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[mediawiki.git] / includes / filebackend / lockmanager / MemcLockManager.php

<?php
/**
 * Version of LockManager based on using memcached servers.
 *
 * 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 LockManager
 */

/**
 * Manage locks using memcached servers.
 *
 * Version of LockManager based on using memcached servers.
 * This is meant for multi-wiki systems that may share files.
 * All locks are non-blocking, which avoids deadlocks.
 *
 * All lock requests for a resource, identified by a hash string, will map to one
 * bucket. Each bucket maps to one or several peer servers, each running memcached.
 * A majority of peers must agree for a lock to be acquired.
 *
 * @ingroup LockManager
 * @since 1.20
 */
class MemcLockManager extends QuorumLockManager {
        /** @var Array Mapping of lock types to the type actually used */
        protected $lockTypeMap = array(
                self::LOCK_SH => self::LOCK_SH,
                self::LOCK_UW => self::LOCK_SH,
                self::LOCK_EX => self::LOCK_EX
        );

        /** @var Array Map server names to MemcachedBagOStuff objects */
        protected $bagOStuffs = array();
        /** @var Array */
        protected $serversUp = array(); // (server name => bool)

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

        /**
         * Construct a new instance from configuration.
         *
         * $config paramaters include:
         *   - lockServers  : Associative array of server names to "<IP>:<port>" strings.
         *   - srvsByBucket : Array of 1-16 consecutive integer keys, starting from 0,
         *                    each having an odd-numbered list of server names (peers) as values.
         *   - memcConfig   : Configuration array for ObjectCache::newFromParams. [optional]
         *                    If set, this must use one of the memcached classes.
         *
         * @param array $config
         * @throws MWException
         */
        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 );
        }

        /**
         * @see QuorumLockManager::getLocksOnServer()
         * @return Status
         */
        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;
        }

        /**
         * @see QuorumLockManager::freeLocksOnServer()
         * @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;
        }

        /**
         * @see QuorumLockManager::releaseAllLocks()
         * @return Status
         */
        protected function releaseAllLocks() {
                return Status::newGood(); // not supported
        }

        /**
         * @see QuorumLockManager::isServerUp()
         * @return bool
         */
        protected function isServerUp( $lockSrv ) {
                return (bool)$this->getCache( $lockSrv );
        }

        /**
         * Get the MemcachedBagOStuff object for a $lockSrv
         *
         * @param string $lockSrv Server name
         * @return MemcachedBagOStuff|null
         */
        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;
        }

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

        /**
         * @return Array An empty lock structure for a key
         */
        protected static function newLockArray() {
                return array( self::LOCK_SH => array(), self::LOCK_EX => array() );
        }

        /**
         * @param $a array
         * @return Array An empty lock structure for a key
         */
        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();
                }
        }

        /**
         * @param $memc MemcachedBagOStuff
         * @param array $keys List of keys to acquire
         * @return bool
         */
        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;
        }

        /**
         * @param $memc MemcachedBagOStuff
         * @param array $keys List of acquired keys
         * @return void
         */
        protected function releaseMutexes( MemcachedBagOStuff $memc, array $keys ) {
                foreach ( $keys as $key ) {
                        $memc->delete( "$key:mutex" );
                }
        }

        /**
         * Make sure remaining locks get cleared for sanity
         */
        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 );
                        }
                }
        }
}