => $this->fetchEntityData( $mtimes ), 'ctime' => $now ];
         *             // Guess time-till-change among the dependencies, e.g. 1/(total change rate)
         *             $ttc = 1 / array_sum( array_map(
         *                 function ( $mtime ) use ( $now ) {
         *                     return 1 / ( $mtime ? ( $now - $mtime ) : 900 );
         *                 },
         *                 $mtimes
         *             ) );
         *             // The time-to-verify should not be overly pessimistic nor optimistic
         *             $ttv = min( max( $ttc, self::MIN_TTV ), self::MAX_TTV );
         *
         *             return $value;
         *         },
         *         [ 'staleTTL' => $cache::TTL_DAY ] // keep around to verify and re-save
         *     );
         * @endcode
         *
         * @see WANObjectCache::getCheckKeyTime()
         * @see WANObjectCache::getWithSetCallback()
         *
         * @param string[] $keys
         * @return float[] Map of (key => UNIX timestamp)
         * @since 1.31
         */
        final public function getMultiCheckKeyTime( array $keys ) {
                $rawKeys = [];
                foreach ( $keys as $key ) {
                        $rawKeys[$key] = self::$TIME_KEY_PREFIX . $key;
                }

                $rawValues = $this->cache->getMulti( $rawKeys );
                $rawValues += array_fill_keys( $rawKeys, false );

                $times = [];
                foreach ( $rawKeys as $key => $rawKey ) {
                        $purge = $this->parsePurgeValue( $rawValues[$rawKey] );
                        if ( $purge !== false ) {
                                $time = $purge[self::$PURGE_TIME];
                        } else {
                                // Casting assures identical floats for the next getCheckKeyTime() calls
                                $now = (string)$this->getCurrentTime();
                                $this->cache->add(
                                        $rawKey,
                                        $this->makePurgeValue( $now, self::HOLDOFF_TTL ),
                                        self::$CHECK_KEY_TTL
                                );
                                $time = (float)$now;
                        }

                        $times[$key] = $time;
                }

                return $times;
        }

        /**
         * 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_TTL_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
                $ok = $this->relayPurge( self::$TIME_KEY_PREFIX . $key, self::$CHECK_KEY_TTL, $holdoff );

                $kClass = $this->determineKeyClassForStats( $key );
                $this->stats->increment( "wanobjectcache.$kClass.ck_touch." . ( $ok ? 'ok' : 'error' ) );

                return $ok;
        }

        /**
         * 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 stale, 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
                $ok = $this->relayDelete( self::$TIME_KEY_PREFIX . $key );

                $kClass = $this->determineKeyClassForStats( $key );
                $this->stats->increment( "wanobjectcache.$kClass.ck_reset." . ( $ok ? 'ok' : 'error' ) );

                return $ok;
        }

        /**
         * 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 (key that is expensive with too many DB dependencies for "check keys"):
         * @code
         *     $catToys = $cache->getWithSetCallback(
         *         // Key to store the cached value under
         *         $cache->makeKey( 'cat-toys', $catId ),
         *         // 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 CatToys::newFromResults( $dbr->select( ... ) );
         *         },
         *         [
         *              // Get the highest timestamp of any of the cat's toys
         *             'touchedCallback' => function ( $value ) use ( $catId ) {
         *                 $dbr = wfGetDB( DB_REPLICA );
         *                 $ts = $dbr->selectField( 'cat_toys', 'MAX(ct_touched)', ... );
         *
         *                 return wfTimestampOrNull( TS_UNIX, $ts );
         *             },
         *             // Avoid DB queries for repeated access
         *             'pcTTL' => $cache::TTL_PROC_SHORT
         *         ]
         *     );
         * @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
         *
         * Example usage (key holding an LRU subkey:value map; this can avoid flooding cache with
         * keys for an unlimited set of (constraint,situation) pairs, thereby avoiding elevated
         * cache evictions and wasted memory):
         * @code
         *     $catSituationTolerabilityCache = $this->cache->getWithSetCallback(
         *         // Group by constraint ID/hash, cat family ID/hash, or something else useful
         *         $this->cache->makeKey( 'cat-situation-tolerability-checks', $groupKey ),
         *         WANObjectCache::TTL_DAY, // rarely used groups should fade away
         *         // The $scenarioKey format is $constraintId:<ID/hash of $situation>
         *         function ( $cacheMap ) use ( $scenarioKey, $constraintId, $situation ) {
         *             $lruCache = MapCacheLRU::newFromArray( $cacheMap ?: [], self::CACHE_SIZE );
         *             $result = $lruCache->get( $scenarioKey ); // triggers LRU bump if present
         *             if ( $result === null || $this->isScenarioResultExpired( $result ) ) {
         *                 $result = $this->checkScenarioTolerability( $constraintId, $situation );
         *                 $lruCache->set( $scenarioKey, $result, 3 / 8 );
         *             }
         *             // Save the new LRU cache map and reset the map's TTL
         *             return $lruCache->toArray();
         *         },
         *         [
         *             // Once map is > 1 sec old, consider refreshing
         *             'ageNew' => 1,
         *             // Update within 5 seconds after "ageNew" given a 1hz cache check rate
         *             'hotTTR' => 5,
         *             // Avoid querying cache servers multiple times in a request; this also means
         *             // that a request can only alter the value of any given constraint key once
         *             'pcTTL' => WANObjectCache::TTL_PROC_LONG
         *         ]
         *     );
         *     $tolerability = isset( $catSituationTolerabilityCache[$scenarioKey] )
         *         ? $catSituationTolerabilityCache[$scenarioKey]
         *         : $this->checkScenarioTolerability( $constraintId, $situation );
         * @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 stale 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: If the key is invalidated (by "checkKeys"/"touchedCallback") less than this
         *      many seconds ago, consider reusing the stale value. 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. This option is not
         *      useful for avoiding traffic spikes in the case of the key simply expiring on account
         *      of its TTL (use "lowTTL" instead).
         *      Default: WANObjectCache::GRACE_TTL_NONE.
         *   - lockTSE: If the key is tombstoned or invalidated (by "checkKeys"/"touchedCallback")
         *      less than this many seconds ago, try to have a single thread handle cache regeneration
         *      at any given time. Other threads will 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. Note that if the key falls out of cache then concurrent
         *      threads will all run the callback on cache miss until the value is saved in cache.
         *      The only stampede protection in that case is from duplicate cache sets when the
         *      callback takes longer than WANObjectCache::SET_DELAY_HIGH_MS milliseconds; consider
         *      using "busyValue" if such stampedes are a problem. Note that the higher "lockTSE" is
         *      set, the higher the worst-case staleness of returned values can be. Also note that
         *      this option does not by itself handle the case of the key simply expiring on account
         *      of its TTL, so make sure that "lowTTL" is not disabled when using this option. Avoid
         *      combining this option with delete() as it can always cause a stampede due to their
         *      being no stale value available until after a thread completes the callback.
         *      Use WANObjectCache::TSE_NONE to disable this logic.
         *      Default: WANObjectCache::TSE_NONE.
         *   - busyValue: Specify a placeholder value to use when no value exists and another thread
         *      is currently regenerating it. This assures that cache stampedes cannot happen if the
         *      value falls out of cache. This also mitigates stampedes when value regeneration
         *      becomes very slow (greater than $ttl/"lowTTL"). If this is a closure, then it will
         *      be invoked to get the placeholder when needed.
         *      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 lets callers make breaking changes to the format
         *      of cached values without causing problems for sites that use non-instantaneous code
         *      deployments. Old and new code will recognize incompatible versions and purges from
         *      both old and new code will been seen by each other. When this method encounters an
         *      incompatibly versioned value at the provided key, a "variant key" will be used for
         *      reading from and saving to cache. The variant key is specific to the key and version
         *      number provided to this method. If the variant key value is older than that of the
         *      provided key, or the provided key is non-existant, then the variant key will be seen
         *      as non-existant. Therefore, delete() calls invalidate the provided key's variant keys.
         *      The "checkKeys" and "touchedCallback" options still apply to variant keys as usual.
         *      Avoid storing class objects, 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
         *   - touchedCallback: A callback that takes the current value and returns a UNIX timestamp
         *      indicating the last time a dynamic dependency changed. Null can be returned if there
         *      are no relevant dependency changes to check. This can be used to check against things
         *      like last-modified times of files or DB timestamp fields. This should generally not be
         *      used for small and easily queried values in a DB if the callback itself ends up doing
         *      a similarly expensive DB query to check a timestamp. Usages of this option makes the
         *      most sense for values that are moderately to highly expensive to regenerate and easy
         *      to query for dependency timestamps. The use of "pcTTL" reduces timestamp queries.
         *      Default: null.
         * @return mixed Value found or written to the key
         * @note Options added in 1.28: version, busyValue, hotTTR, ageNew, pcGroup, minAsOf
         * @note Options added in 1.31: staleTTL, graceTTL
         * @note Options added in 1.33: touchedCallback
         * @note Callable type hints are not used to avoid class-autoloading
         */
        final public function getWithSetCallback( $key, $ttl, $callback, array $opts = [] ) {
                $version = $opts['version'] ?? null;
                $pcTTL = $opts['pcTTL'] ?? self::TTL_UNCACHEABLE;
                $pCache = ( $pcTTL >= 0 )
                        ? $this->getProcessCache( $opts['pcGroup'] ?? self::PC_PRIMARY )
                        : null;

                // Use the process cache if requested as long as no outer cache callback is running.
                // Nested callback process cache use is not lag-safe with regard to HOLDOFF_TTL since
                // process cached values are more lagged than persistent ones as they are not purged.
                if ( $pCache && $this->callbackDepth == 0 ) {
                        $cached = $pCache->get( $this->getProcessCacheKey( $key, $version ), INF, false );
                        if ( $cached !== false ) {
                                return $cached;
                        }
                }

                $res = $this->fetchOrRegenerate( $key, $ttl, $callback, $opts );
                list( $value, $valueVersion, $curAsOf ) = $res;
                if ( $valueVersion !== $version ) {
                        // Current value has a different version; use the variant key for this version.
                        // Regenerate the variant value if it is not newer than the main value at $key
                        // so that purges to the main key propagate to the variant value.
                        list( $value ) = $this->fetchOrRegenerate(
                                $this->makeGlobalKey( 'WANCache-key-variant', md5( $key ), $version ),
                                $ttl,
                                $callback,
                                [ 'version' => null, 'minAsOf' => $curAsOf ] + $opts
                        );
                }

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

                return $value;
        }

        /**
         * Do the actual I/O for getWithSetCallback() when needed
         *
         * @see WANObjectCache::getWithSetCallback()
         *
         * @param string $key
         * @param int $ttl
         * @param callable $callback
         * @param array $opts Options map for getWithSetCallback()
         * @return array Ordered list of the following:
         *   - Cached or regenerated value
         *   - Cached or regenerated value version number or null if not versioned
         *   - Timestamp of the current cached value at the key or null if there is no value
         * @note Callable type hints are not used to avoid class-autoloading
         */
        private function fetchOrRegenerate( $key, $ttl, $callback, array $opts ) {
                $checkKeys = $opts['checkKeys'] ?? [];
                $graceTTL = $opts['graceTTL'] ?? self::GRACE_TTL_NONE;
                $minAsOf = $opts['minAsOf'] ?? self::MIN_TIMESTAMP_NONE;
                $hotTTR = $opts['hotTTR'] ?? self::HOT_TTR;
                $lowTTL = $opts['lowTTL'] ?? min( self::LOW_TTL, $ttl );
                $ageNew = $opts['ageNew'] ?? self::AGE_NEW;
                $touchedCb = $opts['touchedCallback'] ?? null;
                $initialTime = $this->getCurrentTime();

                $kClass = $this->determineKeyClassForStats( $key );

                // Get the current key value and its metadata
                $curTTL = self::PASS_BY_REF;
                $curInfo = self::PASS_BY_REF; /** @var array $curInfo */
                $curValue = $this->get( $key, $curTTL, $checkKeys, $curInfo );
                // Apply any $touchedCb invalidation timestamp to get the "last purge timestamp"
                list( $curTTL, $LPT ) = $this->resolveCTL( $curValue, $curTTL, $curInfo, $touchedCb );
                // Use the cached value if it exists and is not due for synchronous regeneration
                if (
                        $this->isValid( $curValue, $curInfo['asOf'], $minAsOf ) &&
                        $this->isAliveOrInGracePeriod( $curTTL, $graceTTL )
                ) {
                        $preemptiveRefresh = (
                                $this->worthRefreshExpiring( $curTTL, $lowTTL ) ||
                                $this->worthRefreshPopular( $curInfo['asOf'], $ageNew, $hotTTR, $initialTime )
                        );
                        if ( !$preemptiveRefresh ) {
                                $this->stats->increment( "wanobjectcache.$kClass.hit.good" );

                                return [ $curValue, $curInfo['version'], $curInfo['asOf'] ];
                        } elseif ( $this->scheduleAsyncRefresh( $key, $ttl, $callback, $opts ) ) {
                                $this->stats->increment( "wanobjectcache.$kClass.hit.refresh" );

                                return [ $curValue, $curInfo['version'], $curInfo['asOf'] ];
                        }
                }

                // Determine if there is stale or volatile cached value that is still usable
                $isKeyTombstoned = ( $curInfo['tombAsOf'] !== null );
                if ( $isKeyTombstoned ) {
                        // Key is write-holed; use the (volatile) interim key as an alternative
                        list( $possValue, $possInfo ) = $this->getInterimValue( $key, $minAsOf );
                        // Update the "last purge time" since the $touchedCb timestamp depends on $value
                        $LPT = $this->resolveTouched( $possValue, $LPT, $touchedCb );
                } else {
                        $possValue = $curValue;
                        $possInfo = $curInfo;
                }

                // Avoid overhead from callback runs, regeneration locks, and cache sets during
                // hold-off periods for the key by reusing very recently generated cached values
                if (
                        $this->isValid( $possValue, $possInfo['asOf'], $minAsOf, $LPT ) &&
                        $this->isVolatileValueAgeNegligible( $initialTime - $possInfo['asOf'] )
                ) {
                        $this->stats->increment( "wanobjectcache.$kClass.hit.volatile" );

                        return [ $possValue, $possInfo['version'], $curInfo['asOf'] ];
                }

                $lockTSE = $opts['lockTSE'] ?? self::TSE_NONE;
                $busyValue = $opts['busyValue'] ?? null;
                $staleTTL = $opts['staleTTL'] ?? self::STALE_TTL_NONE;
                $version = $opts['version'] ?? null;

                // Determine whether one thread per datacenter should handle regeneration at a time
                $useRegenerationLock =
                        // Note that since tombstones no-op set(), $lockTSE and $curTTL cannot be used to
                        // deduce the key hotness because |$curTTL| will always keep increasing until the
                        // tombstone expires or is overwritten by a new tombstone. Also, even if $lockTSE
                        // is not set, constant regeneration of a key for the tombstone lifetime might be
                        // very expensive. Assume tombstoned keys are possibly hot in order to reduce
                        // the risk of high regeneration load after the delete() method is called.
                        $isKeyTombstoned ||
                        // Assume a key is hot if requested soon ($lockTSE seconds) after invalidation.
                        // This avoids stampedes when timestamps from $checkKeys/$touchedCb bump.
                        ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE ) ||
                        // Assume a key is hot if there is no value and a busy fallback is given.
                        // This avoids stampedes on eviction or preemptive regeneration taking too long.
                        ( $busyValue !== null && $possValue === false );

                // If a regeneration lock is required, threads that do not get the lock will try to use
                // the stale value, the interim value, or the $busyValue placeholder, in that order. If
                // none of those are set then all threads will bypass the lock and regenerate the value.
                $hasLock = $useRegenerationLock && $this->claimStampedeLock( $key );
                if ( $useRegenerationLock && !$hasLock ) {
                        if ( $this->isValid( $possValue, $possInfo['asOf'], $minAsOf ) ) {
                                $this->stats->increment( "wanobjectcache.$kClass.hit.stale" );

                                return [ $possValue, $possInfo['version'], $curInfo['asOf'] ];
                        } elseif ( $busyValue !== null ) {
                                $miss = is_infinite( $minAsOf ) ? 'renew' : 'miss';
                                $this->stats->increment( "wanobjectcache.$kClass.$miss.busy" );

                                return [ $this->resolveBusyValue( $busyValue ), $version, $curInfo['asOf'] ];
                        }
                }

                // Generate the new value given any prior value with a matching version
                $setOpts = [];
                $preCallbackTime = $this->getCurrentTime();
                ++$this->callbackDepth;
                try {
                        $value = $callback(
                                ( $curInfo['version'] === $version ) ? $curValue : false,
                                $ttl,
                                $setOpts,
                                ( $curInfo['version'] === $version ) ? $curInfo['asOf'] : null
                        );
                } finally {
                        --$this->callbackDepth;
                }
                $postCallbackTime = $this->getCurrentTime();

                // How long it took to fetch, validate, and generate the value
                $elapsed = max( $postCallbackTime - $initialTime, 0.0 );

                // Attempt to save the newly generated value if applicable
                if (
                        // Callback yielded a cacheable value
                        ( $value !== false && $ttl >= 0 ) &&
                        // Current thread was not raced out of a regeneration lock or key is tombstoned
                        ( !$useRegenerationLock || $hasLock || $isKeyTombstoned ) &&
                        // Key does not appear to be undergoing a set() stampede
                        $this->checkAndSetCooloff( $key, $kClass, $elapsed, $lockTSE, $hasLock )
                ) {
                        // How long it took to generate the value
                        $walltime = max( $postCallbackTime - $preCallbackTime, 0.0 );
                        $this->stats->timing( "wanobjectcache.$kClass.regen_walltime", 1e3 * $walltime );
                        // If the key is write-holed then use the (volatile) interim key as an alternative
                        if ( $isKeyTombstoned ) {
                                $this->setInterimValue( $key, $value, $lockTSE, $version, $walltime );
                        } else {
                                $finalSetOpts = [
                                        'since' => $setOpts['since'] ?? $preCallbackTime,
                                        'version' => $version,
                                        'staleTTL' => $staleTTL,
                                        'lockTSE' => $lockTSE, // informs lag vs performance trade-offs
                                        'creating' => ( $curValue === false ), // optimization
                                        'walltime' => $walltime
                                ] + $setOpts;
                                $this->set( $key, $value, $ttl, $finalSetOpts );
                        }
                }

                $this->yieldStampedeLock( $key, $hasLock );

                $miss = is_infinite( $minAsOf ) ? 'renew' : 'miss';
                $this->stats->increment( "wanobjectcache.$kClass.$miss.compute" );

                return [ $value, $version, $curInfo['asOf'] ];
        }

        /**
         * @param string $key
         * @return bool Success
         */
        private function claimStampedeLock( $key ) {
                // Note that locking is not bypassed due to I/O errors; this avoids stampedes
                return $this->cache->add( self::$MUTEX_KEY_PREFIX . $key, 1, self::$LOCK_TTL );
        }

        /**
         * @param string $key
         * @param bool $hasLock
         */
        private function yieldStampedeLock( $key, $hasLock ) {
                if ( $hasLock ) {
                        // The backend might be a mcrouter proxy set to broadcast DELETE to *all* the local
                        // datacenter cache servers via OperationSelectorRoute (for increased consistency).
                        // Since that would be excessive for these locks, use TOUCH to expire the key.
                        $this->cache->changeTTL( self::$MUTEX_KEY_PREFIX . $key, $this->getCurrentTime() - 60 );
                }
        }

        /**
         * @param float $age Age of volatile/interim key in seconds
         * @return bool Whether the age of a volatile value is negligible
         */
        private function isVolatileValueAgeNegligible( $age ) {
                return ( $age < mt_rand( self::$RECENT_SET_LOW_MS, self::$RECENT_SET_HIGH_MS ) / 1e3 );
        }

        /**
         * @param string $key
         * @param string $kClass
         * @param float $elapsed Seconds spent regenerating the value
         * @param float $lockTSE
         * @param bool $hasLock
         * @return bool Whether it is OK to proceed with a key set operation
         */
        private function checkAndSetCooloff( $key, $kClass, $elapsed, $lockTSE, $hasLock ) {
                $this->stats->timing( "wanobjectcache.$kClass.regen_set_delay", 1e3 * $elapsed );

                // If $lockTSE is set, the lock was bypassed because there was no stale/interim value,
                // and $elapsed indicates that regeration is slow, then there is a risk of set()
                // stampedes with large blobs. With a typical scale-out infrastructure, CPU and query
                // load from $callback invocations is distributed among appservers and replica DBs,
                // but cache operations for a given key route to a single cache server (e.g. striped
                // consistent hashing).
                if ( $lockTSE < 0 || $hasLock ) {
                        return true; // either not a priori hot or thread has the lock
                } elseif ( $elapsed <= self::$SET_DELAY_HIGH_MS * 1e3 ) {
                        return true; // not enough time for threads to pile up
                }

                $this->cache->clearLastError();
                if (
                        !$this->cache->add( self::$COOLOFF_KEY_PREFIX . $key, 1, self::$COOLOFF_TTL ) &&
                        // Don't treat failures due to I/O errors as the key being in cooloff
                        $this->cache->getLastError() === BagOStuff::ERR_NONE
                ) {
                        $this->stats->increment( "wanobjectcache.$kClass.cooloff_bounce" );

                        return false;
                }

                return true;
        }

        /**
         * @param mixed $value
         * @param float|null $curTTL
         * @param array $curInfo
         * @param callable|null $touchedCallback
         * @return array (current time left or null, UNIX timestamp of last purge or null)
         * @note Callable type hints are not used to avoid class-autoloading
         */
        private function resolveCTL( $value, $curTTL, $curInfo, $touchedCallback ) {
                if ( $touchedCallback === null || $value === false ) {
                        return [ $curTTL, max( $curInfo['tombAsOf'], $curInfo['lastCKPurge'] ) ];
                }

                $touched = $touchedCallback( $value );
                if ( $touched !== null && $touched >= $curInfo['asOf'] ) {
                        $curTTL = min( $curTTL, self::$TINY_NEGATIVE, $curInfo['asOf'] - $touched );
                }

                return [ $curTTL, max( $curInfo['tombAsOf'], $curInfo['lastCKPurge'], $touched ) ];
        }

        /**
         * @param mixed $value
         * @param float|null $lastPurge
         * @param callable|null $touchedCallback
         * @return float|null UNIX timestamp of last purge or null
         * @note Callable type hints are not used to avoid class-autoloading
         */
        private function resolveTouched( $value, $lastPurge, $touchedCallback ) {
                return ( $touchedCallback === null || $value === false )
                        ? $lastPurge // nothing to derive the "touched timestamp" from
                        : max( $touchedCallback( $value ), $lastPurge );
        }

        /**
         * @param string $key
         * @param float $minAsOf Minimum acceptable "as of" timestamp
         * @return array (cached value or false, cache key metadata map)
         */
        private function getInterimValue( $key, $minAsOf ) {
                $now = $this->getCurrentTime();

                if ( $this->useInterimHoldOffCaching ) {
                        $wrapped = $this->cache->get( self::$INTERIM_KEY_PREFIX . $key );

                        list( $value, $keyInfo ) = $this->unwrap( $wrapped, $now );
                        if ( $this->isValid( $value, $keyInfo['asOf'], $minAsOf ) ) {
                                return [ $value, $keyInfo ];
                        }
                }

                return $this->unwrap( false, $now );
        }

        /**
         * @param string $key
         * @param mixed $value
         * @param int $ttl
         * @param int|null $version Value version number
         * @param float $walltime How long it took to generate the value in seconds
         */
        private function setInterimValue( $key, $value, $ttl, $version, $walltime ) {
                $ttl = max( self::$INTERIM_KEY_TTL, (int)$ttl );

                $wrapped = $this->wrap( $value, $ttl, $version, $this->getCurrentTime(), $walltime );
                $this->cache->merge(
                        self::$INTERIM_KEY_PREFIX . $key,
                        function () use ( $wrapped ) {
                                return $wrapped;
                        },
                        $ttl,
                        1
                );
        }

        /**
         * @param mixed $busyValue
         * @return mixed
         */
        private function resolveBusyValue( $busyValue ) {
                return ( $busyValue instanceof Closure ) ? $busyValue() : $busyValue;
        }

        /**
         * 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 ) use ( $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 mixed[] 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 = []
        ) {
                // Load required keys into process cache in one go
                $this->warmupCache = $this->getRawKeysForWarmup(
                        $this->getNonProcessCachedMultiKeys( $keyedIds, $opts ),
                        $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 $ids and it takes the following arguments:
         *          - $ids: a list of entity IDs that require cache regeneration
         *          - &$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 ) use ( $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 mixed[] 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 = []
        ) {
                $checkKeys = $opts['checkKeys'] ?? [];
                unset( $opts['lockTSE'] ); // incompatible
                unset( $opts['busyValue'] ); // incompatible

                // Load required keys into process cache in one go
                $keysByIdGet = $this->getNonProcessCachedMultiKeys( $keyedIds, $opts );
                $this->warmupCache = $this->getRawKeysForWarmup( $keysByIdGet, $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( $keysByIdGet, $curTTLs, $checkKeys, $asOfs );
                foreach ( $keysByIdGet as $id => $key ) {
                        if ( !array_key_exists( $key, $curByKey ) || $curTTLs[$key] < 0 ) {
                                $idsRegen[] = $id;
                        }
                }

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

                $this->warmupCache = [];

                return $values;
        }

        /**
         * Set a key to soon expire in the local cluster if it pre-dates $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
         */
        final 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;
        }

        /**
         * Set a "check" key to soon expire in the local cluster if it pre-dates $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
         */
        final public function reapCheckKey( $key, $purgeTimestamp, &$isStale = false ) {
                $purge = $this->parsePurgeValue( $this->cache->get( self::$TIME_KEY_PREFIX . $key ) );
                if ( $purge && $purge[self::$PURGE_TIME] < $purgeTimestamp ) {
                        $isStale = true;
                        $this->logger->warning( "Reaping stale check key '$key'." );
                        $ok = $this->cache->changeTTL( self::$TIME_KEY_PREFIX . $key, self::TTL_SECOND );
                        if ( !$ok ) {
                                $this->logger->error( "Could not complete reap of check key '$key'." );
                        }

                        return $ok;
                }

                $isStale = false;

                return false;
        }

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

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

        /**
         * Hash a possibly long string into a suitable component for makeKey()/makeGlobalKey()
         *
         * @param string $component A raw component used in building a cache key
         * @return string 64 character HMAC using a stable secret for public collision resistance
         * @since 1.34
         */
        public function hash256( $component ) {
                return hash_hmac( 'sha256', $component, $this->secret );
        }

        /**
         * Get an iterator of (cache key => entity ID) for a list of entity IDs
         *
         * The callback takes an ID string and returns a key via makeKey()/makeGlobalKey().
         * There should be no network nor filesystem I/O used in the callback. The entity
         * ID/key mapping must be 1:1 or an exception will be thrown. If hashing is needed,
         * then use the hash256() method.
         *
         * Example usage for the default keyspace:
         * @code
         *     $keyedIds = $cache->makeMultiKeys(
         *         $modules,
         *         function ( $module ) use ( $cache ) {
         *             return $cache->makeKey( 'module-info', $module );
         *         }
         *     );
         * @endcode
         *
         * Example usage for mixed default and global keyspace:
         * @code
         *     $keyedIds = $cache->makeMultiKeys(
         *         $filters,
         *         function ( $filter ) use ( $cache ) {
         *             return ( strpos( $filter, 'central:' ) === 0 )
         *                 ? $cache->makeGlobalKey( 'regex-filter', $filter )
         *                 : $cache->makeKey( 'regex-filter', $filter )
         *         }
         *     );
         * @endcode
         *
         * Example usage with hashing:
         * @code
         *     $keyedIds = $cache->makeMultiKeys(
         *         $urls,
         *         function ( $url ) use ( $cache ) {
         *             return $cache->makeKey( 'url-info', $cache->hash256( $url ) );
         *         }
         *     );
         * @endcode
         *
         * @see WANObjectCache::makeKey()
         * @see WANObjectCache::makeGlobalKey()
         * @see WANObjectCache::hash256()
         *
         * @param string[]|int[] $ids List of entity IDs
         * @param callable $keyCallback Function returning makeKey()/makeGlobalKey() on the input ID
         * @return ArrayIterator Iterator of (cache key => ID); order of $ids is preserved
         * @throws UnexpectedValueException
         * @since 1.28
         */
        final public function makeMultiKeys( array $ids, $keyCallback ) {
                $idByKey = [];
                foreach ( $ids as $id ) {
                        // Discourage triggering of automatic makeKey() hashing in some backends
                        if ( strlen( $id ) > 64 ) {
                                $this->logger->warning( __METHOD__ . ": long ID '$id'; use hash256()" );
                        }
                        $key = $keyCallback( $id, $this );
                        // Edge case: ignore key collisions due to duplicate $ids like "42" and 42
                        if ( !isset( $idByKey[$key] ) ) {
                                $idByKey[$key] = $id;
                        } elseif ( (string)$id !== (string)$idByKey[$key] ) {
                                throw new UnexpectedValueException(
                                        "Cache key collision; IDs ('$id','{$idByKey[$key]}') map to '$key'"
                                );
                        }
                }

                return new ArrayIterator( $idByKey );
        }

        /**
         * Get an (ID => value) map from (i) a non-unique list of entity IDs, and (ii) the list
         * of corresponding entity values by first appearance of each ID in the entity ID list
         *
         * For use with getMultiWithSetCallback() and getMultiWithUnionSetCallback().
         *
         * *Only* use this method if the entity ID/key mapping is trivially 1:1 without exception.
         * Key generation method must utitilize the *full* entity ID in the key (not a hash of it).
         *
         * Example usage:
         * @code
         *     $poems = $cache->getMultiWithSetCallback(
         *         $cache->makeMultiKeys(
         *             $uuids,
         *             function ( $uuid ) use ( $cache ) {
         *                 return $cache->makeKey( 'poem', $uuid );
         *             }
         *         ),
         *         $cache::TTL_DAY,
         *         function ( $uuid ) use ( $url ) {
         *             return $this->http->run( [ 'method' => 'GET', 'url' => "$url/$uuid" ] );
         *         }
         *     );
         *     $poemsByUUID = $cache->multiRemap( $uuids, $poems );
         * @endcode
         *
         * @see WANObjectCache::makeMultiKeys()
         * @see WANObjectCache::getMultiWithSetCallback()
         * @see WANObjectCache::getMultiWithUnionSetCallback()
         *
         * @param string[]|int[] $ids Entity ID list makeMultiKeys()
         * @param mixed[] $res Result of getMultiWithSetCallback()/getMultiWithUnionSetCallback()
         * @return mixed[] Map of (ID => value); order of $ids is preserved
         * @since 1.34
         */
        final public function multiRemap( array $ids, array $res ) {
                if ( count( $ids ) !== count( $res ) ) {
                        // If makeMultiKeys() is called on a list of non-unique IDs, then the resulting
                        // ArrayIterator will have less entries due to "first appearance" de-duplication
                        $ids = array_keys( array_flip( $ids ) );
                        if ( count( $ids ) !== count( $res ) ) {
                                throw new UnexpectedValueException( "Multi-key result does not match ID list" );
                        }
                }

                return array_combine( $ids, $res );
        }

        /**
         * Get the "last error" registered; clearLastError() should be called manually
         * @return int ERR_* class constant for the "last error" registry
         */
        final public function getLastError() {
                $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();
        }

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

        /**
         * Enable or 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
         */
        final 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
         */
        final 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 Seconds to keep the tombstone around
         * @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->mcrouterAware ) {
                        // See https://github.com/facebook/mcrouter/wiki/Multi-cluster-broadcast-setup
                        // Wildcards select all matching routes, e.g. the WAN cluster on all DCs
                        $ok = $this->cache->set(
                                "/*/{$this->cluster}/{$key}",
                                $this->makePurgeValue( $this->getCurrentTime(), $holdoff ),
                                $ttl
                        );
                } else {
                        // Some other proxy handles broadcasting or there is only one datacenter
                        $ok = $this->cache->set(
                                $key,
                                $this->makePurgeValue( $this->getCurrentTime(), $holdoff ),
                                $ttl
                        );
                }

                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->mcrouterAware ) {
                        // See https://github.com/facebook/mcrouter/wiki/Multi-cluster-broadcast-setup
                        // Wildcards select all matching routes, e.g. the WAN cluster on all DCs
                        $ok = $this->cache->delete( "/*/{$this->cluster}/{$key}" );
                } else {
                        // Some other proxy handles broadcasting or there is only one datacenter
                        $ok = $this->cache->delete( $key );
                }

                return $ok;
        }

        /**
         * @param string $key
         * @param int $ttl Seconds to live
         * @param callable $callback
         * @param array $opts
         * @return bool Success
         * @note Callable type hints are not used to avoid class-autoloading
         */
        private function scheduleAsyncRefresh( $key, $ttl, $callback, $opts ) {
                if ( !$this->asyncHandler ) {
                        return false;
                }
                // Update the cache value later, such during post-send of an HTTP request
                $func = $this->asyncHandler;
                $func( function () use ( $key, $ttl, $callback, $opts ) {
                        $opts['minAsOf'] = INF; // force a refresh
                        $this->fetchOrRegenerate( $key, $ttl, $callback, $opts );
                } );

                return true;
        }

        /**
         * 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
         */
        private 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;
                }

                $popularHitsPerSec = 1;
                // 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 * $popularHitsPerSec) = 1 (by definition)
                // P(refresh) = 1/($refreshWindowSec * $popularHitsPerSec)
                $chance = 1 / ( $popularHitsPerSec * $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 if $value is not false, versioned (if needed), and not older than $minTime (if set)
         *
         * @param array|bool $value
         * @param float $asOf The time $value was generated
         * @param float $minAsOf Minimum acceptable "as of" timestamp
         * @param float|null $purgeTime The last time the value was invalidated
         * @return bool
         */
        protected function isValid( $value, $asOf, $minAsOf, $purgeTime = null ) {
                // Avoid reading any key not generated after the latest delete() or touch
                $safeMinAsOf = max( $minAsOf, $purgeTime + self::$TINY_POSTIVE );

                if ( $value === false ) {
                        return false;
                } elseif ( $safeMinAsOf > 0 && $asOf < $minAsOf ) {
                        return false;
                }

                return true;
        }

        /**
         * @param mixed $value
         * @param int $ttl Seconds to live or zero for "indefinite"
         * @param int|null $version Value version number or null if not versioned
         * @param float $now Unix Current timestamp just before calling set()
         * @param float $walltime How long it took to generate the value in seconds
         * @return array
         */
        private function wrap( $value, $ttl, $version, $now, $walltime ) {
                // Returns keys in ascending integer order for PHP7 array packing:
                // https://nikic.github.io/2014/12/22/PHPs-new-hashtable-implementation.html
                $wrapped = [
                        self::$FLD_FORMAT_VERSION => self::$VERSION,
                        self::$FLD_VALUE => $value,
                        self::$FLD_TTL => $ttl,
                        self::$FLD_TIME => $now
                ];
                if ( $version !== null ) {
                        $wrapped[self::$FLD_VALUE_VERSION] = $version;
                }
                if ( $walltime >= self::$GENERATION_SLOW_SEC ) {
                        $wrapped[self::$FLD_GENERATION_TIME] = $walltime;
                }

                return $wrapped;
        }

        /**
         * @param array|string|bool $wrapped The entry at a cache key
         * @param float $now Unix Current timestamp (preferrably pre-query)
         * @return array (value or false if absent/tombstoned/malformed, value metadata map).
         * The cache key metadata includes the following metadata:
         *   - asOf: UNIX timestamp of the value or null if there is no value
         *   - curTTL: remaining time-to-live (negative if tombstoned) or null if there is no value
         *   - version: value version number or null if the if there is no value
         *   - tombAsOf: UNIX timestamp of the tombstone or null if there is no tombstone
         */
        private function unwrap( $wrapped, $now ) {
                $value = false;
                $info = [ 'asOf' => null, 'curTTL' => null, 'version' => null, 'tombAsOf' => null ];

                if ( is_array( $wrapped ) ) {
                        // Entry expected to be a cached value; validate it
                        if (
                                ( $wrapped[self::$FLD_FORMAT_VERSION] ?? null ) === self::$VERSION &&
                                $wrapped[self::$FLD_TIME] >= $this->epoch
                        ) {
                                if ( $wrapped[self::$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;
                                }
                                $value = $wrapped[self::$FLD_VALUE];
                                $info['version'] = $wrapped[self::$FLD_VALUE_VERSION] ?? null;
                                $info['asOf'] = $wrapped[self::$FLD_TIME];
                                $info['curTTL'] = $curTTL;
                        }
                } else {
                        // Entry expected to be a tombstone; parse it
                        $purge = $this->parsePurgeValue( $wrapped );
                        if ( $purge !== false ) {
                                // Tombstoned keys should always have a negative current $ttl
                                $info['curTTL'] = min( $purge[self::$PURGE_TIME] - $now, self::$TINY_NEGATIVE );
                                $info['tombAsOf'] = $purge[self::$PURGE_TIME];
                        }
                }

                return [ $value, $info ];
        }

        /**
         * @param string[] $keys
         * @param string $prefix
         * @return string[] Prefix keys; the order of $keys is preserved
         */
        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 A collection name to describe this class of key
         */
        private function determineKeyClassForStats( $key ) {
                $parts = explode( ':', $key, 3 );

                return $parts[1] ?? $parts[0]; // sanity
        }

        /**
         * @param string|array|bool $value Possible string of the form "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
         */
        private 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;
                }

                if ( $segments[1] < $this->epoch ) {
                        // Values this old are ignored
                        return false;
                }

                return [
                        self::$PURGE_TIME => (float)$segments[1],
                        self::$PURGE_HOLDOFF => (int)$segments[2],
                ];
        }

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

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

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

        /**
         * @param string $key
         * @param int $version
         * @return string
         */
        private function getProcessCacheKey( $key, $version ) {
                return $key . ' ' . (int)$version;
        }

        /**
         * @param ArrayIterator $keys
         * @param array $opts
         * @return string[] Map of (ID => cache key)
         */
        private function getNonProcessCachedMultiKeys( ArrayIterator $keys, array $opts ) {
                $pcTTL = $opts['pcTTL'] ?? self::TTL_UNCACHEABLE;

                $keysMissing = [];
                if ( $pcTTL > 0 && $this->callbackDepth == 0 ) {
                        $version = $opts['version'] ?? null;
                        $pCache = $this->getProcessCache( $opts['pcGroup'] ?? self::PC_PRIMARY );
                        foreach ( $keys as $key => $id ) {
                                if ( !$pCache->has( $this->getProcessCacheKey( $key, $version ), $pcTTL ) ) {
                                        $keysMissing[$id] = $key;
                                }
                        }
                }

                return $keysMissing;
        }

        /**
         * @param string[] $keys
         * @param string[]|string[][] $checkKeys
         * @return string[] List of cache keys
         */
        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;
        }

        /**
         * @return float UNIX timestamp
         * @codeCoverageIgnore
         */
        protected function getCurrentTime() {
                if ( $this->wallClockOverride ) {
                        return $this->wallClockOverride;
                }

                $clockTime = (float)time(); // call this first
                // microtime() uses an initial gettimeofday() call added to usage clocks.
                // This can severely drift from time() and the microtime() value of other threads
                // due to undercounting of the amount of time elapsed. Instead of seeing the current
                // time as being in the past, use the value of time(). This avoids setting cache values
                // that will immediately be seen as expired and possibly cause stampedes.
                return max( microtime( true ), $clockTime );
        }

        /**
         * @param float|null &$time Mock UNIX timestamp for testing
         * @codeCoverageIgnore
         */
        public function setMockTime( &$time ) {
                $this->wallClockOverride =& $time;
                $this->cache->setMockTime( $time );
        }
}