MemcLockManager.php

Go to the documentation of this file.

<?php
class MemcLockManager extends QuorumLockManager {
        protected $lockTypeMap = array(
                self::LOCK_SH => self::LOCK_SH,
                self::LOCK_UW => self::LOCK_SH,
                self::LOCK_EX => self::LOCK_EX
        );

        protected $bagOStuffs = array();
        protected $serversUp = array(); // (server name => bool)

        protected $session = ''; // string; random UUID

        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 = isset( $config['memcConfig'] )
                        ? $config['memcConfig']
                        : array( 'class' => 'MemcachedPhpBagOStuff' );

                foreach ( $config['lockServers'] as $name => $address ) {
                        $params = array( 'servers' => array( $address ) ) + $memcConfig;
                        $cache = ObjectCache::newFromParams( $params );
                        if ( $cache instanceof MemcachedBagOStuff ) {
                                $this->bagOStuffs[$name] = $cache;
                        } else {
                                throw new MWException(
                                        'Only MemcachedBagOStuff classes are supported by MemcLockManager.' );
                        }
                }

                $this->session = wfRandomString( 32 );
        }

        protected function getLocksOnServer( $lockSrv, array $paths, $type ) {
                $status = Status::newGood();

                $memc = $this->getCache( $lockSrv );
                $keys = array_map( array( $this, 'recordKeyForPath' ), $paths ); // lock records

                // 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 ( $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, $locksHeld, 7*86400 );
                                if ( !$ok ) {
                                        $status->fatal( 'lockmanager-fail-acquirelock', $path );
                                } else {
                                        wfDebug( __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 $paths, $type ) {
                $status = Status::newGood();

                $memc = $this->getCache( $lockSrv );
                $keys = array_map( array( $this, 'recordKeyForPath' ), $paths ); // lock records

                // Lock all of the active lock record keys...
                if ( !$this->acquireMutexes( $memc, $keys ) ) {
                        foreach ( $paths as $path ) {
                                $status->fatal( 'lockmanager-fail-releaselock', $path );
                        }
                        return;
                }

                // Fetch all the existing lock records...
                $lockRecords = $memc->getMulti( $keys );

                // Remove the requested locks from all records...
                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 = self::sanitizeLockArray( $lockRecords[$locksKey] );
                        if ( isset( $locksHeld[$type][$this->session] ) ) {
                                unset( $locksHeld[$type][$this->session] ); // unregister this session
                                if ( $locksHeld === self::newLockArray() ) {
                                        $ok = $memc->delete( $locksKey );
                                } else {
                                        $ok = $memc->set( $locksKey, $locksHeld );
                                }
                                if ( !$ok ) {
                                        $status->fatal( 'lockmanager-fail-releaselock', $path );
                                }
                        } else {
                                $status->warning( 'lockmanager-fail-releaselock', $path );
                        }
                        wfDebug( __METHOD__ . ": released lock on key $locksKey.\n" );
                }

                // Unlock all of the active lock record keys...
                $this->releaseMutexes( $memc, $keys );

                return $status;
        }

        protected function releaseAllLocks() {
                return Status::newGood(); // not supported
        }

        protected function isServerUp( $lockSrv ) {
                return (bool)$this->getCache( $lockSrv );
        }

        protected function getCache( $lockSrv ) {
                $memc = null;
                if ( isset( $this->bagOStuffs[$lockSrv] ) ) {
                        $memc = $this->bagOStuffs[$lockSrv];
                        if ( !isset( $this->serversUp[$lockSrv] ) ) {
                                $this->serversUp[$lockSrv] = $memc->set( __CLASS__ . ':ping', 1, 1 );
                                if ( !$this->serversUp[$lockSrv] ) {
                                        trigger_error( __METHOD__ . ": Could not contact $lockSrv.", E_USER_WARNING );
                                }
                        }
                        if ( !$this->serversUp[$lockSrv] ) {
                                return null; // server appears to be down
                        }
                }
                return $memc;
        }

        protected function recordKeyForPath( $path ) {
                return implode( ':', array( __CLASS__, 'locks', $this->sha1Base36Absolute( $path ) ) );
        }

        protected static function newLockArray() {
                return array( self::LOCK_SH => array(), self::LOCK_EX => array() );
        }

        protected static function sanitizeLockArray( $a ) {
                if ( is_array( $a ) && isset( $a[self::LOCK_EX] ) && isset( $a[self::LOCK_SH] ) ) {
                        return $a;
                } else {
                        trigger_error( __METHOD__ . ": reset invalid lock array.", E_USER_WARNING );
                        return self::newLockArray();
                }
        }

        protected function acquireMutexes( MemcachedBagOStuff $memc, array $keys ) {
                $lockedKeys = array();

                // 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.
                $rounds = 0;
                $start = microtime( true );
                do {
                        if ( ( ++$rounds % 4 ) == 0 ) {
                                usleep( 1000*50 ); // 50 ms
                        }
                        foreach ( array_diff( $keys, $lockedKeys ) as $key ) {
                                if ( $memc->add( "$key:mutex", 1, 180 ) ) { // lock record
                                        $lockedKeys[] = $key;
                                } else {
                                        continue; // acquire in order
                                }
                        }
                } while ( count( $lockedKeys ) < count( $keys ) && ( microtime( true ) - $start ) <= 3 );

                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( array( $path ), self::LOCK_EX );
                                $this->doUnlock( array( $path ), self::LOCK_SH );
                        }
                }
        }
}