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[mediawiki.git] / includes / filerepo / backend / lockmanager / LockManager.php

<?php
/**
 * @defgroup LockManager Lock management
 * @ingroup FileBackend
 */

/**
 * Resource locking handling.
 *
 * 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
 * @author Aaron Schulz
 */

/**
 * @brief Class for handling resource locking.
 *
 * Locks on resource keys can either be shared or exclusive.
 *
 * Implementations must keep track of what is locked by this proccess
 * in-memory and support nested locking calls (using reference counting).
 * At least LOCK_UW and LOCK_EX must be implemented. LOCK_SH can be a no-op.
 * Locks should either be non-blocking or have low wait timeouts.
 *
 * Subclasses should avoid throwing exceptions at all costs.
 *
 * @ingroup LockManager
 * @since 1.19
 */
abstract class LockManager {
        /** @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_EX, // subclasses may use self::LOCK_SH
                self::LOCK_EX => self::LOCK_EX
        );

        /** @var Array Map of (resource path => lock type => count) */
        protected $locksHeld = array();

        /* Lock types; stronger locks have higher values */
        const LOCK_SH = 1; // shared lock (for reads)
        const LOCK_UW = 2; // shared lock (for reads used to write elsewhere)
        const LOCK_EX = 3; // exclusive lock (for writes)

        /**
         * Construct a new instance from configuration
         *
         * @param $config Array
         */
        public function __construct( array $config ) {}

        /**
         * Lock the resources at the given abstract paths
         *
         * @param $paths Array List of resource names
         * @param $type integer LockManager::LOCK_* constant
         * @return Status
         */
        final public function lock( array $paths, $type = self::LOCK_EX ) {
                wfProfileIn( __METHOD__ );
                $status = $this->doLock( array_unique( $paths ), $this->lockTypeMap[$type] );
                wfProfileOut( __METHOD__ );
                return $status;
        }

        /**
         * Unlock the resources at the given abstract paths
         *
         * @param $paths Array List of storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @return Status
         */
        final public function unlock( array $paths, $type = self::LOCK_EX ) {
                wfProfileIn( __METHOD__ );
                $status = $this->doUnlock( array_unique( $paths ), $this->lockTypeMap[$type] );
                wfProfileOut( __METHOD__ );
                return $status;
        }

        /**
         * Get the base 36 SHA-1 of a string, padded to 31 digits
         *
         * @param $path string
         * @return string
         */
        final protected static function sha1Base36( $path ) {
                return wfBaseConvert( sha1( $path ), 16, 36, 31 );
        }

        /**
         * Lock resources with the given keys and lock type
         *
         * @param $paths Array List of storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @return string
         */
        abstract protected function doLock( array $paths, $type );

        /**
         * Unlock resources with the given keys and lock type
         *
         * @param $paths Array List of storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @return string
         */
        abstract protected function doUnlock( array $paths, $type );
}

/**
 * Self-releasing locks
 *
 * LockManager helper class to handle scoped locks, which
 * release when an object is destroyed or goes out of scope.
 *
 * @ingroup LockManager
 * @since 1.19
 */
class ScopedLock {
        /** @var LockManager */
        protected $manager;
        /** @var Status */
        protected $status;
        /** @var Array List of resource paths*/
        protected $paths;

        protected $type; // integer lock type

        /**
         * @param $manager LockManager
         * @param $paths Array List of storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @param $status Status
         */
        protected function __construct(
                LockManager $manager, array $paths, $type, Status $status
        ) {
                $this->manager = $manager;
                $this->paths = $paths;
                $this->status = $status;
                $this->type = $type;
        }

        /**
         * Get a ScopedLock object representing a lock on resource paths.
         * Any locks are released once this object goes out of scope.
         * The status object is updated with any errors or warnings.
         *
         * @param $manager LockManager
         * @param $paths Array List of storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @param $status Status
         * @return ScopedLock|null Returns null on failure
         */
        public static function factory(
                LockManager $manager, array $paths, $type, Status $status
        ) {
                $lockStatus = $manager->lock( $paths, $type );
                $status->merge( $lockStatus );
                if ( $lockStatus->isOK() ) {
                        return new self( $manager, $paths, $type, $status );
                }
                return null;
        }

        function __destruct() {
                $wasOk = $this->status->isOK();
                $this->status->merge( $this->manager->unlock( $this->paths, $this->type ) );
                if ( $wasOk ) {
                        // Make sure status is OK, despite any unlockFiles() fatals
                        $this->status->setResult( true, $this->status->value );
                }
        }
}

/**
 * Version of LockManager that uses a quorum from peer servers for locks.
 * The resource space can also be sharded into separate peer groups.
 *
 * @ingroup LockManager
 * @since 1.20
 */
abstract class QuorumLockManager extends LockManager {
        /** @var Array Map of bucket indexes to peer server lists */
        protected $srvsByBucket = array(); // (bucket index => (lsrv1, lsrv2, ...))

        /**
         * @see LockManager::doLock()
         * @param $paths array
         * @param $type int
         * @return Status
         */
        final protected function doLock( array $paths, $type ) {
                $status = Status::newGood();

                $pathsToLock = array(); // (bucket => paths)
                // Get locks that need to be acquired (buckets => locks)...
                foreach ( $paths as $path ) {
                        if ( isset( $this->locksHeld[$path][$type] ) ) {
                                ++$this->locksHeld[$path][$type];
                        } elseif ( isset( $this->locksHeld[$path][self::LOCK_EX] ) ) {
                                $this->locksHeld[$path][$type] = 1;
                        } else {
                                $bucket = $this->getBucketFromKey( $path );
                                $pathsToLock[$bucket][] = $path;
                        }
                }

                $lockedPaths = array(); // files locked in this attempt
                // Attempt to acquire these locks...
                foreach ( $pathsToLock as $bucket => $paths ) {
                        // Try to acquire the locks for this bucket
                        $status->merge( $this->doLockingRequestBucket( $bucket, $paths, $type ) );
                        if ( !$status->isOK() ) {
                                $status->merge( $this->doUnlock( $lockedPaths, $type ) );
                                return $status;
                        }
                        // Record these locks as active
                        foreach ( $paths as $path ) {
                                $this->locksHeld[$path][$type] = 1; // locked
                        }
                        // Keep track of what locks were made in this attempt
                        $lockedPaths = array_merge( $lockedPaths, $paths );
                }

                return $status;
        }

        /**
         * @see LockManager::doUnlock()
         * @param $paths array
         * @param $type int
         * @return Status
         */
        final protected function doUnlock( array $paths, $type ) {
                $status = Status::newGood();

                $pathsToUnlock = array();
                foreach ( $paths as $path ) {
                        if ( !isset( $this->locksHeld[$path][$type] ) ) {
                                $status->warning( 'lockmanager-notlocked', $path );
                        } else {
                                --$this->locksHeld[$path][$type];
                                // Reference count the locks held and release locks when zero
                                if ( $this->locksHeld[$path][$type] <= 0 ) {
                                        unset( $this->locksHeld[$path][$type] );
                                        $bucket = $this->getBucketFromKey( $path );
                                        $pathsToUnlock[$bucket][] = $path;
                                }
                                if ( !count( $this->locksHeld[$path] ) ) {
                                        unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
                                }
                        }
                }

                // Remove these specific locks if possible, or at least release
                // all locks once this process is currently not holding any locks.
                foreach ( $pathsToUnlock as $bucket => $paths ) {
                        $status->merge( $this->doUnlockingRequestBucket( $bucket, $paths, $type ) );
                }
                if ( !count( $this->locksHeld ) ) {
                        $status->merge( $this->releaseAllLocks() );
                }

                return $status;
        }

        /**
         * Attempt to acquire locks with the peers for a bucket.
         * This is all or nothing; if any key is locked then this totally fails.
         *
         * @param $bucket integer
         * @param $paths Array List of resource keys to lock
         * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
         * @return Status
         */
        final protected function doLockingRequestBucket( $bucket, array $paths, $type ) {
                $status = Status::newGood();

                $yesVotes = 0; // locks made on trustable servers
                $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
                $quorum = floor( $votesLeft/2 + 1 ); // simple majority
                // Get votes for each peer, in order, until we have enough...
                foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
                        if ( !$this->isServerUp( $lockSrv ) ) {
                                --$votesLeft;
                                $status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
                                continue; // server down?
                        }
                        // Attempt to acquire the lock on this peer
                        $status->merge( $this->getLocksOnServer( $lockSrv, $paths, $type ) );
                        if ( !$status->isOK() ) {
                                return $status; // vetoed; resource locked
                        }
                        ++$yesVotes; // success for this peer
                        if ( $yesVotes >= $quorum ) {
                                return $status; // lock obtained
                        }
                        --$votesLeft;
                        $votesNeeded = $quorum - $yesVotes;
                        if ( $votesNeeded > $votesLeft ) {
                                break; // short-circuit
                        }
                }
                // At this point, we must not have met the quorum
                $status->setResult( false );

                return $status;
        }

        /**
         * Attempt to release locks with the peers for a bucket
         *
         * @param $bucket integer
         * @param $paths Array List of resource keys to lock
         * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
         * @return Status
         */
        final protected function doUnlockingRequestBucket( $bucket, array $paths, $type ) {
                $status = Status::newGood();

                foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
                        if ( !$this->isServerUp( $lockSrv ) ) {
                                $status->fatal( 'lockmanager-fail-svr-release', $lockSrv );
                        // Attempt to release the lock on this peer
                        } else {
                                $status->merge( $this->freeLocksOnServer( $lockSrv, $paths, $type ) );
                        }
                }

                return $status;
        }

        /**
         * Get the bucket for resource path.
         * This should avoid throwing any exceptions.
         *
         * @param $path string
         * @return integer
         */
        protected function getBucketFromKey( $path ) {
                $prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
                return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
        }

        /**
         * Check if a lock server is up
         *
         * @param $lockSrv string
         * @return bool
         */
        abstract protected function isServerUp( $lockSrv );

        /**
         * Get a connection to a lock server and acquire locks on $paths
         *
         * @param $lockSrv string
         * @param $paths array
         * @param $type integer
         * @return Status
         */
        abstract protected function getLocksOnServer( $lockSrv, array $paths, $type );

        /**
         * Get a connection to a lock server and release locks on $paths.
         *
         * Subclasses must effectively implement this or releaseAllLocks().
         *
         * @param $lockSrv string
         * @param $paths array
         * @param $type integer
         * @return Status
         */
        abstract protected function freeLocksOnServer( $lockSrv, array $paths, $type );

        /**
         * Release all locks that this session is holding.
         *
         * Subclasses must effectively implement this or freeLocksOnServer().
         *
         * @return Status
         */
        abstract protected function releaseAllLocks();
}

/**
 * Simple version of LockManager that does nothing
 * @since 1.19
 */
class NullLockManager extends LockManager {
        /**
         * @see LockManager::doLock()
         * @param $paths array
         * @param $type int
         * @return Status
         */
        protected function doLock( array $paths, $type ) {
                return Status::newGood();
        }

        /**
         * @see LockManager::doUnlock()
         * @param $paths array
         * @param $type int
         * @return Status
         */
        protected function doUnlock( array $paths, $type ) {
                return Status::newGood();
        }
}