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

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

use Wikimedia\WaitConditionLoop;

/**
 * Storage medium specific cache for storing items (e.g. redis, memcached, ...)
 *
 * This should not be used for proxy classes that simply wrap other cache instances
 *
 * @ingroup Cache
 * @since 1.34
 */
abstract class MediumSpecificBagOStuff extends BagOStuff {
        /** @var array[] Lock tracking */
        protected $locks = [];
        /** @var int ERR_* class constant */
        protected $lastError = self::ERR_NONE;
        /** @var string */
        protected $keyspace = 'local';
        /** @var int Seconds */
        protected $syncTimeout;
        /** @var int Bytes; chunk size of segmented cache values */
        protected $segmentationSize;
        /** @var int Bytes; maximum total size of a segmented cache value */
        protected $segmentedValueMaxSize;

        /** @var array */
        private $duplicateKeyLookups = [];
        /** @var bool */
        private $reportDupes = false;
        /** @var bool */
        private $dupeTrackScheduled = false;

        /** @var callable[] */
        protected $busyCallbacks = [];

        /** @var string Component to use for key construction of blob segment keys */
        const SEGMENT_COMPONENT = 'segment';

        /**
         * @see BagOStuff::__construct()
         * Additional $params options include:
         *   - logger: Psr\Log\LoggerInterface instance
         *   - keyspace: Default keyspace for $this->makeKey()
         *   - reportDupes: Whether to emit warning log messages for all keys that were
         *      requested more than once (requires an asyncHandler).
         *   - syncTimeout: How long to wait with WRITE_SYNC in seconds.
         *   - segmentationSize: The chunk size, in bytes, of segmented values. The value should
         *      not exceed the maximum size of values in the storage backend, as configured by
         *      the site administrator.
         *   - segmentedValueMaxSize: The maximum total size, in bytes, of segmented values.
         *      This should be configured to a reasonable size give the site traffic and the
         *      amount of I/O between application and cache servers that the network can handle.
         * @param array $params
         */
        public function __construct( array $params = [] ) {
                parent::__construct( $params );

                if ( isset( $params['keyspace'] ) ) {
                        $this->keyspace = $params['keyspace'];
                }

                if ( !empty( $params['reportDupes'] ) && is_callable( $this->asyncHandler ) ) {
                        $this->reportDupes = true;
                }

                $this->syncTimeout = $params['syncTimeout'] ?? 3;
                $this->segmentationSize = $params['segmentationSize'] ?? 8388608; // 8MiB
                $this->segmentedValueMaxSize = $params['segmentedValueMaxSize'] ?? 67108864; // 64MiB
        }

        /**
         * Get an item with the given key
         *
         * If the key includes a deterministic input hash (e.g. the key can only have
         * the correct value) or complete staleness checks are handled by the caller
         * (e.g. nothing relies on the TTL), then the READ_VERIFIED flag should be set.
         * This lets tiered backends know they can safely upgrade a cached value to
         * higher tiers using standard TTLs.
         *
         * @param string $key
         * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
         * @return mixed Returns false on failure or if the item does not exist
         */
        public function get( $key, $flags = 0 ) {
                $this->trackDuplicateKeys( $key );

                return $this->resolveSegments( $key, $this->doGet( $key, $flags ) );
        }

        /**
         * Track the number of times that a given key has been used.
         * @param string $key
         */
        private function trackDuplicateKeys( $key ) {
                if ( !$this->reportDupes ) {
                        return;
                }

                if ( !isset( $this->duplicateKeyLookups[$key] ) ) {
                        // Track that we have seen this key. This N-1 counting style allows
                        // easy filtering with array_filter() later.
                        $this->duplicateKeyLookups[$key] = 0;
                } else {
                        $this->duplicateKeyLookups[$key] += 1;

                        if ( $this->dupeTrackScheduled === false ) {
                                $this->dupeTrackScheduled = true;
                                // Schedule a callback that logs keys processed more than once by get().
                                call_user_func( $this->asyncHandler, function () {
                                        $dups = array_filter( $this->duplicateKeyLookups );
                                        foreach ( $dups as $key => $count ) {
                                                $this->logger->warning(
                                                        'Duplicate get(): "{key}" fetched {count} times',
                                                        // Count is N-1 of the actual lookup count
                                                        [ 'key' => $key, 'count' => $count + 1, ]
                                                );
                                        }
                                } );
                        }
                }
        }

        /**
         * @param string $key
         * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
         * @param mixed|null &$casToken Token to use for check-and-set comparisons
         * @return mixed Returns false on failure or if the item does not exist
         */
        abstract protected function doGet( $key, $flags = 0, &$casToken = null );

        /**
         * Set an item
         *
         * @param string $key
         * @param mixed $value
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         */
        public function set( $key, $value, $exptime = 0, $flags = 0 ) {
                list( $entry, $usable ) = $this->makeValueOrSegmentList( $key, $value, $exptime, $flags );
                // Only when all segments (if any) are stored should the main key be changed
                return $usable ? $this->doSet( $key, $entry, $exptime, $flags ) : false;
        }

        /**
         * Set an item
         *
         * @param string $key
         * @param mixed $value
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         */
        abstract protected function doSet( $key, $value, $exptime = 0, $flags = 0 );

        /**
         * Delete an item
         *
         * For large values written using WRITE_ALLOW_SEGMENTS, this only deletes the main
         * segment list key unless WRITE_PRUNE_SEGMENTS is in the flags. While deleting the segment
         * list key has the effect of functionally deleting the key, it leaves unused blobs in cache.
         *
         * @param string $key
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool True if the item was deleted or not found, false on failure
         */
        public function delete( $key, $flags = 0 ) {
                if ( ( $flags & self::WRITE_PRUNE_SEGMENTS ) != self::WRITE_PRUNE_SEGMENTS ) {
                        return $this->doDelete( $key, $flags );
                }

                $mainValue = $this->doGet( $key, self::READ_LATEST );
                if ( !$this->doDelete( $key, $flags ) ) {
                        return false;
                }

                if ( !SerializedValueContainer::isSegmented( $mainValue ) ) {
                        return true; // no segments to delete
                }

                $orderedKeys = array_map(
                        function ( $segmentHash ) use ( $key ) {
                                return $this->makeGlobalKey( self::SEGMENT_COMPONENT, $key, $segmentHash );
                        },
                        $mainValue->{SerializedValueContainer::SEGMENTED_HASHES}
                );

                return $this->deleteMulti( $orderedKeys, $flags );
        }

        /**
         * Delete an item
         *
         * @param string $key
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool True if the item was deleted or not found, false on failure
         */
        abstract protected function doDelete( $key, $flags = 0 );

        public function add( $key, $value, $exptime = 0, $flags = 0 ) {
                list( $entry, $usable ) = $this->makeValueOrSegmentList( $key, $value, $exptime, $flags );
                // Only when all segments (if any) are stored should the main key be changed
                return $usable ? $this->doAdd( $key, $entry, $exptime, $flags ) : false;
        }

        /**
         * Insert an item if it does not already exist
         *
         * @param string $key
         * @param mixed $value
         * @param int $exptime
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants (since 1.33)
         * @return bool Success
         */
        abstract protected function doAdd( $key, $value, $exptime = 0, $flags = 0 );

        /**
         * Merge changes into the existing cache value (possibly creating a new one)
         *
         * The callback function returns the new value given the current value
         * (which will be false if not present), and takes the arguments:
         * (this BagOStuff, cache key, current value, TTL).
         * The TTL parameter is reference set to $exptime. It can be overriden in the callback.
         * Nothing is stored nor deleted if the callback returns false.
         *
         * @param string $key
         * @param callable $callback Callback method to be executed
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $attempts The amount of times to attempt a merge in case of failure
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         */
        public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
                return $this->mergeViaCas( $key, $callback, $exptime, $attempts, $flags );
        }

        /**
         * @param string $key
         * @param callable $callback Callback method to be executed
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $attempts The amount of times to attempt a merge in case of failure
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         * @see BagOStuff::merge()
         */
        final protected function mergeViaCas( $key, callable $callback, $exptime, $attempts, $flags ) {
                $attemptsLeft = $attempts;
                do {
                        $casToken = null; // passed by reference
                        // Get the old value and CAS token from cache
                        $this->clearLastError();
                        $currentValue = $this->resolveSegments(
                                $key,
                                $this->doGet( $key, self::READ_LATEST, $casToken )
                        );
                        if ( $this->getLastError() ) {
                                // Don't spam slow retries due to network problems (retry only on races)
                                $this->logger->warning(
                                        __METHOD__ . ' failed due to read I/O error on get() for {key}.',
                                        [ 'key' => $key ]
                                );
                                $success = false;
                                break;
                        }

                        // Derive the new value from the old value
                        $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );
                        $keyWasNonexistant = ( $currentValue === false );
                        $valueMatchesOldValue = ( $value === $currentValue );
                        unset( $currentValue ); // free RAM in case the value is large

                        $this->clearLastError();
                        if ( $value === false ) {
                                $success = true; // do nothing
                        } elseif ( $valueMatchesOldValue && $attemptsLeft !== $attempts ) {
                                $success = true; // recently set by another thread to the same value
                        } elseif ( $keyWasNonexistant ) {
                                // Try to create the key, failing if it gets created in the meantime
                                $success = $this->add( $key, $value, $exptime, $flags );
                        } else {
                                // Try to update the key, failing if it gets changed in the meantime
                                $success = $this->cas( $casToken, $key, $value, $exptime, $flags );
                        }
                        if ( $this->getLastError() ) {
                                // Don't spam slow retries due to network problems (retry only on races)
                                $this->logger->warning(
                                        __METHOD__ . ' failed due to write I/O error for {key}.',
                                        [ 'key' => $key ]
                                );
                                $success = false;
                                break;
                        }

                } while ( !$success && --$attemptsLeft );

                return $success;
        }

        /**
         * Check and set an item
         *
         * @param mixed $casToken
         * @param string $key
         * @param mixed $value
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         */
        protected function cas( $casToken, $key, $value, $exptime = 0, $flags = 0 ) {
                if ( $casToken === null ) {
                        $this->logger->warning(
                                __METHOD__ . ' got empty CAS token for {key}.',
                                [ 'key' => $key ]
                        );

                        return false; // caller may have meant to use add()?
                }

                list( $entry, $usable ) = $this->makeValueOrSegmentList( $key, $value, $exptime, $flags );
                // Only when all segments (if any) are stored should the main key be changed
                return $usable ? $this->doCas( $casToken, $key, $entry, $exptime, $flags ) : false;
        }

        /**
         * Check and set an item
         *
         * @param mixed $casToken
         * @param string $key
         * @param mixed $value
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         */
        protected function doCas( $casToken, $key, $value, $exptime = 0, $flags = 0 ) {
                // @TODO: the lock() call assumes that all other relavent sets() use one
                if ( !$this->lock( $key, 0 ) ) {
                        return false; // non-blocking
                }

                $curCasToken = null; // passed by reference
                $this->clearLastError();
                $this->doGet( $key, self::READ_LATEST, $curCasToken );
                if ( is_object( $curCasToken ) ) {
                        // Using === does not work with objects since it checks for instance identity
                        throw new UnexpectedValueException( "CAS token cannot be an object" );
                }
                if ( $this->getLastError() ) {
                        // Fail if the old CAS token could not be read
                        $success = false;
                        $this->logger->warning(
                                __METHOD__ . ' failed due to write I/O error for {key}.',
                                [ 'key' => $key ]
                        );
                } elseif ( $casToken === $curCasToken ) {
                        $success = $this->doSet( $key, $value, $exptime, $flags );
                } else {
                        $success = false; // mismatched or failed
                        $this->logger->info(
                                __METHOD__ . ' failed due to race condition for {key}.',
                                [ 'key' => $key ]
                        );
                }

                $this->unlock( $key );

                return $success;
        }

        /**
         * Change the expiration on a key if it exists
         *
         * If an expiry in the past is given then the key will immediately be expired
         *
         * For large values written using WRITE_ALLOW_SEGMENTS, this only changes the TTL of the
         * main segment list key. While lowering the TTL of the segment list key has the effect of
         * functionally lowering the TTL of the key, it might leave unused blobs in cache for longer.
         * Raising the TTL of such keys is not effective, since the expiration of a single segment
         * key effectively expires the entire value.
         *
         * @param string $key
         * @param int $exptime TTL or UNIX timestamp
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants (since 1.33)
         * @return bool Success Returns false on failure or if the item does not exist
         * @since 1.28
         */
        public function changeTTL( $key, $exptime = 0, $flags = 0 ) {
                return $this->doChangeTTL( $key, $exptime, $flags );
        }

        /**
         * @param string $key
         * @param int $exptime
         * @param int $flags
         * @return bool
         */
        protected function doChangeTTL( $key, $exptime, $flags ) {
                if ( !$this->lock( $key, 0 ) ) {
                        return false;
                }

                $expiry = $this->getExpirationAsTimestamp( $exptime );
                $delete = ( $expiry != self::TTL_INDEFINITE && $expiry < $this->getCurrentTime() );

                // Use doGet() to avoid having to trigger resolveSegments()
                $blob = $this->doGet( $key, self::READ_LATEST );
                if ( $blob ) {
                        if ( $delete ) {
                                $ok = $this->doDelete( $key, $flags );
                        } else {
                                $ok = $this->doSet( $key, $blob, $exptime, $flags );
                        }
                } else {
                        $ok = false;
                }

                $this->unlock( $key );

                return $ok;
        }

        /**
         * Acquire an advisory lock on a key string
         *
         * Note that if reentry is enabled, duplicate calls ignore $expiry
         *
         * @param string $key
         * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
         * @param int $expiry Lock expiry [optional]; 1 day maximum
         * @param string $rclass Allow reentry if set and the current lock used this value
         * @return bool Success
         */
        public function lock( $key, $timeout = 6, $expiry = 6, $rclass = '' ) {
                // Avoid deadlocks and allow lock reentry if specified
                if ( isset( $this->locks[$key] ) ) {
                        if ( $rclass != '' && $this->locks[$key]['class'] === $rclass ) {
                                ++$this->locks[$key]['depth'];
                                return true;
                        } else {
                                return false;
                        }
                }

                $fname = __METHOD__;
                $expiry = min( $expiry ?: INF, self::TTL_DAY );
                $loop = new WaitConditionLoop(
                        function () use ( $key, $expiry, $fname ) {
                                $this->clearLastError();
                                if ( $this->add( "{$key}:lock", 1, $expiry ) ) {
                                        return WaitConditionLoop::CONDITION_REACHED; // locked!
                                } elseif ( $this->getLastError() ) {
                                        $this->logger->warning(
                                                $fname . ' failed due to I/O error for {key}.',
                                                [ 'key' => $key ]
                                        );

                                        return WaitConditionLoop::CONDITION_ABORTED; // network partition?
                                }

                                return WaitConditionLoop::CONDITION_CONTINUE;
                        },
                        $timeout
                );

                $code = $loop->invoke();
                $locked = ( $code === $loop::CONDITION_REACHED );
                if ( $locked ) {
                        $this->locks[$key] = [ 'class' => $rclass, 'depth' => 1 ];
                } elseif ( $code === $loop::CONDITION_TIMED_OUT ) {
                        $this->logger->warning(
                                "$fname failed due to timeout for {key}.",
                                [ 'key' => $key, 'timeout' => $timeout ]
                        );
                }

                return $locked;
        }

        /**
         * Release an advisory lock on a key string
         *
         * @param string $key
         * @return bool Success
         */
        public function unlock( $key ) {
                if ( !isset( $this->locks[$key] ) ) {
                        return false;
                }

                if ( --$this->locks[$key]['depth'] <= 0 ) {
                        unset( $this->locks[$key] );

                        $ok = $this->doDelete( "{$key}:lock" );
                        if ( !$ok ) {
                                $this->logger->warning(
                                        __METHOD__ . ' failed to release lock for {key}.',
                                        [ 'key' => $key ]
                                );
                        }

                        return $ok;
                }

                return true;
        }

        /**
         * Delete all objects expiring before a certain date.
         * @param string|int $timestamp The reference date in MW or TS_UNIX format
         * @param callable|null $progress Optional, a function which will be called
         *     regularly during long-running operations with the percentage progress
         *     as the first parameter. [optional]
         * @param int $limit Maximum number of keys to delete [default: INF]
         *
         * @return bool Success; false if unimplemented
         */
        public function deleteObjectsExpiringBefore(
                $timestamp,
                callable $progress = null,
                $limit = INF
        ) {
                return false;
        }

        /**
         * Get an associative array containing the item for each of the keys that have items.
         * @param string[] $keys List of keys; can be a map of (unused => key) for convenience
         * @param int $flags Bitfield; supports READ_LATEST [optional]
         * @return mixed[] Map of (key => value) for existing keys; preserves the order of $keys
         */
        public function getMulti( array $keys, $flags = 0 ) {
                $foundByKey = $this->doGetMulti( $keys, $flags );

                $res = [];
                foreach ( $keys as $key ) {
                        // Resolve one blob at a time (avoids too much I/O at once)
                        if ( array_key_exists( $key, $foundByKey ) ) {
                                // A value should not appear in the key if a segment is missing
                                $value = $this->resolveSegments( $key, $foundByKey[$key] );
                                if ( $value !== false ) {
                                        $res[$key] = $value;
                                }
                        }
                }

                return $res;
        }

        /**
         * Get an associative array containing the item for each of the keys that have items.
         * @param string[] $keys List of keys
         * @param int $flags Bitfield; supports READ_LATEST [optional]
         * @return array Map of (key => value) for existing keys
         */
        protected function doGetMulti( array $keys, $flags = 0 ) {
                $res = [];
                foreach ( $keys as $key ) {
                        $val = $this->doGet( $key, $flags );
                        if ( $val !== false ) {
                                $res[$key] = $val;
                        }
                }

                return $res;
        }

        /**
         * Batch insertion/replace
         *
         * This does not support WRITE_ALLOW_SEGMENTS to avoid excessive read I/O
         *
         * @param mixed[] $data Map of (key => value)
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants (since 1.33)
         * @return bool Success
         * @since 1.24
         */
        public function setMulti( array $data, $exptime = 0, $flags = 0 ) {
                if ( ( $flags & self::WRITE_ALLOW_SEGMENTS ) === self::WRITE_ALLOW_SEGMENTS ) {
                        throw new InvalidArgumentException( __METHOD__ . ' got WRITE_ALLOW_SEGMENTS' );
                }
                return $this->doSetMulti( $data, $exptime, $flags );
        }

        /**
         * @param mixed[] $data Map of (key => value)
         * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         */
        protected function doSetMulti( array $data, $exptime = 0, $flags = 0 ) {
                $res = true;
                foreach ( $data as $key => $value ) {
                        $res = $this->doSet( $key, $value, $exptime, $flags ) && $res;
                }
                return $res;
        }

        /**
         * Batch deletion
         *
         * This does not support WRITE_ALLOW_SEGMENTS to avoid excessive read I/O
         *
         * @param string[] $keys List of keys
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         * @since 1.33
         */
        public function deleteMulti( array $keys, $flags = 0 ) {
                if ( ( $flags & self::WRITE_ALLOW_SEGMENTS ) === self::WRITE_ALLOW_SEGMENTS ) {
                        throw new InvalidArgumentException( __METHOD__ . ' got WRITE_ALLOW_SEGMENTS' );
                }
                return $this->doDeleteMulti( $keys, $flags );
        }

        /**
         * @param string[] $keys List of keys
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants
         * @return bool Success
         */
        protected function doDeleteMulti( array $keys, $flags = 0 ) {
                $res = true;
                foreach ( $keys as $key ) {
                        $res = $this->doDelete( $key, $flags ) && $res;
                }
                return $res;
        }

        /**
         * Change the expiration of multiple keys that exist
         *
         * @param string[] $keys List of keys
         * @param int $exptime TTL or UNIX timestamp
         * @param int $flags Bitfield of BagOStuff::WRITE_* constants (since 1.33)
         * @return bool Success
         * @see BagOStuff::changeTTL()
         *
         * @since 1.34
         */
        public function changeTTLMulti( array $keys, $exptime, $flags = 0 ) {
                $res = true;
                foreach ( $keys as $key ) {
                        $res = $this->doChangeTTL( $key, $exptime, $flags ) && $res;
                }

                return $res;
        }

        /**
         * Decrease stored value of $key by $value while preserving its TTL
         * @param string $key
         * @param int $value Value to subtract from $key (default: 1) [optional]
         * @return int|bool New value or false on failure
         */
        public function decr( $key, $value = 1 ) {
                return $this->incr( $key, -$value );
        }

        /**
         * Increase stored value of $key by $value while preserving its TTL
         *
         * This will create the key with value $init and TTL $ttl instead if not present
         *
         * @param string $key
         * @param int $ttl
         * @param int $value
         * @param int $init
         * @return int|bool New value or false on failure
         * @since 1.24
         */
        public function incrWithInit( $key, $ttl, $value = 1, $init = 1 ) {
                $this->clearLastError();
                $newValue = $this->incr( $key, $value );
                if ( $newValue === false && !$this->getLastError() ) {
                        // No key set; initialize
                        $newValue = $this->add( $key, (int)$init, $ttl ) ? $init : false;
                        if ( $newValue === false && !$this->getLastError() ) {
                                // Raced out initializing; increment
                                $newValue = $this->incr( $key, $value );
                        }
                }

                return $newValue;
        }

        /**
         * Get and reassemble the chunks of blob at the given key
         *
         * @param string $key
         * @param mixed $mainValue
         * @return string|null|bool The combined string, false if missing, null on error
         */
        final protected function resolveSegments( $key, $mainValue ) {
                if ( SerializedValueContainer::isUnified( $mainValue ) ) {
                        return $this->unserialize( $mainValue->{SerializedValueContainer::UNIFIED_DATA} );
                }

                if ( SerializedValueContainer::isSegmented( $mainValue ) ) {
                        $orderedKeys = array_map(
                                function ( $segmentHash ) use ( $key ) {
                                        return $this->makeGlobalKey( self::SEGMENT_COMPONENT, $key, $segmentHash );
                                },
                                $mainValue->{SerializedValueContainer::SEGMENTED_HASHES}
                        );

                        $segmentsByKey = $this->doGetMulti( $orderedKeys );

                        $parts = [];
                        foreach ( $orderedKeys as $segmentKey ) {
                                if ( isset( $segmentsByKey[$segmentKey] ) ) {
                                        $parts[] = $segmentsByKey[$segmentKey];
                                } else {
                                        return false; // missing segment
                                }
                        }

                        return $this->unserialize( implode( '', $parts ) );
                }

                return $mainValue;
        }

        /**
         * Get the "last error" registered; clearLastError() should be called manually
         * @return int ERR_* constant for the "last error" registry
         * @since 1.23
         */
        public function getLastError() {
                return $this->lastError;
        }

        /**
         * Clear the "last error" registry
         * @since 1.23
         */
        public function clearLastError() {
                $this->lastError = self::ERR_NONE;
        }

        /**
         * Set the "last error" registry
         * @param int $err ERR_* constant
         * @since 1.23
         */
        protected function setLastError( $err ) {
                $this->lastError = $err;
        }

        /**
         * Let a callback be run to avoid wasting time on special blocking calls
         *
         * The callbacks may or may not be called ever, in any particular order.
         * They are likely to be invoked when something WRITE_SYNC is used used.
         * They should follow a caching pattern as shown below, so that any code
         * using the work will get it's result no matter what happens.
         * @code
         *     $result = null;
         *     $workCallback = function () use ( &$result ) {
         *         if ( !$result ) {
         *             $result = ....
         *         }
         *         return $result;
         *     }
         * @endcode
         *
         * @param callable $workCallback
         * @since 1.28
         */
        final public function addBusyCallback( callable $workCallback ) {
                $this->busyCallbacks[] = $workCallback;
        }

        /**
         * Determine the entry (inline or segment list) to store under a key to save the value
         *
         * @param string $key
         * @param mixed $value
         * @param int $exptime
         * @param int $flags
         * @return array (inline value or segment list, whether the entry is usable)
         * @since 1.34
         */
        final protected function makeValueOrSegmentList( $key, $value, $exptime, $flags ) {
                $entry = $value;
                $usable = true;

                if (
                        ( $flags & self::WRITE_ALLOW_SEGMENTS ) === self::WRITE_ALLOW_SEGMENTS &&
                        !is_int( $value ) && // avoid breaking incr()/decr()
                        is_finite( $this->segmentationSize )
                ) {
                        $segmentSize = $this->segmentationSize;
                        $maxTotalSize = $this->segmentedValueMaxSize;

                        $serialized = $this->serialize( $value );
                        $size = strlen( $serialized );
                        if ( $size > $maxTotalSize ) {
                                $this->logger->warning(
                                        "Value for {key} exceeds $maxTotalSize bytes; cannot segment.",
                                        [ 'key' => $key ]
                                );
                        } elseif ( $size <= $segmentSize ) {
                                // The serialized value was already computed, so just use it inline
                                $entry = SerializedValueContainer::newUnified( $serialized );
                        } else {
                                // Split the serialized value into chunks and store them at different keys
                                $chunksByKey = [];
                                $segmentHashes = [];
                                $count = intdiv( $size, $segmentSize ) + ( ( $size % $segmentSize ) ? 1 : 0 );
                                for ( $i = 0; $i < $count; ++$i ) {
                                        $segment = substr( $serialized, $i * $segmentSize, $segmentSize );
                                        $hash = sha1( $segment );
                                        $chunkKey = $this->makeGlobalKey( self::SEGMENT_COMPONENT, $key, $hash );
                                        $chunksByKey[$chunkKey] = $segment;
                                        $segmentHashes[] = $hash;
                                }
                                $flags &= ~self::WRITE_ALLOW_SEGMENTS; // sanity
                                $usable = $this->setMulti( $chunksByKey, $exptime, $flags );
                                $entry = SerializedValueContainer::newSegmented( $segmentHashes );
                        }
                }

                return [ $entry, $usable ];
        }

        /**
         * @param int|float $exptime
         * @return bool Whether the expiry is non-infinite, and, negative or not a UNIX timestamp
         * @since 1.34
         */
        final protected function isRelativeExpiration( $exptime ) {
                return ( $exptime !== self::TTL_INDEFINITE && $exptime < ( 10 * self::TTL_YEAR ) );
        }

        /**
         * Convert an optionally relative timestamp to an absolute time
         *
         * The input value will be cast to an integer and interpreted as follows:
         *   - zero: no expiry; return zero (e.g. TTL_INDEFINITE)
         *   - negative: relative TTL; return UNIX timestamp offset by this value
         *   - positive (< 10 years): relative TTL; return UNIX timestamp offset by this value
         *   - positive (>= 10 years): absolute UNIX timestamp; return this value
         *
         * @param int $exptime
         * @return int Expiration timestamp or TTL_INDEFINITE for indefinite
         * @since 1.34
         */
        final protected function getExpirationAsTimestamp( $exptime ) {
                if ( $exptime == self::TTL_INDEFINITE ) {
                        return $exptime;
                }

                return $this->isRelativeExpiration( $exptime )
                        ? intval( $this->getCurrentTime() + $exptime )
                        : $exptime;
        }

        /**
         * Convert an optionally absolute expiry time to a relative time. If an
         * absolute time is specified which is in the past, use a short expiry time.
         *
         * The input value will be cast to an integer and interpreted as follows:
         *   - zero: no expiry; return zero (e.g. TTL_INDEFINITE)
         *   - negative: relative TTL; return a short expiry time (1 second)
         *   - positive (< 10 years): relative TTL; return this value
         *   - positive (>= 10 years): absolute UNIX timestamp; return offset to current time
         *
         * @param int $exptime
         * @return int Relative TTL or TTL_INDEFINITE for indefinite
         * @since 1.34
         */
        final protected function getExpirationAsTTL( $exptime ) {
                if ( $exptime == self::TTL_INDEFINITE ) {
                        return $exptime;
                }

                return $this->isRelativeExpiration( $exptime )
                        ? $exptime
                        : (int)max( $exptime - $this->getCurrentTime(), 1 );
        }

        /**
         * Check if a value is an integer
         *
         * @param mixed $value
         * @return bool
         */
        final protected function isInteger( $value ) {
                if ( is_int( $value ) ) {
                        return true;
                } elseif ( !is_string( $value ) ) {
                        return false;
                }

                $integer = (int)$value;

                return ( $value === (string)$integer );
        }

        /**
         * Construct a cache key.
         *
         * @param string $keyspace
         * @param array $args
         * @return string Colon-delimited list of $keyspace followed by escaped components of $args
         * @since 1.27
         */
        public function makeKeyInternal( $keyspace, $args ) {
                $key = $keyspace;
                foreach ( $args as $arg ) {
                        $key .= ':' . str_replace( ':', '%3A', $arg );
                }
                return strtr( $key, ' ', '_' );
        }

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

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

        /**
         * @param int $flag ATTR_* class constant
         * @return int QOS_* class constant
         * @since 1.28
         */
        public function getQoS( $flag ) {
                return $this->attrMap[$flag] ?? self::QOS_UNKNOWN;
        }

        /**
         * @return int|float The chunk size, in bytes, of segmented objects (INF for no limit)
         * @since 1.34
         */
        public function getSegmentationSize() {
                return $this->segmentationSize;
        }

        /**
         * @return int|float Maximum total segmented object size in bytes (INF for no limit)
         * @since 1.34
         */
        public function getSegmentedValueMaxSize() {
                return $this->segmentedValueMaxSize;
        }

        /**
         * @param mixed $value
         * @return string|int String/integer representation
         * @note Special handling is usually needed for integers so incr()/decr() work
         */
        protected function serialize( $value ) {
                return is_int( $value ) ? $value : serialize( $value );
        }

        /**
         * @param string|int $value
         * @return mixed Original value or false on error
         * @note Special handling is usually needed for integers so incr()/decr() work
         */
        protected function unserialize( $value ) {
                return $this->isInteger( $value ) ? (int)$value : unserialize( $value );
        }

        /**
         * @param string $text
         */
        protected function debug( $text ) {
                if ( $this->debugMode ) {
                        $this->logger->debug( "{class} debug: $text", [ 'class' => static::class ] );
                }
        }
}