BagOStuff.php

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<?php
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;

abstract class BagOStuff implements IExpiringStore, LoggerAwareInterface {
    protected $locks = [];

    protected $lastError = self::ERR_NONE;

    protected $keyspace = 'local';

    protected $logger;

    protected $asyncHandler;

    private $debugMode = false;

    private $duplicateKeyLookups = [];

    private $reportDupes = false;

    private $dupeTrackScheduled = false;

    const ERR_NONE = 0; // no error
    const ERR_NO_RESPONSE = 1; // no response
    const ERR_UNREACHABLE = 2; // can't connect
    const ERR_UNEXPECTED = 3; // response gave some error

    const READ_LATEST = 1; // use latest data for replicated stores
    const READ_VERIFIED = 2; // promise that caller can tell when keys are stale
    const WRITE_SYNC = 1; // synchronously write to all locations for replicated stores
    const WRITE_CACHE_ONLY = 2; // Only change state of the in-memory cache

    public function __construct( array $params = [] ) {
        if ( isset( $params['logger'] ) ) {
            $this->setLogger( $params['logger'] );
        } else {
            $this->setLogger( new NullLogger() );
        }

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

        $this->asyncHandler = isset( $params['asyncHandler'] )
            ? $params['asyncHandler']
            : null;

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

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

    public function setDebug( $bool ) {
        $this->debugMode = $bool;
    }

    final public function getWithSetCallback( $key, $ttl, $callback, $flags = 0 ) {
        $value = $this->get( $key, $flags );

        if ( $value === false ) {
            if ( !is_callable( $callback ) ) {
                throw new InvalidArgumentException( "Invalid cache miss callback provided." );
            }
            $value = call_user_func( $callback );
            if ( $value !== false ) {
                $this->set( $key, $value, $ttl );
            }
        }

        return $value;
    }

    public function get( $key, $flags = 0, $oldFlags = null ) {
        // B/C for ( $key, &$casToken = null, $flags = 0 )
        $flags = is_int( $oldFlags ) ? $oldFlags : $flags;

        $this->trackDuplicateKeys( $key );

        return $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 );

    protected function getWithToken( $key, &$casToken, $flags = 0 ) {
        throw new Exception( __METHOD__ . ' not implemented.' );
    }

    abstract public function set( $key, $value, $exptime = 0, $flags = 0 );

    abstract public function delete( $key );

    public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
        return $this->mergeViaLock( $key, $callback, $exptime, $attempts, $flags );
    }

    protected function mergeViaCas( $key, $callback, $exptime = 0, $attempts = 10 ) {
        do {
            $this->clearLastError();
            $reportDupes = $this->reportDupes;
            $this->reportDupes = false;
            $casToken = null; // passed by reference
            $currentValue = $this->getWithToken( $key, $casToken, self::READ_LATEST );
            $this->reportDupes = $reportDupes;

            if ( $this->getLastError() ) {
                return false; // don't spam retries (retry only on races)
            }

            // Derive the new value from the old value
            $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );

            $this->clearLastError();
            if ( $value === false ) {
                $success = true; // do nothing
            } elseif ( $currentValue === false ) {
                // Try to create the key, failing if it gets created in the meantime
                $success = $this->add( $key, $value, $exptime );
            } else {
                // Try to update the key, failing if it gets changed in the meantime
                $success = $this->cas( $casToken, $key, $value, $exptime );
            }
            if ( $this->getLastError() ) {
                return false; // IO error; don't spam retries
            }
        } while ( !$success && --$attempts );

        return $success;
    }

    protected function cas( $casToken, $key, $value, $exptime = 0 ) {
        throw new Exception( "CAS is not implemented in " . __CLASS__ );
    }

    protected function mergeViaLock( $key, $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
        if ( !$this->lock( $key, 6 ) ) {
            return false;
        }

        $this->clearLastError();
        $reportDupes = $this->reportDupes;
        $this->reportDupes = false;
        $currentValue = $this->get( $key, self::READ_LATEST );
        $this->reportDupes = $reportDupes;

        if ( $this->getLastError() ) {
            $success = false;
        } else {
            // Derive the new value from the old value
            $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );
            if ( $value === false ) {
                $success = true; // do nothing
            } else {
                $success = $this->set( $key, $value, $exptime, $flags ); // set the new value
            }
        }

        if ( !$this->unlock( $key ) ) {
            // this should never happen
            trigger_error( "Could not release lock for key '$key'." );
        }

        return $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;
            }
        }

        $expiry = min( $expiry ?: INF, self::TTL_DAY );

        $this->clearLastError();
        $timestamp = microtime( true ); // starting UNIX timestamp
        if ( $this->add( "{$key}:lock", 1, $expiry ) ) {
            $locked = true;
        } elseif ( $this->getLastError() || $timeout <= 0 ) {
            $locked = false; // network partition or non-blocking
        } else {
            // Estimate the RTT (us); use 1ms minimum for sanity
            $uRTT = max( 1e3, ceil( 1e6 * ( microtime( true ) - $timestamp ) ) );
            $sleep = 2 * $uRTT; // rough time to do get()+set()

            $attempts = 0; // failed attempts
            do {
                if ( ++$attempts >= 3 && $sleep <= 5e5 ) {
                    // Exponentially back off after failed attempts to avoid network spam.
                    // About 2*$uRTT*(2^n-1) us of "sleep" happen for the next n attempts.
                    $sleep *= 2;
                }
                usleep( $sleep ); // back off
                $this->clearLastError();
                $locked = $this->add( "{$key}:lock", 1, $expiry );
                if ( $this->getLastError() ) {
                    $locked = false; // network partition
                    break;
                }
            } while ( !$locked && ( microtime( true ) - $timestamp ) < $timeout );
        }

        if ( $locked ) {
            $this->locks[$key] = [ 'class' => $rclass, 'depth' => 1 ];
        }

        return $locked;
    }

    public function unlock( $key ) {
        if ( isset( $this->locks[$key] ) && --$this->locks[$key]['depth'] <= 0 ) {
            unset( $this->locks[$key] );

            return $this->delete( "{$key}:lock" );
        }

        return true;
    }

    final public function getScopedLock( $key, $timeout = 6, $expiry = 30, $rclass = '' ) {
        $expiry = min( $expiry ?: INF, self::TTL_DAY );

        if ( !$this->lock( $key, $timeout, $expiry, $rclass ) ) {
            return null;
        }

        $lSince = microtime( true ); // lock timestamp

        return new ScopedCallback( function() use ( $key, $lSince, $expiry ) {
            $latency = .050; // latency skew (err towards keeping lock present)
            $age = ( microtime( true ) - $lSince + $latency );
            if ( ( $age + $latency ) >= $expiry ) {
                $this->logger->warning( "Lock for $key held too long ($age sec)." );
                return; // expired; it's not "safe" to delete the key
            }
            $this->unlock( $key );
        } );
    }

    public function deleteObjectsExpiringBefore( $date, $progressCallback = false ) {
        // stub
        return false;
    }

    public function getMulti( array $keys, $flags = 0 ) {
        $res = [];
        foreach ( $keys as $key ) {
            $val = $this->get( $key );
            if ( $val !== false ) {
                $res[$key] = $val;
            }
        }
        return $res;
    }

    public function setMulti( array $data, $exptime = 0 ) {
        $res = true;
        foreach ( $data as $key => $value ) {
            if ( !$this->set( $key, $value, $exptime ) ) {
                $res = false;
            }
        }
        return $res;
    }

    public function add( $key, $value, $exptime = 0 ) {
        if ( $this->get( $key ) === false ) {
            return $this->set( $key, $value, $exptime );
        }
        return false; // key already set
    }

    public function incr( $key, $value = 1 ) {
        if ( !$this->lock( $key ) ) {
            return false;
        }
        $n = $this->get( $key );
        if ( $this->isInteger( $n ) ) { // key exists?
            $n += intval( $value );
            $this->set( $key, max( 0, $n ) ); // exptime?
        } else {
            $n = false;
        }
        $this->unlock( $key );

        return $n;
    }

    public function decr( $key, $value = 1 ) {
        return $this->incr( $key, - $value );
    }

    public function incrWithInit( $key, $ttl, $value = 1, $init = 1 ) {
        $newValue = $this->incr( $key, $value );
        if ( $newValue === false ) {
            // No key set; initialize
            $newValue = $this->add( $key, (int)$init, $ttl ) ? $init : false;
        }
        if ( $newValue === false ) {
            // Raced out initializing; increment
            $newValue = $this->incr( $key, $value );
        }

        return $newValue;
    }

    public function getLastError() {
        return $this->lastError;
    }

    public function clearLastError() {
        $this->lastError = self::ERR_NONE;
    }

    protected function setLastError( $err ) {
        $this->lastError = $err;
    }

    public function modifySimpleRelayEvent( array $event ) {
        return $event;
    }

    protected function debug( $text ) {
        if ( $this->debugMode ) {
            $this->logger->debug( "{class} debug: $text", [
                'class' => get_class( $this ),
            ] );
        }
    }

    protected function convertExpiry( $exptime ) {
        if ( $exptime != 0 && $exptime < ( 10 * self::TTL_YEAR ) ) {
            return time() + $exptime;
        } else {
            return $exptime;
        }
    }

    protected function convertToRelative( $exptime ) {
        if ( $exptime >= ( 10 * self::TTL_YEAR ) ) {
            $exptime -= time();
            if ( $exptime <= 0 ) {
                $exptime = 1;
            }
            return $exptime;
        } else {
            return $exptime;
        }
    }

    protected function isInteger( $value ) {
        return ( is_int( $value ) || ctype_digit( $value ) );
    }

    public function makeKeyInternal( $keyspace, $args ) {
        $key = $keyspace;
        foreach ( $args as $arg ) {
            $arg = str_replace( ':', '%3A', $arg );
            $key = $key . ':' . $arg;
        }
        return strtr( $key, ' ', '_' );
    }

    public function makeGlobalKey() {
        return $this->makeKeyInternal( 'global', func_get_args() );
    }

    public function makeKey() {
        return $this->makeKeyInternal( $this->keyspace, func_get_args() );
    }
}