MemcLockManager.php

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<?php
use Wikimedia\WaitConditionLoop;
 
class MemcLockManager extends QuorumLockManager {
    protected $lockTypeMap = [
        self::LOCK_SH => self::LOCK_SH,
        self::LOCK_UW => self::LOCK_SH,
        self::LOCK_EX => self::LOCK_EX
    ];
 
    protected $cacheServers = [];
    protected $statusCache;
 
    public function __construct( array $config ) {
        parent::__construct( $config );
 
        // Sanitize srvsByBucket config to prevent PHP errors
        $this->srvsByBucket = array_filter( $config['srvsByBucket'], 'is_array' );
        $this->srvsByBucket = array_values( $this->srvsByBucket ); // consecutive
 
        $memcConfig = $config['memcConfig'] ?? [];
        $memcConfig += [ 'class' => MemcachedPhpBagOStuff::class ]; // default
 
        $class = $memcConfig['class'];
        if ( !is_subclass_of( $class, MemcachedBagOStuff::class ) ) {
            throw new InvalidArgumentException( "$class is not of type MemcachedBagOStuff." );
        }
 
        foreach ( $config['lockServers'] as $name => $address ) {
            $params = [ 'servers' => [ $address ] ] + $memcConfig;
            $this->cacheServers[$name] = new $class( $params );
        }
 
        $this->statusCache = new MapCacheLRU( 100 );
    }
 
    protected function getLocksOnServer( $lockSrv, array $pathsByType ) {
        $status = StatusValue::newGood();
 
        $memc = $this->getCache( $lockSrv );
        // List of affected paths
        $paths = array_merge( ...array_values( $pathsByType ) );
        $paths = array_unique( $paths );
        // List of affected lock record keys
        $keys = array_map( [ $this, 'recordKeyForPath' ], $paths );
 
        // Lock all of the active lock record keys...
        if ( !$this->acquireMutexes( $memc, $keys ) ) {
            foreach ( $paths as $path ) {
                $status->fatal( 'lockmanager-fail-acquirelock', $path );
            }
 
            return $status;
        }
 
        // Fetch all the existing lock records...
        $lockRecords = $memc->getMulti( $keys );
 
        $now = time();
        // Check if the requested locks conflict with existing ones...
        foreach ( $pathsByType as $type => $paths ) {
            foreach ( $paths as $path ) {
                $locksKey = $this->recordKeyForPath( $path );
                $locksHeld = isset( $lockRecords[$locksKey] )
                    ? self::sanitizeLockArray( $lockRecords[$locksKey] )
                    : self::newLockArray(); // init
                foreach ( $locksHeld[self::LOCK_EX] as $session => $expiry ) {
                    if ( $expiry < $now ) { // stale?
                        unset( $locksHeld[self::LOCK_EX][$session] );
                    } elseif ( $session !== $this->session ) {
                        $status->fatal( 'lockmanager-fail-acquirelock', $path );
                    }
                }
                if ( $type === self::LOCK_EX ) {
                    foreach ( $locksHeld[self::LOCK_SH] as $session => $expiry ) {
                        if ( $expiry < $now ) { // stale?
                            unset( $locksHeld[self::LOCK_SH][$session] );
                        } elseif ( $session !== $this->session ) {
                            $status->fatal( 'lockmanager-fail-acquirelock', $path );
                        }
                    }
                }
                if ( $status->isOK() ) {
                    // Register the session in the lock record array
                    $locksHeld[$type][$this->session] = $now + $this->lockTTL;
                    // We will update this record if none of the other locks conflict
                    $lockRecords[$locksKey] = $locksHeld;
                }
            }
        }
 
        // If there were no lock conflicts, update all the lock records...
        if ( $status->isOK() ) {
            foreach ( $paths as $path ) {
                $locksKey = $this->recordKeyForPath( $path );
                $locksHeld = $lockRecords[$locksKey];
                $ok = $memc->set( $locksKey, $locksHeld, self::MAX_LOCK_TTL );
                if ( !$ok ) {
                    $status->fatal( 'lockmanager-fail-acquirelock', $path );
                } else {
                    $this->logger->debug( __METHOD__ . ": acquired lock on key $locksKey.\n" );
                }
            }
        }
 
        // Unlock all of the active lock record keys...
        $this->releaseMutexes( $memc, $keys );
 
        return $status;
    }
 
    protected function freeLocksOnServer( $lockSrv, array $pathsByType ) {
        $status = StatusValue::newGood();
 
        $memc = $this->getCache( $lockSrv );
        // List of affected paths
        $paths = array_merge( ...array_values( $pathsByType ) );
        $paths = array_unique( $paths );
        // List of affected lock record keys
        $keys = array_map( [ $this, 'recordKeyForPath' ], $paths );
 
        // Lock all of the active lock record keys...
        if ( !$this->acquireMutexes( $memc, $keys ) ) {
            foreach ( $paths as $path ) {
                $status->fatal( 'lockmanager-fail-releaselock', $path );
            }
 
            return $status;
        }
 
        // Fetch all the existing lock records...
        $lockRecords = $memc->getMulti( $keys );
 
        // Remove the requested locks from all records...
        foreach ( $pathsByType as $type => $paths ) {
            foreach ( $paths as $path ) {
                $locksKey = $this->recordKeyForPath( $path ); // lock record
                if ( !isset( $lockRecords[$locksKey] ) ) {
                    $status->warning( 'lockmanager-fail-releaselock', $path );
                    continue; // nothing to do
                }
                $locksHeld = $this->sanitizeLockArray( $lockRecords[$locksKey] );
                if ( isset( $locksHeld[$type][$this->session] ) ) {
                    unset( $locksHeld[$type][$this->session] ); // unregister this session
                    $lockRecords[$locksKey] = $locksHeld;
                } else {
                    $status->warning( 'lockmanager-fail-releaselock', $path );
                }
            }
        }
 
        // Persist the new lock record values...
        foreach ( $paths as $path ) {
            $locksKey = $this->recordKeyForPath( $path );
            if ( !isset( $lockRecords[$locksKey] ) ) {
                continue; // nothing to do
            }
            $locksHeld = $lockRecords[$locksKey];
            if ( $locksHeld === $this->newLockArray() ) {
                $ok = $memc->delete( $locksKey );
            } else {
                $ok = $memc->set( $locksKey, $locksHeld, self::MAX_LOCK_TTL );
            }
            if ( $ok ) {
                $this->logger->debug( __METHOD__ . ": released lock on key $locksKey.\n" );
            } else {
                $status->fatal( 'lockmanager-fail-releaselock', $path );
            }
        }
 
        // Unlock all of the active lock record keys...
        $this->releaseMutexes( $memc, $keys );
 
        return $status;
    }
 
    protected function releaseAllLocks() {
        return StatusValue::newGood(); // not supported
    }
 
    protected function isServerUp( $lockSrv ) {
        return (bool)$this->getCache( $lockSrv );
    }
 
    protected function getCache( $lockSrv ) {
        if ( !isset( $this->cacheServers[$lockSrv] ) ) {
            throw new InvalidArgumentException( "Invalid cache server '$lockSrv'." );
        }
 
        $online = $this->statusCache->get( "online:$lockSrv", 30 );
        if ( $online === null ) {
            $online = $this->cacheServers[$lockSrv]->set( __CLASS__ . ':ping', 1, 1 );
            if ( !$online ) { // server down?
                $this->logger->warning( __METHOD__ . ": Could not contact $lockSrv." );
            }
            $this->statusCache->set( "online:$lockSrv", (int)$online );
        }
 
        return $online ? $this->cacheServers[$lockSrv] : null;
    }
 
    protected function recordKeyForPath( $path ) {
        return implode( ':', [ __CLASS__, 'locks', $this->sha1Base36Absolute( $path ) ] );
    }
 
    protected function newLockArray() {
        return [ self::LOCK_SH => [], self::LOCK_EX => [] ];
    }
 
    protected function sanitizeLockArray( $a ) {
        if ( is_array( $a ) && isset( $a[self::LOCK_EX] ) && isset( $a[self::LOCK_SH] ) ) {
            return $a;
        }
 
        $this->logger->error( __METHOD__ . ": reset invalid lock array." );
 
        return $this->newLockArray();
    }
 
    protected function acquireMutexes( MemcachedBagOStuff $memc, array $keys ) {
        $lockedKeys = [];
 
        // Acquire the keys in lexicographical order, to avoid deadlock problems.
        // If P1 is waiting to acquire a key P2 has, P2 can't also be waiting for a key P1 has.
        sort( $keys );
 
        // Try to quickly loop to acquire the keys, but back off after a few rounds.
        // This reduces memcached spam, especially in the rare case where a server acquires
        // some lock keys and dies without releasing them. Lock keys expire after a few minutes.
        $loop = new WaitConditionLoop(
            function () use ( $memc, $keys, &$lockedKeys ) {
                foreach ( array_diff( $keys, $lockedKeys ) as $key ) {
                    if ( $memc->add( "$key:mutex", 1, 180 ) ) { // lock record
                        $lockedKeys[] = $key;
                    }
                }
 
                return array_diff( $keys, $lockedKeys )
                    ? WaitConditionLoop::CONDITION_CONTINUE
                    : true;
            },
            3.0 // timeout
        );
        $loop->invoke();
 
        if ( count( $lockedKeys ) != count( $keys ) ) {
            $this->releaseMutexes( $memc, $lockedKeys ); // failed; release what was locked
            return false;
        }
 
        return true;
    }
 
    protected function releaseMutexes( MemcachedBagOStuff $memc, array $keys ) {
        foreach ( $keys as $key ) {
            $memc->delete( "$key:mutex" );
        }
    }
 
    function __destruct() {
        while ( count( $this->locksHeld ) ) {
            foreach ( $this->locksHeld as $path => $locks ) {
                $this->doUnlock( [ $path ], self::LOCK_EX );
                $this->doUnlock( [ $path ], self::LOCK_SH );
            }
        }
    }
}