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[mediawiki.git] / includes / filerepo / backend / FileBackend.php

<?php
/**
 * @defgroup FileBackend File backend
 * @ingroup  FileRepo
 *
 * File backend is used to interact with file storage systems,
 * such as the local file system, NFS, or cloud storage systems.
 */

/**
 * Base class for all file backends.
 *
 * 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 FileBackend
 * @author Aaron Schulz
 */

/**
 * @brief Base class for all file backend classes (including multi-write backends).
 *
 * This class defines the methods as abstract that subclasses must implement.
 * Outside callers can assume that all backends will have these functions.
 *
 * All "storage paths" are of the format "mwstore://<backend>/<container>/<path>".
 * The <path> portion is a relative path that uses UNIX file system (FS) notation,
 * though any particular backend may not actually be using a local filesystem.
 * Therefore, the relative paths are only virtual.
 *
 * Backend contents are stored under wiki-specific container names by default.
 * For legacy reasons, this has no effect for the FS backend class, and per-wiki
 * segregation must be done by setting the container paths appropriately.
 *
 * FS-based backends are somewhat more restrictive due to the existence of real
 * directory files; a regular file cannot have the same name as a directory. Other
 * backends with virtual directories may not have this limitation. Callers should
 * store files in such a way that no files and directories are under the same path.
 *
 * Methods should avoid throwing exceptions at all costs.
 * As a corollary, external dependencies should be kept to a minimum.
 *
 * @ingroup FileBackend
 * @since 1.19
 */
abstract class FileBackend {
        protected $name; // string; unique backend name
        protected $wikiId; // string; unique wiki name
        protected $readOnly; // string; read-only explanation message
        protected $parallelize; // string; when to do operations in parallel
        protected $concurrency; // integer; how many operations can be done in parallel

        /** @var LockManager */
        protected $lockManager;
        /** @var FileJournal */
        protected $fileJournal;

        /**
         * Create a new backend instance from configuration.
         * This should only be called from within FileBackendGroup.
         *
         * $config includes:
         *     'name'        : The unique name of this backend.
         *                     This should consist of alphanumberic, '-', and '_' characters.
         *                     This name should not be changed after use.
         *     'wikiId'      : Prefix to container names that is unique to this wiki.
         *                     It should only consist of alphanumberic, '-', and '_' characters.
         *     'lockManager' : Registered name of a file lock manager to use.
         *     'fileJournal' : File journal configuration; see FileJournal::factory().
         *                     Journals simply log changes to files stored in the backend.
         *     'readOnly'    : Write operations are disallowed if this is a non-empty string.
         *                     It should be an explanation for the backend being read-only.
         *     'parallelize' : When to do file operations in parallel (when possible).
         *                     Allowed values are "implicit", "explicit" and "off".
         *     'concurrency' : How many file operations can be done in parallel.
         *
         * @param $config Array
         * @throws MWException
         */
        public function __construct( array $config ) {
                $this->name = $config['name'];
                if ( !preg_match( '!^[a-zA-Z0-9-_]{1,255}$!', $this->name ) ) {
                        throw new MWException( "Backend name `{$this->name}` is invalid." );
                }
                $this->wikiId = isset( $config['wikiId'] )
                        ? $config['wikiId']
                        : wfWikiID(); // e.g. "my_wiki-en_"
                $this->lockManager = ( $config['lockManager'] instanceof LockManager )
                        ? $config['lockManager']
                        : LockManagerGroup::singleton()->get( $config['lockManager'] );
                $this->fileJournal = isset( $config['fileJournal'] )
                        ? FileJournal::factory( $config['fileJournal'], $this->name )
                        : FileJournal::factory( array( 'class' => 'NullFileJournal' ), $this->name );
                $this->readOnly = isset( $config['readOnly'] )
                        ? (string)$config['readOnly']
                        : '';
                $this->parallelize = isset( $config['parallelize'] )
                        ? (string)$config['parallelize']
                        : 'off';
                $this->concurrency = isset( $config['concurrency'] )
                        ? (int)$config['concurrency']
                        : 50;
        }

        /**
         * Get the unique backend name.
         * We may have multiple different backends of the same type.
         * For example, we can have two Swift backends using different proxies.
         *
         * @return string
         */
        final public function getName() {
                return $this->name;
        }

        /**
         * Check if this backend is read-only
         *
         * @return bool
         */
        final public function isReadOnly() {
                return ( $this->readOnly != '' );
        }

        /**
         * Get an explanatory message if this backend is read-only
         *
         * @return string|bool Returns false if the backend is not read-only
         */
        final public function getReadOnlyReason() {
                return ( $this->readOnly != '' ) ? $this->readOnly : false;
        }

        /**
         * This is the main entry point into the backend for write operations.
         * Callers supply an ordered list of operations to perform as a transaction.
         * Files will be locked, the stat cache cleared, and then the operations attempted.
         * If any serious errors occur, all attempted operations will be rolled back.
         *
         * $ops is an array of arrays. The outer array holds a list of operations.
         * Each inner array is a set of key value pairs that specify an operation.
         *
         * Supported operations and their parameters:
         * a) Create a new file in storage with the contents of a string
         *     array(
         *         'op'                  => 'create',
         *         'dst'                 => <storage path>,
         *         'content'             => <string of new file contents>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>
         *     )
         * b) Copy a file system file into storage
         *     array(
         *         'op'                  => 'store',
         *         'src'                 => <file system path>,
         *         'dst'                 => <storage path>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>
         *     )
         * c) Copy a file within storage
         *     array(
         *         'op'                  => 'copy',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>
         *     )
         * d) Move a file within storage
         *     array(
         *         'op'                  => 'move',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>
         *     )
         * e) Delete a file within storage
         *     array(
         *         'op'                  => 'delete',
         *         'src'                 => <storage path>,
         *         'ignoreMissingSource' => <boolean>
         *     )
         * f) Do nothing (no-op)
         *     array(
         *         'op'                  => 'null',
         *     )
         *
         * Boolean flags for operations (operation-specific):
         * 'ignoreMissingSource' : The operation will simply succeed and do
         *                         nothing if the source file does not exist.
         * 'overwrite'           : Any destination file will be overwritten.
         * 'overwriteSame'       : An error will not be given if a file already
         *                         exists at the destination that has the same
         *                         contents as the new contents to be written there.
         *
         * $opts is an associative of boolean flags, including:
         * 'force'               : Operation precondition errors no longer trigger an abort.
         *                         Any remaining operations are still attempted. Unexpected
         *                         failures may still cause remaning operations to be aborted.
         * 'nonLocking'          : No locks are acquired for the operations.
         *                         This can increase performance for non-critical writes.
         *                         This has no effect unless the 'force' flag is set.
         * 'allowStale'          : Don't require the latest available data.
         *                         This can increase performance for non-critical writes.
         *                         This has no effect unless the 'force' flag is set.
         * 'nonJournaled'        : Don't log this operation batch in the file journal.
         *                         This limits the ability of recovery scripts.
         * 'parallelize'         : Try to do operations in parallel when possible.
         *
         * Remarks on locking:
         * File system paths given to operations should refer to files that are
         * already locked or otherwise safe from modification from other processes.
         * Normally these files will be new temp files, which should be adequate.
         *
         * Return value:
         * This returns a Status, which contains all warnings and fatals that occured
         * during the operation. The 'failCount', 'successCount', and 'success' members
         * will reflect each operation attempted. The status will be "OK" unless:
         *     a) unexpected operation errors occurred (network partitions, disk full...)
         *     b) significant operation errors occured and 'force' was not set
         *
         * @param $ops Array List of operations to execute in order
         * @param $opts Array Batch operation options
         * @return Status
         */
        final public function doOperations( array $ops, array $opts = array() ) {
                if ( $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                if ( empty( $opts['force'] ) ) { // sanity
                        unset( $opts['nonLocking'] );
                        unset( $opts['allowStale'] );
                }
                $opts['concurrency'] = 1; // off
                if ( $this->parallelize === 'implicit' ) {
                        if ( !isset( $opts['parallelize'] ) || $opts['parallelize'] ) {
                                $opts['concurrency'] = $this->concurrency;
                        }
                } elseif ( $this->parallelize === 'explicit' ) {
                        if ( !empty( $opts['parallelize'] ) ) {
                                $opts['concurrency'] = $this->concurrency;
                        }
                }
                return $this->doOperationsInternal( $ops, $opts );
        }

        /**
         * @see FileBackend::doOperations()
         */
        abstract protected function doOperationsInternal( array $ops, array $opts );

        /**
         * Same as doOperations() except it takes a single operation.
         * If you are doing a batch of operations that should either
         * all succeed or all fail, then use that function instead.
         *
         * @see FileBackend::doOperations()
         *
         * @param $op Array Operation
         * @param $opts Array Operation options
         * @return Status
         */
        final public function doOperation( array $op, array $opts = array() ) {
                return $this->doOperations( array( $op ), $opts );
        }

        /**
         * Performs a single create operation.
         * This sets $params['op'] to 'create' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param $params Array Operation parameters
         * @param $opts Array Operation options
         * @return Status
         */
        final public function create( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'create' ) + $params, $opts );
        }

        /**
         * Performs a single store operation.
         * This sets $params['op'] to 'store' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param $params Array Operation parameters
         * @param $opts Array Operation options
         * @return Status
         */
        final public function store( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'store' ) + $params, $opts );
        }

        /**
         * Performs a single copy operation.
         * This sets $params['op'] to 'copy' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param $params Array Operation parameters
         * @param $opts Array Operation options
         * @return Status
         */
        final public function copy( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'copy' ) + $params, $opts );
        }

        /**
         * Performs a single move operation.
         * This sets $params['op'] to 'move' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param $params Array Operation parameters
         * @param $opts Array Operation options
         * @return Status
         */
        final public function move( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'move' ) + $params, $opts );
        }

        /**
         * Performs a single delete operation.
         * This sets $params['op'] to 'delete' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param $params Array Operation parameters
         * @param $opts Array Operation options
         * @return Status
         */
        final public function delete( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'delete' ) + $params, $opts );
        }

        /**
         * Perform a set of independent file operations on some files.
         *
         * This does no locking, nor journaling, and possibly no stat calls.
         * Any destination files that already exist will be overwritten.
         * This should *only* be used on non-original files, like cache files.
         *
         * Supported operations and their parameters:
         * a) Create a new file in storage with the contents of a string
         *     array(
         *         'op'                  => 'create',
         *         'dst'                 => <storage path>,
         *         'content'             => <string of new file contents>
         *     )
         * b) Copy a file system file into storage
         *     array(
         *         'op'                  => 'store',
         *         'src'                 => <file system path>,
         *         'dst'                 => <storage path>
         *     )
         * c) Copy a file within storage
         *     array(
         *         'op'                  => 'copy',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>
         *     )
         * d) Move a file within storage
         *     array(
         *         'op'                  => 'move',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>
         *     )
         * e) Delete a file within storage
         *     array(
         *         'op'                  => 'delete',
         *         'src'                 => <storage path>,
         *         'ignoreMissingSource' => <boolean>
         *     )
         * f) Do nothing (no-op)
         *     array(
         *         'op'                  => 'null',
         *     )
         *
         * Boolean flags for operations (operation-specific):
         * 'ignoreMissingSource' : The operation will simply succeed and do
         *                         nothing if the source file does not exist.
         *
         * Return value:
         * This returns a Status, which contains all warnings and fatals that occured
         * during the operation. The 'failCount', 'successCount', and 'success' members
         * will reflect each operation attempted for the given files. The status will be
         * considered "OK" as long as no fatal errors occured.
         *
         * @param $ops Array Set of operations to execute
         * @return Status
         * @since 1.20
         */
        final public function doQuickOperations( array $ops ) {
                if ( $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                foreach ( $ops as &$op ) {
                        $op['overwrite'] = true; // avoids RTTs in key/value stores
                }
                return $this->doQuickOperationsInternal( $ops );
        }

        /**
         * @see FileBackend::doQuickOperations()
         * @since 1.20
         */
        abstract protected function doQuickOperationsInternal( array $ops );

        /**
         * Same as doQuickOperations() except it takes a single operation.
         * If you are doing a batch of operations, then use that function instead.
         *
         * @see FileBackend::doQuickOperations()
         *
         * @param $op Array Operation
         * @return Status
         * @since 1.20
         */
        final public function doQuickOperation( array $op ) {
                return $this->doQuickOperations( array( $op ) );
        }

        /**
         * Performs a single quick create operation.
         * This sets $params['op'] to 'create' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param $params Array Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickCreate( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'create' ) + $params );
        }

        /**
         * Performs a single quick store operation.
         * This sets $params['op'] to 'store' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param $params Array Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickStore( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'store' ) + $params );
        }

        /**
         * Performs a single quick copy operation.
         * This sets $params['op'] to 'copy' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param $params Array Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickCopy( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'copy' ) + $params );
        }

        /**
         * Performs a single quick move operation.
         * This sets $params['op'] to 'move' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param $params Array Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickMove( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'move' ) + $params );
        }

        /**
         * Performs a single quick delete operation.
         * This sets $params['op'] to 'delete' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param $params Array Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickDelete( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'delete' ) + $params );
        }

        /**
         * Concatenate a list of storage files into a single file system file.
         * The target path should refer to a file that is already locked or
         * otherwise safe from modification from other processes. Normally,
         * the file will be a new temp file, which should be adequate.
         * $params include:
         *     srcs : ordered source storage paths (e.g. chunk1, chunk2, ...)
         *     dst  : file system path to 0-byte temp file
         *
         * @param $params Array Operation parameters
         * @return Status
         */
        abstract public function concatenate( array $params );

        /**
         * Prepare a storage directory for usage.
         * This will create any required containers and parent directories.
         * Backends using key/value stores only need to create the container.
         *
         * $params include:
         *     dir : storage directory
         *
         * @param $params Array
         * @return Status
         */
        final public function prepare( array $params ) {
                if ( $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                return $this->doPrepare( $params );
        }

        /**
         * @see FileBackend::prepare()
         */
        abstract protected function doPrepare( array $params );

        /**
         * Take measures to block web access to a storage directory and
         * the container it belongs to. FS backends might add .htaccess
         * files whereas key/value store backends might restrict container
         * access to the auth user that represents end-users in web request.
         * This is not guaranteed to actually do anything.
         *
         * $params include:
         *     dir       : storage directory
         *     noAccess  : try to deny file access
         *     noListing : try to deny file listing
         *
         * @param $params Array
         * @return Status
         */
        final public function secure( array $params ) {
                if ( $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                $status = $this->doPrepare( $params ); // dir must exist to restrict it
                if ( $status->isOK() ) {
                        $status->merge( $this->doSecure( $params ) );
                }
                return $status;
        }

        /**
         * @see FileBackend::secure()
         */
        abstract protected function doSecure( array $params );

        /**
         * Delete a storage directory if it is empty.
         * Backends using key/value stores may do nothing unless the directory
         * is that of an empty container, in which case it should be deleted.
         *
         * $params include:
         *     dir       : storage directory
         *     recursive : recursively delete empty subdirectories first (@since 1.20)
         *
         * @param $params Array
         * @return Status
         */
        final public function clean( array $params ) {
                if ( $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                return $this->doClean( $params );
        }

        /**
         * @see FileBackend::clean()
         */
        abstract protected function doClean( array $params );

        /**
         * Check if a file exists at a storage path in the backend.
         * This returns false if only a directory exists at the path.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return bool|null Returns null on failure
         */
        abstract public function fileExists( array $params );

        /**
         * Get the last-modified timestamp of the file at a storage path.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return string|bool TS_MW timestamp or false on failure
         */
        abstract public function getFileTimestamp( array $params );

        /**
         * Get the contents of a file at a storage path in the backend.
         * This should be avoided for potentially large files.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return string|bool Returns false on failure
         */
        abstract public function getFileContents( array $params );

        /**
         * Get the size (bytes) of a file at a storage path in the backend.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return integer|bool Returns false on failure
         */
        abstract public function getFileSize( array $params );

        /**
         * Get quick information about a file at a storage path in the backend.
         * If the file does not exist, then this returns false.
         * Otherwise, the result is an associative array that includes:
         *     mtime  : the last-modified timestamp (TS_MW)
         *     size   : the file size (bytes)
         * Additional values may be included for internal use only.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return Array|bool|null Returns null on failure
         */
        abstract public function getFileStat( array $params );

        /**
         * Get a SHA-1 hash of the file at a storage path in the backend.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return string|bool Hash string or false on failure
         */
        abstract public function getFileSha1Base36( array $params );

        /**
         * Get the properties of the file at a storage path in the backend.
         * Returns FSFile::placeholderProps() on failure.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return Array
         */
        abstract public function getFileProps( array $params );

        /**
         * Stream the file at a storage path in the backend.
         * If the file does not exists, a 404 error will be given.
         * Appropriate HTTP headers (Status, Content-Type, Content-Length)
         * must be sent if streaming began, while none should be sent otherwise.
         * Implementations should flush the output buffer before sending data.
         *
         * $params include:
         *     src     : source storage path
         *     headers : additional HTTP headers to send on success
         *     latest  : use the latest available data
         *
         * @param $params Array
         * @return Status
         */
        abstract public function streamFile( array $params );

        /**
         * Returns a file system file, identical to the file at a storage path.
         * The file returned is either:
         * a) A local copy of the file at a storage path in the backend.
         *    The temporary copy will have the same extension as the source.
         * b) An original of the file at a storage path in the backend.
         * Temporary files may be purged when the file object falls out of scope.
         *
         * Write operations should *never* be done on this file as some backends
         * may do internal tracking or may be instances of FileBackendMultiWrite.
         * In that later case, there are copies of the file that must stay in sync.
         * Additionally, further calls to this function may return the same file.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return FSFile|null Returns null on failure
         */
        abstract public function getLocalReference( array $params );

        /**
         * Get a local copy on disk of the file at a storage path in the backend.
         * The temporary copy will have the same file extension as the source.
         * Temporary files may be purged when the file object falls out of scope.
         *
         * $params include:
         *     src    : source storage path
         *     latest : use the latest available data
         *
         * @param $params Array
         * @return TempFSFile|null Returns null on failure
         */
        abstract public function getLocalCopy( array $params );

        /**
         * Check if a directory exists at a given storage path.
         * Backends using key/value stores will check if the path is a
         * virtual directory, meaning there are files under the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * $params include:
         *     dir : storage directory
         *
         * @param $params array
         * @return bool|null Returns null on failure
         * @since 1.20
         */
        abstract public function directoryExists( array $params );

        /**
         * Get an iterator to list *all* directories under a storage directory.
         * If the directory is of the form "mwstore://backend/container",
         * then all directories in the container should be listed.
         * If the directory is of form "mwstore://backend/container/dir",
         * then all directories directly under that directory should be listed.
         * Results should be storage directories relative to the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * $params include:
         *     dir     : storage directory
         *     topOnly : only return direct child dirs of the directory
         *
         * @param $params array
         * @return Traversable|Array|null Returns null on failure
         * @since 1.20
         */
        abstract public function getDirectoryList( array $params );

        /**
         * Same as FileBackend::getDirectoryList() except only lists
         * directories that are immediately under the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * $params include:
         *     dir : storage directory
         *
         * @param $params array
         * @return Traversable|Array|null Returns null on failure
         * @since 1.20
         */
        final public function getTopDirectoryList( array $params ) {
                return $this->getDirectoryList( array( 'topOnly' => true ) + $params );
        }

        /**
         * Get an iterator to list *all* stored files under a storage directory.
         * If the directory is of the form "mwstore://backend/container",
         * then all files in the container should be listed.
         * If the directory is of form "mwstore://backend/container/dir",
         * then all files under that directory should be listed.
         * Results should be storage paths relative to the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * $params include:
         *     dir     : storage directory
         *     topOnly : only return direct child files of the directory (@since 1.20)
         *
         * @param $params array
         * @return Traversable|Array|null Returns null on failure
         */
        abstract public function getFileList( array $params );

        /**
         * Same as FileBackend::getFileList() except only lists
         * files that are immediately under the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * $params include:
         *     dir : storage directory
         *
         * @param $params array
         * @return Traversable|Array|null Returns null on failure
         * @since 1.20
         */
        final public function getTopFileList( array $params ) {
                return $this->getFileList( array( 'topOnly' => true ) + $params );
        }

        /**
         * Invalidate any in-process file existence and property cache.
         * If $paths is given, then only the cache for those files will be cleared.
         *
         * @param $paths Array Storage paths (optional)
         * @return void
         */
        public function clearCache( array $paths = null ) {}

        /**
         * Lock the files at the given storage paths in the backend.
         * This will either lock all the files or none (on failure).
         *
         * Callers should consider using getScopedFileLocks() instead.
         *
         * @param $paths Array Storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @return Status
         */
        final public function lockFiles( array $paths, $type ) {
                return $this->lockManager->lock( $paths, $type );
        }

        /**
         * Unlock the files at the given storage paths in the backend.
         *
         * @param $paths Array Storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @return Status
         */
        final public function unlockFiles( array $paths, $type ) {
                return $this->lockManager->unlock( $paths, $type );
        }

        /**
         * Lock the files at the given storage paths in the backend.
         * This will either lock all the files or none (on failure).
         * On failure, the status object will be updated with errors.
         *
         * Once the return value goes out scope, the locks will be released and
         * the status updated. Unlock fatals will not change the status "OK" value.
         *
         * @param $paths Array Storage paths
         * @param $type integer LockManager::LOCK_* constant
         * @param $status Status Status to update on lock/unlock
         * @return ScopedLock|null Returns null on failure
         */
        final public function getScopedFileLocks( array $paths, $type, Status $status ) {
                return ScopedLock::factory( $this->lockManager, $paths, $type, $status );
        }

        /**
         * Get an array of scoped locks needed for a batch of file operations.
         *
         * Normally, FileBackend::doOperations() handles locking, unless
         * the 'nonLocking' param is passed in. This function is useful if you
         * want the files to be locked for a broader scope than just when the
         * files are changing. For example, if you need to update DB metadata,
         * you may want to keep the files locked until finished.
         *
         * @see FileBackend::doOperations()
         *
         * @param $ops Array List of file operations to FileBackend::doOperations()
         * @param $status Status Status to update on lock/unlock
         * @return Array List of ScopedFileLocks or null values
         * @since 1.20
         */
        abstract public function getScopedLocksForOps( array $ops, Status $status );

        /**
         * Get the root storage path of this backend.
         * All container paths are "subdirectories" of this path.
         *
         * @return string Storage path
         * @since 1.20
         */
        final public function getRootStoragePath() {
                return "mwstore://{$this->name}";
        }

        /**
         * Get the file journal object for this backend
         *
         * @return FileJournal
         */
        final public function getJournal() {
                return $this->fileJournal;
        }

        /**
         * Check if a given path is a "mwstore://" path.
         * This does not do any further validation or any existence checks.
         *
         * @param $path string
         * @return bool
         */
        final public static function isStoragePath( $path ) {
                return ( strpos( $path, 'mwstore://' ) === 0 );
        }

        /**
         * Split a storage path into a backend name, a container name,
         * and a relative file path. The relative path may be the empty string.
         * This does not do any path normalization or traversal checks.
         *
         * @param $storagePath string
         * @return Array (backend, container, rel object) or (null, null, null)
         */
        final public static function splitStoragePath( $storagePath ) {
                if ( self::isStoragePath( $storagePath ) ) {
                        // Remove the "mwstore://" prefix and split the path
                        $parts = explode( '/', substr( $storagePath, 10 ), 3 );
                        if ( count( $parts ) >= 2 && $parts[0] != '' && $parts[1] != '' ) {
                                if ( count( $parts ) == 3 ) {
                                        return $parts; // e.g. "backend/container/path"
                                } else {
                                        return array( $parts[0], $parts[1], '' ); // e.g. "backend/container"
                                }
                        }
                }
                return array( null, null, null );
        }

        /**
         * Normalize a storage path by cleaning up directory separators.
         * Returns null if the path is not of the format of a valid storage path.
         *
         * @param $storagePath string
         * @return string|null
         */
        final public static function normalizeStoragePath( $storagePath ) {
                list( $backend, $container, $relPath ) = self::splitStoragePath( $storagePath );
                if ( $relPath !== null ) { // must be for this backend
                        $relPath = self::normalizeContainerPath( $relPath );
                        if ( $relPath !== null ) {
                                return ( $relPath != '' )
                                        ? "mwstore://{$backend}/{$container}/{$relPath}"
                                        : "mwstore://{$backend}/{$container}";
                        }
                }
                return null;
        }

        /**
         * Get the parent storage directory of a storage path.
         * This returns a path like "mwstore://backend/container",
         * "mwstore://backend/container/...", or null if there is no parent.
         *
         * @param $storagePath string
         * @return string|null
         */
        final public static function parentStoragePath( $storagePath ) {
                $storagePath = dirname( $storagePath );
                list( $b, $cont, $rel ) = self::splitStoragePath( $storagePath );
                return ( $rel === null ) ? null : $storagePath;
        }

        /**
         * Get the final extension from a storage or FS path
         *
         * @param $path string
         * @return string
         */
        final public static function extensionFromPath( $path ) {
                $i = strrpos( $path, '.' );
                return strtolower( $i ? substr( $path, $i + 1 ) : '' );
        }

        /**
         * Check if a relative path has no directory traversals
         *
         * @param $path string
         * @return bool
         * @since 1.20
         */
        final public static function isPathTraversalFree( $path ) {
                return ( self::normalizeContainerPath( $path ) !== null );
        }

        /**
         * Validate and normalize a relative storage path.
         * Null is returned if the path involves directory traversal.
         * Traversal is insecure for FS backends and broken for others.
         *
         * This uses the same traversal protection as Title::secureAndSplit().
         *
         * @param $path string Storage path relative to a container
         * @return string|null
         */
        final protected static function normalizeContainerPath( $path ) {
                // Normalize directory separators
                $path = strtr( $path, '\\', '/' );
                // Collapse any consecutive directory separators
                $path = preg_replace( '![/]{2,}!', '/', $path );
                // Remove any leading directory separator
                $path = ltrim( $path, '/' );
                // Use the same traversal protection as Title::secureAndSplit()
                if ( strpos( $path, '.' ) !== false ) {
                        if (
                                $path === '.' ||
                                $path === '..' ||
                                strpos( $path, './' ) === 0 ||
                                strpos( $path, '../' ) === 0 ||
                                strpos( $path, '/./' ) !== false ||
                                strpos( $path, '/../' ) !== false
                        ) {
                                return null;
                        }
                }
                return $path;
        }
}