MediumSpecificBagOStuff.php

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<?php
use Wikimedia\WaitConditionLoop;
 
abstract class MediumSpecificBagOStuff extends BagOStuff {
    protected $locks = [];
    protected $segmentationSize;
    protected $segmentedValueMaxSize;
 
    protected $maxLockSendDelay = 0.05;
 
    private $duplicateKeyLookups = [];
    private $reportDupes = false;
    private $dupeTrackScheduled = false;
 
    private const SEGMENT_COMPONENT = 'segment';
 
    protected const PASS_BY_REF = -1;
 
    protected const METRIC_OP_GET = 'get';
    protected const METRIC_OP_SET = 'set';
    protected const METRIC_OP_DELETE = 'delete';
    protected const METRIC_OP_CHANGE_TTL = 'change_ttl';
    protected const METRIC_OP_ADD = 'add';
    protected const METRIC_OP_INCR = 'incr';
    protected const METRIC_OP_DECR = 'decr';
    protected const METRIC_OP_CAS = 'cas';
 
    protected const LOCK_RCLASS = 0;
    protected const LOCK_DEPTH = 1;
    protected const LOCK_TIME = 2;
    protected const LOCK_EXPIRY = 3;
 
    public function __construct( array $params = [] ) {
        parent::__construct( $params );
 
        if ( !empty( $params['reportDupes'] ) && $this->asyncHandler ) {
            $this->reportDupes = true;
        }
 
        // Default to 8MiB if segmentationSize is not set
        $this->segmentationSize = $params['segmentationSize'] ?? 8388608;
        // Default to 64MiB if segmentedValueMaxSize is not set
        $this->segmentedValueMaxSize = $params['segmentedValueMaxSize'] ?? 67108864;
    }
 
    public function get( $key, $flags = 0 ) {
        $this->trackDuplicateKeys( $key );
 
        return $this->resolveSegments( $key, $this->doGet( $key, $flags ) );
    }
 
    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, ]
                        );
                    }
                } );
            }
        }
    }
 
    abstract protected function doGet( $key, $flags = 0, &$casToken = null );
 
    public function set( $key, $value, $exptime = 0, $flags = 0 ) {
        $entry = $this->makeValueOrSegmentList( $key, $value, $exptime, $flags, $ok );
        // Only when all segments (if any) are stored should the main key be changed
        return $ok && $this->doSet( $key, $entry, $exptime, $flags );
    }
 
    abstract protected function doSet( $key, $value, $exptime = 0, $flags = 0 );
 
    public function delete( $key, $flags = 0 ) {
        if ( !$this->fieldHasFlags( $flags, 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 ) ) {
            // no segments to delete
            return true;
        }
 
        $orderedKeys = array_map(
            function ( $segmentHash ) use ( $key ) {
                return $this->makeGlobalKey( self::SEGMENT_COMPONENT, $key, $segmentHash );
            },
            $mainValue->{SerializedValueContainer::SEGMENTED_HASHES}
        );
 
        return $this->deleteMulti( $orderedKeys, $flags & ~self::WRITE_PRUNE_SEGMENTS );
    }
 
    abstract protected function doDelete( $key, $flags = 0 );
 
    public function add( $key, $value, $exptime = 0, $flags = 0 ) {
        $entry = $this->makeValueOrSegmentList( $key, $value, $exptime, $flags, $ok );
        // Only when all segments (if any) are stored should the main key be changed
        return $ok && $this->doAdd( $key, $entry, $exptime, $flags );
    }
 
    abstract protected function doAdd( $key, $value, $exptime = 0, $flags = 0 );
 
    public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
        return $this->mergeViaCas( $key, $callback, $exptime, $attempts, $flags );
    }
 
    final protected function mergeViaCas( $key, callable $callback, $exptime, $attempts, $flags ) {
        $attemptsLeft = $attempts;
        do {
            $token = self::PASS_BY_REF;
            // Get the old value and CAS token from cache
            $watchPoint = $this->watchErrors();
            $currentValue = $this->resolveSegments(
                $key,
                $this->doGet( $key, $flags, $token )
            );
            if ( $this->getLastError( $watchPoint ) ) {
                // 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 = $callback( $this, $key, $currentValue, $exptime );
            $keyWasNonexistant = ( $currentValue === false );
            $valueMatchesOldValue = ( $value === $currentValue );
            // free RAM in case the value is large
            unset( $currentValue );
 
            $watchPoint = $this->watchErrors();
            if ( $value === false || $exptime < 0 ) {
                // do nothing
                $success = true;
            } elseif ( $valueMatchesOldValue && $attemptsLeft !== $attempts ) {
                // recently set by another thread to the same value
                $success = true;
            } 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( $token, $key, $value, $exptime, $flags );
            }
            if ( $this->getLastError( $watchPoint ) ) {
                // 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;
    }
 
    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 ]
            );
 
            // caller may have meant to use add()?
            return false;
        }
 
        $entry = $this->makeValueOrSegmentList( $key, $value, $exptime, $flags, $ok );
        // Only when all segments (if any) are stored should the main key be changed
        return $ok && $this->doCas( $casToken, $key, $entry, $exptime, $flags );
    }
 
    protected function doCas( $casToken, $key, $value, $exptime = 0, $flags = 0 ) {
        // @TODO: the use of lock() assumes that all other relevant sets() use a lock
        if ( !$this->lock( $key, 0 ) ) {
            // non-blocking
            return false;
        }
 
        $curCasToken = self::PASS_BY_REF;
        $watchPoint = $this->watchErrors();
        $exists = ( $this->doGet( $key, self::READ_LATEST, $curCasToken ) !== false );
        if ( $this->getLastError( $watchPoint ) ) {
            // 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 ( $exists && $this->tokensMatch( $casToken, $curCasToken ) ) {
            $success = $this->doSet( $key, $value, $exptime, $flags );
        } else {
            // mismatched or failed
            $success = false;
            $this->logger->info(
                __METHOD__ . ' failed due to race condition for {key}.',
                [ 'key' => $key, 'key_exists' => $exists ]
            );
        }
 
        $this->unlock( $key );
 
        return $success;
    }
 
    final protected function tokensMatch( $value, $otherValue ) {
        $type = gettype( $value );
        // Ideally, tokens are counters, timestamps, hashes, or serialized PHP values.
        // However, some classes might use the PHP values themselves.
        if ( $type !== gettype( $otherValue ) ) {
            return false;
        }
        // Serialize both tokens to strictly compare objects or arrays (which might objects
        // nested inside). Note that this will not apply if integer/string CAS tokens are used.
        if ( $type === 'array' || $type === 'object' ) {
            return ( serialize( $value ) === serialize( $otherValue ) );
        }
        // For string/integer tokens, use a simple comparison
        return ( $value === $otherValue );
    }
 
    public function changeTTL( $key, $exptime = 0, $flags = 0 ) {
        return $this->doChangeTTL( $key, $exptime, $flags );
    }
 
    protected function doChangeTTL( $key, $exptime, $flags ) {
        // @TODO: the use of lock() assumes that all other relevant sets() use a lock
        if ( !$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;
    }
 
    public function incrWithInit( $key, $exptime, $step = 1, $init = null, $flags = 0 ) {
        $step = (int)$step;
        $init = is_int( $init ) ? $init : $step;
 
        return $this->doIncrWithInit( $key, $exptime, $step, $init, $flags );
    }
 
    abstract protected function doIncrWithInit( $key, $exptime, $step, $init, $flags );
 
    public function lock( $key, $timeout = 6, $exptime = 6, $rclass = '' ) {
        $exptime = min( $exptime ?: INF, self::TTL_DAY );
 
        $acquired = false;
 
        if ( isset( $this->locks[$key] ) ) {
            // Already locked; avoid deadlocks and allow lock reentry if specified
            if ( $rclass != '' && $this->locks[$key][self::LOCK_RCLASS] === $rclass ) {
                ++$this->locks[$key][self::LOCK_DEPTH];
                $acquired = true;
            }
        } else {
            // Not already locked; acquire a lock on the backend
            $lockTsUnix = $this->doLock( $key, $timeout, $exptime );
            if ( $lockTsUnix !== null ) {
                $this->locks[$key] = [
                    self::LOCK_RCLASS => $rclass,
                    self::LOCK_DEPTH  => 1,
                    self::LOCK_TIME   => $lockTsUnix,
                    self::LOCK_EXPIRY => $lockTsUnix + $exptime
                ];
                $acquired = true;
            }
        }
 
        return $acquired;
    }
 
    protected function doLock( $key, $timeout, $exptime ) {
        $lockTsUnix = null;
 
        $fname = __METHOD__;
        $loop = new WaitConditionLoop(
            function () use ( $key, $exptime, $fname, &$lockTsUnix ) {
                $watchPoint = $this->watchErrors();
                if ( $this->add( $this->makeLockKey( $key ), 1, $exptime ) ) {
                    $lockTsUnix = microtime( true );
 
                    return WaitConditionLoop::CONDITION_REACHED;
                } elseif ( $this->getLastError( $watchPoint ) ) {
                    $this->logger->warning(
                        "$fname failed due to I/O error for {key}.",
                        [ 'key' => $key ]
                    );
 
                    return WaitConditionLoop::CONDITION_ABORTED;
                }
 
                return WaitConditionLoop::CONDITION_CONTINUE;
            },
            $timeout
        );
        $code = $loop->invoke();
 
        if ( $code === $loop::CONDITION_TIMED_OUT ) {
            $this->logger->warning(
                "$fname failed due to timeout for {key}.",
                [ 'key' => $key, 'timeout' => $timeout ]
            );
        }
 
        return $lockTsUnix;
    }
 
    public function unlock( $key ) {
        $released = false;
 
        if ( isset( $this->locks[$key] ) ) {
            if ( --$this->locks[$key][self::LOCK_DEPTH] > 0 ) {
                $released = true;
            } else {
                $released = $this->doUnlock( $key );
                unset( $this->locks[$key] );
                if ( !$released ) {
                    $this->logger->warning(
                        __METHOD__ . ' failed to release lock for {key}.',
                        [ 'key' => $key ]
                    );
                }
            }
        } else {
            $this->logger->warning(
                __METHOD__ . ' no lock to release for {key}.',
                [ 'key' => $key ]
            );
        }
 
        return $released;
    }
 
    protected function doUnlock( $key ) {
        $released = false;
 
        // Estimate the remaining TTL of the lock key
        $curTTL = $this->locks[$key][self::LOCK_EXPIRY] - $this->getCurrentTime();
 
        // Check the risk of race conditions for key deletion
        if ( $this->getQoS( self::ATTR_DURABILITY ) <= self::QOS_DURABILITY_SCRIPT ) {
            // Lock (and data) keys use memory specific to this request (e.g. HashBagOStuff)
            $isSafe = true;
        } else {
            // It is unsafe to delete the lock key if there is a serious risk of the key already
            // being claimed by another thread before the delete operation reaches the backend
            $isSafe = ( $curTTL > $this->maxLockSendDelay );
        }
 
        if ( $isSafe ) {
            $released = $this->doDelete( $this->makeLockKey( $key ) );
        } else {
            $this->logger->warning(
                "Lock for {key} held too long ({age} sec).",
                [ 'key' => $key, 'curTTL' => $curTTL ]
            );
        }
 
        return $released;
    }
 
    protected function makeLockKey( $key ) {
        return "$key:lock";
    }
 
    public function deleteObjectsExpiringBefore(
        $timestamp,
        callable $progress = null,
        $limit = INF,
        string $tag = null
    ) {
        return false;
    }
 
    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;
    }
 
    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;
    }
 
    public function setMulti( array $valueByKey, $exptime = 0, $flags = 0 ) {
        if ( $this->fieldHasFlags( $flags, self::WRITE_ALLOW_SEGMENTS ) ) {
            throw new InvalidArgumentException( __METHOD__ . ' got WRITE_ALLOW_SEGMENTS' );
        }
 
        return $this->doSetMulti( $valueByKey, $exptime, $flags );
    }
 
    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;
    }
 
    public function deleteMulti( array $keys, $flags = 0 ) {
        if ( $this->fieldHasFlags( $flags, self::WRITE_PRUNE_SEGMENTS ) ) {
            throw new InvalidArgumentException( __METHOD__ . ' got WRITE_PRUNE_SEGMENTS' );
        }
 
        return $this->doDeleteMulti( $keys, $flags );
    }
 
    protected function doDeleteMulti( array $keys, $flags = 0 ) {
        $res = true;
        foreach ( $keys as $key ) {
            $res = $this->doDelete( $key, $flags ) && $res;
        }
        return $res;
    }
 
    public function changeTTLMulti( array $keys, $exptime, $flags = 0 ) {
        return $this->doChangeTTLMulti( $keys, $exptime, $flags );
    }
 
    protected function doChangeTTLMulti( array $keys, $exptime, $flags = 0 ) {
        $res = true;
        foreach ( $keys as $key ) {
            $res = $this->doChangeTTL( $key, $exptime, $flags ) && $res;
        }
 
        return $res;
    }
 
    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 {
                    // missing segment
                    return false;
                }
            }
 
            return $this->unserialize( implode( '', $parts ) );
        }
 
        return $mainValue;
    }
 
    private function useSegmentationWrapper( $value, $flags ) {
        if (
            $this->segmentationSize === INF ||
            !$this->fieldHasFlags( $flags, self::WRITE_ALLOW_SEGMENTS )
        ) {
            return false;
        }
 
        if ( is_string( $value ) ) {
            return ( strlen( $value ) >= $this->segmentationSize );
        }
 
        if ( is_array( $value ) ) {
            // Expect that the contained value will be one of the first array entries
            foreach ( array_slice( $value, 0, 4 ) as $v ) {
                if ( is_string( $v ) && strlen( $v ) >= $this->segmentationSize ) {
                    return true;
                }
            }
        }
 
        // Avoid breaking functions for incrementing/decrementing integer key values
        return false;
    }
 
    final protected function makeValueOrSegmentList( $key, $value, $exptime, $flags, &$ok ) {
        $entry = $value;
        $ok = true;
 
        if ( $this->useSegmentationWrapper( $value, $flags ) ) {
            $segmentSize = $this->segmentationSize;
            $maxTotalSize = $this->segmentedValueMaxSize;
            $serialized = $this->getSerialized( $value, $key );
            $size = strlen( $serialized );
            if ( $size > $maxTotalSize ) {
                $this->logger->warning(
                    "Value for {key} exceeds $maxTotalSize bytes; cannot segment.",
                    [ 'key' => $key ]
                );
            } 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;
                $ok = $this->setMulti( $chunksByKey, $exptime, $flags );
                $entry = SerializedValueContainer::newSegmented( $segmentHashes );
            }
        }
 
        return $entry;
    }
 
    final protected function isRelativeExpiration( $exptime ) {
        return ( $exptime !== self::TTL_INDEFINITE && $exptime < ( 10 * self::TTL_YEAR ) );
    }
 
    final protected function getExpirationAsTimestamp( $exptime ) {
        if ( $exptime == self::TTL_INDEFINITE ) {
            return $exptime;
        }
 
        return $this->isRelativeExpiration( $exptime )
            ? intval( $this->getCurrentTime() + $exptime )
            : $exptime;
    }
 
    final protected function getExpirationAsTTL( $exptime ) {
        if ( $exptime == self::TTL_INDEFINITE ) {
            return $exptime;
        }
 
        return $this->isRelativeExpiration( $exptime )
            ? $exptime
            : (int)max( $exptime - $this->getCurrentTime(), 1 );
    }
 
    final protected function isInteger( $value ) {
        if ( is_int( $value ) ) {
            return true;
        } elseif ( !is_string( $value ) ) {
            return false;
        }
 
        $integer = (int)$value;
 
        return ( $value === (string)$integer );
    }
 
    public function getQoS( $flag ) {
        return $this->attrMap[$flag] ?? self::QOS_UNKNOWN;
    }
 
    public function getSegmentationSize() {
        return $this->segmentationSize;
    }
 
    public function getSegmentedValueMaxSize() {
        return $this->segmentedValueMaxSize;
    }
 
    protected function getSerialized( $value, $key ) {
        $this->checkValueSerializability( $value, $key );
 
        return $this->serialize( $value );
    }
 
    private function checkValueSerializability( $value, $key ) {
        if ( is_array( $value ) ) {
            $this->checkIterableMapSerializability( $value, $key );
        } elseif ( is_object( $value ) ) {
            // Note that Closure instances count as objects
            if ( $value instanceof stdClass ) {
                $this->checkIterableMapSerializability( $value, $key );
            } elseif ( !( $value instanceof JsonSerializable ) ) {
                $this->logger->warning(
                    "{class} value for '{cachekey}'; serialization is suspect.",
                    [ 'cachekey' => $key, 'class' => get_class( $value ) ]
                );
            }
        }
    }
 
    private function checkIterableMapSerializability( $value, $key ) {
        foreach ( $value as $index => $entry ) {
            if ( is_object( $entry ) ) {
                // Note that Closure instances count as objects
                if (
                    !( $entry instanceof stdClass ) &&
                    !( $entry instanceof JsonSerializable )
                ) {
                    $this->logger->warning(
                        "{class} value for '{cachekey}' at '$index'; serialization is suspect.",
                        [ 'cachekey' => $key, 'class' => get_class( $entry ) ]
                    );
 
                    return;
                }
            }
        }
    }
 
    protected function serialize( $value ) {
        return is_int( $value ) ? $value : serialize( $value );
    }
 
    protected function unserialize( $value ) {
        return $this->isInteger( $value ) ? (int)$value : unserialize( $value );
    }
 
    protected function debug( $text ) {
        $this->logger->debug( "{class} debug: $text", [ 'class' => static::class ] );
    }
 
    protected function updateOpStats( string $op, array $keyInfo ) {
        $deltasByMetric = [];
 
        foreach ( $keyInfo as $indexOrKey => $keyOrSizes ) {
            if ( is_array( $keyOrSizes ) ) {
                $key = $indexOrKey;
                [ $sPayloadSize, $rPayloadSize ] = $keyOrSizes;
            } else {
                $key = $keyOrSizes;
                $sPayloadSize = 0;
                $rPayloadSize = 0;
            }
 
            // Metric prefix for the cache wrapper and key collection name
            $prefix = $this->determineKeyPrefixForStats( $key );
 
            if ( $op === self::METRIC_OP_GET ) {
                // This operation was either a "hit" or "miss" for this key
                $name = "{$prefix}.{$op}_" . ( $rPayloadSize === false ? 'miss_rate' : 'hit_rate' );
            } else {
                // There is no concept of "hit" or "miss" for this operation
                $name = "{$prefix}.{$op}_call_rate";
            }
            $deltasByMetric[$name] = ( $deltasByMetric[$name] ?? 0 ) + 1;
 
            if ( $sPayloadSize > 0 ) {
                $name = "{$prefix}.{$op}_bytes_sent";
                $deltasByMetric[$name] = ( $deltasByMetric[$name] ?? 0 ) + $sPayloadSize;
            }
 
            if ( $rPayloadSize > 0 ) {
                $name = "{$prefix}.{$op}_bytes_read";
                $deltasByMetric[$name] = ( $deltasByMetric[$name] ?? 0 ) + $rPayloadSize;
            }
        }
 
        foreach ( $deltasByMetric as $name => $delta ) {
            $this->stats->updateCount( $name, $delta );
        }
    }
}