Merge ".mailmap: Correct two contributor names"

[mediawiki.git] / includes / libs / objectcache / WANObjectCache.php

<?php
/**
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */

namespace Wikimedia\ObjectCache;

use ArrayIterator;
use Closure;
use Exception;
use MapCacheLRU;
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use RuntimeException;
use UnexpectedValueException;
use Wikimedia\LightweightObjectStore\ExpirationAwareness;
use Wikimedia\Stats\IBufferingStatsdDataFactory;
use Wikimedia\Stats\StatsFactory;

/**
 * Multi-datacenter aware caching interface
 *
 * ### Using WANObjectCache
 *
 * %WANObjectCache (known as **WANCache**, pronounced whan-cache) improves performance
 * by reducing database load, increasing web server capacity (fewer repeated computations) and
 * providing faster access to data. The data cached here follows a "cache-aside" strategy, with
 * data potentially derived from database rows. Generally speaking, cache data should be treated
 * as equally up-to-date to data from a replica database, and is thus essentially subject to the
 * same replication lag.
 *
 * The primary way to interact with this class is via the getWithSetCallback() method.
 *
 * Each data center has its own cache cluster, with web servers in a given datacenter
 * populating and reading from the local datacenter only. The exceptions are methods delete(),
 * touchCheckKey(), and resetCheckKey(), which also asynchronously broadcast the equivalent
 * purge to other datacenters.
 *
 * To learn how this is used and configured at Wikimedia Foundation,
 * refer to <https://wikitech.wikimedia.org/wiki/Memcached_for_MediaWiki>.
 *
 * For broader guidance on how to approach caching in MediaWiki at scale,
 * refer to <https://wikitech.wikimedia.org/wiki/MediaWiki_Engineering/Guides/Backend_performance_practices>.
 *
 * For your code to "see" new values in a timely manner, you need to follow either the
 * validation strategy, or the purge strategy.
 *
 * #### Strategy 1: Validation
 *
 * The validation strategy refers to the natural avoidance of stale data
 * by one of the following means:
 *
 *   - A) The cached value is immutable.
 *
 *        If you can obtain all the information needed to uniquely describe the value,
 *        then the value never has to change or be purged. Instead, the key changes,
 *        which naturally creates a miss where you can compute the right value.
 *        For example, a transformation like parsing or transforming some input,
 *        could have a cache key like `example-myparser, option-xyz, v2, hash1234`
 *        which would describe the transformation, the version/parameters, and a hash
 *        of the exact input.
 *
 *        This also naturally avoids oscillation or corruption in the context of multiple
 *        servers and data centers, where your code may not always be running the same version
 *        everywhere at the same time. Newer code would have its own set of cache keys,
 *        ensuring a deterministic outcome.
 *   - B) The value is cached with a low TTL.
 *
 *        If you can tolerate a few seconds or minutes of delay before changes are reflected
 *        in the way your data is used, and if re-computation is quick, you can consider
 *        caching it with a "blind" TTL – using the value's age as your method of validation.
 *   - C) Validity is checked against an external source.
 *
 *        Perhaps you prefer to utilize the old data as fallback or to help compute the new
 *        value, or for other reasons you need to have a stable key across input changes
 *        (e.g. cache by page title instead of revision ID). If you put the variable identifier
 *        (e.g. input hash, or revision ID) in the cache value, and validate this on retrieval
 *        then you don't need purging or expiration.
 *
 *        After calling get() you can validate the ID inside the cached value against what
 *        you know. When needed, recompute the value and call set().
 *
 * #### Strategy 2: Purge
 *
 * The purge strategy refers to the approach whereby your application knows that source
 * data has changed and can react by purging the relevant cache keys.
 * The simplest purge method is delete().
 *
 * Note that cache updates and purges are not immediately visible to all application servers in
 * all data centers. The cache should be treated like a replica database in this regard.
 * If immediate synchronization is required, then solutions must be sought outside WANCache.
 *
 * Write operations like delete() and the "set" part of getWithSetCallback(), may return true as
 * soon as the command has been sent or buffered to an open connection to the cache cluster.
 * It will be processed and/or broadcasted asynchronously.
 *
 * @anchor wanobjectcache-deployment
 * ### Deploying WANObjectCache
 *
 * There are two supported ways for sysadmins to set up multi-DC cache purging:
 *
 *   - A) Set up mcrouter as the cache backend, with a memcached BagOStuff class for the 'cache'
 *        parameter, and a wildcard routing prefix for the 'broadcastRoutingPrefix' parameter.
 *        Configure mcrouter as follows:
 *          - Define a "<datacenter>" pool of memcached servers for each datacenter.
 *          - Define a "<datacenter>/wan" route to each datacenter, using "AllSyncRoute" for the
 *            routes that go to the local datacenter pool and "AllAsyncRoute" for the routes that
 *            go to remote datacenter pools. The child routes should use "HashRoute|<datacenter>".
 *            This allows for the use of a wildcard route for 'broadcastRoutingPrefix'. See
 *            https://github.com/facebook/mcrouter/wiki/Routing-Prefix and
 *            https://github.com/facebook/mcrouter/wiki/Multi-cluster-broadcast-setup.
 *          - In order to reroute operations from "down" servers to spare ("gutter") servers, use
 *            "FailoverWithExptimeRoute" (failover_exptime=60) instead of "HashRoute|<datacenter>"
 *            in the "AllSyncRoute"/"AllAsyncRoute" child routes.
 *            The "gutter" pool is a set of memcached servers that only handle failover traffic.
 *            Such servers should be carefully spread over different rows and racks. See
 *            https://github.com/facebook/mcrouter/wiki/List-of-Route-Handles#failoverroute
 *   - B) Set up dynomite as the cache backend, using a memcached BagOStuff class for the 'cache'
 *        parameter. Note that with this setup, all key setting operations will be broadcasted,
 *        rather than just purges. Writes will be 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.
 *
 * Internally, access to a given key actually involves the use of one or more "sister" keys.
 * A sister key is constructed by prefixing the base key with "WANCache:" (used to distinguish
 * WANObjectCache formatted keys) and suffixing a colon followed by a single-character sister
 * key type. The sister key types include the following:
 *
 * - `v`: used to store "regular" values (metadata-wrapped) and temporary purge "tombstones".
 * - `t`: used to store "last purge" timestamps for "check" keys.
 * - `m`: used to store temporary mutex locks to avoid cache stampedes.
 * - `i`: used to store temporary interim values (metadata-wrapped) for tombstoned keys.
 *
 * @ingroup Cache
 * @newable
 * @since 1.26
 */
class WANObjectCache implements
        ExpirationAwareness,
        StorageAwareness,
        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 StatsFactory */
        protected $stats;
        /** @var callable|null Function that takes a WAN cache callback and runs it later */
        protected $asyncHandler;

        /**
         * Routing prefix for operations that should be broadcasted to all data centers.
         *
         * If null, the there is only one datacenter or a backend proxy broadcasts everything.
         *
         * @var string|null
         */
        protected $broadcastRoute;
        /** @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 hashing long strings into key components */
        protected $secret;
        /** @var int Scheme to use for key coalescing (Hash Tags or Hash Stops) */
        protected $coalesceScheme;

        /** @var array<int,array> List of (key, UNIX timestamp) tuples for get() cache misses */
        private $missLog;

        /** @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;

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

        /** Consider regeneration if the key will expire within this many seconds */
        private const LOW_TTL = 60;
        /** Max TTL, in seconds, to store keys when a data source has high replication lag */
        public const TTL_LAGGED = 30;

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

        /** Idiom for getWithSetCallback() meaning "no cache stampede mutex" */
        private const TSE_NONE = -1;

        /** Idiom for set()/getWithSetCallback() meaning "no post-expiration persistence" */
        public const STALE_TTL_NONE = 0;
        /** Idiom for set()/getWithSetCallback() meaning "no post-expiration grace period" */
        public const GRACE_TTL_NONE = 0;
        /** Idiom for delete()/touchCheckKey() meaning "no hold-off period" */
        public const HOLDOFF_TTL_NONE = 0;

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

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

        /** Idiom for get()/getMulti() to return extra information by reference */
        public const PASS_BY_REF = [];

        /** Use twemproxy-style Hash Tag key scheme (e.g. "{...}") */
        private const SCHEME_HASH_TAG = 1;
        /** Use mcrouter-style Hash Stop key scheme (e.g. "...|#|") */
        private const SCHEME_HASH_STOP = 2;

        /** Seconds to keep dependency purge keys around */
        private const CHECK_KEY_TTL = self::TTL_YEAR;
        /** Seconds to keep interim value keys for tombstoned keys around */
        private const INTERIM_KEY_TTL = 2;

        /** Seconds to keep lock keys around */
        private const LOCK_TTL = 10;
        /** Seconds to ramp up the chance of regeneration due to expected time-till-refresh */
        private const RAMPUP_TTL = 30;

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

        /** Min millisecond set() backoff during hold-off (far less than INTERIM_KEY_TTL) */
        private const RECENT_SET_LOW_MS = 50;
        /** Max millisecond set() backoff during hold-off (far less than INTERIM_KEY_TTL) */
        private const RECENT_SET_HIGH_MS = 100;

        /** Consider value generation somewhat high if it takes this many seconds or more */
        private const GENERATION_HIGH_SEC = 0.2;

        /** Key to the tombstone entry timestamp */
        private const PURGE_TIME = 0;
        /** Key to the tombstone entry hold-off TTL */
        private const PURGE_HOLDOFF = 1;

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

        /** Version number attribute for a key; keep value for b/c (< 1.36) */
        public const KEY_VERSION = 'version';
        /** Generation completion timestamp attribute for a key; keep value for b/c (< 1.36) */
        public const KEY_AS_OF = 'asOf';
        /** Logical TTL attribute for a key */
        public const KEY_TTL = 'ttl';
        /** Remaining TTL attribute for a key; keep value for b/c (< 1.36) */
        public const KEY_CUR_TTL = 'curTTL';
        /** Tomstone timestamp attribute for a key; keep value for b/c (< 1.36) */
        public const KEY_TOMB_AS_OF = 'tombAsOf';
        /** Highest "check" key timestamp for a key; keep value for b/c (< 1.36) */
        public const KEY_CHECK_AS_OF = 'lastCKPurge';

        /** Value for a key */
        private const RES_VALUE = 0;
        /** Version number attribute for a key */
        private const RES_VERSION = 1;
        /** Generation completion timestamp attribute for a key */
        private const RES_AS_OF = 2;
        /** Logical TTL attribute for a key */
        private const RES_TTL = 3;
        /** Tomstone timestamp attribute for a key */
        private const RES_TOMB_AS_OF = 4;
        /** Highest "check" key timestamp for a key */
        private const RES_CHECK_AS_OF = 5;
        /** Highest "touched" timestamp for a key */
        private const RES_TOUCH_AS_OF = 6;
        /** Remaining TTL attribute for a key */
        private const RES_CUR_TTL = 7;

        /** Key to WAN cache version number; stored in blobs */
        private const FLD_FORMAT_VERSION = 0;
        /** Key to the cached value; stored in blobs */
        private const FLD_VALUE = 1;
        /** Key to the original TTL; stored in blobs */
        private const FLD_TTL = 2;
        /** Key to the cache timestamp; stored in blobs */
        private const FLD_TIME = 3;
        /** Key to the flags bit field (reserved number) */
        private const /** @noinspection PhpUnusedPrivateFieldInspection */ FLD_FLAGS = 4;
        /** Key to collection cache version number; stored in blobs */
        private const FLD_VALUE_VERSION = 5;
        private const /** @noinspection PhpUnusedPrivateFieldInspection */ FLD_GENERATION_TIME = 6;

        /** Single character component for value keys */
        private const TYPE_VALUE = 'v';
        /** Single character component for timestamp check keys */
        private const TYPE_TIMESTAMP = 't';
        /** Single character component for mutex lock keys */
        private const TYPE_MUTEX = 'm';
        /** Single character component for interim value keys */
        private const TYPE_INTERIM = 'i';

        /** Value prefix of purge values */
        private const PURGE_VAL_PREFIX = 'PURGED';

        /**
         * @stable to call
         * @param array $params
         *   - cache    : BagOStuff object for a persistent cache
         *   - logger   : LoggerInterface object
         *   - stats    : StatsFactory object. Since 1.43, constructing a WANObjectCache object
         *       with an IBufferingStatsdDataFactory stats collector will emit a warning.
         *   - 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]
         *   - broadcastRoutingPrefix: a routing prefix used to broadcast certain operations to all
         *       datacenters; See also <https://github.com/facebook/mcrouter/wiki/Config-Files>.
         *       This prefix takes the form `/<datacenter>/<name of wan route>/`, where `datacenter`
         *       is usually a wildcard to select all matching routes (e.g. the WAN cluster in all DCs).
         *       See also <https://github.com/facebook/mcrouter/wiki/Multi-cluster-broadcast-setup>.
         *       This is required when using mcrouter as a multi-region backing store proxy. [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]
         *   - coalesceScheme: which key scheme to use in order to encourage the backend to place any
         *       "helper" keys for a "value" key within the same cache server. This reduces network
         *       overhead and reduces the chance the single downed cache server causes disruption.
         *       Use "hash_stop" with mcrouter and "hash_tag" with dynomite. [default: "hash_stop"]
         */
        public function __construct( array $params ) {
                $this->cache = $params['cache'];
                $this->broadcastRoute = $params['broadcastRoutingPrefix'] ?? null;
                $this->epoch = $params['epoch'] ?? 0;
                $this->secret = $params['secret'] ?? (string)$this->epoch;
                if ( ( $params['coalesceScheme'] ?? '' ) === 'hash_tag' ) {
                        // https://redis.io/topics/cluster-spec
                        // https://github.com/twitter/twemproxy/blob/v0.4.1/notes/recommendation.md#hash-tags
                        // https://github.com/Netflix/dynomite/blob/v0.7.0/notes/recommendation.md#hash-tags
                        $this->coalesceScheme = self::SCHEME_HASH_TAG;
                } else {
                        // https://github.com/facebook/mcrouter/wiki/Key-syntax
                        $this->coalesceScheme = self::SCHEME_HASH_STOP;
                }

                $this->setLogger( $params['logger'] ?? new NullLogger() );

                if ( isset( $params['stats'] ) && $params['stats'] instanceof IBufferingStatsdDataFactory ) {
                        wfDeprecated(
                                __METHOD__,
                                'Use of StatsdDataFactory is deprecated in 1.43. Use StatsFactory instead.'
                        );
                        $params['stats'] = null;
                }
                $this->stats = $params['stats'] ?? StatsFactory::newNull();

                $this->asyncHandler = $params['asyncHandler'] ?? null;
                $this->missLog = array_fill( 0, 10, [ '', 0.0 ] );
        }

        /**
         * @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 purged by $checkKeys
         *   - b) float (>=0); if $key exists, has a TTL, and is not purged by $checkKeys
         *   - c) float (<0); if $key is tombstoned, stale, or existing but purged 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 key purges 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 exist 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(), which has cache slam avoidance and key
         * versioning features, instead of bare get()/set() calls.
         *
         * Do not use this method on versioned keys accessed via getWithSetCallback().
         *
         * When using the $info parameter, it should be passed in as WANObjectCache::PASS_BY_REF.
         * In that case, it becomes a key metadata map. Otherwise, for backwards compatibility,
         * $info becomes the value generation timestamp (null if the key is nonexistant/tombstoned).
         * Key metadata map fields include:
         *   - WANObjectCache::KEY_VERSION: value version number; null if key is nonexistant
         *   - WANObjectCache::KEY_AS_OF: value generation timestamp (UNIX); null if key is nonexistant
         *   - WANObjectCache::KEY_TTL: assigned TTL (seconds); null if key is nonexistant/tombstoned
         *   - WANObjectCache::KEY_CUR_TTL: remaining TTL (seconds); null if key is nonexistant
         *   - WANObjectCache::KEY_TOMB_AS_OF: tombstone timestamp (UNIX); null if key is not tombstoned
         *   - WANObjectCache::KEY_CHECK_AS_OF: highest "check" key timestamp (UNIX); null if none
         *
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param float|null &$curTTL Seconds of TTL left [returned]
         * @param string[] $checkKeys Map of (integer or cache key => "check" key(s));
         *  "check" keys must also be made with makeKey()/makeGlobalKey()
         * @param array &$info Metadata map [returned]
         * @return mixed Value of cache key; false on failure
         */
        final public function get( $key, &$curTTL = null, array $checkKeys = [], &$info = [] ) {
                // Note that an undeclared variable passed as $info starts as null (not the default).
                // Also, if no $info parameter is provided, then it doesn't matter how it changes here.
                $legacyInfo = ( $info !== self::PASS_BY_REF );

                $now = $this->getCurrentTime();
                $res = $this->fetchKeys( [ $key ], $checkKeys, $now )[$key];

                $curTTL = $res[self::RES_CUR_TTL];
                $info = $legacyInfo
                        ? $res[self::RES_AS_OF]
                        : [
                                self::KEY_VERSION => $res[self::RES_VERSION],
                                self::KEY_AS_OF => $res[self::RES_AS_OF],
                                self::KEY_TTL => $res[self::RES_TTL],
                                self::KEY_CUR_TTL => $res[self::RES_CUR_TTL],
                                self::KEY_TOMB_AS_OF => $res[self::RES_TOMB_AS_OF],
                                self::KEY_CHECK_AS_OF => $res[self::RES_CHECK_AS_OF]
                        ];

                if ( $curTTL === null || $curTTL <= 0 ) {
                        // Log the timestamp in case a corresponding set() call does not provide "walltime"
                        unset( $this->missLog[array_key_first( $this->missLog )] );
                        $this->missLog[] = [ $key, $this->getCurrentTime() ];
                }

                return $res[self::RES_VALUE];
        }

        /**
         * Fetch the value of several keys from cache
         *
         * $curTTLs becomes a map of only present/tombstoned $keys to their current time-to-live.
         *
         * $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. The logic of "check"
         * keys otherwise works the same as in WANObjectCache::get().
         *
         * When using the $info parameter, it should be passed in as WANObjectCache::PASS_BY_REF.
         * In that case, it becomes a mapping of all the $keys to their metadata maps, each in the
         * style of WANObjectCache::get(). Otherwise, for backwards compatibility, $info becomes a
         * map of only present/tombstoned $keys to their value generation timestamps.
         *
         * @see WANObjectCache::get()
         *
         * @param string[] $keys List/map with makeKey()/makeGlobalKey() cache keys as values
         * @param array<string,float> &$curTTLs Map of (key => seconds of TTL left) [returned]
         * @param string[]|string[][] $checkKeys Map of (integer or cache key => "check" key(s));
         *  "check" keys must also be made with makeKey()/makeGlobalKey()
         * @param array<string,array> &$info Map of (key => metadata map) [returned]
         * @return array<string,mixed> Map of (key => value) for existing values in order of $keys
         */
        final public function getMulti(
                array $keys,
                &$curTTLs = [],
                array $checkKeys = [],
                &$info = []
        ) {
                // Note that an undeclared variable passed as $info starts as null (not the default).
                // Also, if no $info parameter is provided, then it doesn't matter how it changes here.
                $legacyInfo = ( $info !== self::PASS_BY_REF );

                $curTTLs = [];
                $info = [];
                $valuesByKey = [];

                $now = $this->getCurrentTime();
                $resByKey = $this->fetchKeys( $keys, $checkKeys, $now );
                foreach ( $resByKey as $key => $res ) {
                        if ( $res[self::RES_VALUE] !== false ) {
                                $valuesByKey[$key] = $res[self::RES_VALUE];
                        }

                        if ( $res[self::RES_CUR_TTL] !== null ) {
                                $curTTLs[$key] = $res[self::RES_CUR_TTL];
                        }
                        $info[$key] = $legacyInfo
                                ? $res[self::RES_AS_OF]
                                : [
                                        self::KEY_VERSION => $res[self::RES_VERSION],
                                        self::KEY_AS_OF => $res[self::RES_AS_OF],
                                        self::KEY_TTL => $res[self::RES_TTL],
                                        self::KEY_CUR_TTL => $res[self::RES_CUR_TTL],
                                        self::KEY_TOMB_AS_OF => $res[self::RES_TOMB_AS_OF],
                                        self::KEY_CHECK_AS_OF => $res[self::RES_CHECK_AS_OF]
                                ];
                }

                return $valuesByKey;
        }

        /**
         * Fetch the value and key metadata of several keys from cache
         *
         * $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.
         *
         * @param string[] $keys List/map with makeKey()/makeGlobalKey() cache keys as values
         * @param string[]|string[][] $checkKeys Map of (integer or cache key => "check" key(s));
         *  "check" keys must also be made with makeKey()/makeGlobalKey()
         * @param float $now The current UNIX timestamp
         * @param callable|null $touchedCb Callback yielding a UNIX timestamp from a value, or null
         * @return array<string,array> Map of (key => WANObjectCache::RESULT_* map) in order of $keys
         * @note Callable type hints are not used to avoid class-autoloading
         */
        protected function fetchKeys( array $keys, array $checkKeys, float $now, $touchedCb = null ) {
                $resByKey = [];

                // List of all sister keys that need to be fetched from cache
                $allSisterKeys = [];
                // Order-corresponding value sister key list for the base key list ($keys)
                $valueSisterKeys = [];
                // List of "check" sister keys to compare all value sister keys against
                $checkSisterKeysForAll = [];
                // Map of (base key => additional "check" sister key(s) to compare against)
                $checkSisterKeysByKey = [];

                foreach ( $keys as $key ) {
                        $sisterKey = $this->makeSisterKey( $key, self::TYPE_VALUE );
                        $allSisterKeys[] = $sisterKey;
                        $valueSisterKeys[] = $sisterKey;
                }

                foreach ( $checkKeys as $i => $checkKeyOrKeyGroup ) {
                        // Note: avoid array_merge() inside loop in case there are many keys
                        if ( is_int( $i ) ) {
                                // Single "check" key that applies to all base keys
                                $sisterKey = $this->makeSisterKey( $checkKeyOrKeyGroup, self::TYPE_TIMESTAMP );
                                $allSisterKeys[] = $sisterKey;
                                $checkSisterKeysForAll[] = $sisterKey;
                        } else {
                                // List of "check" keys that apply to a specific base key
                                foreach ( (array)$checkKeyOrKeyGroup as $checkKey ) {
                                        $sisterKey = $this->makeSisterKey( $checkKey, self::TYPE_TIMESTAMP );
                                        $allSisterKeys[] = $sisterKey;
                                        $checkSisterKeysByKey[$i][] = $sisterKey;
                                }
                        }
                }

                if ( $this->warmupCache ) {
                        // Get the wrapped values of the sister keys from the warmup cache
                        $wrappedBySisterKey = $this->warmupCache;
                        $sisterKeysMissing = array_diff( $allSisterKeys, array_keys( $wrappedBySisterKey ) );
                        if ( $sisterKeysMissing ) {
                                $this->warmupKeyMisses += count( $sisterKeysMissing );
                                $wrappedBySisterKey += $this->cache->getMulti( $sisterKeysMissing );
                        }
                } else {
                        // Fetch the wrapped values of the sister keys from the backend
                        $wrappedBySisterKey = $this->cache->getMulti( $allSisterKeys );
                }

                // List of "check" sister key purge timestamps to compare all value sister keys against
                $ckPurgesForAll = $this->processCheckKeys(
                        $checkSisterKeysForAll,
                        $wrappedBySisterKey,
                        $now
                );
                // Map of (base key => extra "check" sister key purge timestamp(s) to compare against)
                $ckPurgesByKey = [];
                foreach ( $checkSisterKeysByKey as $keyWithCheckKeys => $checkKeysForKey ) {
                        $ckPurgesByKey[$keyWithCheckKeys] = $this->processCheckKeys(
                                $checkKeysForKey,
                                $wrappedBySisterKey,
                                $now
                        );
                }

                // Unwrap and validate any value found for each base key (under the value sister key)
                foreach (
                        array_map( null, $valueSisterKeys, $keys )
                                as [ $valueSisterKey, $key ]
                ) {
                        if ( array_key_exists( $valueSisterKey, $wrappedBySisterKey ) ) {
                                // Key exists as either a live value or tombstone value
                                $wrapped = $wrappedBySisterKey[$valueSisterKey];
                        } else {
                                // Key does not exist
                                $wrapped = false;
                        }

                        $res = $this->unwrap( $wrapped, $now );
                        $value = $res[self::RES_VALUE];

                        foreach ( array_merge( $ckPurgesForAll, $ckPurgesByKey[$key] ?? [] ) as $ckPurge ) {
                                $res[self::RES_CHECK_AS_OF] = max(
                                        $ckPurge[self::PURGE_TIME],
                                        $res[self::RES_CHECK_AS_OF]
                                );
                                // Timestamp marking the end of the hold-off period for this purge
                                $holdoffDeadline = $ckPurge[self::PURGE_TIME] + $ckPurge[self::PURGE_HOLDOFF];
                                // Check if the value was generated during the hold-off period
                                if ( $value !== false && $holdoffDeadline >= $res[self::RES_AS_OF] ) {
                                        // How long ago this value was purged by *this* "check" key
                                        $ago = min( $ckPurge[self::PURGE_TIME] - $now, self::TINY_NEGATIVE );
                                        // How long ago this value was purged by *any* known "check" key
                                        $res[self::RES_CUR_TTL] = min( $res[self::RES_CUR_TTL], $ago );
                                }
                        }

                        if ( $touchedCb !== null && $value !== false ) {
                                $touched = $touchedCb( $value );
                                if ( $touched !== null && $touched >= $res[self::RES_AS_OF] ) {
                                        $res[self::RES_CUR_TTL] = min(
                                                $res[self::RES_CUR_TTL],
                                                $res[self::RES_AS_OF] - $touched,
                                                self::TINY_NEGATIVE
                                        );
                                }
                        } else {
                                $touched = null;
                        }

                        $res[self::RES_TOUCH_AS_OF] = max( $res[self::RES_TOUCH_AS_OF], $touched );

                        $resByKey[$key] = $res;
                }

                return $resByKey;
        }

        /**
         * @param string[] $checkSisterKeys List of "check" sister keys
         * @param mixed[] $wrappedBySisterKey Preloaded map of (sister key => wrapped value)
         * @param float $now UNIX timestamp
         * @return array[] List of purge value arrays
         */
        private function processCheckKeys(
                array $checkSisterKeys,
                array $wrappedBySisterKey,
                float $now
        ) {
                $purges = [];

                foreach ( $checkSisterKeys as $timeKey ) {
                        $purge = isset( $wrappedBySisterKey[$timeKey] )
                                ? $this->parsePurgeValue( $wrappedBySisterKey[$timeKey] )
                                : null;

                        if ( $purge === null ) {
                                // No holdoff when lazy creating a check key, use cache right away (T344191)
                                $wrapped = $this->makeCheckPurgeValue( $now, self::HOLDOFF_TTL_NONE, $purge );
                                $this->cache->add(
                                        $timeKey,
                                        $wrapped,
                                        self::CHECK_KEY_TTL,
                                        $this->cache::WRITE_BACKGROUND
                                );
                        }

                        $purges[] = $purge;
                }

                return $purges;
        }

        /**
         * 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 data was read using "view snapshots" (e.g. innodb REPEATABLE-READ),
         * 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.
         *
         * Consider using getWithSetCallback(), which has cache slam avoidance and key
         * versioning features, instead of bare get()/set() calls.
         *
         * Do not use this method on versioned keys accessed via getWithSetCallback().
         *
         * Example usage:
         * @code
         *     $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *     $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 made with makeKey()/makeGlobalKey()
         * @param mixed $value Value to set for the cache key
         * @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: Highest seconds of replication lag potentially affecting reads used to generate
         *      the value. This should not be affected by the duration of transaction "view snapshots"
         *      (e.g. innodb REPEATABLE-READ) nor the time elapsed since the first read (though both
         *      increase staleness). For reads using view snapshots, only the replication lag during
         *      snapshot initialization matters. Use false if replication is stopped/broken on a
         *      replica server involved in the reads.
         *      Default: 0 seconds
         *   - since: UNIX timestamp indicative of the highest possible staleness caused by the
         *      duration of transaction "view snapshots" (e.g. innodb REPEATABLE-READ) and the time
         *      elapsed since the first read. This should not be affected by replication lag.
         *      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 generation 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
         *   - segmentable: Allow partitioning of the value if it is a large string.
         *      Default: false.
         *   - creating: Optimize for the case where the key does not already exist.
         *      Default: false
         *   - version: Integer version number signifying the format of the value.
         *      Default: null
         *   - walltime: How long the value took to generate in seconds. Default: null
         * @phpcs:ignore Generic.Files.LineLength
         * @phan-param array{lag?:float|int,since?:float|int,pending?:bool,lockTSE?:int,staleTTL?:int,creating?:bool,version?:int,walltime?:int|float,segmentable?:bool} $opts
         * @note Options added in 1.28: staleTTL
         * @note Options added in 1.33: creating
         * @note Options added in 1.34: version, walltime
         * @note Options added in 1.40: segmentable
         * @return bool Success
         */
        final public function set( $key, $value, $ttl = self::TTL_INDEFINITE, array $opts = [] ) {
                $keygroup = $this->determineKeyGroupForStats( $key );

                $ok = $this->setMainValue(
                        $key,
                        $value,
                        $ttl,
                        $opts['version'] ?? null,
                        $opts['walltime'] ?? null,
                        $opts['lag'] ?? 0,
                        $opts['since'] ?? null,
                        $opts['pending'] ?? false,
                        $opts['lockTSE'] ?? self::TSE_NONE,
                        $opts['staleTTL'] ?? self::STALE_TTL_NONE,
                        $opts['segmentable'] ?? false,
                        $opts['creating'] ?? false
                );

                $this->stats->getCounter( 'wanobjectcache_set_total' )
                        ->setLabel( 'keygroup', $keygroup )
                        ->setLabel( 'result', ( $ok ? 'ok' : 'error' ) )
                        ->copyToStatsdAt( "wanobjectcache.$keygroup.set." . ( $ok ? 'ok' : 'error' ) )
                        ->increment();

                return $ok;
        }

        /**
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param mixed $value
         * @param int|float $ttl
         * @param int|null $version
         * @param float|null $walltime
         * @param float|int|bool $dataReplicaLag
         * @param float|int|null $dataReadSince
         * @param bool $dataPendingCommit
         * @param int $lockTSE
         * @param int $staleTTL
         * @param bool $segmentable
         * @param bool $creating
         * @return bool Success
         */
        private function setMainValue(
                $key,
                $value,
                $ttl,
                ?int $version,
                ?float $walltime,
                $dataReplicaLag,
                $dataReadSince,
                bool $dataPendingCommit,
                int $lockTSE,
                int $staleTTL,
                bool $segmentable,
                bool $creating
        ) {
                if ( $ttl < 0 ) {
                        // not cacheable
                        return true;
                }

                $now = $this->getCurrentTime();
                $ttl = (int)$ttl;
                $walltime ??= $this->timeSinceLoggedMiss( $key, $now );
                $dataSnapshotLag = ( $dataReadSince !== null ) ? max( 0, $now - $dataReadSince ) : 0;
                $dataCombinedLag = $dataReplicaLag + $dataSnapshotLag;

                // Forbid caching data that only exists within an uncommitted transaction. Also, lower
                // the TTL when the data has a "since" time so far in the past that a delete() tombstone,
                // made after that time, could have already expired (the key is no longer write-holed).
                // The mitigation TTL depends on whether this data lag is assumed to systemically effect
                // regeneration attempts in the near future. The TTL also reflects regeneration wall time.
                if ( $dataPendingCommit ) {
                        // Case A: data comes from an uncommitted write transaction
                        $mitigated = 'pending writes';
                        // Data might never be committed; rely on a less problematic regeneration attempt
                        $mitigationTTL = self::TTL_UNCACHEABLE;
                } elseif ( $dataSnapshotLag > self::MAX_READ_LAG ) {
                        // Case B: high snapshot lag
                        $pregenSnapshotLag = ( $walltime !== null ) ? ( $dataSnapshotLag - $walltime ) : 0;
                        if ( ( $pregenSnapshotLag + self::GENERATION_HIGH_SEC ) > self::MAX_READ_LAG ) {
                                // Case B1: generation started when transaction duration was already long
                                $mitigated = 'snapshot lag (late generation)';
                                // Probably non-systemic; rely on a less problematic regeneration attempt
                                $mitigationTTL = self::TTL_UNCACHEABLE;
                        } else {
                                // Case B2: slow generation made transaction duration long
                                $mitigated = 'snapshot lag (high generation time)';
                                // Probably systemic; use a low TTL to avoid stampedes/uncacheability
                                $mitigationTTL = self::TTL_LAGGED;
                        }
                } elseif ( $dataReplicaLag === false || $dataReplicaLag > self::MAX_READ_LAG ) {
                        // Case C: low/medium snapshot lag with high replication lag
                        $mitigated = 'replication lag';
                        // Probably systemic; use a low TTL to avoid stampedes/uncacheability
                        $mitigationTTL = self::TTL_LAGGED;
                } elseif ( $dataCombinedLag > self::MAX_READ_LAG ) {
                        $pregenCombinedLag = ( $walltime !== null ) ? ( $dataCombinedLag - $walltime ) : 0;
                        // Case D: medium snapshot lag with medium replication lag
                        if ( ( $pregenCombinedLag + self::GENERATION_HIGH_SEC ) > self::MAX_READ_LAG ) {
                                // Case D1: generation started when read lag was too high
                                $mitigated = 'read lag (late generation)';
                                // Probably non-systemic; rely on a less problematic regeneration attempt
                                $mitigationTTL = self::TTL_UNCACHEABLE;
                        } else {
                                // Case D2: slow generation made read lag too high
                                $mitigated = 'read lag (high generation time)';
                                // Probably systemic; use a low TTL to avoid stampedes/uncacheability
                                $mitigationTTL = self::TTL_LAGGED;
                        }
                } else {
                        // Case E: new value generated with recent data
                        $mitigated = null;
                        // Nothing to mitigate
                        $mitigationTTL = null;
                }

                if ( $mitigationTTL === self::TTL_UNCACHEABLE ) {
                        $this->logger->warning(
                                "Rejected set() for {cachekey} due to $mitigated.",
                                [
                                        'cachekey' => $key,
                                        'lag' => $dataReplicaLag,
                                        'age' => $dataSnapshotLag,
                                        'walltime' => $walltime
                                ]
                        );

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

                // TTL to use in staleness checks (does not effect persistence layer TTL)
                $logicalTTL = null;

                if ( $mitigationTTL !== null ) {
                        // New value was generated from data that is old enough to be risky
                        if ( $lockTSE >= 0 ) {
                                // Persist the value as long as normal, but make it count as stale sooner
                                $logicalTTL = min( $ttl ?: INF, $mitigationTTL );
                        } else {
                                // Persist the value for a shorter duration
                                $ttl = min( $ttl ?: INF, $mitigationTTL );
                        }

                        $this->logger->warning(
                                "Lowered set() TTL for {cachekey} due to $mitigated.",
                                [
                                        'cachekey' => $key,
                                        'lag' => $dataReplicaLag,
                                        'age' => $dataSnapshotLag,
                                        'walltime' => $walltime
                                ]
                        );
                }

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

                $flags = $this->cache::WRITE_BACKGROUND;
                if ( $segmentable ) {
                        $flags |= $this->cache::WRITE_ALLOW_SEGMENTS;
                }

                if ( $creating ) {
                        $ok = $this->cache->add(
                                $this->makeSisterKey( $key, self::TYPE_VALUE ),
                                $wrapped,
                                $storeTTL,
                                $flags
                        );
                } else {
                        $ok = $this->cache->merge(
                                $this->makeSisterKey( $key, self::TYPE_VALUE ),
                                static 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,
                                $this->cache::MAX_CONFLICTS_ONE,
                                $flags
                        );
                }

                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, user-requested purges or cache cleanup scripts might not need
         * to invoke a hold-off period on cache backfills, so they can use HOLDOFF_TTL_NONE.
         *
         * 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 made with makeKey()/makeGlobalKey()
         * @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 ) {
                // Purge values must be stored under the value key so that WANObjectCache::set()
                // can atomically merge values without accidentally undoing a recent purge and thus
                // violating the holdoff TTL restriction.
                $valueSisterKey = $this->makeSisterKey( $key, self::TYPE_VALUE );

                if ( $ttl <= 0 ) {
                        // A client or cache cleanup script is requesting a cache purge, so there is no
                        // volatility period due to replica DB lag. Any recent change to an entity cached
                        // in this key should have triggered an appropriate purge event.
                        $ok = $this->relayNonVolatilePurge( $valueSisterKey );
                } else {
                        // A cacheable entity recently changed, so there might be a volatility period due
                        // to replica DB lag. Clients usually expect their actions to be reflected in any
                        // of their subsequent web request. This is attainable if (a) purge relay lag is
                        // lower than the time it takes for subsequent request by the client to arrive,
                        // and, (b) DB replica queries have "read-your-writes" consistency due to DB lag
                        // mitigation systems.
                        $now = $this->getCurrentTime();
                        // Set the key to the purge value in all datacenters
                        $purge = $this->makeTombstonePurgeValue( $now );
                        $ok = $this->relayVolatilePurge( $valueSisterKey, $purge, $ttl );
                }

                $keygroup = $this->determineKeyGroupForStats( $key );

                $this->stats->getCounter( 'wanobjectcache_delete_total' )
                        ->setLabel( 'keygroup', $keygroup )
                        ->setLabel( 'result', ( $ok ? 'ok' : 'error' ) )
                        ->copyToStatsdAt( "wanobjectcache.$keygroup.delete." . ( $ok ? 'ok' : 'error' ) )
                        ->increment();

                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 Cache key made with makeKey()/makeGlobalKey()
         * @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 purge 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 dependent 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 Cache keys made with makeKey()/makeGlobalKey()
         * @return float[] Map of (key => UNIX timestamp)
         * @since 1.31
         */
        final public function getMultiCheckKeyTime( array $keys ) {
                $checkSisterKeysByKey = [];
                foreach ( $keys as $key ) {
                        $checkSisterKeysByKey[$key] = $this->makeSisterKey( $key, self::TYPE_TIMESTAMP );
                }

                $wrappedBySisterKey = $this->cache->getMulti( $checkSisterKeysByKey );
                $wrappedBySisterKey += array_fill_keys( $checkSisterKeysByKey, false );

                $now = $this->getCurrentTime();
                $times = [];
                foreach ( $checkSisterKeysByKey as $key => $checkSisterKey ) {
                        $purge = $this->parsePurgeValue( $wrappedBySisterKey[$checkSisterKey] );
                        if ( $purge === null ) {
                                $wrapped = $this->makeCheckPurgeValue( $now, self::HOLDOFF_TTL_NONE, $purge );
                                $this->cache->add(
                                        $checkSisterKey,
                                        $wrapped,
                                        self::CHECK_KEY_TTL,
                                        $this->cache::WRITE_BACKGROUND
                                );
                        }

                        $times[$key] = $purge[self::PURGE_TIME];
                }

                return $times;
        }

        /**
         * Increase the last-purge timestamp of a "check" key in all datacenters
         *
         * This method should only be called when some heavily referenced data changes in
         * a significant way, such that it is impractical to call delete() on all the cache
         * keys that should be purged. The get*() method calls used to fetch these keys must
         * include the given "check" key in the relevant "check" keys argument/option.
         *
         * A "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 fetched
         * using get*() calls, that include the "check" key, will be seen as purged.
         *
         * 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 made with makeKey()/makeGlobalKey()
         * @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 ) {
                $checkSisterKey = $this->makeSisterKey( $key, self::TYPE_TIMESTAMP );

                $now = $this->getCurrentTime();
                $purge = $this->makeCheckPurgeValue( $now, $holdoff );
                $ok = $this->relayVolatilePurge( $checkSisterKey, $purge, self::CHECK_KEY_TTL );

                $keygroup = $this->determineKeyGroupForStats( $key );

                $this->stats->getCounter( 'wanobjectcache_check_total' )
                        ->setLabel( 'keygroup', $keygroup )
                        ->setLabel( 'result', ( $ok ? 'ok' : 'error' ) )
                        ->copyToStatsdAt( "wanobjectcache.$keygroup.ck_touch." . ( $ok ? 'ok' : 'error' ) )
                        ->increment();

                return $ok;
        }

        /**
         * Clear the last-purge timestamp of a "check" key in all datacenters
         *
         * Similar to touchCheckKey(), in that keys fetched using get*() calls, that include
         * the given "check" key, will be seen as purged. However, there are some differences:
         *   - 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 over touchCheckKey() is that the "check" keys, which have high TTLs,
         * will only be created when a get*() method actually uses those keys. This is better
         * when a large number of "check" keys must be changed 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 made with makeKey()/makeGlobalKey()
         * @return bool True if the item was purged or not found, false on failure
         */
        final public function resetCheckKey( $key ) {
                $checkSisterKey = $this->makeSisterKey( $key, self::TYPE_TIMESTAMP );
                $ok = $this->relayNonVolatilePurge( $checkSisterKey );

                $keygroup = $this->determineKeyGroupForStats( $key );

                $this->stats->getCounter( 'wanobjectcache_reset_total' )
                        ->setLabel( 'keygroup', $keygroup )
                        ->setLabel( 'result', ( $ok ? 'ok' : 'error' ) )
                        ->copyToStatsdAt( "wanobjectcache.$keygroup.ck_reset." . ( $ok ? 'ok' : 'error' ) )
                        ->increment();

                return $ok;
        }

        /**
         * Method to fetch/regenerate a cache key
         *
         * 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:
         *   - $oldValue: prior cache value or false if none was present
         *   - &$ttl: alterable reference to the TTL to be assigned to the new value
         *   - &$setOpts: alterable reference to the set() options to be used with the new value
         *   - $oldAsOf: generation UNIX timestamp of $oldValue or null if not present (since 1.28)
         *   - $params: custom field/value map as defined by $cbParams (since 1.35)
         *
         * 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-dependent 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 )
         *
         * This applies cache server I/O stampede protection against duplicate cache sets.
         * This is important when the callback is slow and/or yields large values for a 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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *             // 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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *             // Account for any snapshot/replica DB lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             return CatConfig::newFromRow( $dbr->selectRow( ... ) );
         *         },
         *         [
         *             // Calling touchCheckKey() on this key purges 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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *             // 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 purges 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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *             // 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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *                 $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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *             // 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 with makeKey()/makeGlobalKey()
         * @param int $ttl Nominal seconds-to-live for newly computed values. 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 stale due to a purge (by "checkKeys" or "touchedCallback")
         *      less than this many seconds ago, consider reusing the stale value. The odds of a
         *      refresh become 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 value is stale and the "time since expiry" (TSE) is less than the given
         *      number of seconds ago, then reuse the stale value if another such thread is already
         *      regenerating the value. The TSE of the key is influenced by purges (e.g. via delete(),
         *      "checkKeys", "touchedCallback"), and various other options (e.g. "staleTTL"). A low
         *      enough TSE is assumed to indicate a high enough key access rate to justify stampede
         *      avoidance. Note that no cache value exists after deletion, expiration, or eviction
         *      at the storage-layer; to prevent stampedes during these cases, use "busyValue".
         *      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 purge 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.
         * @param array $cbParams Custom field/value map to pass to the callback (since 1.35)
         * @phpcs:ignore Generic.Files.LineLength
         * @phan-param array{checkKeys?:string[],graceTTL?:int,lockTSE?:int,busyValue?:mixed,pcTTL?:int,pcGroup?:string,version?:int,minAsOf?:float|int,hotTTR?:int,lowTTL?:int,ageNew?:int,staleTTL?:int,touchedCallback?:callable} $opts
         * @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 = [], array $cbParams = []
        ) {
                $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( $key, $pcTTL, false );
                        if ( $cached !== false ) {
                                $this->logger->debug( "getWithSetCallback($key): process cache hit" );
                                return $cached;
                        }
                }

                [ $value, $valueVersion, $curAsOf ] = $this->fetchOrRegenerate( $key, $ttl, $callback, $opts, $cbParams );
                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.
                        $this->logger->debug( "getWithSetCallback($key): using variant key" );
                        [ $value ] = $this->fetchOrRegenerate(
                                $this->makeGlobalKey( 'WANCache-key-variant', md5( $key ), (string)$version ),
                                $ttl,
                                $callback,
                                [ 'version' => null, 'minAsOf' => $curAsOf ] + $opts,
                                $cbParams
                        );
                }

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

                return $value;
        }

        /**
         * Do the actual I/O for getWithSetCallback() when needed
         *
         * @see WANObjectCache::getWithSetCallback()
         *
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param int $ttl
         * @param callable $callback
         * @param array $opts
         * @param array $cbParams
         * @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, array $cbParams ) {
                $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;
                $startTime = $this->getCurrentTime();

                $keygroup = $this->determineKeyGroupForStats( $key );

                // Get the current key value and its metadata
                $curState = $this->fetchKeys( [ $key ], $checkKeys, $startTime, $touchedCb )[$key];
                $curValue = $curState[self::RES_VALUE];

                // Use the cached value if it exists and is not due for synchronous regeneration
                if ( $this->isAcceptablyFreshValue( $curState, $graceTTL, $minAsOf ) ) {
                        if ( !$this->isLotteryRefreshDue( $curState, $lowTTL, $ageNew, $hotTTR, $startTime ) ) {
                                $this->stats->getTiming( 'wanobjectcache_getwithset_seconds' )
                                        ->setLabel( 'keygroup', $keygroup )
                                        ->setLabel( 'result', 'hit' )
                                        ->setLabel( 'reason', 'good' )
                                        ->copyToStatsdAt( "wanobjectcache.$keygroup.hit.good" )
                                        ->observe( 1e3 * ( $this->getCurrentTime() - $startTime ) );

                                return [ $curValue, $curState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
                        } elseif ( $this->scheduleAsyncRefresh( $key, $ttl, $callback, $opts, $cbParams ) ) {
                                $this->logger->debug( "fetchOrRegenerate($key): hit with async refresh" );

                                $this->stats->getTiming( 'wanobjectcache_getwithset_seconds' )
                                        ->setLabel( 'keygroup', $keygroup )
                                        ->setLabel( 'result', 'hit' )
                                        ->setLabel( 'reason', 'refresh' )
                                        ->copyToStatsdAt( "wanobjectcache.$keygroup.hit.refresh" )
                                        ->observe( 1e3 * ( $this->getCurrentTime() - $startTime ) );

                                return [ $curValue, $curState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
                        } else {
                                $this->logger->debug( "fetchOrRegenerate($key): hit with sync refresh" );
                        }
                }

                $isKeyTombstoned = ( $curState[self::RES_TOMB_AS_OF] !== null );
                // Use the interim key as a temporary alternative if the key is tombstoned
                if ( $isKeyTombstoned ) {
                        $volState = $this->getInterimValue( $key, $minAsOf, $startTime, $touchedCb );
                        $volValue = $volState[self::RES_VALUE];
                } else {
                        $volState = $curState;
                        $volValue = $curValue;
                }

                // During the volatile "hold-off" period that follows a purge of the key, the value
                // will be regenerated many times if frequently accessed. This is done to mitigate
                // the effects of backend replication lag as soon as possible. However, throttle the
                // overhead of locking and regeneration by reusing values recently written to cache
                // tens of milliseconds ago. Verify the "as of" time against the last purge event.
                $lastPurgeTime = max(
                        // RES_TOUCH_AS_OF depends on the value (possibly from the interim key)
                        $volState[self::RES_TOUCH_AS_OF],
                        $curState[self::RES_TOMB_AS_OF],
                        $curState[self::RES_CHECK_AS_OF]
                );
                $safeMinAsOf = max( $minAsOf, $lastPurgeTime + self::TINY_POSITIVE );
                if ( $this->isExtremelyNewValue( $volState, $safeMinAsOf, $startTime ) ) {
                        $this->logger->debug( "fetchOrRegenerate($key): volatile hit" );

                        $this->stats->getTiming( 'wanobjectcache_getwithset_seconds' )
                                ->setLabel( 'keygroup', $keygroup )
                                ->setLabel( 'result', 'hit' )
                                ->setLabel( 'reason', 'volatile' )
                                ->copyToStatsdAt( "wanobjectcache.$keygroup.hit.volatile" )
                                ->observe( 1e3 * ( $this->getCurrentTime() - $startTime ) );

                        return [ $volValue, $volState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
                }

                $lockTSE = $opts['lockTSE'] ?? self::TSE_NONE;
                $busyValue = $opts['busyValue'] ?? null;
                $staleTTL = $opts['staleTTL'] ?? self::STALE_TTL_NONE;
                $segmentable = $opts['segmentable'] ?? false;
                $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 purge.
                        // This avoids stampedes when timestamps from $checkKeys/$touchedCb bump.
                        (
                                $curState[self::RES_CUR_TTL] !== null &&
                                $curState[self::RES_CUR_TTL] <= 0 &&
                                abs( $curState[self::RES_CUR_TTL] ) <= $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 && $volValue === 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 ) {
                        // Determine if there is stale or volatile cached value that is still usable
                        // @phan-suppress-next-line PhanTypeMismatchArgumentNullable False positive
                        if ( $this->isValid( $volValue, $volState[self::RES_AS_OF], $minAsOf ) ) {
                                $this->logger->debug( "fetchOrRegenerate($key): returning stale value" );

                                $this->stats->getTiming( 'wanobjectcache_getwithset_seconds' )
                                        ->setLabel( 'keygroup', $keygroup )
                                        ->setLabel( 'result', 'hit' )
                                        ->setLabel( 'reason', 'stale' )
                                        ->copyToStatsdAt( "wanobjectcache.$keygroup.hit.stale" )
                                        ->observe( 1e3 * ( $this->getCurrentTime() - $startTime ) );

                                return [ $volValue, $volState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
                        } elseif ( $busyValue !== null ) {
                                $miss = is_infinite( $minAsOf ) ? 'renew' : 'miss';
                                $this->logger->debug( "fetchOrRegenerate($key): busy $miss" );

                                $this->stats->getTiming( 'wanobjectcache_getwithset_seconds' )
                                        ->setLabel( 'keygroup', $keygroup )
                                        ->setLabel( 'result', $miss )
                                        ->setLabel( 'reason', 'busy' )
                                        ->copyToStatsdAt( "wanobjectcache.$keygroup.$miss.busy" )
                                        ->observe( 1e3 * ( $this->getCurrentTime() - $startTime ) );

                                $placeholderValue = $this->resolveBusyValue( $busyValue );

                                return [ $placeholderValue, $version, $curState[self::RES_AS_OF] ];
                        }
                }

                // Generate the new value given any prior value with a matching version
                $setOpts = [];
                $preCallbackTime = $this->getCurrentTime();
                ++$this->callbackDepth;
                // https://github.com/phan/phan/issues/4419
                $value = null;
                try {
                        $value = $callback(
                                ( $curState[self::RES_VERSION] === $version ) ? $curValue : false,
                                $ttl,
                                $setOpts,
                                ( $curState[self::RES_VERSION] === $version ) ? $curState[self::RES_AS_OF] : null,
                                $cbParams
                        );
                } finally {
                        --$this->callbackDepth;
                }
                $postCallbackTime = $this->getCurrentTime();

                // How long it took to generate the value
                $walltime = max( $postCallbackTime - $preCallbackTime, 0.0 );

                $this->stats->getTiming( 'wanobjectcache_regen_seconds' )
                        ->setLabel( 'keygroup', $keygroup )
                        ->copyToStatsdAt( "wanobjectcache.$keygroup.regen_walltime" )
                        ->observe( 1e3 * $walltime );

                // 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 )
                ) {
                        // If the key is write-holed then use the (volatile) interim key as an alternative
                        if ( $isKeyTombstoned ) {
                                $this->setInterimValue(
                                        $key,
                                        $value,
                                        $lockTSE,
                                        $version,
                                        $segmentable
                                );
                        } else {
                                $this->setMainValue(
                                        $key,
                                        $value,
                                        $ttl,
                                        $version,
                                        $walltime,
                                        // @phan-suppress-next-line PhanCoalescingAlwaysNull
                                        $setOpts['lag'] ?? 0,
                                        // @phan-suppress-next-line PhanCoalescingAlwaysNull
                                        $setOpts['since'] ?? $preCallbackTime,
                                        // @phan-suppress-next-line PhanCoalescingAlwaysNull
                                        $setOpts['pending'] ?? false,
                                        $lockTSE,
                                        $staleTTL,
                                        $segmentable,
                                        ( $curValue === false )
                                );
                        }
                }

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

                $miss = is_infinite( $minAsOf ) ? 'renew' : 'miss';
                $this->logger->debug( "fetchOrRegenerate($key): $miss, new value computed" );

                $this->stats->getTiming( 'wanobjectcache_getwithset_seconds' )
                        ->setLabel( 'keygroup', $keygroup )
                        ->setLabel( 'result', $miss )
                        ->setLabel( 'reason', 'compute' )
                        ->copyToStatsdAt( "wanobjectcache.$keygroup.$miss.compute" )
                        ->observe( 1e3 * ( $this->getCurrentTime() - $startTime ) );

                return [ $value, $version, $curState[self::RES_AS_OF] ];
        }

        /**
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @return bool Success
         */
        private function claimStampedeLock( $key ) {
                $checkSisterKey = $this->makeSisterKey( $key, self::TYPE_MUTEX );
                // Note that locking is not bypassed due to I/O errors; this avoids stampedes
                return $this->cache->add( $checkSisterKey, 1, self::LOCK_TTL );
        }

        /**
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param bool $hasLock
         */
        private function yieldStampedeLock( $key, $hasLock ) {
                if ( $hasLock ) {
                        $checkSisterKey = $this->makeSisterKey( $key, self::TYPE_MUTEX );
                        $this->cache->delete( $checkSisterKey, $this->cache::WRITE_BACKGROUND );
                }
        }

        /**
         * Get sister keys that should be collocated with their corresponding base cache keys
         *
         * The key will bear the WANCache prefix and use the configured coalescing scheme
         *
         * @param string[] $baseKeys Cache keys made with makeKey()/makeGlobalKey()
         * @param string $type Consistent hashing agnostic suffix character matching [a-zA-Z]
         * @param string|null $route Routing prefix (optional)
         * @return string[] Order-corresponding list of sister keys
         */
        private function makeSisterKeys( array $baseKeys, string $type, ?string $route = null ) {
                $sisterKeys = [];
                foreach ( $baseKeys as $baseKey ) {
                        $sisterKeys[] = $this->makeSisterKey( $baseKey, $type, $route );
                }

                return $sisterKeys;
        }

        /**
         * Get a sister key that should be collocated with a base cache key
         *
         * The keys will bear the WANCache prefix and use the configured coalescing scheme
         *
         * @param string $baseKey Cache key made with makeKey()/makeGlobalKey()
         * @param string $typeChar Consistent hashing agnostic suffix character matching [a-zA-Z]
         * @param string|null $route Routing prefix (optional)
         * @return string Sister key
         */
        private function makeSisterKey( string $baseKey, string $typeChar, ?string $route = null ) {
                if ( $this->coalesceScheme === self::SCHEME_HASH_STOP ) {
                        // Key style: "WANCache:<base key>|#|<character>"
                        $sisterKey = 'WANCache:' . $baseKey . '|#|' . $typeChar;
                } else {
                        // Key style: "WANCache:{<base key>}:<character>"
                        $sisterKey = 'WANCache:{' . $baseKey . '}:' . $typeChar;
                }

                if ( $route !== null ) {
                        $sisterKey = $this->prependRoute( $sisterKey, $route );
                }

                return $sisterKey;
        }

        /**
         * Check if a key value is non-false, new enough, and has an "as of" time almost equal to now
         *
         * If the value was just written to cache, and it did not take an unusually long time to
         * generate, then it is probably not worth regenerating yet. For example, replica databases
         * might still return lagged pre-purge values anyway.
         *
         * @param array $res Current value WANObjectCache::RES_* data map
         * @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
         * @param float $now Current UNIX timestamp
         * @return bool Whether the age of a volatile value is negligible
         */
        private function isExtremelyNewValue( $res, $minAsOf, $now ) {
                if ( $res[self::RES_VALUE] === false || $res[self::RES_AS_OF] < $minAsOf ) {
                        return false;
                }

                $age = $now - $res[self::RES_AS_OF];

                return ( $age < mt_rand( self::RECENT_SET_LOW_MS, self::RECENT_SET_HIGH_MS ) / 1e3 );
        }

        /**
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
         * @param float $now Fetch time to determine "age" metadata
         * @param callable|null $touchedCb Function to find the max "dependency touched" UNIX timestamp
         * @return array<int,mixed> Result map/n-tuple from unwrap()
         * @phan-return array{0:mixed,1:mixed,2:?float,3:?int,4:?float,5:?float,6:?float,7:?float}
         * @note Callable type hints are not used to avoid class-autoloading
         */
        private function getInterimValue( $key, $minAsOf, $now, $touchedCb ) {
                if ( $this->useInterimHoldOffCaching ) {
                        $interimSisterKey = $this->makeSisterKey( $key, self::TYPE_INTERIM );
                        $wrapped = $this->cache->get( $interimSisterKey );
                        $res = $this->unwrap( $wrapped, $now );
                        if ( $res[self::RES_VALUE] !== false && $res[self::RES_AS_OF] >= $minAsOf ) {
                                if ( $touchedCb !== null ) {
                                        // Update "last purge time" since the $touchedCb timestamp depends on $value
                                        // Get the new "touched timestamp", accounting for callback-checked dependencies
                                        $res[self::RES_TOUCH_AS_OF] = max(
                                                $touchedCb( $res[self::RES_VALUE] ),
                                                $res[self::RES_TOUCH_AS_OF]
                                        );
                                }

                                return $res;
                        }
                }

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

        /**
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param mixed $value
         * @param int|float $ttl
         * @param int|null $version Value version number
         * @param bool $segmentable
         * @return bool Success
         */
        private function setInterimValue(
                $key,
                $value,
                $ttl,
                ?int $version,
                bool $segmentable
        ) {
                $now = $this->getCurrentTime();
                $ttl = max( self::INTERIM_KEY_TTL, (int)$ttl );

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

                $flags = $this->cache::WRITE_BACKGROUND;
                if ( $segmentable ) {
                        $flags |= $this->cache::WRITE_ALLOW_SEGMENTS;
                }

                return $this->cache->set(
                        $this->makeSisterKey( $key, self::TYPE_INTERIM ),
                        $wrapped,
                        $ttl,
                        $flags
                );
        }

        /**
         * @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 must be the result of WANObjectCache::makeMultiKeys()
         *   - b) The $callback argument expects a function that returns an entity value, using
         *        boolean "false" if it does not exist. The callback takes the following arguments:
         *         - $id: ID of the entity to query
         *         - $oldValue: prior cache value or false if none was present
         *         - &$ttl: reference to the TTL to be assigned to the new value (alterable)
         *         - &$setOpts: reference to the new value set() options (alterable)
         *         - $oldAsOf: generation UNIX timestamp of $oldValue or null if not present
         *   - 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, $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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *             // 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 that yields entity generation callbacks
         * @param array $opts Options map similar to that of getWithSetCallback()
         * @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 = []
        ) {
                // Batch load required keys into the in-process warmup cache
                $this->warmupCache = $this->fetchWrappedValuesForWarmupCache(
                        $this->getNonProcessCachedMultiKeys( $keyedIds, $opts ),
                        $opts['checkKeys'] ?? []
                );
                $this->warmupKeyMisses = 0;

                // The required callback signature includes $id as the first argument for convenience
                // to distinguish different items. To reuse the code in getWithSetCallback(), wrap the
                // callback with a proxy callback that has the standard getWithSetCallback() signature.
                // This is defined only once per batch to avoid closure creation overhead.
                $proxyCb = static function ( $oldValue, &$ttl, &$setOpts, $oldAsOf, $params )
                        use ( $callback )
                {
                        return $callback( $params['id'], $oldValue, $ttl, $setOpts, $oldAsOf );
                };

                // Get the order-preserved result map using the warm-up cache
                $values = [];
                foreach ( $keyedIds as $key => $id ) {
                        $values[$key] = $this->getWithSetCallback(
                                $key,
                                $ttl,
                                $proxyCb,
                                $opts,
                                [ 'id' => $id ]
                        );
                }

                $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 function that returns a map of (ID => new value),
         *        using boolean "false" for entities that could not be found, for all entity IDs in
         *        $ids. The callback takes the following arguments:
         *          - $ids: list of entity IDs that require value generation
         *          - &$ttls: reference to the (entity ID => new TTL) map (alterable)
         *          - &$setOpts: reference to the new value set() options (alterable)
         *   - 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 = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
         *             // Account for any snapshot/replica DB lag
         *             $setOpts += Database::getCacheSetOptions( $dbr );
         *
         *             // Load the rows for these files
         *             $rows = array_fill_keys( $ids, false );
         *             $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 that yields entity generation callbacks
         * @param array $opts Options map similar to that of getWithSetCallback()
         * @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'] ?? [];
                $minAsOf = $opts['minAsOf'] ?? self::MIN_TIMESTAMP_NONE;

                // unset incompatible keys
                unset( $opts['lockTSE'] );
                unset( $opts['busyValue'] );

                // Batch load required keys into the in-process warmup cache
                $keysByIdGet = $this->getNonProcessCachedMultiKeys( $keyedIds, $opts );
                $this->warmupCache = $this->fetchWrappedValuesForWarmupCache( $keysByIdGet, $checkKeys );
                $this->warmupKeyMisses = 0;

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

                // Find out which keys are missing/deleted/stale
                $now = $this->getCurrentTime();
                $resByKey = $this->fetchKeys( $keysByIdGet, $checkKeys, $now );
                foreach ( $keysByIdGet as $id => $key ) {
                        $res = $resByKey[$key];
                        if (
                                $res[self::RES_VALUE] === false ||
                                $res[self::RES_CUR_TTL] < 0 ||
                                $res[self::RES_AS_OF] < $minAsOf
                        ) {
                                $idsRegen[] = $id;
                        }
                }

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

                $method = __METHOD__;
                // The required callback signature includes $id as the first argument for convenience
                // to distinguish different items. To reuse the code in getWithSetCallback(), wrap the
                // callback with a proxy callback that has the standard getWithSetCallback() signature.
                // This is defined only once per batch to avoid closure creation overhead.
                $proxyCb = function ( $oldValue, &$ttl, &$setOpts, $oldAsOf, $params )
                        use ( $callback, $newValsById, $newTTLsById, $newSetOpts, $method )
                {
                        $id = $params['id'];

                        if ( array_key_exists( $id, $newValsById ) ) {
                                // Value was already regenerated 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 ];
                                $result = $callback( [ $id ], $ttls, $setOpts );
                                if ( !isset( $result[$id] ) ) {
                                        // T303092
                                        $this->logger->warning(
                                                $method . ' failed due to {id} not set in result {result}', [
                                                        'id' => $id,
                                                        'result' => json_encode( $result )
                                                ] );
                                }
                                $newValue = $result[$id];
                                $ttl = $ttls[$id];
                        }

                        return $newValue;
                };

                // Get the order-preserved result map using the warm-up cache
                $values = [];
                foreach ( $keyedIds as $key => $id ) {
                        $values[$key] = $this->getWithSetCallback(
                                $key,
                                $ttl,
                                $proxyCb,
                                $opts,
                                [ 'id' => $id ]
                        );
                }

                $this->warmupCache = [];

                return $values;
        }

        /**
         * @see BagOStuff::makeGlobalKey()
         * @since 1.27
         * @param string $keygroup Key group component, should be under 48 characters.
         * @param string|int ...$components Additional, ordered, key components for entity IDs
         * @return string Colon-separated, keyspace-prepended, ordered list of encoded components
         */
        public function makeGlobalKey( $keygroup, ...$components ) {
                return $this->cache->makeGlobalKey( $keygroup, ...$components );
        }

        /**
         * @see BagOStuff::makeKey()
         * @since 1.27
         * @param string $keygroup Key group component, should be under 48 characters.
         * @param string|int ...$components Additional, ordered, key components for entity IDs
         * @return string Colon-separated, keyspace-prepended, ordered list of encoded components
         */
        public function makeKey( $keygroup, ...$components ) {
                return $this->cache->makeKey( $keygroup, ...$components );
        }

        /**
         * 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 argument expects a function that returns the key for an entity ID 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. Use
         * the hash256() method for any hashing. The callback takes the following arguments:
         *   - $id: An entity ID
         *   - $cache: This WANObjectCache instance
         *
         * Example usage for the default keyspace:
         * @code
         *     $keyedIds = $cache->makeMultiKeys(
         *         $modules,
         *         function ( $module, $cache ) {
         *             return $cache->makeKey( 'example-module', $module );
         *         }
         *     );
         * @endcode
         *
         * Example usage for mixed default and global keyspace:
         * @code
         *     $keyedIds = $cache->makeMultiKeys(
         *         $filters,
         *         function ( $filter, $cache ) {
         *             return self::isCentral( $filter )
         *                 ? $cache->makeGlobalKey( 'example-filter', $filter )
         *                 : $cache->makeKey( 'example-filter', $filter )
         *         }
         *     );
         * @endcode
         *
         * Example usage with hashing:
         * @code
         *     $keyedIds = $cache->makeMultiKeys(
         *         $urls,
         *         function ( $url, $cache ) {
         *             return $cache->makeKey( 'example-url', $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
         * @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 utilize 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_fill_keys( $ids, true ) );
                        if ( count( $ids ) !== count( $res ) ) {
                                throw new UnexpectedValueException( "Multi-key result does not match ID list" );
                        }
                }

                return array_combine( $ids, $res );
        }

        /**
         * Get a "watch point" token that can be used to get the "last error" to occur after now
         *
         * @return int A token that the current error event
         * @since 1.38
         */
        public function watchErrors() {
                return $this->cache->watchErrors();
        }

        /**
         * Get the "last error" registry
         *
         * The method should be invoked by a caller as part of the following pattern:
         *   - The caller invokes watchErrors() to get a "since token"
         *   - The caller invokes a sequence of cache operation methods
         *   - The caller invokes getLastError() with the "since token"
         *
         * External callers can also invoke this method as part of the following pattern:
         *   - The caller invokes clearLastError()
         *   - The caller invokes a sequence of cache operation methods
         *   - The caller invokes getLastError()
         *
         * @param int $watchPoint Only consider errors from after this "watch point" [optional]
         * @return int BagOStuff:ERR_* constant for the "last error" registry
         * @note Parameters added in 1.38: $watchPoint
         */
        final public function getLastError( $watchPoint = 0 ) {
                $code = $this->cache->getLastError( $watchPoint );
                switch ( $code ) {
                        case self::ERR_NONE:
                                return self::ERR_NONE;
                        case self::ERR_NO_RESPONSE:
                                return self::ERR_NO_RESPONSE;
                        case self::ERR_UNREACHABLE:
                                return self::ERR_UNREACHABLE;
                        default:
                                return self::ERR_UNEXPECTED;
                }
        }

        /**
         * Clear the "last error" registry
         * @deprecated Since 1.38, hard deprecated in 1.43
         */
        final public function clearLastError() {
                wfDeprecated( __METHOD__, '1.38' );
                $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|string|null $mtime UNIX timestamp; null if none
         * @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 ) {
                // handle fractional seconds and string integers
                $mtime = (int)$mtime;
                if ( $mtime <= 0 ) {
                        // no last-modified time provided
                        return $minTTL;
                }

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

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

        /**
         * @internal For use by unit tests only
         * @return int
         * @since 1.30
         */
        final public function getWarmupKeyMisses() {
                // Number of misses in $this->warmupCache during the last call to certain methods
                return $this->warmupKeyMisses;
        }

        /**
         * Set a sister key to a purge value in all datacenters
         *
         * This method should not wait for the operation to complete on remote datacenters
         *
         * Since older purge values can sometimes arrive after newer ones, use a relative expiry
         * so that even if the older value replaces the newer value, the TTL will greater than the
         * remaining TTL on the older value (assuming that all purges for a key use the same TTL).
         *
         * @param string $sisterKey A value key or "check" key
         * @param string $purgeValue Result of makeTombstonePurgeValue()/makeCheckKeyPurgeValue()
         * @param int $ttl Seconds to keep the purge value around
         * @return bool Success
         */
        protected function relayVolatilePurge( string $sisterKey, string $purgeValue, int $ttl ) {
                if ( $this->broadcastRoute !== null ) {
                        $routeKey = $this->prependRoute( $sisterKey, $this->broadcastRoute );
                } else {
                        $routeKey = $sisterKey;
                }

                return $this->cache->set(
                        $routeKey,
                        $purgeValue,
                        $ttl,
                        $this->cache::WRITE_BACKGROUND
                );
        }

        /**
         * Remove a sister key from all datacenters
         *
         * This method should not wait for the operation to complete on remote datacenters
         *
         * @param string $sisterKey A value key or "check" key
         * @return bool Success
         */
        protected function relayNonVolatilePurge( string $sisterKey ) {
                if ( $this->broadcastRoute !== null ) {
                        $routeKey = $this->prependRoute( $sisterKey, $this->broadcastRoute );
                } else {
                        $routeKey = $sisterKey;
                }

                return $this->cache->delete( $routeKey, $this->cache::WRITE_BACKGROUND );
        }

        /**
         * @param string $sisterKey
         * @param string $route Key routing prefix
         * @return string
         */
        protected function prependRoute( string $sisterKey, string $route ) {
                if ( $sisterKey[0] === '/' ) {
                        throw new RuntimeException( "Sister key '$sisterKey' already contains a route." );
                }

                return $route . $sisterKey;
        }

        /**
         * Schedule a deferred cache regeneration if possible
         *
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param int $ttl Seconds to live
         * @param callable $callback
         * @param array $opts
         * @param array $cbParams
         * @return bool Success
         * @note Callable type hints are not used to avoid class-autoloading
         */
        private function scheduleAsyncRefresh( $key, $ttl, $callback, array $opts, array $cbParams ) {
                if ( !$this->asyncHandler ) {
                        return false;
                }
                // Update the cache value later, such during post-send of an HTTP request. This forces
                // cache regeneration by setting "minAsOf" to infinity, meaning that no existing value
                // is considered valid. Furthermore, note that preemptive regeneration is not applicable
                // to invalid values, so there is no risk of infinite preemptive regeneration loops.
                $func = $this->asyncHandler;
                $func( function () use ( $key, $ttl, $callback, $opts, $cbParams ) {
                        $opts['minAsOf'] = INF;
                        try {
                                $this->fetchOrRegenerate( $key, $ttl, $callback, $opts, $cbParams );
                        } catch ( Exception $e ) {
                                // Log some context for easier debugging
                                $this->logger->error( 'Async refresh failed for {key}', [
                                        'key' => $key,
                                        'ttl' => $ttl,
                                        'exception' => $e
                                ] );
                                throw $e;
                        }
                } );

                return true;
        }

        /**
         * Check if a key value is non-false, new enough, and either fresh or "gracefully" stale
         *
         * @param array $res Current value WANObjectCache::RES_* data map
         * @param int $graceTTL Consider using stale values if $curTTL is greater than this
         * @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
         * @return bool
         */
        private function isAcceptablyFreshValue( $res, $graceTTL, $minAsOf ) {
                if ( !$this->isValid( $res[self::RES_VALUE], $res[self::RES_AS_OF], $minAsOf ) ) {
                        // Value does not exists or is too old
                        return false;
                }

                $curTTL = $res[self::RES_CUR_TTL];
                if ( $curTTL > 0 ) {
                        // Value is definitely still fresh
                        return true;
                }

                // Remaining seconds during which this stale value can be used
                $curGraceTTL = $graceTTL + $curTTL;

                return ( $curGraceTTL > 0 )
                        // Chance of using the value decreases as $curTTL goes from 0 to -$graceTTL
                        ? !$this->worthRefreshExpiring( $curGraceTTL, $graceTTL, $graceTTL )
                        // Value is too stale to fall in the grace period
                        : false;
        }

        /**
         * Check if a key is due for randomized regeneration due to near-expiration/popularity
         *
         * @param array $res Current value WANObjectCache::RES_* data map
         * @param float $lowTTL Consider a refresh when $curTTL is less than this; the "low" threshold
         * @param int $ageNew Age of key when this might recommend refreshing (seconds)
         * @param int $hotTTR Age of key when it should be refreshed if popular (seconds)
         * @param float $now The current UNIX timestamp
         * @return bool
         */
        protected function isLotteryRefreshDue( $res, $lowTTL, $ageNew, $hotTTR, $now ) {
                $curTTL = $res[self::RES_CUR_TTL];
                $logicalTTL = $res[self::RES_TTL];
                $asOf = $res[self::RES_AS_OF];

                return (
                        $this->worthRefreshExpiring( $curTTL, $logicalTTL, $lowTTL ) ||
                        $this->worthRefreshPopular( $asOf, $ageNew, $hotTTR, $now )
                );
        }

        /**
         * 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, 1_000_000_000 ) <= 1_000_000_000 * $chance );
        }

        /**
         * Check if a key is nearing expiration and thus due for randomized regeneration
         *
         * If $curTTL is greater than the "low" threshold (e.g. not nearing expiration) then this
         * returns false. If $curTTL <= 0 (e.g. value already expired), then this returns false.
         * Otherwise, the chance of this returning true increases steadily from 0% to 100% as
         * $curTTL moves from the "low" threshold down to 0 seconds.
         *
         * The logical TTL will be used as the "low" threshold if it is less than $lowTTL.
         *
         * This method uses deadline-aware randomization in order to handle wide variations
         * of cache access traffic without the need for configuration or expensive state.
         *
         * @param float $curTTL Approximate TTL left on the key
         * @param float $logicalTTL Full logical TTL assigned to the key; 0 for "infinite"
         * @param float $lowTTL Consider a refresh when $curTTL is less than this; the "low" threshold
         * @return bool
         */
        protected function worthRefreshExpiring( $curTTL, $logicalTTL, $lowTTL ) {
                if ( $lowTTL <= 0 ) {
                        return false;
                }
                // T264787: avoid having keys start off with a high chance of being refreshed;
                // the point where refreshing becomes possible cannot precede the key lifetime.
                $effectiveLowTTL = min( $lowTTL, $logicalTTL ?: INF );

                // How long the value was in the "low TTL" phase
                $timeOld = $effectiveLowTTL - $curTTL;
                if ( $timeOld <= 0 || $timeOld >= $effectiveLowTTL ) {
                        return false;
                }

                // Ratio of the low TTL phase that has elapsed (r)
                $ttrRatio = $timeOld / $effectiveLowTTL;
                // Use p(r) as the monotonically increasing "chance of refresh" function,
                // having p(0)=0 and p(1)=1. The value expires at the nominal expiry.
                $chance = $ttrRatio ** 4;

                return ( mt_rand( 1, 1_000_000_000 ) <= 1_000_000_000 * $chance );
        }

        /**
         * Check that a wrapper value exists and has an acceptable age
         *
         * @param array|false $value Value wrapper or false
         * @param float $asOf Value generation "as of" timestamp
         * @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
         * @return bool
         */
        protected function isValid( $value, $asOf, $minAsOf ) {
                return ( $value !== false && $asOf >= $minAsOf );
        }

        /**
         * @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()
         * @return array
         */
        private function wrap( $value, $ttl, $version, $now ) {
                // 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;
                }

                return $wrapped;
        }

        /**
         * @param array|string|false $wrapped The entry at a cache key (false if key is nonexistant)
         * @param float $now Unix Current timestamp (preferably pre-query)
         * @return array<int,mixed> Result map/n-tuple that includes the following:
         *   - WANObjectCache::RES_VALUE: value or false if absent/tombstoned/malformed
         *   - WANObjectCache::KEY_VERSION: value version number; null if there is no value
         *   - WANObjectCache::KEY_AS_OF: value generation timestamp (UNIX); null if there is no value
         *   - WANObjectCache::KEY_TTL: assigned logical TTL (seconds); null if there is no value
         *   - WANObjectCache::KEY_TOMB_AS_OF: tombstone timestamp (UNIX); null if not tombstoned
         *   - WANObjectCache::RES_CHECK_AS_OF: null placeholder for highest "check" key timestamp
         *   - WANObjectCache::RES_TOUCH_AS_OF: null placeholder for highest "touched" timestamp
         *   - WANObjectCache::KEY_CUR_TTL: remaining logical TTL (seconds) (negative if tombstoned)
         * @phan-return array{0:mixed,1:mixed,2:?float,3:?int,4:?float,5:?float,6:?float,7:?float}
         */
        private function unwrap( $wrapped, $now ) {
                // https://nikic.github.io/2014/12/22/PHPs-new-hashtable-implementation.html
                $res = [
                        // Attributes that only depend on the fetched key value
                        self::RES_VALUE => false,
                        self::RES_VERSION => null,
                        self::RES_AS_OF => null,
                        self::RES_TTL => null,
                        self::RES_TOMB_AS_OF => null,
                        // Attributes that depend on caller-specific "check" keys or "touched callbacks"
                        self::RES_CHECK_AS_OF => null,
                        self::RES_TOUCH_AS_OF => null,
                        self::RES_CUR_TTL => 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;
                                }
                                $res[self::RES_VALUE] = $wrapped[self::FLD_VALUE];
                                $res[self::RES_VERSION] = $wrapped[self::FLD_VALUE_VERSION] ?? null;
                                $res[self::RES_AS_OF] = $wrapped[self::FLD_TIME];
                                $res[self::RES_CUR_TTL] = $curTTL;
                                $res[self::RES_TTL] = $wrapped[self::FLD_TTL];
                        }
                } else {
                        // Entry expected to be a tombstone; parse it
                        $purge = $this->parsePurgeValue( $wrapped );
                        if ( $purge !== null ) {
                                // Tombstoned keys should always have a negative "current TTL"
                                $curTTL = min( $purge[self::PURGE_TIME] - $now, self::TINY_NEGATIVE );
                                $res[self::RES_CUR_TTL] = $curTTL;
                                $res[self::RES_TOMB_AS_OF] = $purge[self::PURGE_TIME];
                        }
                }

                return $res;
        }

        /**
         * @param string $key Cache key in the format `<keyspace>:<keygroup>[:<other components>]...`
         *  as formatted by WANObjectCache::makeKey() or ::makeKeyGlobal.
         * @return string The key group of this cache key
         */
        private function determineKeyGroupForStats( $key ) {
                $parts = explode( ':', $key, 3 );
                // Fallback in case the key was not made by makeKey.
                // Replace dots because they are special in StatsD (T232907)
                return strtr( $parts[1] ?? $parts[0], '.', '_' );
        }

        /**
         * Extract purge metadata from cached value if it is a valid purge value
         *
         * Valid purge values come from makeTombstonePurgeValue()/makeCheckKeyPurgeValue()
         *
         * @param mixed $value Cached value
         * @return array|null Tuple of (UNIX timestamp, hold-off seconds); null if value is invalid
         */
        private function parsePurgeValue( $value ) {
                if ( !is_string( $value ) ) {
                        return null;
                }

                $segments = explode( ':', $value, 3 );
                $prefix = $segments[0];
                if ( $prefix !== self::PURGE_VAL_PREFIX ) {
                        // Not a purge value
                        return null;
                }

                $timestamp = (float)$segments[1];
                // makeTombstonePurgeValue() doesn't store hold-off TTLs
                $holdoff = isset( $segments[2] ) ? (int)$segments[2] : self::HOLDOFF_TTL;

                if ( $timestamp < $this->epoch ) {
                        // Purge value is too old
                        return null;
                }

                return [ self::PURGE_TIME => $timestamp, self::PURGE_HOLDOFF => $holdoff ];
        }

        /**
         * @param float $timestamp UNIX timestamp
         * @return string Wrapped purge value; format is "PURGED:<timestamp>"
         */
        private function makeTombstonePurgeValue( float $timestamp ) {
                return self::PURGE_VAL_PREFIX . ':' . (int)$timestamp;
        }

        /**
         * @param float $timestamp UNIX timestamp
         * @param int $holdoff In seconds
         * @param array|null &$purge Unwrapped purge value array [returned]
         * @return string Wrapped purge value; format is "PURGED:<timestamp>:<holdoff>"
         */
        private function makeCheckPurgeValue( float $timestamp, int $holdoff, ?array &$purge = null ) {
                $normalizedTime = (int)$timestamp;
                // Purge array that matches what parsePurgeValue() would have returned
                $purge = [ self::PURGE_TIME => (float)$normalizedTime, self::PURGE_HOLDOFF => $holdoff ];

                return self::PURGE_VAL_PREFIX . ":$normalizedTime:$holdoff";
        }

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

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

        /**
         * @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 ) {
                        $pCache = $this->getProcessCache( $opts['pcGroup'] ?? self::PC_PRIMARY );
                        foreach ( $keys as $key => $id ) {
                                if ( !$pCache->has( $key, $pcTTL ) ) {
                                        $keysMissing[$id] = $key;
                                }
                        }
                }

                return $keysMissing;
        }

        /**
         * @param string[] $keys Cache keys made with makeKey()/makeGlobalKey()
         * @param string[]|string[][] $checkKeys Map of (integer or cache key => "check" key(s));
         *  "check" keys must also be made with makeKey()/makeGlobalKey()
         * @return array<string,mixed> Map of (sister key => value, or, false if not found)
         */
        private function fetchWrappedValuesForWarmupCache( array $keys, array $checkKeys ) {
                if ( !$keys ) {
                        return [];
                }

                // Get all the value keys to fetch...
                $sisterKeys = $this->makeSisterKeys( $keys, self::TYPE_VALUE );
                // Get all the "check" keys to fetch...
                foreach ( $checkKeys as $i => $checkKeyOrKeyGroup ) {
                        // Note: avoid array_merge() inside loop in case there are many keys
                        if ( is_int( $i ) ) {
                                // Single "check" key that applies to all value keys
                                $sisterKeys[] = $this->makeSisterKey( $checkKeyOrKeyGroup, self::TYPE_TIMESTAMP );
                        } else {
                                // List of "check" keys that apply to a specific value key
                                foreach ( (array)$checkKeyOrKeyGroup as $checkKey ) {
                                        $sisterKeys[] = $this->makeSisterKey( $checkKey, self::TYPE_TIMESTAMP );
                                }
                        }
                }

                $wrappedBySisterKey = $this->cache->getMulti( $sisterKeys );
                $wrappedBySisterKey += array_fill_keys( $sisterKeys, false );

                return $wrappedBySisterKey;
        }

        /**
         * @param string $key Cache key made with makeKey()/makeGlobalKey()
         * @param float $now Current UNIX timestamp
         * @return float|null Seconds since the last logged get() miss for this key, or, null
         */
        private function timeSinceLoggedMiss( $key, $now ) {
                // phpcs:ignore Generic.CodeAnalysis.AssignmentInCondition.Found
                for ( end( $this->missLog ); $miss = current( $this->missLog ); prev( $this->missLog ) ) {
                        if ( $miss[0] === $key ) {
                                return ( $now - $miss[1] );
                        }
                }

                return null;
        }

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

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

/** @deprecated class alias since 1.43 */
class_alias( WANObjectCache::class, 'WANObjectCache' );